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MIKE user manualYuri Beletsky <beletsky@lco.cl>

Revision HistoryRevision 1.0 2015-01-01 YB

Table of Contents1. General overview .................................................................................................. 22. Optical design ....................................................................................................... 3

2.1. Dichroic ....................................................................................................... 32.2. Cameras ...................................................................................................... 52.3. Injection optics ............................................................................................ 52.4. Prisms and Gratings ................................................................................... 62.5. Bonding triplets and prism .......................................................................... 72.6. Glass fabrication and coatings .................................................................... 82.7. Mechanical structure ................................................................................... 8

3. Overview .............................................................................................................. 114. Basic information ................................................................................................. 125. Resolution and binning ........................................................................................ 126. CCD detectors ..................................................................................................... 137. Efficiency and Exposure Times ........................................................................... 148. MIKE calibration unit ........................................................................................... 16

8.1. Internal comparison source ....................................................................... 168.2. Internal incandescent lamp ....................................................................... 168.3. External comparison source ...................................................................... 17

9. Spectrograph efficiency ....................................................................................... 1710. Scattered light ................................................................................................... 1711. Observing .......................................................................................................... 18

11.1. Preparing observing catalogue ............................................................... 1811.2. MIKE data acquisition GUI ...................................................................... 1911.3. Slit selection ............................................................................................ 2111.4. Atmospheric Dispersion .......................................................................... 2411.5. Data calibration ....................................................................................... 25

12. Data Reduction Software .................................................................................. 28

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1. General overview

General goals for the design of MIKE were to produce a high efficiency, high-resolution spectrograph with complete optical wavelength coverage (320nm – 1µm)while minimizing complexity, size, and cost. Given these goals, there are severaladvantages to the double-beam arrangement. In addition to the obvious efficiencyprovided by taking two spectra simultaneously, the red/blue wavelength split allowsthe parallel channels to be optimized for higher throughput and optimal dispersion inseveral ways. First, we have used prisms rather than gratings for cross-dispersionbecause of the higher throughput they provide; dual beams allows us to use a highlydispersive prism on the red side, which would be opaque below 400nm. On the blueside, fused silica prisms can be used with negligible losses. Second, the dual–beamconfiguration allows us to use gratings with different rulings on the red and blue sides,minimizing the variation in the linear free spectral range and optimizing the blazeangle for each side. Finally, the limited red and blue spectral ranges gives us bettercontrol over the image quality and chromatic aberration in the refracting cameras, andbetter efficiency in anti-reflection coatings and detectors. The double–pass systemalso provides several cost, complexity, and size advantages. It allows the instrumentto be relatively compact and minimizes the number of large customized optics thatneed to be fabricated, not to mention bonded and mounted. In addition, the dualpass configuration minimizes the number of moving parts by limiting the possibleadjustments to only camera focal position, grating angles (red and blue independently),and slit geometry. As the entire echelle format on each side fits on a single 2k x 4k CCD,adjustments to the grating positions should be infrequent. The historical antecedents tothe optical layout of MIKE are the echelle spectrographs at the 2.1m Struve reflector atMcDonald observatory (see McCarthy et al.) and at the Las Campanas 2.5 m du PontTelescope (Shectman)) both of which incorporate prisms and cameras in a double-passarrangement. Both of these instruments, however, use simpler cameras and injectionoptics to convert the beam from the telescope focal ratio to the proper focal ratio forcollimation through the main (camera) optics. MIKE also includes an astatic supportto minimize flexure that is based on a cantilevered counterweight design used on theMcDonald echelle. The designs for the MIKE cameras started from a doublet-triplet-doublet arrangement similar to the camera and collimator designs for COSMIC (seeKells et al). All lens surfaces in MIKE are spherical. The optical layout of the instrumentis illustrated in Figure 1. The red and blue optical paths are traced individually for clarityand displayed at the same scale. The only optical elements common to the two opticalpaths are the slit and dichroic. Photographs of the assembled instrument are shownin Figure 1.

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2. Optical design

2.1. Dichroic

The first optical element in MIKE beyond the slit is a 4mm thick glass window witha dichroic coating on the first surface that reflects blue light and transmits red.The crossover wavelength is at 455nm with 80% reflectance by 440nm and 80%transmission by 480nm. The crossover wavelength was chosen to allow similar opticalquality in both cameras, dictated by dn/dλ of standard optical glasses, and also tominimize the number of astronomically interesting spectral features that would fall nearthe dichroic split. With the dichroic in place, spectra can be taken with the red andblue channels simultaneously. However, it is also possible to use the red or blue side

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alone by replacing the dichroic with a mirror or a clear glass window. The dichroic ispositioned in the optical path so that the incidence angle of the beam is 30 degrees fromnormal. Angles less than 45 degrees are preferable to minimize polarization effectsand to keep the crossover interval to a minimum. The back surface of the dichroic slideis AR coated. The ideal AR coating would be optimized to work best in the crossoverregion, work well into the red, and work poorly (or not at all) in the blue. The logic forthis is that in the narrow crossover range, some light (say 50%) will get through thedichroic and <5% of that could unfortunately be reflected back towards the dichroic-coated surface from the AR coated back side. This light can then be reflected againby the dichroic, and a small ghost image will appear in the red-side spectra at the farblue range of its operation. Likewise, ghosts will appear on the blue side as well. Thecurrent dichroic has a uniform AR coating, and the described effect is visible in spectrawe have taken of the sun and comparison sources in the lab. The dichroic is trivial toreplace and other dichroics will be available shortly.

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2.2. Cameras

The red camera is F/3.3 and consists of a singlet-triplet-singlet lens with one calciumfluoride element in the center of the triplet. The remaining elements are standard Oharaglasses. The blue camera is F/3.6 and consists of a singlet- triplet-triplet lens withcalcium fluoride in the center of both triplets. The remaining elements are all Oharai-line glasses with good transmission at 330nm. The prescription for both camerasincludes a field flattener as the dewar window and sits 10mm in front of the CCD. Allcamera lenses are smaller than 220mm with no vignetting. Both cameras have a 150mm diameter pupil and were designed with an external stop at roughly the spacing tothe center of the echelle grating (400mm on the red side, 600mm on the blue side).Ray traces through the cameras are shown in Figures 1. The total transmission throughthe red side should be roughly 60-70% based on reported glass, RTV, and AR coatingefficiencies, and grating reflectivity. The total transmission on the blue side is predictedto be 73-90% at wavelengths longer than 340nm, falling to 40% and then 5% at 330nmand 320nm, respectively. Both lenses were optimized over a field 35mm in radius. Themerit function was defined to constrain rms spot sizes every 100nm over each band-pass but it allows lateral color as cross dispersion makes this irrelevant to the cameraperformance. The spot diagrams in Figure 5 illustrate the final image quality (includinginjection optics) for both optical trains. The average rms spot radius for the camerasalone (once through) is 2.8μm for the red and 2.9μm for the blue. The average rmsspot radius through the entire optical system is 5.2μm on the red side, and 5.9μm onthe blue side.

2.3. Injection optics

The conversion from F/11 (telescope output) to F/3.3 and F/3.6 for the camera/collimators is accomplished using a set of small injection optics, which immediatelyfollow the dichroic in the red and blue optical paths. Again, this allows the two sets ofinjection optics to be optimized in transmission and image quality for the red and bluewavelength ranges. The injection optics on both sides consists of a field lens and atriplet with negative power. As in the cameras, the central element of both triplets iscalcium fluoride. The design of the injection optics was constrained significantly by thepackaging of the instrument. On the blue side, packaging was especially difficult, asthe dichroic reflects the beam back towards the mounting flange and limits the spaceallowed for the blue camera. For this reason, we elected to cross the ingoing andoutgoing beams as shown in Figure 1, which gives us roughly an extra 30mm of pathlength between the dichroic and the blue camera. Special care was taken to eliminatepossible reflections back towards the CCD off the injection triplet by tilting and offsetting

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it to constraining the angle of the incident (and possibly reflected) rays. The data wehave taken in the lab shows that we did successfully eliminate ghosts. The injectionoptics essentially form a virtual image of the slit in the focal plane of each camera andsubstitute for the field flattener (dewar window) in the first pass through the cameras.The triplet and a flat mirror occupy less than half of the focal plane of the each camera,as can be seen in Figures 1 and 4. The optical axes of the cameras are offset by about30mm from the optical axes of the injection optics. Light goes through the red injectiontriplet relatively on-axis, while the chief ray goes through the blue injection triplet roughly6mm off-axis. The alignment of the blue side is therefore slightly more sensitive thanthe red side.

2.4. Prisms and Gratings

MIKE uses one replicated echelle grating from Richardson Grating Lab on each side.The red side uses a 6x12 inch R2 grating with 52.6 gr/mm and a 63.5 deg blaze angle.With our 500 mm focal length camera, the free spectral range of the reddest order(number 31, centered at 10977A) is 66 mm long, which does not quite fit on a 2k x4k x15μm CCD. However, coverage is complete blueward of 10470A (order 33). Thegrating can be positioned to obtain, for example, orders 31 to 69 (10977-4931A) ororders 39 to 73 (8725-4661A). Cross-dispersion is provided by a single prism madeof Ohara PBM2 glass with an apex angle of 47 deg. The blue side uses a 6x16 inchR2.6 grating with 52.7 gr/mm and 69.0 deg blaze. With the 550mm focal length camera,the reddest order (number 74 at 4790A) is 39 mm long and fits easily on the blue side2k x 4k x15μm CCD. Two fused silica prisms with 38 deg apex angles provide theequivalent of a single 62 deg apex angle prism on this side. The spacing between orders74 to 107 (central wavelength 3313A) is 29mm, which just fits onto the CCD (~31mmwide). The blue camera will transmit and form good images down to at least order110 (3222A). These bluest wavelengths can be obtained if the grating is appropriatelypositioned. The echelle format for the two sides is shown in Figure 6. Minimum orderseparation is 6 arcsec on both the red and blue sides (1arcsec = 105μm on the redside, 115μm on the blue side). Spectra taken in the lab demonstrate that there is verylittle scattered light in the images, so that the spacing between orders is more thanadequate for sky subtraction (see discussion below). Both camera designs form imageswith >95% encircled energy within 1 pixel (15μm), so resolution is limited by slit size.For a minimum slit width of 0.35 arcsec , the maximum resolving power is 54,000 on thered side and 71,000 on the blue side. The resolving power is 19,000 and 25,000, redand blue respectively, with a 1arcsec slit. All three prisms are more than 8 inches thickat the base. In order to obtain the glass for these prisms, we ordered glass blanks ofroughly one half the required thickness and had them fabricated in two halves with one

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surface of each half- prism normal to the cylindrical axis of the blank. We then bondedthe two halves together using optical RTV, as described below.

2.5. Bonding triplets and prism

All triplets and prisms have been bonded with Sylgard 184, which is an optical RTVmade by Dow Corning. We prepared and tested a 10mm sample of this RTV andfound it to have roughly 90% transmission efficiency at 310nm and essentially 100%transmission at longer wavelengths. The RTV bonds in the camera triplets are 0.009inches thick. Plastic shim stock (0.006in) held in place with double-sided scotch tape(0.003in)was used control the RTV thickness at three places around the edge of theelements. Finite element analysis of our triplets indicates that this thickness providesadequate stress relief for thermal changes between elements. To test the adhesionstrength of the glass-RTV bonds, we conducted torture tests on bonded pieces ofstandard window glass. We found no evidence of adhesion failures for temperaturedifferentials of more than 50 deg C between the panes for bond layers 0.009 inches

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thick. The median operating temperature in the Magellan domes is about 10 deg Cwith a range about the median of about 10 deg. The RTV will set in about 2 daysat room temperature, but takes as long as 2 weeks at 15 deg C. As a compromisebetween timely setting and median operating temperature, we bonded all of the tripletsat 15 deg C. A humidity- controlled cold room was built at Carnegie Observatories forthis purpose. The prisms were bonded at room temperature, since differential thermalexpansion is not a problem along a flat joint between glasses of the same type. For theprisms, scotch tape spacers were used to assure that the bond layers were uniformly0.003 inches thick. The triplets in the small injection optics were bonded at 15 deg Cwith 0.003 inch spacers.

2.6. Glass fabrication and coatings

All of our lenses and prisms were figured and polished by Harold Johnson Optical Labs.All lenses were figured to stock test-plate radii and were fabricated to specificationswithout incident. Newport Thin Films provided coatings on four of the red cameraelements. Spectrum Thin Films coated the remaining elements, including the bluecamera, prisms, and all small injection optics. The coatings on the red and blue opticshave average reflectivity less than 1.0% and 0.75%, respectively.

2.7. Mechanical structure

The outer structure of MIKE is made from aluminum tooling plate (3⁄4-inch and 1⁄2-inchthick) with all joints screwed together and keyed to prevent light leaks. The strategywith this approach was to make the structure modular and allow easy changes toindividual plates should the need or desire arise. The red box was fabricated at Rettig

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Machine (Redlands, CA). The rest of the structure was fabricated at Martinez & Turek(Rialto, CA) and by Robert Storts at OCIW. The main box of the structure housestwo independent optical benches (red and blue). These are each mounted on flexuresthat allow the entire optical bench to move ±1mm along the optical axis, providingthe focus adjustment for the cameras. The position of the cameras can be adjustedremotely by rotating an eccentric cam drive with a motorized worm gear mountedunder the optical benches to focus the instrument. The cameras are thermally sensitive,as expected, due to the changes in refractive index with temperature (dn/dT). Tocompensate changes of up to 5 degrees in temperature, the optical benches eachinclude invar plates that are fixed at one end to the structure and at the other end tothe optical benches. These provide partial passive focus compensation via the thermalexpansion of the aluminum. Two separate boxes house dispersion elements: the redbox contains the single red prism and the red R2 grating; the blue box houses the twoblue prisms and blue R2.6 grating. Although the gratings and prisms were not intendedto be changed during the life of the spectrograph, the structure is modular enough thatchanges could be made to the red and blue boxes individually if the motivation to do sowere significantly great. All of the small optics are mounted on two small plates about1 ft square (see Figures 2 and 3), which are removable from the main perpendicularflange at the dewar end of the main box. The dewars also mount onto these plates. Theblue plate contains all the optics and mechanics upstream of the dichroic as well as theblue injection optics and the red field lens. It also contains a single electrical connectorthrough which the slit motor drive, shutter controls, and flipper- mounted, comparison-source mirror are controlled. The red plate contains the injection optics for the red side.The small optics can all be easily removed, replaced and adjusted without opening thecover to the main box that houses the cameras. In addition, the field lenses and dichroicare all mounted on a small block (about 5x3 inches), which can be removed withoutremoving the blue dewar or the rest of the blue plate. This module can be swappedout for other modules, which will include copies of the field lenses and will allow thedichroic to be replaced by a window or mirror by the instrument support staff at theobservatory in a few minutes.

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The camera elements are all held in simple kinematic mounts which have two radialdefining pads made of silicone rubber and a spring preloaded floating pad at the topof the mount. Axially, the elements are constrained by a fixed ring on one side and aspring preloaded ring on the other. The mounting plates are fixed to the optical bencheswith vertical gusset plates and are reinforced for lateral stability. Push/pull screws andslotted holes allow enough control for lateral alignment. Height and tilt are adjustedwith shims. Prisms are similarly constrained radially. Tabs are in place to prevent theprisms from falling but provide no axial constraint. All contact to glass is exclusivelythrough silicone pads (60 shore), which are fixed to the aluminum with contact-cementor double-stick tape. The radial constraint on the triplets is applied only to the outerelements; no contact is made directly with the calcium fluoride. All small optics are heldin custom mounts and constrained axially only with nylon- tipped set screws and springplungers. The gratings are also kinematically mounted with spring preloads against

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opposing nylon defining pads. The positions of the gratings are controlled manuallywith altitude and azimuth screws. Dial gauges indicate the position in both directions.All optics (gratings included) can be removed from their cells should the need arise.

3. Overview

MIKE is a double echelle spectrograph. The first optical element in the spectrograph isa dichroic which reflects (transmits) light into the blue (red) arms of the spectrograph.Each side has its own shutter and CCD, so the two sides can be used simultaneouslywith independent exposure times. The basic specifications of each side are listed inthe table below. The parameters which the observer can select are exact wavelengthcoverage and the slit width.

The spectrograph delivers full wavelength coverage from about 3350-5000 (blue) and4900-9500A (red) in its standard configuration. This configuration is set by the crossover wavelength of the dichroic (4950A). As the observer, you can choose to have thegrating angle adjusted to select a different set of orders to go bluer on the blue side(down to 3200A) or redder on the red side (up to 10,000A). But it is not possible to usea different dichroic, gratings, or prisms. These elements are permanent.

Because MIKE uses prisms for cross dispersion, the separation between orderschanges as a function of wavelength. The smallest separation is about 6". The longestslit that can be used for full wavelength coverage is therefore 5" long. Several differentslit widths are available, as discussed below. Choose a slit width that gives you theresolution you want.

A small blue box in the control room allows the observer to remotely control internallamps (ThAr and incandescent) , the slit position, and the focus of the cameras. Thefirst time user should let the Instrument Specialist focus the spectrograph.

The CCDs and their electronics were provided and are maintained by Ian Thompsonand Greg Burley. All information related to upgrades, linearity, and detailed behaviorhas been provided by Ian Thompson.

MIKE also has a fiber system which was built by Mario Mateo (UM) and Alex Athey(UM, OCIW). The fiber-fed mode is not yet available for use by guest observers withoutexplicit permission and assistance from Mateo.

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4. Basic information

Table 1. An example table

BLUE arm RED Arm

effective focal ratio f/3.9 f/3.6

CCD pixel scale, "/pix 0.12 0.13

A/pixel (unbinned) 0.02 0.05

Detector 2048x4096 (15µm pix) 2048x4096 (15µm pix)

Gain, e-/DN 0.47 1.00

Readnoise, e-/pix 2 3.5

Dark current, DN/pix/hour 5 2

CCD efficiency

Wavelength range,A 3200-5000 4900-100000

Resolution, (0.35" slit) 830000 650000

Resolution, (1.00" slit) 280000 220000

Echelle grating R2.4 R2

Prism (cross-disperser) Fused Silica(2prisms) PBM2 (1 prism)

5. Resolution and binning

The cameras make very good images and can easily resolve a 0.35" slit (the smallestslit available). That means that the number of pixels per resolution element of yourspectra will be limited only by the slit width you use (smaller slits mean higher resolution,linearly related). If you are observing very faint objects, it pays to bin the data in thespectral and spatial directions.

The scale of the CCD detectors is about 8.2 pixels/" (BLUE arm) and 7.5 pixels/" (REDarm). So if you are using a slit which is wider than 0.5 arcseconds there is not muchpoint in taking unbinned data. In other words, you should bin by in the spectral (y)direction so that slit width is Nyquist sampled. In addition to significant gains in readouttime, your data will be less effected by readnoise, which does not increase significantlywith binning. It is very common to use MIKE binned 2x2 or 3x2 on both sides.

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6. CCD detectors

Each MIKE CCD detector system uses a 2K x 4K back-illuminated device, with 15 umpixels and two output amplifiers. At present, the red channel uses a deep depletion E2VCCD44-82 device with a red-enhanced anti-reflection coating, while the blue channeluses a MIT Lincoln Labs CCID-20 with a blue enhanced coating. Each detector ismounted in a rectangular aluminum housing. The dewar window on each side is thefield flattener of the camera. The CCD housing is coupled to an IR-Labs ND-5 cryostat,with the CCD control electronics box attached to the side of the dewar (see photo).Each CCD is liquid nitrogen cooled and has roughly 15 hour hold time in the horizontalposition. Inside the housing, the CCD is connected to a rigid-flex preamp board, whichbrings the signals out on a flex cable to a hermetic connector. The two detector systemsare mounted side-by-side on the instrument (see photo) and operate independently.That is, they have independent shutters, exposure times and readout times

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CCD linearity. Data taken with both the red and blue sides should belinear up to the digital saturation of the A/D converters.

7. Efficiency and Exposure Times

The plot below shows the source flux (in AB mag) from which we detect 1 e-/sec/A. Theslit-to-detector efficiency is roughly 37% (~4500) on the blue side and 20% (~6500A)on the red side. (Measurement and calibration courtesy of Scott Burles and KristinBurgess, MIT.)

This plot is based on data taken through a 2" slit in 0.7" seeing, so not much light waslost at the slit. If you use a slit width which is matched to the seeing (1" slit in 1" seeing),you should expect to lose about 25% of the light at the slit.

This plot is all you need to estimate the exposure times for your program. To be clear,here is an example of how to use this plot:

Flux of target star: V=15 mag

Desired SNR: 50 per pixel (i.e. ~ 2500 e-/pixel)

# Angstroms/pixel: 0.05 A/pixel red side, w/o binning in spectral

direction (y)

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seeing, slit, airmass: seeing=0.8" , slit=0.7" (% into slit relative

to plot ~ 0.7), airmass=1.0

count rate ~5500A: 1 e-/sec/A * 0.05 A/pixel * 10^(0.4*(18.4-15))

* 0.7 = 0.77 e-/pixel/sec

required exp. time: 2500 e-/pixel / 0.77 e-/pixel/sec = 3250

sec.

Readnoise per pixel is small (4-5 DN/pixel), so this simple estimate is approximatelycorrect. Remember that this calculation has to do with the integrated flux over the spatialextent of the source (i.e. peak flux in the spatial direction near 1000 could correspondto a total flux of 2500 DN integrated over all pixels at the same wavelength once youextract the spectrum). Also remember that this calculation assumed 0.8" seeing andan airmass is near 1. You should adjust accordingly if you have greater slit loses (badseeing) or are at high airmass.

If you are observing faint objects that are going to give you a relatively small number ofcounts per pixel in your chosen exposure times, then readnoise will obviously be moreimportant. In this case, you should definitely opt for on-chip binning. As we discussbelow, the scale of the CCD detectors is 8.2 pix/" (blue) and 7.5 pix/" (red). So if youare using a slit which is wider than 0.5", you should bin to minimize amplifier noise.For example:

Flux of target star: V=18 mag

Desired SNR: 20 per pixel (i.e. ~ 400 e-/pixel)

# Angstroms/pixel: 0.1 A/pixel red side, w/ x2 binning in

spectral direction (y)

seeing, slit, airmass: seeing=0.6", slit=0.7" (% into slit relative

to plot ~ 95%), airmass=1

count rate ~5500A: 1 e-/sec/A * 0.1 A/pixel * 10^(0.4*(18.4-18)) *

0.95 = 0.14 e-/pixel/sec

required exp. time: 400 e-/pixel / 0.14 e-/pixel/sec = 46 min.

CCD Linearity: Data taken with both the red and blue sides should be linear up tothe digital saturation of the A/D converters, with the exception of blue-side data takenbetween May 2004 and Sept 20, 2005 (using Lincoln Labs CCD, installed May 2004).

Until Sept 20, 2005, two linearity problems affected the Lincoln Labs CCD thatwas installed in May 2004. These problems were measured and reported by Ian

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Thompson. First, for reasons apparently having to do with the electronics, individualpixels appeared to saturate at about 8,000 DN. For all unbinned observations (1x1readout), saturation set in around 8,000 DN. A discussion of this problem (the full-welllimit) was posted by Dave Osip to the "News & Forums" page (copied here). Second,the amplifiers appear to saturate at about 16,000 DN. For all binned observations,saturation appeared to set in at around 16,000 DN. A plot showing the effect of theamplifier saturation is available here (plot, Ian Thompson). As of Sept 2005, Ian reportsthat both of these problems have been fixed.

8. MIKE calibration unit

8.1. Internal comparison source

MIKE has an internal Thorium Argon lamp. The vast majority of the time, and certainlyfor the standard set-up, this is what you want to use for arcs. It can be turned on and offfrom the remote control box in the data room. The switch labeled "comparison lamp"will both turn on the lamp and move a flipper-mounted mirror into place to direct thelight onto the slit. A one second exposure is okay for the blue side and most of thered side. You will notice that there are strong lines which become very saturated inthe orders around 7500A. If you display the images with the right stretch, you’ll seethat the saturation isn’t as bad as it looks. These regions do not pose a calibrationproblem. However, you may also notice that the reddest 10 orders or so have fairly fewstrong lines. This is a bigger problem. Be sure to look closely at these orders to be sureyou’re getting the counts you need. When binned 2x2, a 1 sec exposure is adequate,but a 5 sec exposure gives you more lines. Wavelength and order maps can be foundon the main MIKE web site at LCO. It is very important not to leave the internal ThArlamp on when it is not needed because it has a lifetime of only a few hundred hoursand external sources will not be bright enough for blue-side calibration. The requiredexposure time is nice and short, so please get into the habit of turning it off promptlywhen the exposure shutters close. It is not possible to accidentally leave the arc lampon during an exposure because the arc light floods the slit viewer with light. It’s hard toignore and it is impossible to place an object on the slit in this state.

8.2. Internal incandescent lamp

There is also an internal flat field lamp which is controlled from the box in the controlroom. You can turn on this lamp simply by flipping the arc-lamp switch down instead ofup. The label of the switch indicates this. This lamp works very well for flats on the red

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side, with or without the diffuser slide (milky flat slide). It is not very bright on the blueside. Methods for taking flats are discussed further below.

8.3. External comparison source

It is also possible to use external calibration lamps for the red side, although it isextremely unusual to use these and probably not necessary. If you want to use themfor some reason (for above 9500A), you need to move the flat field screen into placeusing the corresponding GUI (discussed further below).

9. Spectrograph efficiency

The plot below shows the source flux (in AB mag) from which we detect 1 e-/sec/A. Theslit-to-detector efficiency is roughly 37% (~4500) on the blue side and 20% (~6500A)on the red side. (Measurement and calibration courtesy of Scott Burles and KristinBurgess, MIT.)

This plot is based on data taken through a 2" slit in 0.7" seeing, so not much light waslost at the slit. If you use a slit width which is matched to the seeing (1" slit in 1" seeing),you should expect to lose about 25% of the light at the slit.

10. Scattered light

Scattered light is quite low in MIKE (see the Figures below, for blue- and red- partsaccordingly).

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11. Observing

11.1. Preparing observing catalogue

Observers at Magellan telescopes are highly encouraged to prepare ObservingCatalogues before observing run. The catalogue is a simple text (ASCII) files whichcontains all necessary information for targets acquisition, such as object’s names,coordinates, equinox, etc.. Here is the example of the catalogue file:

Field#1: reference number (can use alphanumeric characters or a decimal

number)

Field#2: object name (can use alphanumeric characters, plus, minus,

period, underscore, no whitespace) Field#3: RA given in hh:mm:ss.s or in

decimal hours

Field#4: Dec given in ±dd:mm:ss or in decimal degrees

Field#5: equinox given in yyyy.y

Field#6: proper motion in RA given in seconds of time per year (s.ss)

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Field#7: proper motion in Dec given in arcseconds per year (s.ss)

Field#8: instrument rotator offset angle given in degrees (dd.d)

Field#9: instrument rotator offset mode

Field#10: RA of guide probe#1 given in hh:mm:ss.s or decimal hours

Field#11: Dec of guide probe#1 given in ±dd:mm:ss or decimal degrees

Field#12: equinox of guide probe#1 positions given in yyyy.y

Field#13: RA of guide probe#2 given in hh:mm:ss.s or decimal hours

Field#14: Dec of guide probe#2 given in ±dd:mm:ss or decimal degrees

Field#15: equinox of guide probe#2 positions given in yyyy.y

Field#16: epoch of proper motion observations given in yyyy.y

All fields are whitespace delimited; tabs will be converted to spaces. You can addadditional information that you desire beyond the required data, but should place acharacter at the beginning of the extra data to mark it as commentary. The catalog maycontain header and commentary lines which do not contain observing targets; theselines should begin with a character. For MIKE instrument rotator offset angle (Field#8)should be set to: 0 , and instrument rotator offset mode (Field#9) should be set to: GRV

11.2. MIKE data acquisition GUI

The data control window is shown below. To start the window, type "mike" in an xtermon the data control computer. An instrument configuration window will appear, alsoshown below. The observer can enter his/her name and then select three cameraconfigurations, blue camera only, read camera only, or both blue and red camera. Thereis an entry for off-line (simulator) operation of the GUI, this is for testing purposes only.Next the observer should select the number of pixels of overscan at the end of eachrow and the number of bias lines at the end of a readout that should be appended to aframe. (128 in x and y is good.) Finally the observer can select the telescope, and againan online or offline operation. If the telescope is online then telescope coordinates andvarious rotator angles are read from the telescope TCS.

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The very top portion of the acquisition window has two pull down menus, the firstcontrols the window status (select Exit GUI to exit the window) the second has an entryfor user name and another to select the path to the data. This path can be changedwithout having to restart the window. (You can set up any directory structure you likeunder ~/DATA/.) These are followed by a bar graph showing the current data diskcapacity, and a display of the current UT.

The top half of the window shows information relevant to BOTH the red and blueside, like telescope coordinates, dome temperatures, airmass, etc. Exposures for

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simultaneous operation of both cameras with all exposure parameters identical arecontrolled by the Exptime, Start, Pause, Abort, and Loop buttons in the upper rightportion of the top half of the GUI. The temperatures of the CCD’s are given in the tophalf of the window. The current set points are -125 C, and these numbers will turn redif the temperature rises above the set point by more than 5 C.

Entries of numerical values for image number and exposure time will cause thebackground in these windows to turn red, the values are entered into the programwhen you hit the return key. Other character fields (white boxes for comments, settingsfor camera focus values, grating angles, slit-size) can be simply edited. Entries for Xbinning, Y binning, full or subrastered readout, and speed are controlled with pop upmenus, indicated by the presence of a small circle in the right side of a menu. Tworeadout speeds are available, a fast, high-noise mode (full frame readout 95 seconds,readnoise approximately 4.7 electrons), and a slow, low-noise mode (full frame readout160 seconds, readnoise approximately 3.7 electrons). Note that the read times in allcases will be significantly shorter when a camera readout is binned. When exposuresare running, the start buttons turn yellow in the exposing cameras. Start/Pause/Abortbuttons are beige when those functions are available.

Cameras can also be operated together using the top half of the window or completelyindependently with the control buttons in the lower left (blue) and lower right (red)portion of the window. Exposure times can be different, exposures can be startedasynchronously, and the number of exposures in a loop can be different. Note alsothat exposure time can be changed during an exposure! Just move the cursor over the"Exp. Time" box in the top half of the window, type a new number, and hit return. Thisis the easiest way to abort an exposures. You can abort the readout also, if you like,using the abort button.

There are two message windows at the bottom of the GUI. These give the status of anindividual readout, both the elapsed read time and the number of lines read and thetotal number of lines to be read.

The data taking system is pretty robust at this point. We don’t know of anything youshould particularly avoid doing. If you do find a way to break the GUI, please tell theTelescope Operator and make sure to include it in the Observer’s Report.

11.3. Slit selection

MIKE has a polished reflecting slit plate with a variety of slits machined into it. Behindthe slit plate is a fixed plate which blocks the light from all of the slits except the one

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which is positioned in front of a single hole at the center of the field. The slit plate ismounted on a motorized stage so that any of the slits can be positioned in front of thesingle hole in the blocking plate. The telescope operator will know where the blockingplate is located on the slit viewing camera. He will mark on the slit viewer the positionwhere the desired slit should be located for observing and will help you position theslit. The pixel scale on the slit viewing camera is 0.067" per pixel, so you can see theposition of the slit with good resolution.

The easiest way to select a slit during the day is by using the internal flat field lampto illuminate the slit plate. Because this is an internal lamp, the state of the telescopeis irrelevant (i.e. the mirror covers can be closed). The lamp will flood the slit viewingcamera with very bright light, and so it will need to be adjusted for large dynamic range(using the "span" command on the slit viewing camera). It is basically not possible to dothis without a little experience, so you should ask the Instrument Specialist or TelescopeOperator for help the selecting the slit you want to use during the afternoon at the startof your run. Once you can see the slits in the slit viewer, just use the switches on thecontrol box to move the slit plate to the right or left and select a slit. The switches arewell labeled. There is a handy knob for controlling the speed of the slit motion.

There is no encoder to identify the exact position of the slit plate. We are working on this,however it is relatively easy to move the slit and return it to within 1 pixel in the dispersiondirection just by eye in the slit viewing camera (see below). Because standard arcs tellyou the wavelength position of the spectra, it is not a problem to move the slit betweenexposures or use multiple slits during a run. Arcs should be taken every 30-60 minutes,anyway, as discussed below.

This is what you will see in the slit viewer (left) and a schematic drawing of the slitviewer plate (right):

Figure 1. slit viewer

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Figure 2. schematic drawing

Available slits from left to right on the slit viewer screen are as

follows:

Aperture Pairs (separation 3"):

1 0.35 x 0.35 (for focusing)

2 1.00 x 0.35

3 1.00 x 0.50

4 1.00 x 0.70

5 1.00 x 1.00

6 1.50 x 0.35

7 1.50 x 0.50

8 1.50 x 0.70

9 1.50 x 1.00

10 1.50 x 1.50

11 2.00 x 0.35

12 2.00 x 0.50

13 2.00 x 0.70

14 2.00 x 1.00

15 2.00 x 1.50

16 2.00 x 2.00

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Single Slits:

17 0.35 x 5.00

18 0.50 x 5.00

19 0.70 x 5.00

20 1.00 x 5.00

21 1.50 x 5.00

22 2.00 x 5.00

Most of the available slits are in fact pairs of apertures. You can use these to observein "A-B" mode, in which you would obtain one exposure with the star in the top aperturefollowed by a second exposure with the star in the bottom aperture. One aperture isthen "object" and one is "sky" in the "A" exposure, and swapped in the "B" exposure.An easily-identified spatial portion of the slit is dedicated to sky and the object this way.This lets you bin the data very aggressively in the cross-dispersion (x) direction; youcan bin by 4 with the 2 arcsecond apertures. Binning is limited by the 1" gap betweenthe slits. Because the 1.5"-long aperture pairs are 1.5" apart, you can bin be largervalues with these apertures. A-B mode is especially useful when observing very faintobjects because it is possible to locate and sky subtract very faint objects this way.

MIKE does not currently have an atmospheric dispersioncorrector. (We are in the process of adding one.) So if you areobserving at airmasses higher than 1.2, you will have some light atblue wavelengths dispersed out of the short apertures. Also, if youare working an a crowded field, it may not be possible to use thepaired apertures and always get a clean measurement of sky.

11.4. Atmospheric Dispersion

MIKE was designed to be mounted at the Nasmyth or auxiliary port. We expect that thepreferred observing strategy will be to use MIKE in a gravity invariant mode in whichit is mounted on the East Nasmyth platform and does not rotate with the instrumentrotator. This is the only mode available now. The virtue of this mode is obviously thestability of the instrument. The down side is that the slit orientation on the sky cannotbe changed. The only potential difficulty this poses for point sources has to do withatmospheric dispersion away from zenith. For that reason, we have positioned MIKEat a 30 angle to the platform. At this angle, atmospheric dispersion lies exactly alongthe slit when observing at a zenith angle of 30 deg.

The figure below shows the atmospheric dispersion (3500-9000A) across and along theslit as a function of zenith angle for MIKE in its standard position (tilted by 30 degrees)

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on the Nasmyth platform. At a zenith angle of 30 degrees, the dispersion is entirelyalong the slit with a magnitude of about 1.1 arcsec. The dispersion across the slit issmall (<0.3 arcsec) for all zenith angles less than 40 deg. At higher zenith angles (>50deg), atmospheric dispersion across the slit is significant. With the 5"–long slits, mostof the light should get into the slit at zenith angles less than 60 deg. If you are using oneof the aperture pairs (2"–long slits or shorter) and are interested in the full wavelengthrange, then it is probably better to avoid zenith angles greater than about 40 deg.

11.5. Data calibration

Bias:

Occasional jumps in the bias level occur but can be subtracted out nicely by using anaverage of the overscan region at the end of each row (x=2048:2176,y=*, unbinned).The highly repeatable bias ramp which occurs at the beginning of each row can beeasily subtracted using the overscan at the end of each column (x=*, y=4096:4224,unbinned). Using these overscan regions is more effective than an using a combinedbias frame. For that reason, 0-second exposures (bias frames) are not very useful andare not recommended.

Dark:

The new Lincoln Labs blue CCD (installed May 2004) has some dark current (~5 DN/hr, depending on running temperature) which will add noise to the exposure but doesnot appear to have significant 2-d structure over the CCD. Check the dark level during

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your run and keep this in mind when estimating the signal to noise ratio of your finalexposure.

Flats:

It is a very good idea to take flats with the diffuser for standard pixel-to-pixel flat fieldingcalibration. You also want to take some flats (either internal lamp or sky flats 0-15 minbefore sunset) without the diffuser. These can be used for an illumination correctionand to trace the edges of the orders. Take an arc at the same time to provide an easyway of identifying the offset between the order position in the flat and the order positionin each program exposure. (The orders can move by fractions of a pixel in both thedispersion direction and the cross-dispersion direction due to temperature changes inthe spectrograph.)

Because this is an echelle, different colors of light are imaged on the slit in differentareas. You want a flat that changes color with position. You wouldn’t want to take a flatwithout the slit in place. But you do want to fill the CCD with light (even between orders!)in order to get a good flat field image for pixel-to-pixel sensitivity corrections. To do this,a diffusing glass slide can be positioned in the optical path just downstream of the slitfor taking "milky flats." This "milky flat" slide will blur the slit image into a roughly 40-50pixel smudge. A 40-50 pixel smudge is a good size because it means that there is notmuch light shared between orders in the flat, but the gaps are well illuminated. This letsyou flat field at the edges of the orders.

The diffusing slide is mounted on a sliding post and can be positioned in or out of theoptical beam manually. The post sticks through the blue adapter plate below and leftof the blue dewar (between the blue and red dewars) and has a brass knob on theend. When the knob is pushed in (flush with the adapter plate), the diffuser is out of thebeam. When the slide is pulled out, the diffuser is in position for milky flats. There is adetent at each position. Push and pull firmly and slowly. The brass knob is well labeled.

To take good flats, you need a light source which supplies a lot of photons and doesnot have strong spectral features. The internal incandescent lamp is fine on the redside of MIKE, but it does not supply much light for the blue side. For that reason, weSTRONGLY suggest that you take some milky flats during twilight using an O or B star.We have found that a star with V=4-5 mag gives very good flats in about 25 secondsper exposure. It is a good idea to take 1/2 of the flats with the star at one end of theslit, and 1/2 with the star at the other end of the slit. That way, the inter-order regionis better illuminated than if you only position the star in the center of the slit. The beststars for this purpose are stars with high rotational velocities — high v*sin(i). We have

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left a catalog of such stars with the Telescope Operators ("high_vsini_obscat.cat"). Thestars on this list have high enough Doppler shifts that all spectral features are verybroad and can easily be median smoothed out with a 1x31 kernel to produce a verynice flat field image. It is relatively easy to take 10 exposures in twilight per night. Theycan be combined over a run if you have multiple nights. We have found the flats tobe pretty stable.

Wavelength calibration:

MIKE has an internal Thorium Argon lamp. The vast majority of the time, and certainlyfor the standard set-up, this is what you want to use for arcs. It can be turned on and offfrom the remote control box in the data room. The switch labeled "comparison lamp"will both turn on the lamp and move a flipper-mounted mirror into place to direct thelight onto the slit. A one second exposure is okay for the blue side and most of thered side. You will notice that there are strong lines which become very saturated in theorders around 7500A. If you display the images with the right stretch, you’ll see thatthe saturation isn’t as bad as it looks. These regions do not pose a calibration problem.However, you may also notice that the reddest 10 orders or so have fairly few stronglines. This is a bigger problem. Be sure to look closely at these orders to be sure you’regetting the counts you need. When binned 2x2, a 1 sec exposure is adequate, but a 5sec exposure gives you more lines. Wavelength and order maps can be found on themain MIKE web site at LCO.

The internal ThAr lamp can be used to take all the arcs you should need during yourrun. We often bin by 2 in y (spectral direction) on both sides, and find that a 1 secexposure gives adequate flux (even at >7600A) to obtain wavelength solutions withresiduals <0.04 pix rms with a standard IRAF reduction. However, we find 5 sec arcsgive us more lines to work with at the red end. We therefore recommend about a 5-10sec exposure time for the internal ThAr arcs depending on your binning. An arc linemap is available at on the MIKE web site at LCO.

It is very important not to leave the internal ThAr lamp on when it isnot needed because it has a lifetime of only a few hundred hours andexternal sources will not be bright enough for blue-side calibration.The required exposure time is nice and short, so please get into thehabit of turning it off promptly when the exposure shutters close. It isnot possible to accidentally leave the arc lamp on during an exposurebecause the arc light floods the slit viewer with light. It’s hard toignore and it is impossible to place an object on the slit in this state.

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12. Data Reduction Software

MIKE has a dedicated pipeline developed by Dan Kelson (OCIW). The pipeline canbe downloaded from the OCIW Software Repository at http://obs.carnegiescience.edu/Code Otherwise the data reduction is straightforward and similar to a usual echellespectra reduction. One can use standard packages, like Iraf, MIDAS, etc.. For theIRAF users there is "mtools" package written by Jack Baldwin for dealing withthe tilted slits in MIKE spectra. It also includes a simple cosmic-ray zapper thatworks on the MIKE 2D images, before you extract the spectra. The packagescan be downloaded from here: http://www.lco.cl/telescopes-information/magellan/instruments/mike/idl-tools/iraf-mtools-package