ARC 3.5m | TripleSpec

Table of Contents

1. Introduction2. Usage Overview - What to Expect3. Sensitivity and Performance3.1. Spectrograph3.1.a. GD153B - an empirical case stud3.1.b. The Limits of Detectability3.2. Slit Viewer4. Practical Observing Guide4.1. Observing Step-by-step4.2. Source and Guide Star Acquisition4.2.a. Background Subtraction4.2.b. Raw Masked Frames4.3. Spectrum Acquisition4.4. Seeing and Slit Exchange4.5. Fowler sampling, read noise, and minimum integration time4.5.a. Fowler Sampling4.5.b. Saturation and Integration Time4.5.c. Exposure sequence details4.6. Dark Current and Frame Offset Level4.7. Linearity and Saturation Level4.8. Focusing4.9. Spectrograph Frames under the Microscope4.10. Array Persistence and Faint Targets

5. TUI for TripleSpec5.1. Guiding5.1.a. Binning5.1.b. Sub-framing5.1.c. Background Subtraction5.2. TUI Spectrograph Control5.3. TUI Spectrograph Nod Script5.4. Data Directories5.5. Registering Guider frames with Spectrograph Frames6. Calibration6.1. Flat fielding and Wavelength Calibration6.2. Telluric and Flux calibration6.3. Why no Dark Frames??7. Data reduction - TspectoolAppendix0.1. Revision History0.2. Timely Updates

1. Introduction

This document describes TripleSpec from a user's perspective. The current version is marginally sufficient to support shared riskobserving in Q3 2008. Ultimately this document will serve as the formal user documentation for TripleSpec.

TripleSpec is a cross-dispersed near-infrared spectrograph that provides simultaneous continuous wavelength coverage from 0.95-2.46um in five spectral orders. The instrument is described in more detail in (Wilson et al. 2004). Users of TripleSpec should referencethis publication in papers that incorporate TripleSpec results.

The primary configuration of the instrument delivers a spectral resolution of R=3500 in a 1.1 arcsecond slit at 2.1 pixels per slit on thespectrograph array. Slits with 0.7", 1.5", and 1.7" are also available. The instrument contains two independent infrared arrays. Oneprovides a 2048x1024 pixel view of the cross dispersed spectrum. The second provides a 1024x1024 view of a 4'x4' region of the sky,including the spectrograph entrance slit, in the Ks (2.16um) band.

Slits

0.7x43" (120 microns projected width) 1.1x43" (186 microns)1.5x43" (261 microns)1.7x43" (290 microns)

Spectral Coverage 0.95-2.46um

Spectral Resolution

5000(?TBD) for 0.7" slit (undersampled and limited by optical performance)3500 for 1.1" slit (2.1 spectral pixels per slit)2800 for 1.5" slit2500 for 1.7" slit

Gain 3.5 e-/ADU +/- 20%

Read Noise 18 electrons / sqrt(Nfowler)

Dark Current 0.05 e-/s

Well/saturation Depth 50000 DN = 180,000 electrons

Minimum Integration time (Nfowler * 0.8 + 0.3) sec on sky(2*Nfowler * 0.8 +0.3) sec to estimate saturation

Spectrograph saturation magnitude 4th (defocus for brighter objects)

Background limited exposure time ~200+ sec

Slit viewer/ guider pixel scale (unbinned) 0.245" / pixel(175 pixel slit length)

Spectrograph spatial pixel scale 0.39"/pixel (110 pixel slit length)

Guider/slitviewer bandpass Ks only - fixed filter

Faintest practical source for acquisition in the slit viewer

Ks ~ 17

Spectrograph continuum sensitivityJ, H, Ks = 17.0, 16.0, 15.5 5-sigma in one hour with 3 pixel spectral smoothing.0.003 Janskys(using 1.1" slit in good seeing)

TripleSpec mounted at the NA2 focus

A spectrum of the dome floor lights through theprotective plastic cover on TripleSpec. Thewavelengths at the ends of the orders arelabeled in the image. The orders, from top tobottom, are 3rd through 7th.

A slitviewer/guider image, which encompassesa 4'x4' field. The slit is offset in the field butnearly centered on the optical axis of thetelescope. This configuration enables the useof the rotator to search for guide stars in therare event that one is not available in thedefault field of view.

2. Usage Overview: What to expect

Unlike visible wavelength spectroscopy, substantial airglow dominates the near-infrared portion of the spectrum, particularly atwavelengths longward of 1.5um. Beginning at 2.0um and longward a significant ambient thermal radiation component begins tocontribute Poisson noise. The spectrum below illustrates these effects on a faint (J, H, Ks ~ 14 mag) object. The animation shows thestar observed at two slit positions as is typical for TripleSpec observations. Evident are bright airglow emission lines filling the slit,particularly in the H-band (2nd from top) order. Wavelength increases to the left, and the rising thermal emission in the K-band order(top) is evident as well. Scattered high dark current pixels pepper the array. Pixels that blink on are off are cosmic ray hits. Theexposure time for this image is 120 seconds.

Typically spectra are acquired at two slit positions and subtracted to suppress airglow line emission and thermal emission. Thedifference spectrum below shows the enhanced visibility of the spectrum in such a difference. Residual airglow lines are still presentdue to temporal variability in airglow, even on timescales of a few minutes. For longer integration near zenith, the small amount offlexure in the instrument (max 1 pixel) can also influence the self-subtraction of the airglow lines. Increasing Poisson noise from K-bandthermal background is evident at the longest wavelengths (upper left).

3. Sensitivity and Performance

3.1. Spectrograph

3.1.a. GD153 - an empirical case study

GD153 is a DA white dwarf with an effective temperature of 39,000K. The infrared portion of the spectrum of this object is approximatelyRayleigh-Jeans. GD153 has a V-band Johnson magnitude of 13.35. The UKIRT faint standards list reports the star (FS33 in the UKIRT list)as 14.24 at K with H-K = -0.078 and J-K=-0.223. The figure at left below shows the counts in units of DN/s based on eight 120 secondsspectra of this object. In this figure the flux in different orders has been merged to give a total system response as a function of wavelength.The adjacent figure gives the S/N per spectral pixel for this object in the full 8x120= 960s integration. Evident is the efficiency function ofeach of the orders, peaking near the center of the order. The third figure shows the calbrated response for this object in Janskys.

Based on the observation of this 14th magnitude white dwarf SNR=5 can be achieved in one hour in a three pixel spectral bin at magnitudesK=15.5, H=16.0, J=17.0.

Response in units of DN/s for GD153 (J=14.0,Ks=14.2). Counts have been binned where thesource is detected in multiple orders.

SNR per pixel in 8x120s of integration onGD153. Calibrated GD153 spectrum in Janskys.

3.1.b. The limits of detectability

The images below shows the K-band detection and extraction of a faint red source in one hour of integration time. During that hour thesource was observed in twelve five-minute integrations, alternating in slit position ABBAABBAABBA. The count rate of 0.07 DN/scorresponds to a K=17.5 mag continuum. With three pixel spectral smoothing the continuum has an SNR=0.5 per smoothed spectralbin. In order to produce a spectrum bright enough for extraction, the five minute integrations had to be binned into 30 minute stacks atthe "A" and "B" positions. Overall, this spectrum may not be very useful scientifically, but it does illustrate the limits of detecting andextracting a source spectrum under good conditions at the 3.5-meter.

Note that although the extracted continuum K-band magnitude is 17.5, the reported source K-band magnitude was 15.8. Theobservers struggled to guide on this faint target and the lower than expected detected magnitude is likely due to losses experienced inattempting to keep the light going down the slit.

The image result of differencing two 6x5 minutestacks of "A" vs. "B" position frames. Theextracted source magnitude is consistent with a17.5 mag Ks-band continuum.

The SNR per sets of 3 binned spectral pixels inthe H and K spectral orders. The K-band SNRhere is about 0.5 per 3 pixel bin.

The extracted spectrum in the H and K-bandorders in units of DN/s. The spikes in thespectra are residual airglow lines contaminatingthe spectrum.

3.2. Slit viewer

The sensitivity of the slit viewer dictates the faintest source that can be observed directly and placed on the slit. Below this sensitivitythreshold observers will have to depend on blind offsetting to place a source on the slit. In general, if a source is too faint to be seen inthe slit viewer its continuum will be difficult to detect in the spectrograph. The integration time for the slit viewer is adjustable, but 30seconds represents a maximum practical integration for positioning the source and for guiding. Longer integration times are possible,but the delay between exposures makes guiding and/or positioning the source on the slit tedious. In addition, since the slit vieweroperates at Ks-band, significant thermal background accumlates during an exposure and will eventually saturate the array. The time tosaturation is a strong function of ambient temperature. As a reference point, a recent observer found that the array was saturating in 45seconds under warm (60F) conditions. Given blackbody emission at ambient temperature, the time to saturation will be four timeslonger at 32F. . The slit viewer is thus about a magnitude more sensitive under winter conditions than during the summer for the sameintegration time. Observers with extremely faint targets may wish to consider this fact in scheduling observations.

Once again GD153, K=14.2, provides a fiducial for slit viewer sensitivity. The frames below show, first, a raw guider frame with a bright

source (not GD153) on the slit at the "B" position. The second figure shows a raw guider frame on a field containing many detectablefaint sources (including GD153 just above the slit) - all of which are difficult to see in this view. As outlined below (see sections 4.2.a.and 5.1.c), collecting a background frame, shifting the field-of-view and subtracting that frame from subsequent frames removes allcommon-mode signals and provides a cleaner view of the sky. In this view (on the far right, with integration time of 15 seconds) theK=14.2 mag GD153 (just above the slit) is well-detected. The faintest stars readily visible are about 2.5 magnitudes fainter than GD153or around K=16.5. With 30 second guider integrations it will be possible to see K=17 sources and place them on the slit. Longer guiderintegration times are likely to be unwieldy.

Boresite guiding requires sufficient spilled light to enable the guider to track the star (e.g. the leftmost figure below). The faintest startthat provides sufficient spilled light has yet to be determined, but is probably in the range of Ks=13-14.

Guider sensitivity will depend on conditions - seeing and ambient temperature in particular. The example frames were obtained on anight with T=0C and sub-arcsecond K-band seeing and thus represent nearly the ultimate performance for this channel. At T=15C thesystem will be approximately 0.7 mag less sensitive than at T=0C under similar seeing conditions due to the increase in thermalbackground.

A guide frame from the observation of a brightcalibrator. The target is at the "B" position on

the slit. The dark features in the frame areeither non-responsive pixels (circular region tothe right) or defects in the surface of the silicon

wafer mirror (e.g. above the slit).

A raw guider frame on a faint object. Field starsare present in this image, but hard to discerndue to the pixel-to-pixel response variationscombined with significant thermal illumination.The 14th mag target is visible, but is nearly lostbecause it falls near the reflective defect justabove the slit.

The same frame as in the adjacent figure, butthis time a displaced frame of identicalintegration time has been subtracted. Thesubtraction removes the systematics of theillumination and reveals the faint stars. Thetelescope focus was slightly off optimal for thisexposure. The slitviewer/guider hasastigmatism. The good news is that defocus isreadily evident and easily distinguishable frompoor seeing. The bad news is that defocus isreadily evident.

4. Practical Observing Guide

Given that the slitviewer/guider operates at Ks band and that the spectrograph has high spectral resolution, the sky is dark enough forsource acquisition and sensitive spectroscopy when twilight is quite bright to the naked eye. Sunlight begins to interfere with fiveminute spectral integrations when the Sun is 6 degrees below the horizon. The Sun typically reaches this position 30 minutes aftersunset or before sunrise. A TripleSpec night begins early and ends late. Users should be prepared for initial source acquisition shortlyafter sunset.

4.1. Observing Step-by-step

1. Move telescope to target. Typically the source will appear within a few arcsecond of the slit. Since the field of view is 4'x4' it isunlikely the target will fall outside the field of view. (Note that the slit mirror has a few defects, one of which is close to theprimary slit (1.1 arcsec)). On occasion, the target may land on the defect (or in the slit itself) rendering the source invisible if thetarget is faint. If the desired source is not evident, a few arcsecond offset may turn it up.

If the source is faint it may be necessary to detect it in a difference between two guider frames. See section 5.1.c forexplicit instructions on how to initiate background subtraction from the TUI TripleSpec guide window.

2. Place target in the "A" slit position.Ctrl-left click if not already selected with green circle and marks on the target, then click Center Sel button to offset thetelescope to the marked hotspot (which should be placed at the A position).By convention the A position is about 1/3 slit length from the left edge of the slit as seen in the guider display. Similarly the"B" position is about 1/3 slit length in from the right end of the slit.The displacement between the A and B positions isabout 20 arcseconds.

3. Set the number of Fowler samples, integration time, and source name in the TripleSpec instrument/expose windows. Shorterintegrations provide better immunity to airglow line variation, however, the background limit - set by the inter-line airglowcontinuum, is reached only after about 3 minutes of integration. Five minutes of frame integration time is a practical and usefulupper limit.

4. EitherSelect an ABBA nod sequence (which should automate the positioning of the source at the alternating slit positions) orExpose on the "A" position and manually offset and integrate for an ABBA sequence.

5. Repeat as desired.

4.2. Source and Guide Star Acquisition

For bright sources, positioning a target on the slit is straightforward. Once a source is identified in the slit viewer, click Center Selbutton to move the target to the "hotspot" location on the slit. Click Guide. The spilled light out of both sided of the slit should providesymmetrical guiding in boresite mode and integration can begin.

For faint sources attention will be required to maximize the source signal in the slit viewer images - which to 0th order is accomplishedby increasing the frame integration time. There are two routes to obtaining optimal SNR in the slit viewer. The best practice remains tobe determined.

4.2.a. Background subtraction

In the background subtraction mode a single frame is buffered and subsequently subtracted from each incoming guiderframe. The subtraction removes all of the common mode structure that contaminates a raw frame and makes faint sourcesvisible at the expense of providing a (+) and (-) image of each source. The telescope must be offset a few arcsecondsfollowing the acquisition of the background image otherwise sources will subtract from themselves making them invisible.Implementing the background subtraction mode with TUI is described below in Section 5.1.

4.2.b. Raw masked frames

The masks applied by the TUI guider have a pixel-to-pixel scaling that effectively flat fields the incoming frames. Typically,thermal infrared dominated frames are not simply flat-fielded because of structure arising from non-sky emission (considerthe glow from a speck of dust on the window). For the sake of source acquisition and guiding such features are more of anannoyance and it may be possible to acquire even the faintest sources if the flat fielding is effective (and avoid the sqrt(2)noise penalty entailed by background subtraction). By the time of shared-risk observing it may be possible to do faintsource acquisition on the masked (proc) guider frames.

4.3. Spectrum acquisition

The spectra above show that there is substantial airglow contamination across the TripleSpec bandpass. If consecutive exposuresplace a point source at two well-separated in-slit positions, subtracting these two spectra will, to first order, suppress the airglow lineflux while maintaining the full signal from the target. The airglow line intensity can vary substantially even in the course of severalminutes. In order to get good subtraction of the airglow lines integration times of less than 5 minutes are desirable. (In theory, theairglow should be removed in data processing as the slit region outside the source is fit and subtracted from the source. In practice theangle of the slit varies with respect to the dispersion direction making clean subtraction difficult - thus the desire to suppress/minimizethe airglow signal.)

4.4. Seeing and Slit Exchange

TripleSpec has four available slits. The primary TripleSpec design implements a 2.3 pixel wide slit which corresponds to a cross-slitspatial dimension of 1.1 arcseconds. The three other slits are 0.7, 1.5, and 1.7 arcseconds wide. These four slits reside on a gold-coated silicon wafer slit mirror supported on an 8 position geneva gear mechanism. A pulldown menu in the TripleSpec TUIConfiguration window permits selection of slits (or selection of blocked positions halfway between slits).

Due to the substantial airglow across the TripleSpec bandwidth there is a sensitivity penalty in addition to the resolution penaltyincurred when using wider slits. The airglow lines become broader thus covering more of the spectrum. More flux is admitted into theinstrument overall yielding more scattered light. If seeing allows, the 1.1 arcsec slit is optimal for observation. Although the 0.7 arcsecslit is even better in this regard, it subtends only 1.5 pixels and is undersampled. The figures below graphically illustrate theimprovement in uncomtaminated spectral coverage vs. resolution.

Comparison of airglow contamination and itsinfluence on SNR at R=500 vs R=3000.TripleSpec resolution does not go down to 500in any of the slits. This figure is just illustrativeof the effects of resolution on SNR.

Evaluation of spectral coverage of airglowcontamination as a function of spectralresolution. (see Martini and DePoy, SPIE,2000).

The table below shows raw (2x2 binned) guide frames in each of the slits. The patterns from the slit mirror defects/mounts are evidentin each case.

0.7 arcsec 1.1 arcsec 1.5 arcsec 1.7 arcsec

4.5. Fowler sampling, read noise, and minimum integration time

4.5.a. Fowler sampling and read noise

The HAWAII-2 spectrograph array can be read non-destructively multiple times during the course of an integration. Fowlersampling refers to conducting a burst of N readouts at the beginning of an integration and an equal burst of N readouts atthe end of integration in order to suppress read noise (FowlerN). Read noise suppression can be important for TripleSpec,since the dark current is 0.05 e-/s and the inter-airglow line continuum is weak. For correlated double sampling (one readat the beginning of an integration and one at the end - also ``Fowler1") TripleSpec read noise is observed to be 5DN or 17electrons. The TripleSpec TUI configuration menu permits the user to select a range for the number of Fowler samples.The TripleSpec read noise is observed to improve, as expected, with the square root of the number of Fowler samples.With Fowler8 the effective read noise is about 7 electrons. After the collection of 100 dark current or sky electrons thesefactors will dominate the read noise.

4.5.b Integration time and saturation

Integration/exposure times for direct readout devices can be non-intuitive since there is no shuttering and the conversionof the first and last pixels in one read of the device are staggered in time. For the TripleSpec HAWAII-2 array theelectronics require 792 milliseconds to address all pixels on the array (in 16 128x1024 stripes of pixels read out inparallel). As an example, consider a Fowler1 integration. The array is reset, establishing the saturation level and the firstpixel of first readout (actually 16 all at once) is read. 792 milliseconds later the last pixels of the read arrive. The imagecaptured is thus staggered in time from one end of a stripe to the other by 0.8s. An 0.3 second delay is enforced prior tothe second readout in order to avoid spurious ``shading" across the image. If the second readout begins immediately afterthis 0.3 sec delay (thus realizing the minimum integration time) the first pixel will be read out 1.1s after it was read the firsttime, and so on for all of the pixels on the array. Despite the fact that 1.9s was invested in acquiring the data, the imageproduced by differencing the two readouts has an on-sky exposure time of 1.1s. The beginning of the last readout started

1.1s after reset while the end occurred 1.9s after reset - the saturation threshold varies across the chip! Saturation shouldbe considered including all of the readout time, not just the on-sky integration time. For Fowler1 the minimum on-skyintegration time is 1.1s, but saturation should be presumed to be estimated from a virtual 1.9s exposure.

Generalizing to FowlerN, N*0.8s is required for the first readout sequence as well as for the second readout sequence. Atminimum integration time, currently with an 0.3 second delay between the last pixel of the first sequence and the first pixelof the last sequence, 0.3+2*0.8*N seconds are required to execute the entire sequence while 0.3+0.8*N seconds ofintegration are obtained on sky (which can be seen also by subtracting pairwise the first read of the first group from thefirst read of the second group, and so on).

TripleSpec software accounts for the readout time in all Fowler modes such that the on-sky time will be the requestedduration. This value will also appear as EXPTIME in the frame FITS header (another keyword INTDELAY provides theimplemented delay interval between the last pixel of the first read and the first pixel of the last read).

The choice of N dictates the minimum on-sky integration time. A typical value is Fowler8 - enabling integrations as shortas 6.7 seconds. Virtually all TripleSpec targets, including calibrator stars and calibration lamps, are observed withintegration times longer than 10 sec. N=8 is the recommended Fowler setting for all TripleSpec data acquisition, unlessexposures with duration less than 7 seconds are required.

The HAWAII-1 slitviewer/guider array could be read out in FowlerN mode, however the level of thermal background on thearray produces thermal Poisson noise far in excess of the system read noise (also of order 17 electrons). In the interest ofdynamic range and efficiency, the HAWAII-1 array only operates in Fowler1 (CDS) mode.

4.5.c Exposure sequence details

Matt Nelson has provided the following detailed breakdown of the exposure sequence:

1 - ICC receives exposure request2 - ICC calculates and sets exposure time (SET cmd to controller)3 - Controller finishes loop in Continuous Reset, Processes commandand replies to ICC.4 - ICC initiates exposure in controller5 - Controller finishes loop in Cont Reset, Breaks out of loop to expose6 - Controller does full pixel by pixel reset of array7 - Controller Delays for reset settling8 - Read-1 reads are made 9 - Controller waits for calculated Integration time10 - Controller waits for 400mS for Array outputs to stabilize11 - Read-2 reads are made12 - ICC finished scavenging last Read-2 read, builds frame andwrites it to disc.13 - ICC replies "done" to hub

Guesses about timing.

1-4 should be relatively fast. The line loops in continuous reset arequite quick so I would expect this sequence to finish in < 10mS

5-6 was never timed by me. What I recall from the pixel clocks when I was developing the DSP code is that the reset pixelclock was running about 1/3 of a normal readout pixel clock. So I'd guess ball park 200-300 mSec for this

7 50mS8 N*790mS9 Exp Time - N*790ms10 400mS11 N*790mS

12-13 Unknown but fairly quick. Most of the frame data are scavenged and averaged while the pixels are still being read. itis just the recovery time of the last frame, subtraction of the Read1/2 the writing of the frame to disc. I'd estimate 100mS-200mS nominal timing.

Of course what is missing is the time required for the hub to cycle back around to requesting the next in the frame series.I'm certain the APO staff would have a good estimate. As a summary, the ICC and controller are probably using up250+50+400+150mS = 850mS of time beyond the time spend during integration + readout. So for a rough estimate ofinstrument cycle time beyond the requested integration time 850+790*N mS should be close.

4.6. Dark Current and Frame Offset Level

The dark current in the spectrograph HAWAII-2 array has been measured on the mountain to be of order 0.05 e-/s or 15 e- in a 300sexposure. To first order dark current is unobservable in a 300s exposure, particularly because electronic offsets (e.g. the thermal drift ofthe cold output transistors) can be much larger than the few DN of dark current (e.g. a random 300s frame was observed to have anoffset level of 130DN - virtually all electronic). The focal plane is quite dark between the orders. Raw frames may have a positive ornegative offset in this region that is not due to electrons in the wells. Typically the subtraction of two consecutive frames (as is naturalin processing ABBA observations) will surpress much of this electronic offset.

4.7. Linearity and Saturation

The spectrograph array saturates at a level of 52,000DN. Measurable (but small) linearity becomes apparent by a count level of20,000DN. The plot below summarizes a continuum linearity test observing a constant background level at various integration times. Alinear fit was made to the points having integration times between 2 and 8 seconds (count levels between 4000 and 13000 DN) whichrepresents the most linear and reliably measured portion of the curve. The table below summarizes the quantiative non-linearity fromthis fit. The lines highlighted in green contributed to the linear fit. The short integration time points are deviant because the integrationtime offset was not precisely determined for these data.

Seconds Deviation Counts from Linear---------------------------------1.21 1.9% 21272.21 0.5% 39453.21 -0.2% 57744.21 -0.2% 75785.21 -0.1% 93717.21 0.1% 1294210.21 -0.5% 1826315.21 -0.9% 2709120.21 -1.1% 3595025.21 -2.5% 4426030.21 -6.5% 50976

4.8. Focusing

Simply put, good focus is obtained by making the TripleSpec guider images round. The guider optical train has astigmatism that entersquickly as the telescope goes out of focus. The good news is that this astigmatism is in the guider optics and not in the spectrographoptics, so the astigmatism serves as a focus tool without influencing the quality of the star image that is actually going down the slit.Seen a different way, the guider image can look poor and astigmatic (within limits) yet the image is still optimal for the spectrograph.The position angle of the astigmatic image is a guide to the direction to move the focus (soon to be documented by the obs specs). Inpractice, it is probably better to adjust the guider images on the fly to remove any evidence of astigmatism rather than to focus using ascript that drives the telescope well out of focus.

Update based on limited observations on UT080907: If the image appears elongated more-or-less parallel to the slit the focus needs tobe made more negative in order to return the image to a circular shape.

K~10 stars make for good focus targets.

4.9. Spectrograph Frames under the Microscope

The image below shows a aggressive stretch of a deep (5 minute) TripleSpec spectrograph exposure. Evident is the thermal emissionthat covers the third order (K-band) and even a little of the long wavelength end (left side) of the fourth order). Atmospheric airglow isapparent in all orders, with the worst airglow appearing in the 4th order (H-band). This particular exposure was obtained in a densefield and multiple source spectra appear in the slit.

In addition to these "external" sources of light, the image also contains features resulting from electronics and internal scattering withinthe spectrograph.

Electronic ghosting: The most evident feature is the electronic crosstalk that appears when a bright source fills many rows/columns inone of the two array quadrants. This effect is most evident for the bright K-band order where emission filling the rows in in the left-

hand quadrant produces an electronic ghost appearing as a vertical stripe of constant intensity in the right-hand quadrant. At a lowerlevel, some of the bright spectral lines on the right-hand quadrant produce electronic ghost lines that cross the entire left-handquadrant at constant intensity.

Optical ghosting: The brightest emission, specifically the K-band thermal emission, can be reflected about a point that is somewhatclose to the center of the 2048x1024 array. This reflection produces an inverted stripe of emission that is evident in the right-handquadrant just above the 6th order. This reflected strip ends in an intense bar this is acutually an image of some of the surfacecomponents and wirebonds on the detector wafer. Fainter reflection stripes are visible at a couple of other locations on the array.

Electronic quadrant offset/shading: At the level of a few DN the "bias" level on the detector can drift or be offset from one quadrant toanother, producing a faint discontinuity across the quadrant boundaries.

All of these effects are repeatable from frame to frame and largely subtract out in the difference between two frames. The one placewhere residual features remain is the "stripe" electronic ghosts produced when observing bright standards. In this case, the level ofghosting is small compared with the source intensity, so the ghosts are of little consequence.

4.10. Array Persistence and Faint Targets

Bright sources produce an after-image on the array. This "persistence" image can linger for up to an hour following the observation ofan extremely bright source. If a faint target is observed directly after a bright calibrator (e.g. K=7) the first few 5-minute exposures onthe faint target may be contaminated with the spectrum of the calibrator (as a faint positive source in both the "A" and "B" positions).Observers should be careful to select fainter standards prior to faint source observations and should be aware of possible persistencecontamination during data reduction.

5. Observing with TUI

Like DIS, TripleSpec uses independent TUI windows to position the source on the slit and acquire the spectra.

5.1. TripleSpec guiding with TUI

Choose "TSpec Slitviewer" from the Main TUI "Guide" pull-down menu. This guider window behaves functionally like any of the otherTUI guider windows. Useful reference include:

Guiding with TUI Users GuideDIS Slitview Camera description

Guider Match Scripts:

On Newton there is a script that can be run from the institutional accounts that will match up the times od the slitviewerimages to your science images and create a log of each science frame with the nearest guide image and the range ofguide frames if a range exists. This script is called: tcam_match. For more details on using this script please see: GuiderMatch info.

5.1.a. Binning

Although the guider hardware always reads a complete 1024x1024 frame, the displayed image can be software binned to save onbandwidth. Users will find that setting Bin=2 will produce the cleanest and most workable images. In particular, systematic pixelcalibration errors lead to vertical "jailbarring" in the images that can be time variable. This effect is visible at Bin=1, but averages out atBin=2. This advantage alone makes Bin=2 preferable for seeing fainter sources.

5.1.b. Sub-framing

The TripleSpec guider will deliver portions of the full instrument frame, once again saving on bandwidth for remote observers. The"Window" area on the guider screen permits the user to drag the frame edges to a desired location relative to a full frame. Pushing the"Full" button returns to a full-frame display. Alternatively, one can zoom in on the guider display with the mouse and then push the"View" button to adopt the current view as the window area.

5.1.c. Background Subtraction

When background subtraction is engaged the existing frame is stored in a buffer and then subtracted from all subsequent incomingframes. The guider screen provides two buttons to control background subraction. The text on the left side of the background subtractbar indicates the current state (e.g. "Bkgnd Sub On" or "Bkgnd Sub Off"). When background subtract is turned on the next frame toarrive will be the first frame that is background subtracted. In order to see sources the telscope must be offset between the framebuffered for background subtraction and the subsequent frames (otherwise the sources will subtract from themselves).

On/Off: The button that reads "Off" or "On" indicates the guider state that will be obtained if the button is pushed. If the buttonsays "Off" it means the background subtract mode is ON and pushing the button will turn it off. If background subtract is turnedon the frame number that is being buffered and subtracted from each subsequent frame appears to the right of the buttons andthe left side of the bar will read "Bkgnd Sub On".New: Pressing the "New" button will update the frame that is stored in the background buffer for subtraction from subsequentframes.

Note that pushing the "On" button in background subtract mode does not start a new guider integration. One must engage thebackground subtract mode and then press "Expose" if the guider is not running.

Typical sequence after slewing to a new source:

Decide on your guider integration time and acquire a frame to serve as the background image with "Expose".Choose Bkgnd Sub = On by pressing the "On" button.Move the telescope >= 5 arcsec.Press "Expose" for a single frame or engage manual guiding to view a steady stream of incoming frames.Identify the target star and move it into the slit.

Important tip:Background subtraction only works well if the saved background frame is "fresh". Even after a few minutes a mismatchcan develop between the saved background frame and the actual background due to airglow, temperature, and flexure. Backgroundframes must also have an identical integration time to the incoming frames. A new background frame is required (by either stoppingand restarting background subtraction or by pressing the "New" button) if

the integration time is changed.the telescope is slewed to a new source.significant non-celestial structure is visible in the guider frames due to a drift in the background level (usually after the savedbackground has become stale by several minutes).

5.2. TUI TripleSpec spectrograph control

Choose "Tspec" from the Main TUI "Inst" pull-down menu.

5.3. TUI TripleSpec Nod Script

Choose "Tspec Nod" from the Main TUI "Scripts" pull-down menu. Most TripleSpec data are obtained in slit-nodded mode where the sourceis positioned alternatively on the "left" (A) and "right" (B) portions of the slit. Pairs of the frames are subsequently subtracted to produce bias-free frames for data reduction.

The nod script is particularly useful for long integration on science targets so that users don't sit idle if they miss the end of a long integration.Since the objective of nodding is to permit airglow and background subtraction on several minute timescales (the time it takes those things tochange) nodding can add inefficiency to short integrations (e.g. standard stars) since the nod and settle time tends to be longer than theframe-to-frame time. For bright sources a set of frames (typically 5) can be taken at the "A" position, the telescope should be manuallynodded to the "B" position and another set of 5 frames taken there.

5.4. Data directories

Spectra: /export/images/tspec/PROGRAM/UTDATE/Guide frames: /export/images/tcam/UTDATE/

5.5. Registering Guider Frames with Spectrograph Frames

Users select the names of their spectrograph data files while the guider frames accumulate serially through the night - t0001.fits, t0002.fits,....The FITS header for the spectrograph files contains a keyword (SVFILE) that contains the guider frame acquired just prior to the start of thespectrograph integration.

The TripleSpec TUI guiderwindow. Functions/controls aresimilar to guiders forDIS/Echelle. Remote users willwant to have 2x2 binningactivated to minimize downloadtime and to clean up odd-evencolumn striping. The platescaleof the array is fine enough thatthere is little penalty inresolution for running in thebinned mode. Sub-framing isavailable to improve responsetime for remote observing. Thedetailed operation of thiswindow is described in Section5.1.

Main TripleSpec instrument TUIwindow showing configuration. Inthis window the number of Fowlersamples can be set and the slitcan be rotated to the desiredposition. Tip-Tilt mode refers to afuture mode of the instrumentwhere the spectrum can besteered at the sub-pixel level by apiezo-electric stage under one ofthe fold mirrors in the system.Note that the array power buttonshould be left alone. It will likelybe removed in future versions ofthe GUI. Users should find thesystem fully powered up andready for operation after startingTUI. The "environment" buttonprovides instrument internaltemperatures and pressure.

TSpec Exposure window:Integration times are set andexposures initiated in thiswindow. The "type" radio buttonssimply set a FITS keyword forthe recorded data. The systemcan take multiple exposures,``#Exp" at the push of onebutton. This function isparticularly useful for brightstandards where a set of 5exposures can be takenconsecutively at the "A" slitposition followed by 5 exposuresat the "B" position to completethe observation. ``Filename" isthe root file extension. Forexample, if "xxx" is chosen forthe file name the resulting fits filewill be "xxx.yyyy.fits" where"yyyy" is an incrementing framenumber that increases steadilythrough the night.

Nod Script window: Obtainedfrom the "Scripts" pull-downmenu, this window has all of thefunctionality of the Exposewindow, but also includes a"cycles" option which currentlydrives the telescope in an"ABBA" slit position seqeuencefor each cycle requested. Ateach "A" or "B" position "#Exp"exposures will be taken. Forexample, Cycles=2 with #Exp=4will yield 32 frames - anABBAABBA sequence with 4exposures at each position.

6. Calibration

6.1. Flat fielding and wavelength calibration

Like any spectrograph, TripleSpec requires both continuum and line illumination for flat-field and spectral calibration. The Triplespectooldata reduction code currently uses airglow lines for spectral calibration. Spectral lamp observations are not necessary, but manyobservers may like the security of having traditional lamp spectra in reserve. In order to use the airglow calibration method some of thespectra must be free of source flux in the middle of the slit. Since the natural observation procedure is to offset the source between twooff-center slit positions, standard TripleSpec observations naturally provide for airglow wavelength calibration. In general, observersshould be sure to have a few observations with only airglow at the slit center for wavelength calibration.

A typical calibration sequence to be conducted once each night (possibly once per run) consists of ten 60 second exposures that include:

No illumination - only continuum thermal radiation from the telescope and dome evident at the longest wavelengths.Bright quartz lamp on for continuum illumination.On Feb 9, 2012 the Bright and Dim Quartz truss lamps were upgraded on the telescope. This changed the recommended exposuretimes for flat fields for some instruments, but the TSPEC recommendations remain the same.

Not needed are:

Neon and Argon lamps on for spectral calibration - (observers may wish to acquire a few such frames as a backup).Darks - No darks are required for TripleSpec data analysis since all reductions are done differentially between pairs of images, both ofwhich contain the same "dark" bias and dark current contributions (see 6.3.)

6.2. Telluric and flux calibration

7th-9th magnitude A0V stars are ideal calibrators for most TripleSpec observations. The Triplespectool pipeline and this calibration method

are described in:

Cushing, Vacca & Rayner 'Spextool: A Spectral Extraction Package for SpeX, a 0.8-5.5 Micron Cross-Dispersed Spectrograph' -URLVacca, Cushing & Rayner 'A Method of Correcting Near-Infrared Spectra for Telluric Absorption' - URL

TripleSpec observers that make use of Mike Cushing's Triplespectool should refer to the tool as a 'modified version of Spextool' in the papertext and reference the above papers.

Aside from broad hydrogen line absorption, A0V stars have few other intrinsic features. The overall A0V spectrum can be well modeled and isfit and removed from the target spectrum in one step of the Triplespectool data pipeline.

Note that for solar-system targets observed in reflected sunlight a G2V standard is reqired instead of an A0V standard. Direct division by theG2V standard will provide correction both for telluric absorption and for the solar absorption spectrum with no modeling require. This option isavailable in the Triplespectool pipeline.

For proper telluric absorption correction (which can be time variable due to the changing water vapor content of the atmosphere) it isimportant to observe an A0V calibrator close in time, in airmass, and ideally in angular separation from the science target.

The files below contain thousands of potential A0V and G2V standards to aid in selection of a calibrator .

A0V standardsG2V standards

6.3. Why no Dark Frames?

The TripleSpec slit wheel drive has 8 positions. Four of these positions have spectrograph slits. The other four positions in between the slitsblock all light from entering the spectrograph. Technically it is thus possible to take true dark frame, but in practice true darks are notnecessary. Although this statement seems counter to standard observational practice, it turns out that dark current and frame bias arenaturally accounted for in the differential procedures that underlie standard TripleSpec data acquisition. In particular, all spectra are obtainedin differential mode - either A/B positions on the slit or on-slit vs. adjacent sky. In each case, data processing takes place by operating on thedifference of pairs of frames. The underlying dark current and bias (and thermal background) are subtracted automatically. Similarly, whenobserving calibration lamps an equivalent length exposure is taken with the lamps off so that dome thermal emission is the only source ofillumination. When pairs of lamp-on vs. lamp-off frames are subtracted the dark current, bias, and thermal backround all are removed leavingonly the source spectrum.

7. Data reduction - Triplespectool

TripleSpec includes a dedicated spectral extraction routine - Triplespectool - that converts TripleSpec images to one-dimensional telluric-absorption-corrected absolute-calibrated spectra with flexibility for selecting spectral apertures for both point and extended sources. This linkpoints to the latest available version of a Tspectool installation and use guide (TSpecTool_guide) as well as the actual software packages.

Triplespectool is based on the Spextool package developed by Michael Cushing for use with the Spex instrument at the IRTF. TripleSpecobservers who make use of Mike Cushing's Triplespectool should refer to the tool as a 'modified version of Spextool' in the paper text andreference the above papers.

Cushing, Vacca & Rayner 'Spextool: A Spectral Extraction Package for SpeX, a 0.8-5.5 Micron Cross-Dispersed Spectrograph' -URLVacca, Cushing & Rayner 'A Method of Correcting Near-Infrared Spectra for Telluric Absorption' - URL

Appendix

0.1. Revision History

0.2 - first issue 0.2a - Serious swap of J and Ks sensitivities in the table below fixed 0.3/0.3a - Shared-risk user input accommodated including enhanced description of background subtract procedure. 0.3b - Added quantitative guidance for on-the-fly focusing based on astigmatism; added a section on persistencecontaminating faint source observations (4.10) 0.4 - Updated calibrations section to account for the fact that Triplespectool now performs wavelength calibration based onairglow lines. 0.5 - Updated version of TSpecTool_guide available. 0.5b - Included section on detailed breakdown of events during an exposure sequence and resulting timing. 0.5c - "-0" problem in G2V and A0V standards files corrected and noted that it is possible to saturate the slit viewer in longexposures. 0.6 - Updated TSpecTool and TSpecTool_guide

0.2. Updates

2008JUL24: The rotator restriction on TripleSpec has been removed.