ASTR_4170Special Topics: Photometry

& Filter Systems

Day-5

Announcements•This week: Dark Night Observing on Tues. 9/8 & Thur. 9/10

•First “Astro-group” meeting. - Friday, 9/11; 2:30-3:30 E-109

Definitions & Terms -1•Photometric Conditions:•Differential Photometry•Absolute Relative Photometry•Absolute Photometry•Standard Star•Zenith Angle•Airmass•Atmospheric Extinction

Observing Programs• Goals – what is it you want to do?• What does your science need be successful?

• Sky conditions• Time, spatial resolution of measurements =>

Instrumentation• Precision, accuracy of measurements ==> ExpTime, Stds• Filters

• Okay, you have telescope time, and the weather isn't photometric: what do you do?

• Backup program? Can you do something useful?• Secondary science? Can you help another program?• Fun projects? Test case for new program, pretty

pictures ...

Sky Conditions• What sky conditions do you need?

• “photometric” -vs- Observable conditions, and how do you know?

• Photometric: Used to be “is it clear”? Clear to an weather forecaster is not necessarily clear to an astronomer. Clear to a spectroscopist is not necessarily clear to a photometrist.

• What level of “photometricity” do you need?• If there is a bit of moon, use it. A lot of moon – easy.• If it's dark, how do you tell?

• Weather pages• All-Sky Cameras• Rasicam

Time Resolution• (Assuming an imaging camera):• You need to know the cycle time of the CCD

• Preparation and flush time• Read time• Storage time

• Then, what is the magnitude of your object and what precision do you need in the data? How long does the exposure need to be to reach this?

• Can the expTime plus overhead meet the science needs?• Yes – continue• No – is there a trade-off you can make?

Telescopes

Telescopes

Spatial Resolution• (Assuming the APSU imaging camera and telescope):• You need to know the “plate scale” of the CCD

• Effective FL of telescope: 0.5m, f/6.2 ==> 3.1m

• = 66.537 sec/mm = 0.066537 sec/m • Apogee U16000 camera has 7.4 micron square pixels

and an imaging area of 4872 X 3248 pixels (15.8Mpix)• So, each pixel is 7.4m X 0.066537 sec/ m = 0.492 sec• So the imaging area is 39.98 arcmin X 26.65 arcmin

• Is the camera oriented in the direction you need?

Measurements• (Assuming the APSU imaging camera):

• Thermoelectric cooling system• Capable of 45°C below ambient (theory) –

project.

• Dark current doubling temperature is 7° (need to test). Needs to be systematically measured – project.

• CTE = 0.99999• Full well = 30,000 electrons• Linearity (needs to be established, tested – project)• 16 electron read noise (need to verify)• DC < 0.5 nA/cm2

Measurements

CCD specifications• (Assuming the APSU imaging camera):

• 1,1 is LL corner• Grade 2 chip:

• < 300 major bright/dark pixel defects• < 3000 minor pixel defects• < 30 cluster defects• < 4 column defects

Filters• The APSU imaging camera:

• SDSS: ugriz• Johnson_Cousins: UBVRCIC• DDO-51 (narrow-band to study MgH)

• Eventually we hope to order:• Stromgren set (uvby,Hb)• RGB(?) for astrophotography• Washington set (Cluster studies)

• WIYN 0.9m:• UBVRI, ugriz, Ha redshift set• Plus any 4-inch filter at KPNO (aperture priority)

Exposure Times• (These can be calculated - project):• Driven by the science needs.

• For “1% photometry” (0.01 magnitude uncertainty) you need 10,000 photons above the sky background (after read noise, after thermal noise, bias subtraction and flat fielding (plus fringe correction…)

• 1/sqrt(10,000) = 0.01• What you get out of the CCD amplifier is ADUs (or DNs), so you have to

account for the gain of the system when calculating photons.• This will get you the Poisson uncertainty of the star measurement.• You will gather more uncertainty in the extinction calculations, system

transfer equations …• Uncertainties add in quadrature.

• In essence, start with what you need to get the end-product and work backwards through each step. So, if you need 1% in the end, you probably want to make sure your science targets start at 0.5% (0.005 mag; 40,000 photons)

Exposure Times• What limits exposure times on each end?

• Short end – illumination correction : function of the shutter shape, size, and open/close motions. This affects the uniform illumination across the chip.

• APSU camera (need to test and verify): should be ~1 second or less for 1%• WIYN_0.9m: 3-5 seconds for 1%

• Long end:• telescope tracking: you want round images, certainly point-like, not

trailed.• Cosmic ray build-up. These nasty buggers will affect your photometry.

They can be cleaned, the automated cleaning routines are pretty good at this. A way to help this is to take 3 exposures (slightly longer than 1/3 of the total time) and co-add them. (or more exposures – but you pay overhead in readout time and noises).

Exposure Times• Illumination shutter map:

Standard Stars• If you’re doing a photometric program, how many

standard stars do you need?• This also science driven … and it’s something you figure

out through experience.• First question, is it CLEAR?

• If not, standards are a waste of time, you can’t correct for clouds (traditionally – this is changing however).

• If you are a “traditional” galaxy observer, and 10% is good, then 3-ish standards evening and morning are fine.

• If you’re trying absolute relative “all-sky” photometry at the 1% level; you’ll probably spend ~1/3 of the time on standards. The quality you get ultimately determines how good your reduced data will be.

Standard Stars• What is a standard star anyway?• A standard star is one that has been observed A LOT and

is (hopefully) known to be non-variable, therefore will produce repeatable magnitudes for each observation.

• You select them based on: • Brightness – bright enough that you’re not spending a

large amount of exposure time (vs targets); but not so bright that they risk saturation or push you to a non-linear portion of the CCD response curve.

• Color – you want standards of varied colors so you can calculate 2nd order extinctions and system transformations. Hopefully you can get several/field.

Standard Stars• Where do you get the standard values?• In this day and age – on-line. Several people have spent

years doing this work for the community and data are on-line (for the most part).

• UBVRI: A.U. Landolt – the main works were 1992, 1983, 1973, though he has come out with extensions to his network since then. Landolt 1992, AJ, 104, 340

• ugriz: J.A. Smith & D.L. Tucker: northern system is in print Smith et al. 2002, AJ, 123, 2121; southern extension is on-line, northern extension (and southern network) are in prep for publication.

• Others: SOFA site

Standard Systems• A good review of most of the common systems in use

today was written by Mike Bessell for the Annual Reviews in Astronomy & Astrophysics.

• The Asiago database

Standard Stars• If you’re not doing a photometric program, how many

standard stars do you need?• None.

• What if you wanted to do a photometric program, but there are clouds?

If it’s observable, use the time to shoot the field(s) that you wanted and come back on a photometric night to standardize it.

For star clusters, we do this a lot to go deep and to search for variables.For standard star establishment, we do it to search for variables.

Differential Programs• If I don’t need standard stars, what do I do?• Take data!• Exposure time rules still apply,• Does the science need still dictate the exposure cadence?