SSD Technical/Training Series - Photometry I

Brad Whitmore
10-11am, Thursday
November 5, 1998
Auditorium

SSD Technical/Training Series - Photometry I- III Outline

November 5 - Photometry I - Brad Whitmore

1. MOTIVATION (Big Science from small measurements)
   • H-R diagram
   • Age of the universe from Cepheid variables
2. PRACTICAL MEASUREMENTS
   • Using APPHOT (walk through the WFPC2 Tutorial)
   • Homework (due Nov. 11)
3. BASICS
   • Types of photometry
   • Magnitude systems
   • Zeropoints

November 12 - Photometry II - Brad Whitmore

1. MORE BASICS
   • Second part of the homework (due Nov. 18)
   • Zeropoints
   • Aperture corrections
   • Optimal aperture sizes
   • Contamination
   • Error estimates
2. THE NITTY GRITTY
   • Color terms
   • spatial PSF variations
   • Charge Transfer Efficiency (CTE)
   • Long vs. short problem

December 10 - Photometry III -

1. STIS PHOTOMETRY - Harry Ferguson
2. NICMOS PHOTOMETRY - Mark Dickinson

PHOTOMETRY I

1. MOTIVATION

Many people think that photometry is dull, like filling out your tax forms.

NOT SO !

Photometry provides a way to quantify the data and answer fundamental questions about the universe.

It allows us to find the hidden order behind the chaos.

(Painstaking yes, dull no)

• Example 1: By measuring the brightness of stars in a star cluster I can:
  • check theories on how stars evolve
  • determine the metallicity
  • determine the distance
  • determine the age of the cluster (universe)
  • figure 1, figure 2, figure3

• Example 2. By measuring how the brightness of a certain type star varies with time in a galaxy I can:

 

• determine the period

v

luminosity

v

distance

v

Hubble constant (using velocity)

v

age of the universe

Figure 4, figure 5, figure 6, figure 7.

• Relevant to SSD since a large fraction of our users do photometry, and much of our calibration work is photometry.

2. ASSIGNMENT # 1

DUE:	Wednesday, Nov. 10 (e-mail to Brad, he will compile results and show at Nov. 11 class, anonymously)
MEASURE:	F814W magnitude of star at u2g40o09t.c0h[1][499,447]. (using the VEGAMAG system, 2 pixel aperture, sky in annulus from radius of 5 - 10 pixels, no further corrections at this stage; i.e., don't make aperture corrections, etc. since we will not have discussed yet).

Please don't compare notes with each other until after the next class.

NOTE: You can download the data from the archives or from:

http://ftp.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_phot/

3. REFERENCES (most important first):

• Section 4 of "WFPC2 Data Analysis - A Tutorial" (from WFPC2 WWW page under advisories)
• Chapter 28 of the "HST Data Handbook", Version 3, Oct. 1997
• Casertano article in 1997 Calibration Workshop (p. 327)
• Holtzman-2 (PASP, 107, 1065 - photometry paper, SEE SECTION 9 COOKBOOK)
• Holtzman-1 (PASP, 107, 156 - general WFPC2 paper)
• ISR 95-04 "Demonstration Analysis Script for Performing Aperture Photometry" (Somewhat out of date)

4. PRACTICAL MEASUREMENTS

• Using APPHOT (walk through the WFPC2 Tutorial referenced above)

NOTE: You can get a text version of the tutorial to help you build your script if you like from:

/data/whimbrel2/tutorial_c4.txt

CHEATSHEET:

The division by seconds can be done by either:

• dividing the original image by the exposure time
• adjusting the zeropoint using -2.5 log(exposure time), or
• using the program that does the conversion for you.

NOTE: We will be using the later approach (i.e., PHOT)

WARMUP EXERCISE:	Say you have a star with 100 counts (i.e., 100 DN) observed with gain=7 (i.e., ~700 electrons were deposited in the CCD).
	If the star had been observed with a gain = 15, it would have ~50 DN per second.
	p. 28-7 from HST Data Handbook says that the zeropoint for gain = 7 is 22.545 and for gain = 14 is 22.545 - .745 = 21.80

Hence:

The 2 do not match exactly since the difference in the gain ratio between gain = 7 and gain = 14 is not exactly 2.0000, (or the adjustment above would have been -2.5 log(2.0000) = 0.753 instead of 0.745.

5. TYPES OF PHOTOMETRY (very briefly):

• aperture photometry (as used in this course)
  • number of counts within a given aperture
  • generally subtract off a sky component based on a measurement in an annulus around the star
  • need to add an aperture correction to correct for light outside the aperture
  • for WFPC2, generally works as well, or better than psf-fitting photometry.
• PSF-fitting photometry
  • designed for crowded regions where the PSFs of different stars overlap
  • for WFPC2, often does not work as well as simple aperture photometry, probably due to the undersampled and variable nature of the PSF
  • most common package is DAOPHOT (Stetson, 1987, PASP, 99, 191), but DoPHOT also popular.
• surface photometry
  • for extended sources such as galaxies
  • units are magnitudes per arcsecond**2
  • use stsdas.analysis.isophote
• narrow-band photometry
  • main issue here is that the emission line does not fill the full wavelength range of the filter the way a continuum source does.
  • see FAQ at:

    http://www.stsci.edu/ftp/instrument_news/WFPC2/Wfpc2_faq/wfpc2_nrw_phot_faq.html

HENCE: By definition, a star that yields 1 DN in 1 second has a magnitude = zeropoint.

• Systems based on Vega (i.e., Vega defined ~ 0.00 in all filters).
  • Jonhnson (UBVRI)
  • Cousins (RI)
  • The Landolt system (Johnson UBV and Cousins RI) system is the most popular.
  • The VEGAMAG system used in SYNPHOT.
  • Other are Stromgren (uvby), Washington system, etc, see p. 108 of the SYNPHOT manual.
• ABMAG
  • Based on a spectrum with constant flux per unit frequency.
  • Moderate usage, seems to be increasing.
• STMAG
  • Based on a spectrum with constant flux per unit wavelength.
  • Rarely used.

NOTE: STMAG and ABMAG are designed to roughly match Johnson V, but as you get farther away in wavelength they can be very different (e.g., at I STMAG is about 1.2 mag different; see Table 8.1 of the WFPC2 Instrument Handbook.

NOTE: The WFPC2 equivalents of the Landolt UBVRI system are the F336W, F439W, F555W, F675W, and F814W filters. Since these are not (can not) be exactly the same as the filter/response combination used to define the UBVRI system, we will need to make corrections which are a function of the spectrum of the objects. These are generally called "color terms", and will be discussed in Photometry II.

There are three basic ways to get your zeropoints:

1. Do it yourself (based on reliable photometry of some object(s) in your image).
2. Use the value in the header of your image deposited there by SYNPHOT (but be CAREFUL that you understand the STMAG system since that is what it is given in).
3. Use the value from the Data Handbook, Table 28.1.

NOTE: Many people still use the original Holtzman-2 (table 7 + equation 8), which is generally in good agreement, but can be pretty far off at large or small wavelength.

In general, method 3 is the easiest and most accurate way to go.

FINAL RANDOM NOTE:

"Why is HST photometry so much better than ground-based ?"

• Smaller Point Spread Function (PSF) means that you can use smaller apertures (often by a factor of 10), and hence have much less background light to contend with (e.g., up to a factor of 100).
• The sky background is fainter