Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.stsci.edu/hst/training/events/Photometry/SSD981105/photometry1.html
Дата изменения: Unknown Дата индексирования: Sun Dec 23 19:37:48 2007 Кодировка: Поисковые слова: http www.stsci.edu science goods |
SSD Technical/Training Series - Photometry I- III Outline
November 5 - Photometry I - Brad Whitmore
November 12 - Photometry II - Brad Whitmore
December 10 - Photometry III -
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 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
- 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). |
3. REFERENCES (most important first):
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 whole idea is to measure the instrumental counts and convert them to astronomical
magnitudes using the formula: Magnitude = -2.5 log (counts/second) + zeropoint |
1 count | = | 1 DN (i.e., counts and Data Number are used interchangeably) |
Gain | = | 7 means that for every ~7 electrons, 1 DN is recorded |
= | 14 means that for every ~14 electrons, 1 DN is recorded | |
HISTORICAL NOTE: Sometimes gain=14 is called gain 15, (e.g, in header), just to confuse you. | ||
NOTE: The real gains are slightly different (see 4.12 in WFPC2 handbook), and vary from chip to chip. |
The division by seconds can be done by either:
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:
gain = 7 | gain = 14 | |
mag = -2.5 log(100) +22.545 | -2.5 log(50) + 21.80 | |
= 17.545 |
|
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):
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.
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:
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 ?"