Part 3: Measure the Images
Step 3 Updated 2002 Dec 1!!
This part is the most technical of the 5, and will likely take the most time. However, it is in fact rather repetitious once you get into it, so it picks up pace as you get into it. One caution here is not to give in to the temptation to try to cut corners. If you get off track it can take a while to get back.
In each image you will see the target star CY Aquarii as the brightest object near the center of the field, surrounded by a number of fainter stars. We will try to use these stars as "comparison stars" to do differential photometry on CY Aqr. So far as we know at the outset (and will learn more about them as we proceed), these stars are not likely to be variable, and so will measure the brightness of CY Aqr relative to them. We do this because some (maybe most) of the data were taken during conditions of variable atmospheric transparency, specifically thin cirrus clouds. By doing "differential photometry" we can remove the affects of thin clouds as over such relatively small fields of view, since the effect of thin clouds is to diminish the light of all stars on the image by the same amount. (Another way of saying this is that clouds should not have any structure on the scale of a few arcminutes, which experience suggests is true in general at the levels we are concerned with here).
This is why we will be doing our analysis without an absolute flux calibration. Further, since differential photometric monitoring is one of the YALO project's primary missions, only very rarely are standard stars observed. Differential photometry is a very powerful technique as we will see, as it lets us get information from variable targets even during observing conditions that do not permit absolute photometry.
In addition to CY Aqr and companion field stars, there are other things including detector blemishes that will appear in all frames, and bright pixels which mark the passage of a cosmic ray through the CCD during the exposure. These leave behind little bright spots that look different from stars, and which may be ignored unless they land right on a star.
Our goals for this part are to make the following measurements:
The outcome of this part will be to compile a table of raw photometric measurements for each image of CY Aquarii and two nearby comparison stars. These measurements will form the basis of the differential photometry analysis in Part 4.
All measurements you take during this step must be recorded in your lab notebooks.
Step 1: Identify the Variable and Comparison Stars
Choose an image from among your data set (other than the very first image) and display it in XVista using the TV command. Using the finding chart you created in Part 1, verify that your images are of CY Aqr and vicinity. Identify the stars on your images and on the finding chart.
The image scale of the ANDICAM CCD detector is 0.6 arcseconds/pixel when the images are binned 2x2 as we have done for these observations.
You will use these names for the comparison stars for all subsequent measurements. Make an annotated finding chart from your "reference" image in this step. You will include this image in your notebook as part of your final writeup. You should label the orientation of the image on the sky (showing which way is North and East is sufficient), the image scale, and neatly label the target star (CY Aqr), and the two comparison stars (C1 and C2). Also note the UTC date and time of the observation.
Step 2: Measure the Seeing and Sky Levels
Start by using your "reference" image from Step 1 above.
tvstar scale=0.6and put the cursor onto the image of the variable star. If you hit the "C" key, it will plot the radial brightness distribution of the stellar image, showing pixel intensity as a function of radial distance from the center of the image. Since each pixel is at a different radius, the points should scatter along in radius in at apparently random intervals.
Now, starting with the first image in your data set, do the following:
Now repeat this procedure for every 5th image in your data set (i.e., do this for images 1, 5, 10, 15, 20, and 25). This will give you an idea of the typical seeing and sky levels during the observations. (Doing it for all 25 galaxies is excessive).
Now you are ready to begin measuring the brightnesses of CY Aqr and two comparison stars on each of your 25 images.
Step 3: Estimate the size of the photometry and sky apertures to use.
From your table of seeing measurements from Step 2, compute the mean seeing for the data set.
For example, if you measure the typical FWHM to be 2.63 pixels, the sky radius should be
sky = 2.63 * 1.7 = 4.47which you should round up to 5 pixels.
For example, the star aperture has a radius of 5 pixels. The inner radius has been adopted to be 11 pixels, and the outer radius is then
r_outer = sqrt(r_star2 + r_inner2)For r_star=5 and r_inner=11, this gives r_outer=12.1, which you would then round up to 13 pixels, so that you have an annulus width of 2 pixels. Thus,
Star Aperture Radius = 5 pixels Sky Annulus R_inner = 11 pixels R_outer = 13 pixelswould be the parameters for the photometry apertures in this example. Your values, of course, may be different.
Step 4: Build a Template Star List
The outcome of Step 3 was to select the size of the object and sky apertures to be used for measuring the star and sky brightnesses on all of your images. It is essential to use a common aperture for all of your measurements.
Examining calibration images acquired with the ANDICAM over the last few months, the CCD gain and readout noise are:
gain = 3.6 electrons/ADUThese values are quite stable from night to night (actually from month to month), so we will adopt these for our noise model of the ANDICAM CCD detector.
readout noise = 11 electrons (rms)
In XVista, start with the first image:
markstar newThis starts the MARKSTAR command and activates the interactive TV display cursor.
save phot=cyaqr.phoThis will create the file "cyaqr.pho" with the star list for the first image. This will be our "template" star list.
Step 5: Measure the sky-subtracted counts for CY Aqr and the two comparison stars
Now that you have created a template star list, you can measure the brightness of CY Aqr and the two comparison stars on all of your images.
In this step, you will use the XVista command aperstar which uses the star aperture and sky annulus to estimate the sky-subtracted counts in the stars. aperstar does all of the dirty work of measuring and subtracting the sky levels, and estimating the noise in the measurements.
Follow these steps:
markstar auto exitYou should see squares plotted around your variable and comparison stars, along with some other info printed on the screen (e.g., the shift in rows and columns between the image and the template).
aperstar 1 star=5 sky=11,13 gain=3.6 ronoise=11You will see a printout like this
524.60 870.69 4.068E+05 + / - 3.81E+02 157.37 612.34 8.181E+03 + / - 1.83E+02 ...The values in the table are as follows:
Rcen Ccen Counts + / - SigmaWhere the first two numbers are the measured centroid of the star in row and column coordinates, "Counts" is the sky-subtracted instrumental counts of the star in ADU, and "Sigma" is the estimated uncertainty in Counts in ADU, estimated using a standard CCD (Poisson + Readout Noise) noise model with the gain and readout noise given above. APERSTAR does all of the dirty work of computing the sky, counting the pixels in the star and sky apertures, and performing the formal noise estimates.
Now you can quit XVista, you're done with the images.
Step 6: Get the UTC and HJD times for your observations
Using the observing logs, compute the UTC time at mid-exposure for each of your images. Write these down in standard hh:mm:ss.s format.
In order to merge the data from different nights, we need to convert the UTC times at mid-exposure into Heliocentric Julian Dates. This puts the data all on the same timescale, removing effects due to the motion of the Earth around the Sun between observations taken on different nights.
For this step you will use the hjdcalc program. It works from the Unix command prompt as follows:
hjdcalc ccyy-mm-dd hh:mm:ss RA DecWhere
ccyy-mm-dd = UTC Date (e.g., 2000-09-15) from the observing log hh:mm:ss = UTC time at mid-exposure (UTC log time + 10 seconds) RA = Right Ascension of the object in hh:mm:ss format DEC = Declination of the object in dd:mm:ss formatBoth RA and DEC are in J2000.0 equinox. You need the RA and DEC you got from your catalog search in Part 1; do not rely on the RA and DEC given in the observing log. The printout from hjdcalc will be something like this. The command
hjdcalc 2000-09-25 04:53:25.3 22:37:48 01:30:49Will print out
Object RA=22:37:48.0 DEC=01:30:49.0 (J2000.0) UTC 2000-09-25 04:53:25.30 JD 2451812.703765 MJD 51812.203765 HJD 2451812.709105 (deltaJD=0.005340)The first two lines above just give you back your RA, DEC, and UTC Date/time. This lets you make sure it got them OK. The following three lines give
JD = Julian Date at mid-exposure (i.e., at Earth) MJD = Modified Julian Date = JD - 2400000.5 (you can ignore this) HJD = Heliocentric JD at mid-exposure deltaJD = difference between the JD and HJD in daysThe only thing that changes for your 25 observations is the UTC time at mid-exposure; the RA, DEC, and UT calendar date (2000-xx-xx) are all the same.
Record the just the HJD for each of your observations to the nearest 0.00001 days (= nearest 0.8 seconds)
Step 7: Compile your results into a single table
Now bring all the pieces above together and compile a single "master" table of your measurements. This constitutes the preliminary photometric results that will then be analyzed in the next part.
Your table of results should be formatted as follows:
Image UTCmid HJDmid Var ErrV C1 Err1 C2 Err2where:
Image = name of the image (e.g., ccd000925.0014) UTCmid = UTC time at mid-exposure HJDmid = Heliocentric Julian Date at mid-exposure Var = sky-subtracted signal of the Variable in ADU ErrV = uncertainty in Var in ADU C1 = sky-subtracted signal of comparison star 1 in ADU Err1 = uncertainty in C1 in ADU C2 = sky-subtracted signal of comparison star 2 in ADU Err2 = uncertainty in C2 in ADU
Because you all have different computer resources, I am going to demand the following "uniform format" just to save me time compiling the master data from all of your measurements:
Flat ASCII text format with no TAB characters.
It's that simple. No Excel, no SigmaPlot, no Kaleidograph, no nuthin'. I want flat, vanilla, just plain and boring-as-it-gets ASCII text with spaces between the values. Use just enough spaces to keep the columns lined up and readable, but be sparing. Please don't make me have to do extra work to decipher your data files. Thank you.
Step 8: Submit your preliminary photometry results
Deadline: Thursday, 2002 Dec 5
Now that you have your preliminary photometry results ready, please submit them to me via email to Prof. Pogge (firstname.lastname@example.org) on or before 5pm on Thursday, 2002 December 5.
Be sure to indicate your name in the email. I will confirm receipt of the email when I receive it.
This milestone has two purposes
You're done with the raw measurements! On to the final analysis steps...
Go back to Part 2: Retrieve your CCD Data
Return to the CY Aqr Lab Main Page
Copyright © Richard W. Pogge, All Rights Reserved.