Astronomy 350 - CY Aqr Lab Part 2

Astronomy 350 Au2000

CCD Differential Photometry of CY Aquarii

In this part, you will retrieve all of the CCD images for your night, and gather all the pieces you need to measure the brightnesses of the variable star and its nearby "comparison" stars. The data have already been through the basic YALO reduction pipeline, and thus have already had the following data-reduction procedures performed:

The DC bias level was measured using the "overscan" columns on the images and subtracted.
The images have been divided by a suitable B-band flat field image taken on the afternoon before the night your observations were taken.

The result of the basic pipeline processing is a set of reduced CCD images showing the region of the sky around CY Aquarii in the B-band for a roughly 20 minute period on a particular night.

All notes you take during this step must be written into your lab notebooks.

Step 1: Retrieve your data

The data for your assigned night are packed into compressed Unix "tape archive" or "tar" files, stored on the main data disk on regulus in the Astronomy 350 Lab Data directory: /home/regulus/ast350/LabData/CYAqr/. For each of you, the tar file will be filed by your last name with the .tgz file extension. For example, student "Pogge" would have their data in a file named

   /home/regulus/ast350/LabData/CYAqr/pogge.tgz

The assignments are as follows:

        Person     Date Observed
     --------------------------- 
      Adkins       2000 Sep 25  
      Durben       2000 Sep 26  
      Gero         2000 Sep 27  
      Grilliot     2000 Sep 28  
      Heinrichs    2000 Sep 29  
      Lundell      2000 Sep 30  
      McFerran     2000 Oct 04  
      Payne        2000 Oct 05  
      Pitts        2000 Oct 06  
      Reed         2000 Oct 07  
      Scaife       2000 Oct 09
      Shepherd     2000 Oct 08  
      Sutherland   2000 Oct 09
      Walker       2000 Oct 11
     ---------------------------

The date above is for the "Observing Day", which at YALO is the date of the start of the night in local time (in this case, Chilean Standard Time). The "Observing Day" begins at Local Noon. This is not the UT date of the observations; that information must be derived separately from the observing logs (or the image headers).

Copy your tar file into your personal directory. For example:

   cp /home/regulus/ast350/LabData/CYAqr/pogge.tgz .

And you are ready to begin the next step.

Step 2: Unpack the "tarball" of data for your night

A compressed tar file is known colloquially as a "tarball". It contains all of your CCD image files in a single, self-contained archive file for ease of copying. To actually get at your files, you need to first unpack the tarball. Follow these steps:

Create a "working" directory to hold the 25 images that make up your data set:
```
   mkdir working
```
Go into your working directory and unpack your tarball:
```
   cd working
   tar xzf ../pogge.tgz
```
The last command above uses the Unix "tar" command to extract ("x") and uncompress ("z") the tar file ("f") stored in "../pogge.tgz". After a short delay (it takes a little under a minute to extract and uncompress the 25 images).
Verify that the images unpacked correctly by typing:
```
   ls -l
```
to see a listing of your images, with their sizes. You should each have 25 files.

The CCD image files will all have names like:

   ccd000925.0014.fits

These are the names given to the files by the ANDICAM data-taking system. The filename format is as follows:

   ccd    = data are from the CCD detector channel of ANDICAM
   000925 = date of the observation, in YYMMDD format.  In this
             example, this image was taken on observing day 2000 Sept 25.
   0014   = sequence number of the image (i.e., this was image #14
             taken on observing day 2000 Sept 25)
   .fits  = the file is a FITS format CCD image.

More precise information is contained in the data headers and online observing logs. These data include the UTC time and date of the observation, telescope and instrument data, etc.

After unpacking the files, you should move your tarball of the original data in a "safe" directory so that if there is a problem later on and you need to retrieve the original data, you can just unpack it again as above and start over from a known step.

Step 3: Retrieve the data log and observing report for your night

Using the Web, go to the YALO Observing Web Page at OSU:

http://www.astronomy.ohio-state.edu/YALO

and follow the "Nightly Observing Log Archive" link to find the observing logs from your observing night. Scroll down to find the log entries for "CY Aqr"

For example, for the log for the observations for Observing Date 2000 Sept 24, you would see entries like this:

Project  ImType Object           RA         Dec       ExpTime Filter  SecZ LST      UTDate     UTC        JD           Filename           [Logged@UT]
...
O39901   OBJECT CY Aqr           22:37:47.9 +01:30:49    30.0 B       1.32 00:27:11 2000-09-25 04:53:10.3 2451812.7036 ccd000924.0021     [04:54:05]
O39901   OBJECT CY Aqr           22:37:47.9 +01:30:49    30.0 B       1.33 00:28:09 2000-09-25 04:54:07.9 2451812.7043 ccd000924.0022     [04:55:02]
...

and so forth. The heading of this table is at the top of the log file.

Copy the entries for CY Aqr for your night into a text file in your working directory for future reference. You will need this information to perform various tasks that follow.

Also retrieve the nightly observer's report for that night by returning to the YALO main page, following the "Observer's Reports" link and then downloading the the observer's report for your night. This file will tell you the weather and approximate seeing conditions on the night that your data were acquired.

Step 4: Compile a brief observing summary

Using the logs, make a brief narrative summary of the observations for your night. In this summary, provide the following information:

The date of the observing, and the name of the on-site observer.
The approximate conditions (seeing and weather) during the observations as reported by the on-site observer.
The starting and ending times of the observations. This will be the UTC time of the first image until the UT time the last image was recorded in the observing log. Also record the range of airmass (given as "SecZ", the Secant of the zenith distance, in the Observing Logs).
Estimate the total elapsed time required to execute the observations. Do this by looking at the amount of time that elapsed between when the last observation was logged just before the first CY Aqr image in the log, and when the last CY Aqr image was logged.
Estimate the "observing efficiency" by computing the ratio of the amount of time spent actually collecting photons (e.g., the number of images acquired multiplied by the exposure time) to the amount of time needed to execute the observations.

This information will eventually become part of your final write-up for this lab.

Return to the CY Aqr Lab Main Page

Advance to Part 3: Measure the Images

Go back to Part 1: Finding Catalog Data

Updated: 2002 November 20 [rwp]