Observing A Star

In most cases, we will obtain several frames of the same star, at slightly different positions on the chip and in different filters, to create an "observation set". For example, if your observing plan calls for observing the star SW And at 10:30pm, you might obtain 3 V and 3 I frames of the star in close succession, and we would refer to these as a "set".

1) chose a star to observe (see Planning Your Observations) and get out its finder chart.

2) point the telescope to the star's coordinates. They may be preset in the telescopes coordinate table. If not, feel free to place them there, but record on the finder which number the coords are in the table.

3) take a 10sec exposure (flash camera icon) to be sure you are pointed correctly. In the dialog box,

• set exposure time to 10 sec,
• select Exposure Mode = Grab,
• select Frame Mode = Light Frame Only,
• select Resolution = 1024x1024, and
• hit OK.

Can you identify the variable and comparison stars relative to the finder chart? Is the variable star near the center of the chip? Are all the comparison stars in the field of view? If not, move the telescope using OFFSET -- for scale, remember, the CCD has a 20x20 arcmin field of view, ie, 1200x1200 arcsec).

4) once you are happy with the pointing, run into the dome and ensure that the slit is not partially or fully blocking the telescope. If, after some experience, you find the AUTODOME is working correctly, you can skip this step.

5) On the finder chart, there are instructions on how to take an observation set for this particular star. For example,

• V 150sec
• I 50sec
• offset 9" E and 9" N
• I 35sec
• V 110sec
• offset 9" E and 9" N
• V 80sec
• I 25sec
• one bias frame.

6) Set the filter wheel to V (filter button 3). Take the first observation (flash camera icon).

• set exposure time to 150 sec,
• select Exposure Mode = Grab,
• select Frame Mode = Light Frame Only,
• select Resolution = 1024x1024, and
• hit OK.

7) Check the sky level; if it is bright (>6000 counts), you may want to shorten the exposure times and take more exposures through each filter (or, dawn may be coming on, or you may be pointed near a bright moon... check in the dome if the sky level seems odd).

8) Save the image to disk (floppy disk icon).

NOTE: For consistency and to avoid confusion, we will name our variable star files with the name listed on the finder chart. In this case, the star "SW And" appears as "sw_ and2v1", where the 2v1 indicates that it is the first observation through the V filter in the second set (eg, we obtained six frames of SW And a few hours ago, now we are obaining a second set of frames on this night). The bias frame would be called "sw_and2z1".

9) Go to Step6 and take the next frame in the sequence, or, if required, move the telescope using the OFFSET command on the telescope PC. If you have finished the sequence, go to Step10.

10) Record comments in your log : Starname, UT/Local Time, Sidereal Time, CCD Temperature, Exposure/Filters, Comments (eg, moon & sky conditions, problems, errors in file naming, etc). Put a cross on your observing plan sheet for this star at the time of observation, so you don't accidentally re-observe it. Go to the next star in your observing plan.

IMAGE ANOMALIES: sometimes something goes wrong, and your images look funny. Here is a nice web page devoted to describing anomalous images, identifying what went wrong, and how it can be corrected. Please inform Dr. Layden or Dr. Laird if you cannot correct the problem.

Updated 1999 June 03