Some notes for observing at the Swope 1-m telescope Kevin Krisciunas, 9 January 2012 How can one know the right exposures for a particular supernova? You cannot know ahead of time. But during the course of a night I display EVERY image I get of a field that is supposed to have a supernova in it. Of course the simple way is just to type display
1 in the iraf window. This will return two numbers (z1 and z2) which "display" has used to scale the image. And the image is displayed in the ds9 window. But, often a supernova is buried in a host galaxy. So try this once you have "z1" from one iteration of display. Say z1 was 500. Try display zs- zr- z1=500 z2=1500 then hit and iraf will prompt you for a window (1:16). Then you can use the right hand mouse button to adjust the scaling on the ds9 display of the image, and you will probably see the supernova. Make frequent use of imexam and the "r" command. This will give you a radial plot of a star located where you have the cursor in the ds9 window. If your standard star (or supernova) gives more than 22,000 counts in such a radial plot, then you are getting to the non-linear realm of the chip. If you get more than 32,000 counts, you are saturating, and the exposure time was definitely too long. ________ If you take twilight skyflats, you have to offset the telescope between exposures. Why? Because as it gets darker after sunset, the stars appear in the images and by offsetting the telescope you can get rid of these stars when the filter by filter median skyflats are made. But you need 3-5 images, not just 1 or 2. At the telescope control system keyboard type RBB 30 DBB 30 Then shortly before a skyflat exposure you type "NN" for example on the TCS control keyboard. Within 1-3 minutes after sunset one can usually start taking u-band skyflats and not get more than 22,000 counts. When an exposure is done you can type imstat in the iraf window and you will be given the mean counts of the image. These should be 9000 to 20000 for skyflats and domeflats. Though I've been taking domeflats too, I prefer skyflats for data reduction. After sunset but before it gets too dark I can usually do 5 skyflats in u (exposure times 3-5 sec) 5 in B (exposure times 3-8 sec?) then 3 in V (exposure times 10 to ??? sec) (3 in g) or (3 in r) or (3 in i) There isn't time to get skyflats in all 6 filters before it gets too dark. Of course, as the sky gets darker, you have to increase the exposure times of the skyflats. I get the mean counts of the skyflat and domeflat frames using the "imstat" command. One can do them all by typing imstat ccd* ____________________ For the proper calibration of photometry one also has to observe standard star fields on clear nights. (If the sky has obvious clouds, then the only real use of a standard star field would be for doing a pointing test or a focus test.) Here's a good plan for observing on a photometric night: skyflats (exposure time 3 seconds at start, maybe 180 seconds at end) pointing check If you use coordinates from the Astronomical Alamanac they might be 2012.5 coordinates, so you have to type "MP 2012.5" on the TCS keyboard. When done with pointing check don't forget to type "MP 2000.0" for regular observing. Go to a standards field. Do 7 exposure focus image with subraster off. Filter is V. Don't forget to turn on subraster when done determining focus. If it is a clear night, observe the standards field in all 6 filters (BVugri). A photometric night might then have a sequence of pointings like this: standards field 1 or more supernovae standards field 90-120 minutes after previous standards field 1 or more SNE standards field 90-120 minutes after previous standards field 1 or more SNE standards field 90-120 minutes after previous standards field 1 or more SNE standards field 90-120 minutes after previous standards field Ending the night with a standards field. Why start and end the night with standards and do them every 90-120 minutes? So that one can demonstrate from those images how constant was the transparency of the sky. _______________ Other things to consider: If the supernovae were observed over a wide range of airmass (1.0 to 2.2), then for best calibration of the SN images you should observe standards over the same range of airmass. Why? So that the atmospheric extinction coefficients can be determined in all the filters. Even under clear sky conditions these coefficients can range by +/- 40 percent. Make liberal use of "imstat" and "display". When first setting on a new location make sure you're pointing at the right place by comparing a displayed image with the finder at the CSP2 website. No sense using up 45 minutes of telescope time making 6 images of the wrong place on the sky. When you display an image, look for a cosmic ray hit close to the location of the supernova. These are often spikes that take up only 1 pixel, while a star takes up many pixels. A surface plot of an image can be obtained with imexam and its "s" command. If there is a cosmic ray hit very close to a supernova, it's often best just to take another image. How close is "close"? 15 pixels is not a problem. But 5 pixels could make for problems. Another reason to display and inspect every image is that maybe you forget to change the focus, change the filter, or turn the guider back on. Of course, you don't want to waste a lot of time doing this. One can inspect an image while the next image is being taken. That uses up no telescope time. ------------------- I wasn't quite explicit in the notes I sent you earlier today. 1) If there are some clouds, then your only option is domeflats and SN images. 2) If it's nice and clear one can also take twilight skyflats. And if it's clear, you should really observe standards too, not just supernovae.