Generally, techniques for observing in the near-IR (0.8-2.5 mm) are similar to those used for optical CCDs. However there are a few important differences. First, we recommend pointing up in the I filter if you are going to be using optical finder charts. Often the difference in the longer wavelength filters is significant (especially at short exposure times) and can be confusing.
NOTE: The
NIRIM optics are slightly chromatically sensitive, so it may be beneficial to
change the array stage location of wavebands as well as changing the plate
scale position. However, chromatic
aberration has not yet been detected at MLO or at WIYN. If you do
note any aberration, please let us know.
Dark frames: Move the filter wheels to the cold plate position (it's actually a metal plate in the filter wheel). You must integrate on a dark frame the same amount of time that you integrate on your exposure. For example, if you expose on a star field for 20 seconds, you must integrate on the cold plate for 20 seconds. You cannot integrate on the cold plate for 1 second and multiply by 20 because there are non-linear contributions to the dark current (e.g. amplifier glow).
Sky subtraction: At the longer wavelengths, around 2 mm, the sky emission is so bright that it is difficult or impossible to see your object without subtracting sky. Strategies for sky subtraction are different for point sources (stars) and extended sources (nebulae and galaxies).
For stars, you can move your object around the array, then take a median of all your exposures and you will have a nice sky frame. This sky frame should be subtracted from each exposure (the IRAF script nproc allows for a sky image specification). This method will only work over relatively short intervals of time, perhaps less than an hour, but depending on the sky conditions, the time scale for change may be much less than an hour. Note: Sky subtractions must be at the same exposure time as your source exposure. If you do a sky subtraction, you do not need to subtract a dark frame as well.
For extended objects like galaxies, you will need to find a good off-source position that is at least free of emission, and if possible, free of very many stars. As with all off-positions, you want to be far enough away to be off your source, but close enough to your source so that the sky you are measuring is a reasonable approximation to the sky at your source. Note: As noted above for stars, sky subtractions must be at the same exposure time as your source exposure. If you do a sky subtraction, you do not need to subtract a dark frame as well.
There is a positional dithering script at WIYN that will move the telescope in a pattern. That pattern is selectable between point sources or extended sources. It also allows your personal favorite dither patterns to be read in as ascii files. See Section 6, titled “The Dither Script at WIYN” by Gene McDougall, for details.
Flat fields: Twilight sky flats or dome flats can be used. We do not have enough experience yet at WIYN to recommend one of these over the other. Note that dome flats are preferable for the optical imager. Feedback is appreciated on your experience with these two methods. See Appendix 6 for suggested exposure times and dome lamp intensities.
Flux calibration: For photometry, we are using the standards located on the IRTF web page http://irtf.ifa.hawaii.edu/IRrefdata/ph_catalogs.html. (We need to get a copy for the WIYN web pages). For broad band imaging (J,H,K'), you will need to use faint standards. About 8-9 magnitudes is the brightest star you want to use for flux standards. Brighter stars will saturate the array. The bandpass of NIRIM filters are not necessarily the same as in those used for the various catalogs located on the IRTF web page, and we cannot claim that high precision photometry is achievable. Again, feedback is very welcome.