s the (relatively) new Apogee CCD camera. Unfortunately, we don’t have operating instructions for the Apogee written up yet.) This handout is written, for the most part, as if you were using the 14” telescope.
Before coming to the observatory, you should read through the instructions for operating the telescope.
You should bring the relevant pages to the observatory with you when you observe.
3.2. Dark Frames
Thermal electrons (ie electrons which get excited due to the non-zero temperature of the CCD chip) get collected by the CCD just like electrons liberated by photons striking the CCD, and dark and light electrons obviously cannot be distinguished. The number of thermal electrons collected increases linearly with the exposure time. Consequently, one needs a dark frame with an exposure identical to the exposure on the object (called the light frame). This must be subtracted from the exposure of your object. Note that since any exposure has a bias oﬀset, subtraction of the dark frame from the light frame also automatically does the bias subtraction.
Note that you must keep a record of all the exposure times for useful exposures, and then be sure that you have obtained and saved dark frames of identical exposure time. By the way, since no light enters the telescope for a dark frame, obviously the ﬁlter does not matter for a dark frame — all that matters is the exposure time. So a dark frame can be subtracted from any frame of identical exposure time, regardless of which ﬁlter was used.
Actually, the 14” CCD observing software will usually take both light and dark frames and automatically subtract the dark frame from the light frame. If this is done, you do not subtract a dark frame during the later reduction stages, since it has already been done.
However, since telescope time in good weather is hard to get, we might prefer to obtain the dark frames after astronomical observations in order to complete the astronomical observations as quickly as possible, in case the weather turns bad or the nebula sets. If this method is chosen, this means that you must be sure to record the exposure times for all useful exposures and then be sure to obtain dark frames for each of those exposure times.
The risk with this method is that the rate at which dark electron accumulate with time may vary, for example if the temperature of the CCD varies; if this happens, then you will not get a good subtraction of the dark current.
To recap, if object (i.e. light) frames are taken with automatic dark-frame subtraction, then do not subtract dark frames during later data reduction. If dark frames are not acquired and subtracted automatically, you must be sure to obtain manually dark frames of identical exposure time to your object frames, and subtract the dark frames from the object frames during your data reduction.
3.3. Telescope Focus: Observing a Bright Star
The optimum telescope focus position varies from night to night due to thermal expansion of the telescope tube. It can also vary with the elevation of the telescope. You need to ﬁnd a relatively bright star (say magnitude 5 or 6), preferably not too far from your target. You want a star which is bright enough so that the exposure time is 2 to 4 seconds so that guiding is not needed, yet does not saturate. Take a series of exposures that range from a clearly out-of-focus setting, through good focus and on to out-of-focus on the other side of the focal point. Write down the focus setting from the micrometer and include it in your write-up. Do this for each of the ﬁlters you will use, since the optimum focus can vary between ﬁlters. Make sure the star is well exposed (more than 1000 counts), but it is essential to avoid saturation of the central pixel(s). While the CCD can record numbers up to 216 = 65536, the response becomes non-linear much before this, at around 16000 counts (it’s hard to give an exact number since the bias has been subtracted, and thus you don’t see the total counts).
The best focus setting is the one which gives the sharpest stellar images. Normally, astronomers ﬁnd the 