ethod 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 stellar image which has the smallest full width at half maximum intensity (FWHM). To do this, zoom in on the star to a very high zoom. Then using the cursor, ﬁnd the brightest pixel (again, make SURE it is not saturated!), and subtract the background brightness. Next, moving along a row, ﬁnd the pixel with the brightness (after subtracting background) closest to half of the brightest pixel; and write down its column number (x number); then go to the other side and again ﬁnd the pixel at half intensity; the diﬀerence in column number is the FWHM. Check to see that you get a similar number (to within a pixel) in the opposite direction. Keep in mind that as the focus improves, more and more light will be concentrated in fewer and fewer pixels, which may consequently saturate; keep checking, and adjust exposure time accordingly.
Keep in mind that you must adjust the stretch of the image displayed in order to see the essential brighter part of the stellar image. In other words, the maximum in the display stretch should be set maybe 10% above the value of the brightest pixel; the minimum should be around zero.
You do not need to save the focus images.
Although if you know the optical layout of the telescope you can determine how the optics inverts the image, it is generally unclear how the CCD is oriented and read out.
Relative to north, the observed image may be rotated by an unknown angle; in addition, it is possible for the image to be inverted (ie top to bottom or left to right). The simplest way to determine the image orientation is to (1) set the guide speed to ”5 = B:1” (2) begin an exposure of a bright star centered in the CCD frame. (3) Wait about 30 seconds. (4) Then guide the telescope to the North (by pushing the North guide button for about 10 seconds).
(5) Next, drive the telescope to the east for about 30 seconds and stop the exposure. The resulting image should show an L-shaped pattern, with a knob at one end of the L. The knob is where the star was when you started the exposure; the side of the L start