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This lesson explains how to generate useful calibration images, how to process them and how to use them.
These are the easiest calibration frames to take. They are 0 second exposures with the camera shutter closed. Since the main imaging camera (SXVR-H36) has not got a shutter the dome must be kept dark, with the primary mirror cover on, for the bias frames to be useful.
Bias frames, like any other sort of exposure, are collected using ''MaxIm''. Since they are required in great numbers for averaging a preset called 'Bias Run' has been created for the purpose of gathering bias frames. It takes 40 bias frames with 1x1 binning and 40 bias frames with 2x2 binning. Of course, if this is not what the observer requires he may modify the run or create his own preset. The images should be stored in a sensibly named folder in the 'MaxIm Dl 5' folder in 'My Documents' according to the date they were taken on.
When the imager is satisfied with the bias frames collected, which can be opened in MaxIm for inspection, he must set them for calibration. In MaxIm, click ''Process'' at the top of the window, then ''Set Calibration''. The ''Set Calibration'' window should appear. Click the ''Browse'' button in the ''Source Folder'' section and direct MaxIm to the folder which contains the bias frames and click ''OK''. Now click the ''Auto-Generate (Clear Old)'' button. MaxIm will scan the folder for calibration images and present them in groups in the middle of the window. Now click the ''Replace w/ Masters'' button. This combines the bias frames into one which makes calibration faster. Click ''OK'' when this is finished.
Now when images are calibrated in MaxIm they will be bias subtracted. However, this is not enough to get a clean image. Different frames will be required.
Dark frames are also easy to gather so long as the observer has enough time. For dark frames to work they must have the same exposure time as the light images they will be calibrating. As a result, many different dark frames will need to be taken. They will have differing exposure times and binning. Since dark current is also temperature dependent, the dark exposures must be taken at various different stages of camera cooling, although the most useful ones will likely be the ones taken when the camera temperature has stabilised.
The same procedure for preparing the dome for bias frames must be followed for collecting dark frames. A preset for dark frames has been created in MaxIm which the imager is free to use or alter. The frames will be stored in a similar fashion to the bias frames and should be checked to make sure that no light leaked into the CCD. Collecting dark frames can take a long time and at least four of each type of dark frame are needed. It is best if the observer knows what sort of dark frames will be needed in advance so he can modify the preset in good time.
To add dark frames to the calibration process go to ''Process'' -> ''Set Calibration'' -> ''Browse''. Indicate which folder has the dark frames and then click on ''Auto-Generate (Keep Old)''. This will keep any other calibration frames (e.g. the bias frames from before) and add the dark ones. At this point ''Replace W/Masters'' can be clicked. Click ''OK'' when finished.
Now when images are calibrated in MaxIm they will be dark and bias subtracted. The dark frames used are automatically bias subtracted themselves. The final calibration image is the 'flat field frame'.
The idea of a flat field frame is that it is an image of a uniformly illuminated, flat object. Flat field frames can be collected in two ways; sky flats and dome flats. Sky flats are images taken of the twilight sky. This can be tricky to gather due to the changes to the exposure time as the sun sets and the order in which flats must be obtained for the different filters. Dome flats can be obtained by pointing the telescope at a white screen in the dome. They can be collected at any time of day and may be collected in any order and for any exposure time.
The procedure for collecting sky flats is complicated and not well understood by the author. However, the ART has the capability of automating the collection of sky flats at dawn and dusk. Students should speak to a real astronomer if they want to know how to take good flat field frames if they do not already know how to do so.
Dome flats are somewhat easier to gather. The main difficulty is getting the dome evenly illuminated. The current setup for dome flats has the telescope in its home position and its tracking turned off. '''To turn off the telescope's tracking go to ''TheSky'', click ''Telescope'', then ''Options'' and finally uncheck ''Tracking''.''' Slew the dome so that the telescope is pointing at the centre of the white screen mounted on the inside of the rotating dome. To illuminate the screen evenly, switch on all the low level light fluorescent tubes around the dome. Hopefully, the telescope will not cast a bad shadow on the screen. Students can experiment with the position of the telescope and dome to see what the optimum orientation is for collecting truly flat field frames.
Flat field frames should be collected so that the image isn't saturated anywhere. Imagers should aim for about 30000 counts across the image in each filter. The luminance filter will require the shortest exposure time at a given binning and the blue filter will require the longest. Imagers should experiment as necessary and then collect at least four suitable flat fields for each filter in each binning setting.
Flat field frames need to be both bias and dark subtracted so the calibration frames gathered earlier can be used. This process is handled automatically. Perform ''Set Calibration'' again to generate a 'Master Flat' for each filter and binning setting. Now all the calibration images have been gathered. Whenever real light frames are gathered during an observing session these master frames can calibrate the raw images.
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