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Dark Frames for STAR200 Guided Images My optimum exposure length from my site is about 600secs. I arrived at this value by taking exposures and measuring the background value. At 600 secs I usually have a background value of about 15000. I also have a standard guiding exposure of 2 seconds binned 2x2, this is because seeing is not usually great from my site amd 2 seconds is enough to average out the seeing and give a fairly accurate centroid of the guide star. I would advise everyone to find a ‘standard’ exposure, because that makes dark frames much easier to make and re-use. Once every other month I set aside a whole night to make dark frames and the procedure I use is this:1) Connect everything together as if you are on an imaging night, but use a night of extremely poor seeing, wind or if you have an observatory, bad weather. 2) Then leaving the cap over the telescope, take an image of a couple of seconds. 3) Mark a box – the same size as you normally would on any part of the image. 4) Start guiding with the guide exposure set to your standard – for me that is 2.0 seconds at 2x2 binning, but on the guide window uncheck the x any y boxes so no guiding commands go to the telescope (I usually have my telescope in land mode for this). 5) Take a series of images – I usually take 8 hours worth, the whole night. Now you have a library of dark frames that you can use for the next month or two. Deep Sky Unguided or Externally Guided Images, Planetary Images Again I would suggest taking these on bad nights, use exactly the same exposure and binning settings and take at least as many as you would images. If you use 200 x 20 second frames to make an image, then you will have a nice smooth background when you average them together, by subtracting only one dark frame, you will reintroduce random noise into the image from the dark frame! This is why I always say use at least as many calibration frames as you do imaging frames. Flat Frames There are many ways to take flat frames, but I will only go over the way I use as you do not need a light box or anything. This is probably not the best way to produce flats, but it works for me and is a good introduction to taking and using flat fields. Deep Sky, Self Guided or Unguided hi res (1x1) No matter which method you used to take your image and dark frames, I have found the best mode for flats is hi res interlaced. Flats must be taken at the same focus position as the images and must also be taken with the camera in the exact same place, for this reason you can’t make libraries of flats – they must be taken on the same night of imaging (unless you have a permanent setup and the camera stays fixed on the telescope). My telescope stays in the same focus position night after night so I am able to fix the camera on and take flats in evening twilight before the imaging session, if however I am changing the focal length or have used the telescope visually in between sessions then they will have to be taken at dawn. Dusk 1) Aim the telescope straight up, switch off the motor drives and take an exposure for 1 second in hi res interlaced mode. Keep doing so until it gets dark enough so that you get a background reading of 20000. At this point take a sequence of 50 images, uncheck new window on the image tab and make sure they are being autosaved. 2) As the exposures are being taken, the background value will be falling as it it getting darker, click the exposure +0.1 seconds every now and again to compensate to keep the background value around the 20000. Towards the end you will probably be adding 0.5 seconds to each exposure to keep up, and you will probably finish with exposures of about 10 seconds. 3) Now we have all the flats, we need to take darks for them. Keeping everything identical, put the cover on the telescope, now take a sequence of exposures of the average exposure time – say 3 seconds. There we have it, if you take the flats in the morning then the procedure needs to be reversed, take test exposures of 10 seconds until you get a background of 20000 and work downwards instead.  Planetary Binned 2x2 The procedure is identical to above, but use 2x2 binned mode. Calibration Procedure – Preprocessing MX7C I keep changing my calibration routine and now believe the following procedure to be the best for me. This calibration routine uses 3 different programs, Astroart, Sigma-Clip Beta 10 (available free from http://www.gralak.com/Sigma/index.html ), and AIP4WIN (not essential, but saves a lot of time). To follow this example you will need MX7C image frames, dark frames, flat frames and flat dark frames. 1) In AstroArt select Tools / Preprocessing. Put all of your dark frames into the images box, on the options tab deselect ‘confirm each image’. This will create a master dark frame – save it as that. 2) Now do the same with you dark flats and create a flat dark master. 3) You need to subtract the master dark individually from all the images, but without stacking / averaging them. This is where I use AIP4WIN as it can do this automatically saving them as it goes. But you can do this in AstroArt by selcting Tools / Preprocessing and putting the master dark into the dark frames box and all the images in the images box, then in the options tab you need to select ‘keep’. This will perform all the subtractions and leave the images on the AstroArt desktop, but not saved, you will need to do this manually. 4) Goto Tools / Plugins and select your colour synthesis plugin of choice – I use MX5/7 Colour Sythesis v1.2 (Steve Hill). I then batch convert all the images into LRG&B. My settings are Colour – Empirical, Sample 3x3 and Fat Pixels, and in Luminance Mean 2x2. everything else is unchecked. 5) In Sigma-Clip Beta 10, open all the flat frames. Normalise them (set all the backgrounds to the same) and save the average of the medians as a 16 bit integer. In AstroArt subtract the master flatdark frame from this to create a flatmaster. 6) Colour synthesise this identically as you did the image frames. 7) Using the Tools / Preprocessing box again, select all the L images frames and put them into the images box, select the L flat frame and put that in the flat box. Open an L image and put a square around a brightish star that is on its own to use as an alignment star. On the options tab of the preprocessing box, select auto alignment, select average for the images. 8) This will create your calibrated luminance, Now do the same with the B images and B flat and G’s and the R’s. There you have it, we can now start the fun part of image processing | ||