Quote:
Originally Posted by alistairsam
what is ADU? "So if your camera is a 16bit camera, shoot to reach 10k or 20k ADU."
"whatever duration will give you approx 1/3 of your camera full well." is this 1/3rd of the exposure time for the image frames?
also, what are the lightboxes used for? i've seen a few black boxes with white insides at the end of a few scopes at snake valley.
for dark frames, is it acceptable to just cover the ota instead of the camera lens opening?
I've tried my hand with deep sky stacker so far.
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ADU = analog to digital units. Basically when you take an exposure photons strike various pixels on your sensor. When a photon strikes a pixel an electron is released and stored in a well (one for each pixel). When you finish the exposure the electronics in your camera read the pixel values (count the number of electrons in the well of each pixel) and produce an image based on these values (e.g. brighter areas = more electrons in each well). This conversion of the analog signal (photons releasing electrons into the well) is converted to a digital signal via the analog to digital converter present in your camera. So in a nut shell 1 photon = 1 electron in the well = 1 ADU. You can see the ADU count if you open the information window on prgrams like MaximDL or CCDstack etc. They will be given as average, max, min values etc.
Now using a mono CCD as an example it is fairly easy to understand what is going on. These camera's have a analog to digital converters that are capable of delivering images with 16 bit depth. This means it can produce 65535 shades of grey. Pixels with low or 0 ADU counts will be black (or close to) whilst pixels with 65535 ADU counts will be white. Now not many chips are actually capable of producing this depth as the well size varies for different types. A general rule here is that small pixels have small wells whilst large pixels have large wells. What this means is if you have a CCD chip with 5 x 5 um pixels chances are the wells will be full before the 65535 ADU value is reached (e.g. it may only have 45000 ADU well capacity). This means they become saturated more quickly and limits the time you can expose for. Different chip manufactures generally state well depth as part of the specifications for each chip and this is useful to know. DSLR camera's only have 12 - 14 bit converters (less dynamic range e.g. fewer shades of grey).
Now darks are easy to take and as you have mentioned it is fine to just put the cap on the scope and shoot away. But different types of cameras have different methods. Your dark frames should be at least the same exposure duration as your light frames (The actual images you take). Some sources state they work better when you expose your darks for 5 x as long as your lights. Darks are also temperature dependant and if there is a difference of a few degrees between the temp that the darks and lights were shot at they probably won't work that well. It should then follow that if you are using an uncooled DSLR that the darks must be taken at the same time as the lights (e.g. on the night). I use a cooled mono CCD camera which can be set to an exact temperature so it is possible to take one set of darks and store them in a library which can be used to calibrate all images taken for at least a year afterward. My max exposure is about 15 mins so I took 10 dark frames at -20 degrees for 75 mins each and combined them into a master darkframe. I use the master dark to calibrate all my pics even when the exposure is only 5 mins or even as little as 30 secs.
Some people have terrible trouble getting good flats. As Marc has stated you should aim for 1/3 to 1/2 full well depth of the chip which is why it is useful to know what it is. To get good flats you need to have a totally uniform light source which is why many people use a light box to do these. My light box has a varible voltage setting so I can increase/decrease light intensity to get my 1/3 full well. When using mono camera's with filters, each filter will let different amounts of light through (e.g. luminance = lots of light so settings are low for a short exposure whilst SII only lets in a little light so high settings for a longer exposure to achieve 1/3 saturation) so it is useful to be able to adjust for different filters no only for exposure time but also light intensity. For a one shot colour camera (e.g.DSLR) you only need to set the light intensity once.
Basically all you need to do is take a heap of bias shots (20 - 40) with the camera/scope covered at the absolute minimum exposure time your camera is capable of and combine them into a master bias frame. You then point your scope at a uniform light source (e.g. light box) and take subs until you get between 1/3 - 1/2 ADU saturation. Once you have found the setting take about 10 - 15 subs (they are usually only 2 - 6 sec long) and combine them into a master. You then use these along with your master bias and dark frames to calibrate each light sub (your actual picture). Once this is done you can stack your light frames to make an image. With my camera I have found an ADU value of about 23000 works best for flats. Flats always need to be taken on the night as the dust, focus, temp etc must be exactly the same as your image whether you have a cooled CCD or not. You should always calibrate your subs before you do anything else (e.g. deconvolution, debayering, stacking, noise filter etc) it is the very first step in processing.
Whilst darkframes will remove uneven reponse from individual pixels (e.g. hot pixels) flat frames will remove uneven lighting across the image (e.g. dust on the lens/filters/focal reducers/chip and vignetting). If done properly the result is a much cleaner image (better contrast and detail) to start your processing.
Hope this helps
Mark