by Dawie Venter, March 2017
Both photos which won awards at ScopeX 2016 were taken with a Sky watcher 8 "SCT Schmidt-Cassegrain Telescope, Focal Length 2000 mm F/Ratio F/12. Camera Canon 60 D Full Spectrum DSLR Modification, basically, this the same as an off the shelf Canon 60 Da.
Messier 8 Lagoon Nebula and NGC 4845 a spiral galazy
Any telescope with an F ratio above F6 is normally considered a visual scope and not good enough for astro photography. Well not necessarily, if you take a different approach in acquiring the images. A few tips that I have learned over the years…
Try to get up to 30 minutes of exposure time.
Two ways to do this: take 30 pictures of the object at an exposure time of 1 minute per picture; or 60 exposures at 30 second, but you will need a faster scope if you typically go below 1 minute exposures. Why do astrophotographers prefer to use faster telescopes like the modern astrograph telescopes at a faster focal ratio of F 4? The advantage of a faster focal ratio is that you need less time to achieve the same result. Tracking over 30 seconds is normally a problem with poor polar alignment. After you acquired your 30 images you will also need at least 5 dark frames.
Good idea is to take your dark frames right after your night’s shooting - this will then create darks at the same CCD chip temperature and ISO setting that you used for your normal pictures (which we call light frames). You can also add Bias and Flat files to the stacking process, but I normally skip this step by just cleaning my telescope lens the night before I go out to achieve the same result.
I use a program called "deep sky stacker" (free software) to stack my normal photos on top of each other and combining it with the dark frames to remove any electronic sensor noise created in the capturing process due to heat on the CCD chip. Second tip is I always use Camera RAW files. The problem with JPG files is that they are pre-processed by the camera itself and we don’t want that - we want to have the data as pure as possible for processing later in your favourite post processing program.
I normally use 2 techniques in my post processing software to enhance my photos: levels and curves.
Raw File unprocessed and Processed
You first uncompress the data by moving the levels slides, use auto at first then manual later.
After this you head over to the curves setting.
And the same thing, auto first and manual adjustments later.
Jump between levels and curves and repeat the process until you are satisfied with the result.