Flat Frames

Thought I'd share this one. I never applied any calibration to any of my images but I did look into the whole thing. I found that the hardest calibration images to get were flat frames i.e. construct a light box, put a white sheet on the wall of the obs or stick a white T shirt over the scope. All require fairly even illumination plus the T shirt adds fluff etc to the scope.

While doing a building survey I picked up an offcut of white perspex 5mm thick which when held up to a window showed that light was getting through it. As an experiment I placed the perspex on top of the 10" SCT pointed at the zenith and then took some flats at twilight. I applied the flat fields to some old luminance images to get an exposure time that seemed to get results. When it got a wee bit darker so that I could at least image a star I took one luminance frame and dark and bias frames.

Images below show the before and after calibration result:








The really good thing about it is that no DIY is required, no fluff etc on scope and the perspex diffuses the light enough to get even illumination. PLUS its cheap.

This seems to work very well. Thanks for posting this one.
I am going to try using a light diffusing film, the type used in LCD displays for giving uniform backlighting. Essentially, they are designed to spread light out as evenly as possible. It should be possible to make a lens cap to go over the SCT.
I have really struggled to get decent flats i.e. length/ strength of exposure , setting up the dslr etc.. Has anyone got links to a good site?

Kieran,

After an all-nighter, the only thing on our minds is a mad dash for the bedroom so as to get some sleep. However, an investment of an additional twenty minutes or so to get our bias and flat frames will reap serious rewards when we sit down to process our data the next day.

There is no substitute for good and complete technique and I am happy to see you post these results since they highlight very well the need and justification for a complete reduction of our data!

You will NEVER catch me without a proper set of bias frames, dark frames and flat frames ... even if it means staying up an extra hr or more following an all-nighter.

Anthony.