So you thought you knew how flats work?

[MarkS]

I'm beginning to understand why flats don't always seem to work properly.  One of the reasons is that they invariably contain stray light i.e. unfocused light that has bounced around inside the scope and reached the CCD.  The light frames that you take also contain stray light - but almost certainly with different characteristics.

An interesting statistic I worled out is that if you are imaging a 2x2 degree patch of sky then this represents only 1/5000 of the area of sky in front of your telescope.  The whole of that area of sky is pouring light into the front of scope.  Where does it all go?  Scary thought!

If you're interested, here are some articles I found quite enlightening:

A lovely example of stray moonlight causing problems:
http://www.astroinformatics.de/index.php?option=com_content&view=article&id=139

Apparently black anodisation is pretty reflective for IR light:
https://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

Even the professionals have problems:
http://casu.ast.cam.ac.uk/surveys-projects/vst/technical/scattered-light-and-illumination-correction

[Mac]

Interesting read,

Just a thought, how much would the stray light be reduced if you added a baffle at the front
which was flocked.

it would act two fold, first as secondary or primary dew shield for the scope,  as well as then reducing the initial reflections before they entered the scope.

If you paint it with this stuff,
http://www.popsci.com/technology/article/2011-11/new-super-black-material-absorbs-99-percent-all-light-dares-strike-it

Then all stray light should be absorbed,
The 1% that is reflected would be absorbed on the next contact. Not too sure on the cost or availability,
failing that.

http://www.edmundoptics.com/lab-production/general-tools/light-absorbing-black-out-material/1502


Mac.

[MarkS]

Agreed - every imaging scope needs a well flocked dew/light shield. 

I have certainly noticed a big difference since I started using a front shield on the Tak - that's why I became very interested in this question.  My flats work so much better (take flats with the dewshield on, of course).  It certainly givies shielding from those rays that enter the scope at an oblique angle and go straight up the Newtonian's focusing tube

One possible problem is that when condensation starts forming on the inside of the dew/light shield it can become quite reflective which leaves you in almost as bad a position as you started.