Dougnuts from flats

[Singlin]

After doing a screen stretch and automatic background extraction on my Master luminance I am getting dougnuts in the image.

Is this caused by faulty flats?

Attached is  one of my Luminance flats

 

L_1309L.fit

Edited March 21, 2020 by Singlin

[Singlin]

Attached is my stretched Master Light frame where you can see a doughnut in the bottom right corner and one in the top left corner.

light_FILTER_L_BINNING_1_integration_ABE.tif

[almcl]

The first file doesn't seem to be a flat - it's got stars in it.  Is it the correct file?  If so, that may be part of the problem.  Also the two images are rotated relative to each other.

As well the 'dust bunnies' (I can see 5 or 6 rather than 2) look awfully symmetrical.  Might have another cause?  Some great data though, well worth persisting with!

 

Edited March 21, 2020 by almcl

[Singlin]

Sorry, I have ammended the flat file.

[almcl]

That new flat seems to have two extremely circular artefacts, upper right and lower left, but I wonder if they are caused by dust or by something else, perhaps a reflection or other anomaly within the imaging train?  

Without details of the imaging set up (scope, camera, filters, reducer/flattener &c) it's difficult to say but the dust might be as much as 10mm from the sensor, which seems a bit unlikely...

Could the flat have been rotated relative to the lights?

[Merlin66]

I assume you're using the ATik 383L.

Doughnuts like that would indicate dust on the front cover plate of the camera....

I find with my solar imaging it's good to test the camera separate from the usual filters etc. 

Just set up a scope, cover the objective with a clear plastic bag and fit a Barlow/ PM etc to get as high an f ratio as possible (higher f ratios shows more dust/ motes especially on the sensor surface)

I get good (well bad - if you know what I mean!) results at around f19 on my ED80 - seems to show up the smallest dust mote!!!

I clean the sensor and the cover plate with IPA and KimiWipes.

Well worth doing, then at least you know the camera is 100% clean - any subsequent "dust bunnies" must originate in the front filters etc etc.

 

[Singlin]

Thanks for the input.

I will double clean and check today.

I would have thought that my flats would have removed them though.