Strange Gradient after applying Flat Frames

I also experience this strange flat effect too. However, I learn that if I took new flats for every astrophotography night, and strictly followed workflow in the tutorial video, this problem seem to be lessen. 

Kijja

I've seen that too. It looks like vignetting. That particular tool in app has never worked for me, so I take it into Lightroom and correct for vignetting and it goes away

Thanks for your replies. The light pollution correction works fine for me but I was just wondering if someone could put me in the right direction where this remaining gradient comes from. Also I fear, when using the LPC tool it might wipe out some possible faint ifn that is still in the data so I would like to go down to the source of this. The flats look exactly like they should look and if I calibrate a single sub it looks nice and flat. However when I then stack the images it there is still some irregular shaped vignetting left on the right hand side of the image. I just can't find any sensible source for that in my OTA or in my calibration frames.

Best Frank

Quick update: 

I tried several different ADU values for my flats ranging from 12k to 45k with 30k and 45k being a smidgen better. However I think as the vignetting differs so much from the actual flats I was wondering if it could have something to do my sensor glow which at full stretch looks something like this:

I tried calibrating without the Bias frame and just the darks / bpm / flat but the result is still not a lot better.

I really have no clue as to where else to search. As mentioned before the exact same pattern occurs at different angles and bearings of the scope.

Update #2: Seems I found the issue. It is the flats with my ASI. Sorry to bother you... Actually it seems the exposure was to short for my flats. once I dimmed my flat box and added some paper between the telescope and the flatpanel and was able to get above 0.5s for the flats the results got a lot better. Not perfect but quite a lot better. I aimed for the same 30k ADU but just with a longer exposure. Not sure if there is a rule of thumb for that. If anyone knows anything I would greatly appreciate your feedback 🙂 

Thanks, Frank

Hi Frank @foschmitz,

Yes, exposing flats with very short exposure times can give all sorts of problems, I know that for a fact. Please try to make the flat exposures at least 0,5 sec and preferably longer to achieve even better correction. Some DSLRS have this problem as well like newer Nikons.

The actual gradient that you reported intitially might have come from LNC in integration, that could be the source to introduce this artificially, but then again, it shouldn't either. Perhaps you can test once more without LNC enabled to check if that makes a difference?

Is everything now working fine in calibration?

Cheers,

Mabula

Posted by: Kijja

I also experience this strange flat effect too. However, I learn that if I took new flats for every astrophotography night, and strictly followed workflow in the tutorial video, this problem seem to be lessen. 

 

Kijja

@kijja,

Yes, shooting new flats if you can will always be better, but sometimes it's hard and you lack time to do so.. If you make flats however , try to have longer exposrue times than a fraction of a second to improve your flats 😉

Flats with very short exposure times can and often will lead to bad flat-field calibration.

Mabula

Posted by: gdwats@comcast.net

I've seen that too. It looks like vignetting. That particular tool in app has never worked for me, so I take it into Lightroom and correct for vignetting and it goes away

@gregwrca,

Vignetting is to be corrected with flats like Frank is trying to do. In that case the correct vignetting tool is redundant. That tool is only for data sets that completely lack flats, only then the tool will be of value to you 😉

Vignetting correction on data without flats actually can be done very well with the tool, (dust bunnies aside, you really need real flats to correct those ... ):

Mabula