Strange thing happening w flats?

[vineyard]

Hello,

Slightly scratching my head about this.  No doubt missing s/t obvious so apologies in advance if so!

Here's w-i-p (only about 5h44) on Leo Triplet.  APP w flats, darks & dark-flats.  Just DBE & then HT in PI.  JPG'd for size.

What is that strange oval thing in the middle.  I'm attaching a FITS of a single flat image - some bunnies etc do show up in that area of the flat FITs (you can see the other bunnies too).  But how come this central bit doesn't get calibrated out?  (In fact I did a test of stacking just a small number of lights and the oval is there, but more faintly - ie stacking more & more lights seems to make it worse).  What could it be that doesn't get calibrated out like other bunnies & instead gets more pronounced as more lights are stacked?

Thank you for any light (if you'll forgive the pun)!

 

Flat neg10g125o16_Flat_0.041_secs_2022-01-24T15-16-30_001.fits MF-IG_125.0-E_0.041495s-ZWO_CCD_ASI294MC_Pro-4144x2822--all_sessions.fits

Edited February 8, 2022 by vineyard
Added flat FITS & masterflat

[ONIKKINEN]

Not sure whats going on in your flats, but it looks like it could be dew forming on the sensor? Or could be something else entirely, but heres what i see in your flats using histogram stretch and false colour rainbow rendering to make details easily visible:

red:

Green:

Blue:

Flats have a very uneven profile in terms of vignetting. Why is that? I have not imaged with refractors so if this is something thats common with refractors im sure someone will point that out (or if you even used a refractor, but no diffraction spikes so that is my guess). But if i got flats like these with my newtonian something would be horribly wrong. Green looks OK to me but red is square and blue is very off center, dont know what to make of this.

The reason why i think dew might be to blame is that in the red channel your image center is actually not the brightest part of the image. Its difficult to see but there are loads of cold pixels that have lower values than slightly outside this area. I think the shape is also somewhat similar to the bright spot in your stacked image. Since the stacked image shows the issue as brighter than should it would mean that the flats must have a darker than should spot in the same place, which looks to be the case. If you had dew during lights as well the center spot would have been calibrated out and both lights and flats would have this shape in them. I am guessing something dewed over while or before taking the flats, but not for the duration of your lights.

If its too difficult to see i took some measurements with a 150x150 pixel area selection and took the red values in median ADU: center, should be brightest: 18650, slightly off center: 18700.

Im not sure why the other 2 channels look so different, and dont seem to have this kind of issue though. Not sure if dew could somehow only shadow the red channel? Probably not now that i think of it, but its a guess.

Edited February 8, 2022 by ONIKKINEN

[vineyard]

Thanks @ONIKKINEN that's v thoughtful reply.  Yes that's a refractor.  Not sure it's dewing b/c I took the flats indoors from a light box.  But, I did have the light box not quite perpendicular to the objective, and it was also set back some (rather than flush with it).  I didn't think it would make a difference (since I believe sometimes people just point their scope at a panel mounted on the wall of their obs?).  But I'll try with that & see what happens - will report back.

Thanks again - you've given me a pointer to explore further.

[david_taurus83]

Think what @ONIKKINEN means there about dew is frosting over the sensor due to the tec cooler on the camera. Maybe time to refresh the dessicant tablets?

[vineyard]

Yes it was a problem w the flats  I noticed that when I switched on the lightbox, that central pattern wouldn't be there but would then start to appear, and then reduce gradually.  I just waited and eventually it disappeared.  So either its something weird with the light sources in the box (highly unlikely) or its dew on the sensor.  I learned something new there b/c I didn't appreciate that that could happen - luckily it went away quite quickly but yes maybe time to recharge the desiccant tablets (have seen the videos on YT) - is it worth getting one of the ZWO cooled camera dew heaters or is that overkill?  There isn't that much humidity here normally.

Here's a quick w-i-p DBE+HT jpg without the halo, which means the new flats will also work with all the other data I took this last month (phew!)

Thanks again for the help @ONIKKINEN& @david_taurus83 🙏🏾

[david_taurus83]

What kind of light source is it? Is it a bulb? Could it be warming up the sensor window temporarily causing dew until the temp goes back down again?

[vineyard]

57 minutes ago, david_taurus83 said:

What kind of light source is it? Is it a bulb? Could it be warming up the sensor window temporarily causing dew until the temp goes back down again?

Its a Huion LED Panel Pad (my better half uses it for her art & making work but I, ahem, borrow it occasionally  ).

[ONIKKINEN]

How long did you wait from bringing the scope inside to taking flats? Dew can take an hour or more to go away.

I would guess its the outside of the sensor window thats dewing up rather than the inside, unless your camera is old and the desiccant is really saturated.

[vineyard]

4 hours ago, ONIKKINEN said:

How long did you wait from bringing the scope inside to taking flats? Dew can take an hour or more to go away.

I would guess its the outside of the sensor window thats dewing up rather than the inside, unless your camera is old and the desiccant is really saturated.

I took the flats much later (days).  It may have been that the camera chills v quickly indoors & that sudden drop draws out some moisture?  Although I leave the camera permanently attached to the back of the flattener.

The camera is old (pre-owned 294MCPro that I bought almost 2y ago) so I probably should remove the desiccants and zap them in the microwave!