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Intergalactic Wanderer - is this overcompensation from the flats?


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Hi all,

This is NGC 2419 taken in good conditions (no Moon, Bortle 4).

I don't understand why the cluster and stars in the middle have a slightly darker ring around them. I can't shift it entirely, using ABE, DBE, or the light pollution tool in APP.

Honeyview_argh.thumb.jpg.85496b2683805bddd855e5ab2409d3be.jpg

Is this over-correction of the flats? Or was the sky just like this anyway ie high cloud perhaps?

This is with an ASI533MC, cooled to -10C, through a 130PDS Newtonian with 0.9x coma corrector, 50 darks, 50 dark flats, 25 flats, about 2 hours' worth of total exposure, 60s subs, gain 101 and offset 50.

Any takers?

Thanks
Brendan
 

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Hi all,

Well the story continues.

Here are two more examples. Weird gradients that don't appear in the subs and, to my eye, seem similar but not identical in each image. I've tried stacking with just bias, with darks, in APP, in PI, always the same result, very difficult to shift (if at all) using ABE or DBE. These have just had ABE applied and a quick EZ Soft Stretch. 

771371221_NGC2683argh.thumb.jpg.a3a68d157df35293f53624dd7fc98dfc.jpg

488980195_NGC2403argh.thumb.jpg.d866a50339d2482e779a130cdd531cfa.jpg

I think it's a fairly recent problem. I don't know what's causing it, and it bothers me.

Any ideas?

Thanks, Brendan

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Pretty sure its some kind of gremlin from a gradient removal process, but thats just a guess without seeing the linear data. You could post a linear stacked file for others to have a look at the data and maybe something more than a guess would appear.

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1 hour ago, BrendanC said:

Good idea - problem is the files are quite large, but here you go. Stacked in PI but saved as FITS to reduce the size. About 100MB each.

Happy to provide anything else that might help. :)

NGC 2403.fit 99.29 MB · 0 downloads NGC 2683.fit 101.3 MB · 0 downloads

Ill have a look later today when i get off work 👍

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Hey, thanks!

In the meantime, here's one of them stacked with just flats...

1825586913_justflats.thumb.jpg.33edc4addd5761b8c28c0cfb2efabe3b.jpg

... and just lights alone.

1420851463_justlights.thumb.jpg.09de2c61e403e2a36ad7fe018175d011.jpg

 

Both have just had ABE and EZ Soft Stretch applied so they're visible.

So I'm wondering whether something has gone terribly wrong my with flats somewhere along the line? I have recently taken quite a few images over an unprecedentedly clear few nights in the UK, so it's entirely possible I've mixed them up.

Edited by BrendanC
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@BrendanC While you'd want to resolve any calibration / light leak issues, I managed to get DBE to clean up NGC 2403 OK - but not perfect of course.

Here it is after some (mainly) careful DBE. 

Left - Original - Unlinked Auto STF
Centre - DBE + Auto STF,
Right - DBE + Super Auto STF

image.thumb.png.251f042856d4352da38386d8215c0372.png

Then SPCC, a little SCNR (although I usually don't need that with SPCC), then EZ soft stretch. 

image.png.7f4d12cd63dd47126f83a1a9b9d2a836.png

At least if you can't get the issue resolved for the data you've already captured, you can probably salvage the above images to some degree - they all look great so I hope you get a satisfactory resolution 🤞

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Hey, thanks! Really appreciate you taking the time to do this.

I've kind of managed to fix it too in the meantime, but I'd very much like to fix it at source. I'm working on the theory that my flats don't seem centrally aligned, whereas my lights do, so something isn't right about how my flats panel is located on my scope. Or something. 

Anyway, thanks again. :)

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Agree with @geeklee and came to a similar conclusion that the background is fixable. But it looks like for these 2 stacks you posted there is some issue with flats calibration. Looks very familiar as a newtonian imager, and the cause is a wild guess at best but something mechanical rather than software/flats exposure or something like that. Something between the primary mirror and camera sensor shifts very slightly between taking flats and lights, causing a wonky background. But its not too bad, definitely brute-forceable.

I tried with Siril, i very much prefer it for background removal to DBE, especially if the gradient is difficult. Below is a false colour rendering and super stretched version that shows the initial condition nicely:

2023-01-26T21_20_48.png.397d16056d74d6930fd378cfdb5dada6.png

The gradient is not perfectly linear, and it does look like there is a "ring" around the galaxy. This looks like its somehow flats/light leak/internal reflection related (like from lack of blackening/flocking). But its not nearly as bad as it could be.

Then the fixed version where i placed the samplers manually while carefully avoiding any stars in them. The cluster of stars and the big bright star should be treated as not background in this case, and so no samplers close to them.

2023-01-26T21_23_09.png.5376d286498abfefe01b2f56e47498ac.png

It still looks a bit spotty in this rendering mode, but this is just for visualization and in reality the image is very clean and its not something anyone will notice in a finished image.

Same story for the other image, no need to screenshot those i think.

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Absolutely brilliant analysis, and far ahead of anything I could have done. Thank you so much!

So it does look like there's a mismatch between the flats and the subs, and this is what someone else has told me since too. I use a Lacerta panel, but it was intended for a 150PDS, not a 130PDS, so I'm wondering whether I need to do something to try and make sure it's absolutely central (I thought it was because I cut out some of the inner foam ring to position it, but maybe not). Also, I'm going to look into covering around the panel with a scarf or something, in case there's light leakage into the focuser tube (which is covered anyway, but I may as well try it).

The scope is flocked, so I doubt there will be internal reflections bouncing around.

Very interesting what you say about Siril. I've found that APP's light pollution removal tool is superior to ABE or DBE, but I'd rather not rely on APP as I'm transitioning to PI (I just have a rental license for APP). I've used Sirilic for stacking but found the Siril GUI a bit opaque. If its background removal is as good as you say, perhaps I should take another look. Pity PI's 'industry standard' tools aren't up to the job!

Great to know I'm not absolutely way off, and the situation is salvageable in software. I've since processed them and managed to get something out of them, but I'd much rather fix this at source. The data looked good to me, and the guiding was excellent. The camera is still in the scope with the same orientation so I might even have a go at creating some new flats (both images used the same flats), which might be a way forward.

Finally - and this is the million dollar question - is there anything else you can think of that could be causing this mismatch? No worries if you think I've covered everything. I really, really appreciate you taking the time out to help me here. I'm sure you know (as we all do) about the love/hate relationship we have with astrophotography. Ecstasy when it works, agony when it doesn't!

Cheers, Brendan

 

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28 minutes ago, BrendanC said:

Absolutely brilliant analysis, and far ahead of anything I could have done. Thank you so much!

So it does look like there's a mismatch between the flats and the subs, and this is what someone else has told me since too. I use a Lacerta panel, but it was intended for a 150PDS, not a 130PDS, so I'm wondering whether I need to do something to try and make sure it's absolutely central (I thought it was because I cut out some of the inner foam ring to position it, but maybe not). Also, I'm going to look into covering around the panel with a scarf or something, in case there's light leakage into the focuser tube (which is covered anyway, but I may as well try it).

The scope is flocked, so I doubt there will be internal reflections bouncing around.

Very interesting what you say about Siril. I've found that APP's light pollution removal tool is superior to ABE or DBE, but I'd rather not rely on APP as I'm transitioning to PI (I just have a rental license for APP). I've used Sirilic for stacking but found the Siril GUI a bit opaque. If its background removal is as good as you say, perhaps I should take another look. Pity PI's 'industry standard' tools aren't up to the job!

Great to know I'm not absolutely way off, and the situation is salvageable in software. I've since processed them and managed to get something out of them, but I'd much rather fix this at source. The data looked good to me, and the guiding was excellent. The camera is still in the scope with the same orientation so I might even have a go at creating some new flats (both images used the same flats), which might be a way forward.

Finally - and this is the million dollar question - is there anything else you can think of that could be causing this mismatch? No worries if you think I've covered everything. I really, really appreciate you taking the time out to help me here. I'm sure you know (as we all do) about the love/hate relationship we have with astrophotography. Ecstasy when it works, agony when it doesn't!

Cheers, Brendan

 

The flat panel orientation or some slight gap here and there issue is completely irrelevant, there is so much light going through the right way, the mirrors, to the camera that any little off axis shine is buried in it and so has no effect. If it did, there would be another type of issue with the flats, namely overcorrection like in the example where you had stacked with only flats. In fact i use an A4 sized flat panel (20€ amazon tracing panel for kids 🤪) that doesn't cover the entire front of my 240mm diameter tube and there is a gap to the sky but since there is so much light everywhere it doesn't matter and flats calibration works every time (and yes the panel also illuminates the focuser tube which undoubtedly leaks but also no issue here).

Your focuser is probably the weakest link and if you want some peace of mind and a better experience using the scope you might want to look into replacing it with something sturdier. Any considerable option will cost more than the 130PDS did when new so maybe a silly upgrade. Or the primary mirror moving too much in its cell, even during the night which its not supposed to do.

I used to get flats issues like this, but have not in a long time. I cant point to a single fix that did it as i fixed a whole bunch of things in my VX8. Namely swapped the secondary spider for a sturdier one, changed the focuser to a Baader diamond steeltrack, reinforced the tube with some tube clamps (i dont think the 130PDS has issues with this, since its a short and stubby tube with decent tube rings), flocked everything, made sure the mirror sits well in its cell, blackened mirror edges for both primary and secondary, and finally splurged on a Helmerichs carbon tube but actually at that point all the issues were fixed so its not a suggestion for you at all. Also, i take flats every single time i touch the setup in any way so at least once per night. More than once if i change camera orientation or remove the camera for some reason.

So if you take flats in every session and these issues continue, time to look into some upgrades. I would change the focuser first, the camera has to shift only maybe 20 pixels in relation to the primary mirror for these flats issues to arise, and a focuser that is only ok will not do the trick.

 

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Fabulous.

The focuser hasn't been a problem at all (SkyWatcher autofocuser with HiTec DC Focus controller) so I'd be reluctant to change that.

I recently had pinched optics so moved the primary clips out a bit, and I'm wondering whether the mirror is now too loose. So, I'll check on that, plus be very rigorous about taking flats. Hopefully that will resolve the issue.

Thank you again. :) :) :)

Edited by BrendanC
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