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Issues with SCT calibration frames

Celestron EdgeHD 8" Calibration Artifacts
Jan Erik Vallestadfielderdatomb18Tony Gondolaandrea tasselli
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
I've recently been trying my Edge 8HD for DSO targets with the Celestron reducer. I have now encountered an issue with calibration.

Flats are taken the same way I do them for my other rigs (flawlessly). Several layers of a white t-shirt and then an evenly illuminated sketchpad on top. The flats I used for this particular image were around 1-1.5 s long, using the Asiair auto-flats function, and they were calibrated with equal darkflats. 

This has been working very well with lenses and refractors, but now I have rings in my image after stacking. Is this caused by internal reflections from the baffle tube?

There seem to be an issue with correcting the vignetting as well.

(Ignore the OAG obstruction):

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tomb18 avatar
tomb18 avatar
Been there done that.
Yes it's internal reflections. It can help to have very short exposures for the flats but even then. It's pretty useless
.
There is no issue with narrow band though. The reflections do not show up.

I have tried everything, flat panels, and sky flats. None have worked.

Try a couple thousand adus instead of the usual 32000.
They will still be there though.
Only solution is to get rig of the reducer.
Tom
andrea tasselli avatar
andrea tasselli avatar
Welcome to internal reflections. The "rings" is an image way out of focus of exit pupil of the SCT. This happens. First off, try moving the pick-up prism away from the current position, that should remove issues with overcorrection of the flat and, possibly, the internal reflection.  Check for any glittering surface at a glancing angle including the FR in your optical train. It doesn't mind if it is painted black, it may still give off reflections at a glancing angle.
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andrea tasselli avatar
andrea tasselli avatar
Ah, forgot about that, messing around with the corrector might not have helped there.
fielderda avatar
fielderda avatar
I've used a flat panel (Spike-a-Flat Fielder) for the past 10 years with my EDGE HD8 and f/7 reducer, and never had any problems.  Using NINA, Flats are typically 4-7 sec for all filters including NB.  Ditto for Hyperstar or Native f/10.

I concur that you're probably getting internal reflections.
You could try using a tablet or laptop to get a flat white for doing flat.  Hope you solve your problem!
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tomb18 avatar
tomb18 avatar
It seems to me that some people have this issue and others not.
I tried for months to rectify this with no luck. So many different ways, changed filter wheels all kinds of things.
I use a asi6200MM so I can crop RGB images. In my case cropping to APC sized images salvages the image in RGB. Narrowband is no problem full frame.
Maybe this issue comes from something outside of the reducer? I us a moonlite chl focuser. Perhaps that?
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
I guess the scope goes back to the drawing board for now. The corrector plate was aligned using factory markings as well as two I made myself, so I don't think that's an issue.

The reducer could be a problem, or something in the tube. I'm not using a filter/filterwheel nor a external focuser. So the optical path from the reducer and back should be good, apart from the OAG obstruction of course. I'll check my flats tomorrow and perhaps try to adjust things a bit.

At least its been great for planetary work after the cleanup/upgrades so far.
tomb18 avatar
tomb18 avatar
Also quite good full frame in narrow band.
Paul Larkin avatar
Paul Larkin avatar
Hi, Jan Erik.
This may have no bearing on what you are seeing, but I'll offer it anyway.

I used to get (sometimes) terrible results like this and put it down to internal reflections etc. as per everyone else's comments which are probably all spot on.

That said, I ran the same subs and flats I was using in DeepSkyStacker throughPixInsight and I got infinitely better results.
That is not to say that PI is necessarily better than DSS, but it is notable that what I had assumed were internal reflections problems with my C8 all but disappeared with a different stacking programme/algorithms.

You did not mention which stacking software you were using.  Either way, it is worth some experimenting with an alternative to see what, if anything, changes.
Cheers.
Paul
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José María Moreno Santiago avatar
José María Moreno Santiago avatar
Es posible que el sistema de flats que tienes no tenga la suficiente holgura en los bordes y esquinas para un diametro de 200 mm del C8.
A mí me pasó algo parecido y lo resolví para esa combinacion de C8+reducer 0,7x mediante un panel Geoptik 30B305 de campo plano de 260 mm, para la exposisición uso Generador de flats de NINA al 30% de ADU aproximadamente, ningún problema, calibrado perfecto
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
Paul Larkin:
Hi, Jan Erik.
This may have no bearing on what you are seeing, but I'll offer it anyway.

I used to get (sometimes) terrible results like this and put it down to internal reflections etc. as per everyone else's comments which are probably all spot on.

That said, I ran the same subs and flats I was using in DeepSkyStacker throughPixInsight and I got infinitely better results.
That is not to say that PI is necessarily better than DSS, but it is notable that what I had assumed were internal reflections problems with my C8 all but disappeared with a different stacking programme/algorithms.

You did not mention which stacking software you were using.  Either way, it is worth some experimenting with an alternative to see what, if anything, changes.
Cheers.
Paul

Hi,

Thanks, I appreciate any advice. At first I ran my subs through ASIStudio as I wasn't on my processing computer, which gave horrible results. Not only rings but waves as well. The images I shared here were processed through PI though, and were better - but still had rings unfortunately.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
José María Moreno Santiago:
Es posible que el sistema de flats que tienes no tenga la suficiente holgura en los bordes y esquinas para un diametro de 200 mm del C8.
A mí me pasó algo parecido y lo resolví para esa combinacion de C8+reducer 0,7x mediante un panel Geoptik 30B305 de campo plano de 260 mm, para la exposisición uso Generador de flats de NINA al 30% de ADU aproximadamente, ningún problema, calibrado perfecto

My spanish is not the best so I'm sorry if I misunderstood  

The panel I'm using is big enough to cover the entire scope, so I don't think illumination is part of the problem. As someone else mentioned exposure time could be though, I simply used the Asiair auto flats functionality which I usually do with my other setups.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
Just a thought but I use a astrozap 8" SCT dew shield. The shield is flocked on the inside but I have noticed that it collects a lot of ice crystalas. As such I have removed it in order to take flats to avoid dropping it onto the corrector. Could this be part of the issue, as the ice perhaps reflects light?

Here's one of the current master flats:


A single sub, original to the left and with a quick pass of ADBE to the right:


I might do a stack without flats just out of interest, although I do not expect much from it.

EDIT: The flats have not corrected the few dust motes correctly either it seems.
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Mau_Bard avatar
Mau_Bard avatar
I am not an SCT owner, yet I had a similar experience with a Newtonian, and the main cause was in my case structural flexure. 
I know this for sure, because I matched flats taken with the tube at various inclinations and their difference was showing rings.
Maybe you have some element flexing, for instance the mirror or the focuser?

Hope this may help, 
Mau
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
I am not an SCT owner, yet I had a similar experience with a Newtonian, and the main cause was in my case structural flexure. 
I know this for sure, because I matched flats taken with the tube at various inclinations and their difference was showing rings.
Maybe you have some element flexing, for instance the mirror or the focuser?

Hope this may help, 
Mau

Thanks, I don't have a focuser in the imaging train, it's mounted externally on the knob itself. But I am going to redo flats with the dew shield on as that may have an effect on the results.
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Tony Gondola avatar
Tony Gondola avatar
Just a couple of thoughts on this:

Flats should done with the same exact same setup as the lights, so removing the dew shield will only make things worse.

Exposure time doesn't matter unless you have one of the few sensors that have problems with short exposure times.

The fact that the dust motes were not removed indicates to me that it's a software/calibration issue.
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fielderda avatar
fielderda avatar
Just in case image rotation hasn't been discussed, the flats must be taken with the same image rotation configuration as the target itself.
I also noticed that there is a dark zone at the bottom of your flat. Is there something that might be interfering with the light path?
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
Tony Gondola:
Just a couple of thoughts on this:

Flats should done with the same exact same setup as the lights, so removing the dew shield will only make things worse.

Exposure time doesn't matter unless you have one of the few sensors that have problems with short exposure times.

The fact that the dust motes were not removed indicates to me that it's a software/calibration issue.

That's where I'm at at the moment. I've realised that removing the shield was a bad idea, so I'll be re-shooting flats with it on tonight. Even if the data might be unsalvageable I might find the solution. Stacking without flats gave slightly better results.
TwilightMoons Astronomy avatar
TwilightMoons Astronomy avatar
The dew shield helps a lot to prevent that sideways light from reflecting inside the tube. At one point I just took an image of the blue sky for my flat because I could not find the fine-weave white shirt I usually used. I had the dew shield in front because I was also using the Hyperstar.

I forgot I was at park and pointed north. I had to redo my flats because I had an image of Polaris on my flat. Oops. 

But once I repositioned and did it again, I had good results without internal reflections. Same for f7 and f10 - the dew shield reduced the side light and kept things more "collimated."
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Tony Gondola avatar
Tony Gondola avatar
Just in case image rotation hasn't been discussed, the flats must be taken with the same image rotation configuration as the target itself.
I also noticed that there is a dark zone at the bottom of your flat. Is there something that might be interfering with the light path?

Yes, that's part of the exact same setup.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
Just in case image rotation hasn't been discussed, the flats must be taken with the same image rotation configuration as the target itself.
I also noticed that there is a dark zone at the bottom of your flat. Is there something that might be interfering with the light path?

Yes, I'm aware thanks. I mentioned that in the original post, that's just the OAG obstructing a bit.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
TwilightMoons Astronomy:
The dew shield helps a lot to prevent that sideways light from reflecting inside the tube. At one point I just took an image of the blue sky for my flat because I could not find the fine-weave white shirt I usually used. I had the dew shield in front because I was also using the Hyperstar.

I forgot I was at park and pointed north. I had to redo my flats because I had an image of Polaris on my flat. Oops. 

But once I repositioned and did it again, I had good results without internal reflections. Same for f7 and f10 - the dew shield reduced the side light and kept things more "collimated."

Yes, the shield is always on - but for the sake of stability I removed before taking flats without thinking things through. I'm not sure it will fix the entire problem it should at least improve it a bit.


The only issue with the dew shield is that it collects a lot of dew, as such frost builds up both inside and out, which also reflects light.
​​​​
Tony Gondola avatar
Tony Gondola avatar
That's a problem. You could take flats right at the end of your session but if the state of the inside of the dew shield is changing through the night then not all of your lights will flat calibrate the same. Have you tried a dew heater to keep the frost from forming?
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TwilightMoons Astronomy avatar
TwilightMoons Astronomy avatar
If you get dew inside the shield, consider flying in some black flocking material. I found that it tends to resist all but the heaviest dew. Also, more dew heater straps to keep it warm.
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Jan Erik Vallestad avatar
Jan Erik Vallestad avatar
TwilightMoons Astronomy:
If you get dew inside the shield, consider flying in some black flocking material. I found that it tends to resist all but the heaviest dew. Also, more dew heater straps to keep it warm.


As I mentioned in an earlier post I use the Aztrosap 8" SCT dew shield, it has flocking inside and actually is a huge part of the problem. Th e shield is quite long and the frost is building up quite far down into it. It's slightly wobbly if you put pressure to it which is why I was hesitant to put a flat panel on top, well that and the fact that I would have gotten droplets on my corrector.

I haven't run my dew ring at more than max 40% after the upgrades I made to my scope, but I had to max it after a session as I actually had some water drops on there. As you can see from the image the frosting stretches even down to the tube itself in certain directions. 

Tony Gondola:
That's a problem. You could take flats right at the end of your session but if the state of the inside of the dew shield is changing through the night then not all of your lights will flat calibrate the same. Have you tried a dew heater to keep the frost from forming?

That's what I've done for the two outings I've had for DSO work so far, but alas it's a bit more complicated than my other rigs. The images otherwise look great though, so I'm not disheartened. Just another hurdle to overcome.

I have thought about putting some reflectix on it just for good measure, I currently do not have a dew heater large enough to fit. But I do so on my refractor with good success, the dew shield on my Esprit is metal though so it conducts heat. I might consider getting the Astrozap flexiheat (with built in heating) or maybe the celestron dew shield and a dew heater in stead. The cost never ends with this scope 

This particular info from Astrozap encourages me to go with the Celestron metal shield and large diameter dew heater as I'm an Asiair user. It should probably be fine, but if warranties are voided I'm getting a bit cold feet: