36 replies to this topic

[pbkoden]

I've been struggling with a strange ring artifact in my images and thought it was due to my neighbors shining a light on my telescope all night long. I was planning to create a light shield, but after some digging tonight I think the problem may be something else. The ring even shows up in my flats if I stretch and prod them enough. Here is the research.

 

First, a stack of uncalibrated images with a simple dynamic background extraction:

 

 

Then, my flat with a background extraction and some stretching to bring out the details:

 

 

And finally, a stack of calibrated images with a simple dynamic background extraction:

 

 

This ring is a nightmare to try and get out of my images. I need to run dynamic background extraction multiple times with dozens of sample points to try and get rid of it. Any ideas on what it could be?

 

Has anyone seen anything like this? It's present in all my filters except narrowband. My best guess is internal tube reflections. The inside of my tube is coated with a matte black paint, but it's not flocked or baffled. I have a lot of ambient light in my normal imaging area (did I mention my neighbors?). Showing up in the flats makes sense too as I sit an led tracing tablet on the end of the scope to take my flats. Some of the light would bounce off the inside tube walls.

[Thirteen]

I just wrote this in another thread today.  But my advice is to check for internal reflections.   Remove the camera only, point the scope at an evenly illuminated surface, and sight down the scope.   You can even take a cell phone pic to look at.   If you see any reflections ringing the optical path or light leaks, you can bet that is the source of your problem.   This method makes the offending surface pretty obvious.   Let us know what you find. 

Edited by Thirteen, 04 April 2017 - 08:50 PM.

[Jon Rista]

This is a great resource to help you identify and resolve these kinds of issues:

 

http://diffractionli...eur-telescopes/

[pbkoden]

With the camera removed, I see one real possible issue. Light shines around the inside of my coma corrector. The inside is ribbed and black, but it lets quite a bit of light in. See below image with my phone. The outside blue-grey ring is the inside surfaces of the coma corrector. Could this be my culprit? In which case, how the heck do I fix that??

 

The other thing I see is the gap between the focuser tube and the focuser body. Should I try and seal this gap up?

 

 

[NiteGuy]

That has me really scratching my head. Only time I've seen anything resembling this, it was the residual from a severe dewing problem but these images have some artifacts that just don't quite seem to fit that scenario.

[NiteGuy]

Just saw the last picture you posted, looking down the tube. Yes, even in the so-called dark-of-night, that stray light can readily be picked up by today's sensitive cameras. Block or baffle every tiny light leak you can find, no matter how insignificant, then try some test shots and let us know what happens.

[Jon Rista]

With the camera removed, I see one real possible issue. Light shines around the inside of my coma corrector. The inside is ribbed and black, but it lets quite a bit of light in. See below image with my phone. The outside blue-grey ring is the inside surfaces of the coma corrector. Could this be my culprit? In which case, how the heck do I fix that??

 

The other thing I see is the gap between the focuser tube and the focuser body. Should I try and seal this gap up?

 

 

Yeah, that's it.  

 

The problem in these cases is most often DIFFUSE reflections...and that is definitely what you are seeing there. The microbaffling probably works better with your actual light frames...however, since we usually acquire flats with much brighter light, and because the light for the flats is not collimated, the diffuse reflection can be much brighter, and it will usually reflect at different angles (collimated light for the light frames, non-collimated light for the flats). That results in a missmatch between the field structure in the flat and the field structure in the light, resulting in only partial correction, undercorrection or overcorrection in some parts of the field. 

 

You would want to paint that matte black, the dullest you can find, which should help.

[Thirteen]

I do believe this gives you some things to chase down.  I'd be as concerned about the gap in the focuser for taking flats.  You may want to drape a cloth over the focuser/camera since that leak path is probably brighter while you are taking flats than it is while it's out imaging.  

 

As for how do deal with the coma corrector, do you have access to the area that's reflecting or not?

[pbkoden]

This is inside the corrector between lenses. I would have to take the corrector completely apart to paint it. I'm not sure I'm up for that.

[Thirteen]

hmm. Perhaps just take the coma corrector off and taking a set of flats to isolate it as the issue?

[Jon Rista]

This is inside the corrector between lenses. I would have to take the corrector completely apart to paint it. I'm not sure I'm up for that.

Ah, yeah, I wouldn't do that. Those lenses would have been collimated, and any paint on rims or anything like that could result in miss collimation. Bummer. 

 

Also, Jason is right about the gap. Any gap will let in light, and that could definitely affect the flats, which again would result in missmatched field structure. 

[pbkoden]

I played with a lot of things tonight. I pointed my scope at the ceiling in a dim room and took a bunch of images. Once taken, I did a dynamic background extraction with a simple grid of samples, and used STF on the resulting image. I always get the same light circle in the middle of the frame (thought it gets bigger when I rack the focuser out). I covered the focuser with a blanket, I covered the mirror end of the tube, I racked the focuser way out and shielded the opening for any ambient light to cut way down on off-axis light hitting the coma corrector. Same circle. I took the camera out of the scope and pointed it at the ceiling by itself, no circle.

 

Any other ideas?

 

Here is a test frame from tonight (they all look the same). Is this normal for an uncorrected image? I would assume a somewhat even gradient other than the bias signal and the dust bunnies.

 

 

 

And here is a stacked calibrated RGB image. Even with a bunch of light pollution, no one should have gradients like these should they? It's the same stinking circle in the middle of the frame.

 

 

[Peter in Reno]

I am sure you already did this but I want to ask. Did you calibrate each flat sub with Master Bias before creating Master Flat? Also, did you calibrate each light sub with Master Dark (but do NOT use Master Bias) as well as with Master Flat before integration R, G, and B?

 

Peter

[pbkoden]

My calibration process is normally as follows per Kayron Mercieca's pre-processing workflow at Light Vortex Astronomy:

• Darks are calibrated with bias
• Flats are calibrated with bias and darks
• Lights are calibrated with bias, darks, and flats

I've used the same process for over a year and it's worked great until I got my new astrograph and started having this fixed pattern issue.

 

During my tests today however, no calibrations were used at all. This is just straight-shot images.

[Jon Rista]

• Flats are calibrated with bias and darks

This is wrong. Especially if you are using the same darks you use for the lights. Totally wrong. 

 

You only need to calibrate the flats with EITHER a master bias, or a master dark, not both. If you choose to use a master dark, you need to create it from flat-dark frames that are explicitly mached to the flat frames temperature and exposure. You are using a QSI683, and I can't imagine you need to use flat darks, so you should only be calibrating your flats with a master bias. 

[pbkoden]

Does it make a difference that I have dark scaling (optimization) turned on for the darks and 90% of the time there is no correlation and it doesn't do it anyways?

[Peter in Reno]

My calibration process is normally as follows per Kayron Mercieca's pre-processing workflow at Light Vortex Astronomy:

• Darks are calibrated with bias
• Flats are calibrated with bias and darks
• Lights are calibrated with bias, darks, and flats

I've used the same process for over a year and it's worked great until I got my new astrograph and started having this fixed pattern issue.

 

During my tests today however, no calibrations were used at all. This is just straight-shot images.

I didn't read the whole thread carefully until I saw that you just started with a brand new scope. I was wondering what was different about this as compared to the same successful process you've been having for the past year. So let's look at your new scope.

 

I assume your new astrograph is "TS 8" f/4 ONTC Newtonian with Moonlite CRL Focuser and GPU f/4 Aplanatic Coma Corrector" as shown in your signature. Is this your scope? Is this your Coma Corrector?

 

This is a very fast scope and if you use the Coma Corrector, does it require proper back focus distance? I see from TS web site that it says Coma Corrector's back focus distance should be 55mm between the Coma Corrector and CCD/CMOS focal plane. If your back focus is not correct, it could cause to show dark ring at the center of the image.

 

What was your previous scope that you have been successful capturing and processing images?

 

I think you need to resolve your new telescope equipment issue first before dealing with processing like getting rid of the dark ring at the center of image and flats. If you still cannot resolve it, I would contact Teleskopic-Express by e-mail for assistance.

 

Peter

[pbkoden]

Peter,

 

I went from an 8" f/4 Orion, to the 8" f/4 TS scope. And I agree that something with the new scope/focuser is at the root of my issue. I'm just trying to figure out what it is..

 

I have great images with the same camera, coma corrector (including spacing), etc. But when I switched from the Orion to the TS it looks like I picked up this issue. It's not present through my narrowband filters, so I assume it is something light leak or reflection related.

 

I would like to rule out my own idiocy before I go to TS for some support, but that is the next step after I troubleshoot everything that I *can* do. Maybe tonight I will wrap a black plastic contractor bag around the whole scope other than the front opening. That should rule out a hell of a lot of light leak possibilities.

 

Phil

[Peter in Reno]

You can wrap aluminum foil around the focuser and camera (don't block cooling vents) and the rear cell of the scope. Aluminum foil will hold better than plastic bag.

 

Peter

[RedLionNJ]

As already suggested (but I'll say it again, anyway) - you have FAR too much going on in those flats. You might reasonably expect a symmetrical, round gradient. Maybe a few donuts. Perhaps even a hair or other small linear feature. But yours go way beyond that. Eliminate the internal reflections, they're killing you. Flock, baffle, use black paint, whatever. It may be a significant effort, but it will be worth it.

[pbkoden]

I should clarify here, those flats are stretched within an ounce of their life!

 

The unmodified flats look a completely smooth medium grey with a barely perceptible darkening in the corners.

[Jon Rista]

Does it make a difference that I have dark scaling (optimization) turned on for the darks and 90% of the time there is no correlation and it doesn't do it anyways?

No. You should not use darks to calibrate the flats if you are also using bias. Flats from that camera only need bias calibration, honestly. You could very well be forcing a missmatch between your flats and lights if you are dual calibrating (you won't get black clipping, like you would with the lights, but it can skew the master flat so it doesn't work properly anymore.) 

[Peter in Reno]

My calibration process is normally as follows per Kayron Mercieca's pre-processing workflow at Light Vortex Astronomy:

• Darks are calibrated with bias
• Flats are calibrated with bias and darks
• Lights are calibrated with bias, darks, and flats

I've used the same process for over a year and it's worked great until I got my new astrograph and started having this fixed pattern issue.

 

During my tests today however, no calibrations were used at all. This is just straight-shot images.

I also use dark scaling (Optimization) and I follow the PixInsight tutorial:

 

http://www.pixinsigh...ames/index.html

 

This uses older version PI interface but still works. This tutorial does not calibrate dark subs with Master Bias like you do so I suggest to follow PI tutorial. Master Bias subtraction of dark subs will take place with the Image Calibration tool when you do light subs calibration.

 

Jon, if you look at the link I provided, both Master Bias and Master Dark are used for dark scaling for flat subs calibration.

 

Peter

[pfile]

i should say that i have seen this exact problem with all 3 of my OTAs. all 3 of them have a reducer in the optical path.

 

i'm starting to think that it is simply stray light, possibly IR light from nearby security cameras, that messes up the lights but is not reproducable in the flats. i find that there's a correlation between exposure length and the "rings" problem; specifically high background ADU follows the problem. as such i don't really see it with narrowband subs, and it's worst with L subs. i have not experimented with removing the reducers.

 

i flocked the living bejezus out of the focuser, reducer, and extension tubes and it had no effect on the problem. if the problem lies in the OTA itself there's not much i can do at least on the AT10 and AT6 as they have baffles. for the TMB perhaps i could flock the inside of the telescope and see if anything changes. my camera's been out of commission for almost a month, so i have not been able to work on this problem...

 

rob

[pbkoden]

Peter, here is my master flat for my latest evening of imaging with STF applied to show additional contrast. It's a very clean looking flat with a couple donuts from dust.

 

 

 

And Peter and Jon, I understand calibrating my flats with a scaled dark is essentially unnecessary when the exposure is so short, but I don't see how it can hurt at all. I'm not doubling up on any calibration. The bias is just the "bias" signal, and the dark is just the thermal signal (which admittedly, there is essentially zero). But I will change my process to skip this step in the future. I just did a test, and out of 15 flat frames, 10 skipped the dark subtraction because there was no correlation.

 

I'll spend my weekend trying to nail down my issue and report back if I can figure anything out.