QHY268M + astronomik OIII + Esprit 100

[mgutierrez]

Hi all,

I'm searching someone who shares this setup with me: QHY268M camera, Astronomik OIII 6nm 36mm filter (non-MaxFR) and a Esprit100.

I'm experiencing a weird issue that I cannot fix

Here is a only dark calibrated master OIII:

 

Note the white gradient towards the corners. My master flat looks like a regular one: darker in the corners and lighter towards the center. But since the gradients are reversed between them, the flat calibrated OIII master is even worse, with more white gradient.

My master flat may be bad or not. I don't know and honestly I think it's not important: the white gradient is present on my uncalibrated master, and I think it's not normal (or it is?!)

This filter was replaced by astronomik a couple of months ago for a similar issue. Similar, cause the gradient was mostly present on one lateral; my guess is that the other lateral had some spot on the filter. So I think it's not a filter issue.

I also swapped the OIII position within the fw, just in case. Same result.

Shooting different objects at different altitude and completely different position, makes no difference.

What do you think?

m

PS: sorry if I posted in the wrong forum

[teoria_del_big_bang]

I do not have the exact same setup but do have same scope and camera but not even sure the setup really matters.
But I do not really understand your issue.
I am no expert and still learning but I think we would need to see a full image not just a preview and ideally upload the raw image.
Also must have at least a flat or better still a master flat.

I think whatever the camera you are going to get gradients and vignetting, and a start to remove those is calibrating with good flats.

Calibrating an image  with only a dark is not going to remove gradients or vignetting , all cameras taking images of feint nebulas will show gradients and vignetting when stretching uncalibrated images.

Even after good calibration you will still have some gradients that then can be removed during the processing.

From what I see on the image in your post it looks to be mostly vignetting that is normal and will be greatly reduced with good calibration using flats.  It looks bad but an auto-stretched image without proper calibration will look bad for vignetting.

Like I say I am no expert but from that one image it is difficult to say if there is an issue there or not

Steve
 

[mgutierrez]

Hi Steve,

Thanks for the reply mate

You can find here some files: https://www.dropbox.com/sh/6ajza48ha3vkcpg/AAD-F6PtCeUilew4x189jBPRa?dl=0

There is a single light, the master flat, the non flat calibrated master light and also the calibrated one. The gradient on the single light is not so evident. After stacking, it is revealed completely.

I always have some kind of vignetting on all of my lights with different filters. What makes this vignetting special, is that is reversed. That is, corners are lighter, not darker as usual. My flats, as you can see, look like a normal flat: corners are darker. But since the lights have an opposite gradient, what the calibration does is to exaggerate these gradients.

I want to focus on the lights. If that gradient is normal, then I would focus on taking proper flats (sky-flats, a different flat panel, etc.). But to me, those gradient present on the single lights should not be there. That's basically my question.

m

[newbie alert]

It's a slightly over correcting flats...

Also your stars look abit strange, are you using the flatener?

[mgutierrez]

1 minute ago, newbie alert said:

It's a slightly over correcting flats...

Also your stars look abit strange, are you using the flatener?

forget about the flats. Do you think the "reverse vignetting" on the single light is normal? (the gradient is more prominent in the stack)

PS: thanks for pointing out about the stars. Yes, I do use the flattener. I usually have a quite flat field. This time I rotated the cam so I may messed it up; no worries

[newbie alert]

I'm pretty sure on the flats, do a stack without them and the edges where they show now as light will be dark

So for me that means your flats are over correcting

its a 16 bit camera so what's the adu that you're aiming for?

 

[teoria_del_big_bang]

Hi Miguel, Is this an issue you think you have had all along because lately you have had some magnificent images and not mentioned any issues.
Or is it just this target because as I say I am no expert but this is not an easy target and is a very faint object.
When I look at the single frame in PI there is actually very little signal there.
So I could be wrong here but if I were to attempt this target with NB filters I think I would be going for more than 180 seconds exposures so that the signal is far greater than the noise. Not that that should affect any vignetting (I wouldn't think). So I know you have a fair number of frames to stack (40) and usually 180 secs * 40 would be good for this camera maybe this target on NB requires longer exposures.
I really cannot say why the reverse vignetting but if you look at the actual pixel values of background  in the corners compared to the pixels of background closer to middle there is very little difference I think it is just the low signal means that PI is really boosting the stretch to see the nebulosity and so any changes in pixel values no matter how small look bad to the eyes.
Sorry I cannot help further but probably I am not experienced enough on this.
 

Steve

Edited September 19, 2022 by teoria_del_big_bang

[mgutierrez]

21 minutes ago, newbie alert said:

I'm pretty sure on the flats, do a stack without them and the edges where they show now as light will be dark

So for me that means your flats are over correcting

its a 16 bit camera so what's the adu that you're aiming for?

 

No. That's the problem. The stack without flats shows lighter corners. Download the file ending in NOFLATS from the dropbox link

[Stuart1971]

1 hour ago, mgutierrez said:

forget about the flats. Do you think the "reverse vignetting" on the single light is normal? (the gradient is more prominent in the stack)

PS: thanks for pointing out about the stars. Yes, I do use the flattener. I usually have a quite flat field. This time I rotated the cam so I may messed it up; no worries

So the single light with the reverse vignetting is straight from the camera with no calibration at all…? Is that correct….?

[mgutierrez]

8 minutes ago, teoria_del_big_bang said:

Hi Miguel, Is this an issue you think you have had all along because lately you have had some magnificent images and not mentioned any issues.
Or is it just this target because as I say I am no expert but this is not an easy target and is a very faint object.
When I look at the single frame in PI there is actually very little signal there.
So I could be wrong here but if I were to attempt this target with NB filters I think I would be going for more than 180 seconds exposures so that the signal is far greater than the noise. Not that that should affect any vignetting (I wouldn't think). So I know you have a fair number of frames to stack (40) and usually 180 secs * 40 would be good for this camera maybe this target on NB requires longer exposures.
I really cannot say why the reverse vignetting but if you look at the actual pixel values of background  in the corners compared to the pixels of background closer to middle there is very little difference I think it is just the low signal means that PI is really boosting the stretch to see the nebulosity and so any changes in pixel values no matter how small look bad to the eyes.
Sorry I cannot help further but probably I am not experienced enough on this.
 

Steve

I'm having this issue from the beginning. I deal with it with post processing, but I would like to get rid off it. Many times makes the post processing quite difficult. It has nothing to do with the target. Longer exposures makes more evident the issue. In any, case, being well exposed, the final stack will reveal the gradient independently of the individual exposure time.

[mgutierrez]

2 minutes ago, Stuart1971 said:

So the single light with the reverse vignetting is straight from the camera with no calibration at all…? Is that correct….?

That's it. Well, only calibrated with darks to remove hot pixels (without darks, the result is obviously the same)

[teoria_del_big_bang]

21 minutes ago, mgutierrez said:

I'm having this issue from the beginning. I deal with it with post processing, but I would like to get rid off it. Many times makes the post processing quite difficult. It has nothing to do with the target. Longer exposures makes more evident the issue. In any, case, being well exposed, the final stack will reveal the gradient independently of the individual exposure time.

True, yes that's correct now I think about it.
And yes something does not make any sense.
If there was an issue causing the lighter corners on the lights, such as light leak, internal reflections in the imaging train, or just the way the filters are working then why would the flats be as you would expect being darker at the corners and outer edge, you would expect the flats (if evenly luminated) to show the same.

Steve
 

[teoria_del_big_bang]

32 minutes ago, mgutierrez said:

That's it. Well, only calibrated with darks to remove hot pixels (without darks, the result is obviously the same)

You say obviously the same is that definite ?
Are we sure the master dark is not affecting anything other than hot pixels etc ?
Light leak whilst taking darks could be an issue , just a thought.
 

Steve

[mgutierrez]

1 minute ago, teoria_del_big_bang said:

You say obviously the same is that definite ?
Are we sure the master dark is not affecting anything other than hot pixels etc ?
Light leak whilst taking darks could be an issue , just a thought.
 

Steve

absolutely sure steve. I was going, in fact, to upload the non-dark version but I saw some hot pixels and I didn't want you to think I'm a bad boy ;-D

[teoria_del_big_bang]

Just now, mgutierrez said:

absolutely sure steve. I was going, in fact, to upload the non-dark version but I saw some hot pixels and I didn't want you to think I'm a bad boy ;-D

Ha Ha, yes I too do not like looking at an image with hot pixels but we all have them 🙂 
I will be following thread with interest but I am at a star camp in South of UK tomorrow and not even packed my scope up yet or anything else so must get on and pack things into the car. Typical UK weather though looking at forecast looks like they may not be that much imaging actually going on 😞 
We really should host these things n Spain I think in future 🤣

Steve

[Adam J]

On 18/09/2022 at 22:52, mgutierrez said:

Hi all,

I'm searching someone who shares this setup with me: QHY268M camera, Astronomik OIII 6nm 36mm filter (non-MaxFR) and a Esprit100.

I'm experiencing a weird issue that I cannot fix

Here is a only dark calibrated master OIII:

 

Note the white gradient towards the corners. My master flat looks like a regular one: darker in the corners and lighter towards the center. But since the gradients are reversed between them, the flat calibrated OIII master is even worse, with more white gradient.

My master flat may be bad or not. I don't know and honestly I think it's not important: the white gradient is present on my uncalibrated master, and I think it's not normal (or it is?!)

This filter was replaced by astronomik a couple of months ago for a similar issue. Similar, cause the gradient was mostly present on one lateral; my guess is that the other lateral had some spot on the filter. So I think it's not a filter issue.

I also swapped the OIII position within the fw, just in case. Same result.

Shooting different objects at different altitude and completely different position, makes no difference.

What do you think?

m

PS: sorry if I posted in the wrong forum

Over correction is normally caused by poorly calibrated flat frames. Did you correctly match dark flats to the flats in temperature offset gain and exposure? 

[mgutierrez]

8 minutes ago, Adam J said:

Over correction is normally caused by poorly calibrated flat frames. Did you correctly match dark flats to the flats in temperature offset gain and exposure? 

I will summarize the question: do you think is normal to have bright corners instead darker ones in a single light?

[Adam J]

3 hours ago, mgutierrez said:

I will summarize the question: do you think is normal to have bright corners instead darker ones in a single light?

No its not normal for that to happen i have a Esprit 100 and I have never seen anything like that. If that is not calibrated at all then I would think that its an issue with the filter, I don't have first hand experience as I use mounted filters but I know that people used to get something like that with the older V1 ZWO 36mm filters that did not have edge blackening applied. Maybe take a look at the filter in question and see if there is any chipping of discontinuity in the edge coating. 

Adam

[mgutierrez]

1 minute ago, Adam J said:

No its not normal for that to happen i have a Esprit 100 and I have never seen anything like that. If that is not calibrated at all then I would think that its an issue with the filter, I don't have first hand experience as I use mounted filters but I know that people used to get something like that with the older V1 ZWO 36mm filters that did not have edge blackening applied. Maybe take a look at the filter in question and see if there is any chipping of discontinuity in the edge coating. 

Adam

Thanks for the reply. I also think is not normal. These filters have already blackening edge. Also, this filter was replaced by astronomik because of a similar issue (see 1st post). My guess is that for some reason I cannot understand, I'm meeting the requirements for a perfect storm.

[scotty38]

Is there a possibility it's a light leak past the filters which is "different" for real images versus flats hence the effect when stacked? I use the Buckeye filter masks with my filters and it made a big difference to the reflections I was seeing.

May be different for you but just wondered if it was worth a thought that's all.

[mgutierrez]

1 hour ago, scotty38 said:

Is there a possibility it's a light leak past the filters which is "different" for real images versus flats hence the effect when stacked? I use the Buckeye filter masks with my filters and it made a big difference to the reflections I was seeing.

May be different for you but just wondered if it was worth a thought that's all.

yes, I thought about reflections. But this is the second filter I try. I also guess that it should affect to other filters as well; at least not only to OIII. Furthermore, this filters have already a blackening edge. I'm in contact with another user and starting to think that this is the "normal" behaviour of this filter and the trick is to take the proper flats. Honestly, I don't know how a light with white corners can be considered normal, but now I'm considering it.

[scotty38]

8 minutes ago, mgutierrez said:

yes, I thought about reflections. But this is the second filter I try. I also guess that it should affect to other filters as well; at least not only to OIII. Furthermore, this filters have already a blackening edge. I'm in contact with another user and starting to think that this is the "normal" behaviour of this filter and the trick is to take the proper flats. Honestly, I don't know how a light with white corners can be considered normal, but now I'm considering it.

Just to be clear I wasn't suggesting you had reflections and the blackened edges won't stop light getting past if there is a small gap. That's what the Buckeye masks prevent. If this is a problem for you I guess it's not unreasonable that one filter/slot in the filter wheel could be affected more than another. Like I say just a thought.

[mgutierrez]

7 minutes ago, scotty38 said:

Just to be clear I wasn't suggesting you had reflections and the blackened edges won't stop light getting past if there is a small gap. That's what the Buckeye masks prevent. If this is a problem for you I guess it's not unreasonable that one filter/slot in the filter wheel could be affected more than another. Like I say just a thought.

yep, sure, I got your point, thanks!

[mgutierrez]

hi all,

just a quick update. Switching to Astronomik MaxFR version of OIII filter, has led to a HUGE improvement:

27min of an uncalibrated OIII stack (same object). Left, the maxfr version. Right, the older one. Note the brigther corners on the right image.

Watch the corresponding calibrated stacks (on the left the maxfr):

 

The difference is evident. Note, however, that maxfr calibrated stack has still some gradient. But that's a completely different history. I expect that kind of gradient: bad flats (OIII flats are specially tricky), light pollution (OIII filter is more sensitive to lp compared to Ha or SII), etc. I consider that's completely normal. In fact, a standard ABE or DBE gets rid of it completely. Not the same with the other stack. Is a pain to deal with it in post-processing.

Well, I just wanted to share with you my findings.

m

[ollypenrice]

Well, inverse vignetting is certainly straight from the camera is certainly a new problem on here, so far as I can remember. There does seem to be a curse on OIII filters in general. I've had two Astronomik and two Baader and all were poor.

Olly