ASI1600 and 130PDS - about to give up, need help

[vlaiv]

Out of interest I calibrated master flat green with master flat luminance and got this:

Apart from far corners (different level of vignetting for filters) - all I can really see is dust difference.

In my opinion - flats work.

2 minutes ago, BrendanC said:

So, the state of play now is:

• The scope isn't up to the job
• The flats are over-correcting
• The flats need to be >1 sec

I don't think that flats are over correcting in traditional sense.

When I wipe LP gradients (linear ones) - this is what remains. I don't really see how that corresponds to flats. What we see here is simply not related to flats in my view - remember - flats have significant vignetting.

For that reason - flats don't need to be long - these work just fine.

It is scope? I would say 99% yes - some sort of internal reflection or light leak or something strange.

[BrendanC]

2 minutes ago, vlaiv said:

It is scope? I would say 99% yes - some sort of internal reflection or light leak or something strange.

Thanks again Vlaiv. Given what I've been going through, I'm thinking it's probably the 'something strange' option.

What I still don't understand is why I was doing fairly long shoots with the DSLR without this problem.

Next shoot, I swear I'm going to wrap the OTA in foil, to try and mitigate any sort of possible leak, and take it from there.

[inFINNity Deck]

14 minutes ago, BrendanC said:

Also, if I have to resort to putting sheets of paper in front of the light panel to achieve this, will inconsistencies in the paper cause artefacts in the flats?

Normally not as the scope will not be focused to the light panel. That foam cap I use to dampen the light has structure and inconsistencies as well, but they do not appear to affect the flats.

Are you taking flat-darks as well (so darks to correct the flats) and is the camera cooled while taking the flats? I normally take flats, darks, and biases at night to ensure there is no ambient light affecting the subs.

Nicolàs

[ollypenrice]

• The scope isn't up to the job     Edited: I don't think it's the scope but Vlaiv is right about light leaks and reflections. My doubts come from having seen so many threads about over-correcting flats. They afflict all kinds of optics.
• The flats are over-correcting     I feel you can also be sure of this.
• The flats need to be >1 sec       Has to be worth a try but also leave a multi-second delay between each flat in the sequence. This also has to be worth a try.
• Also, if I have to resort to putting sheets of paper in front of the light panel to achieve this, will inconsistencies in the paper cause artefacts in the flats?  No, the paper is too far out of focus. Think of that primary mirror sitting in the middle of the light path. You don't see it. However, if it's going to worry you just stand by the panel during the flats sequence and rotate it from time to time.
• What do people think my chances would be of getting better results with an ASI533MC which is looking like an increasingly attractive alternative?  Possibly quite good because this problem is almost certainly arising somewhere in the camera-readout-software-calibration-stacking parts of the operation. All of these will experience change if you change the camera but you risk finding that the problem may persist, especially if you stick with the same manufacturer whose software may have common factors between models.

Olly

Edited June 6, 2022 by ollypenrice

[vlaiv]

1 minute ago, BrendanC said:

Thanks again Vlaiv. Given what I've been going through, I'm thinking it's probably the 'something strange' option.

What I still don't understand is why I was doing fairly long shoots with the DSLR without this problem.

Next shoot, I swear I'm going to wrap the OTA in foil, to try and mitigate any sort of possible leak, and take it from there.

Here is an idea you can try in the mean time.

Take your luminance files and split them into two groups - first half and second half

Use same calibration masters on each group, but stack each group separately.

After finishing - post both results here (just for inspection) - but more importantly look if they look differently when heavily stretched.

If problem is with calibration files - both stacks must show same pattern of these strange gradients (when LP is subtracted), but if things are different after LP removal - then cause is something that changes due the course of the evening.

Light leak on OTA can be both stationary and moving. Stationary will be light source moving together with OTA as it tracks the sky and that is less likely.

In any case - we will narrow things down a bit if we examine two stacks.

[vlaiv]

1 minute ago, ollypenrice said:

The flats are over-correcting     I feel you can also be sure of this.

Why do you insist that this is the case? I just posted images above showing that gradients don't resemble vignetting on flats even a bit.

 

[ollypenrice]

10 minutes ago, vlaiv said:

Why do you insist that this is the case? I just posted images above showing that gradients don't resemble vignetting on flats even a bit.

 

Because the image has a dark centre and bright corners. Here I quickly measured the background sky values in Photoshop.

I'll put money (or would if I had any 😁) on this being the inverse of what a flatless stack would produce. 

Olly

[vlaiv]

7 minutes ago, ollypenrice said:

Because the image has a dark centre and bright corners. Here I quickly measured the background sky values in Photoshop.

This image is wiped by StarTools - why don't you examine original stack that is still linear - distribution of bright and dark patches is rather different.

[Elp]

Doesn't kappa sigma stacking help a bit with uneven lit subs? Obviously if there's a lot of them then the image will still have the issue.

[ollypenrice]

39 minutes ago, vlaiv said:

 

When I wipe LP gradients (linear ones) - this is what remains. I don't really see how that corresponds to flats. What we see here is simply not related to flats in my view - remember - flats have significant vignetting.

 

Here we see four bright corners and a bright middle, exactly what we expect from over-correcting flats. Obviously the wipe has had a strange effect on them but it's all there - bright corners and centre.

I can't download the files but it would be helpful to see a stack (with flats) stretched but with no other intervention and, likewise, a stack without flats given the same stretch.

It's always worth remembering in any trouble shooting operation that there may be more than one defective component so I don't say it's only over-correcting flats but I think they are playing a part.

Olly

[vlaiv]

39 minutes ago, ollypenrice said:

Here we see four bright corners and a bright middle, exactly what we expect from over-correcting flats. Obviously the wipe has had a strange effect on them but it's all there - bright corners and centre.

On the other hand - I'm seeing something completely different:

Zones of "over correction"

You can't have both bright, medium and dark parts of flat over correct

Over correction looks like inverse of flat so it should have following pattern:

very white right edge and corners (as it is very dark in flat itself) and dark central part.

In above stretched image - no corners are lighter then the rest of the image.

[david_taurus83]

I found this helpful diagram as explained to me by a previous forum member (Oddsocks). This is why I recommend moving the light source for the flats away from the OTA as much as possible.

 

[vlaiv]

17 minutes ago, david_taurus83 said:

This is why I recommend moving the light source for the flats away from the OTA as much as possible.

And why flocking and baffling is very important. If scope is flocked and baffled well - then any ray that reflects more than once or twice is severely attenuated compared to principal rays and its signal falls below noise floor - and hence flats work.

However - if you have anything shiny in path of the light - it will create problems.

[BrendanC]

... and I'm starting to get that panicky drowning feeling again.

@vlaiv, I'm going to do as you suggest - split the lum files into first and second half, stack with same calibs, and share here. Simple next step, will take me forward, and allow me to decide on the next step (which could be a different camera altogether, this is driving me crazy).

In the meantime I've just been a bit ruthless with removing subs that looked a bit too bright using PixInsight's blink feature, which were overwhelmingly from the first, what, 30 minutes at least of each of the three nights' data I have in total. This is from about 2.5 hours lum, and 1.25 hours each of R, G and B. The results are a bit better, but as with all my other images, they're in spite of the data rather than because of it. I will however be putting them back IN to the stack for the above test.

 

Edited June 6, 2022 by BrendanC

[BrendanC]

Right.

These were stacked in Siril because it's soooooo much faster than APP.

First half of lum subs in 1.fit, second half in 2.fit, same calibration files used for both.

There is quite a difference.

What does this tell me? That there is a light leak? Or that the sky is sufficiently different on one side of the sky than the other? Or that I have internal light problems? Or my flats are over-correcting? Etc.

@ollypenrice and @vlaiv, over to you guys cos you're the gurus helping me through all this.

 

1.fit 2.fit

[ollypenrice]

3 minutes ago, BrendanC said:

Right.

These were stacked in Siril because it's soooooo much faster than APP.

First half of lum subs in 1.fit, second half in 2.fit, same calibration files used for both.

There is quite a difference.

What does this tell me? That there is a light leak? Or that the sky is sufficiently different on one side of the sky than the other? Or that I have internal light problems? Or my flats are over-correcting? Etc.

@ollypenrice and @vlaiv, over to you guys cos you're the gurus helping me through all this.

 

1.fit 31.26 MB · 0 downloads 2.fit 31.26 MB · 0 downloads

62 MB at 2.2 MB per second with my internet...  

I would do as David/Oddsocks suggested and move the flats light source further away. Oddsocks' diagram is very potent and, though I'd forgotten it, I remember being impressed by the idea at the time.

I'd still like to see two simple JPEGS: a log stretch with flats and a log stretch without. Vlaiv and I are not seeing the same things in the files we have seen so far.

Olly

[vlaiv]

Well, they are more similar than different - but there is one feature that makes clear distinction between the two:

Notice illumination in bottom right corner - top image is dark and bottom is bright. This is with LP gradient removed from both images so it is not due to that.

Difference of two images clearly shows that there is difference in overall illumination:

(images are not aligned - that is why we have "emboss" effect on stars and target - but look at background only - it should be even if two have same background - but it is not - right lower corner shows great difference).

[BrendanC]

So does this mean that the bottom right corner is showing an external light source leak?

[vlaiv]

1 minute ago, BrendanC said:

So does this mean that the bottom right corner is showing an external light source leak?

My conclusion is that there is some sort of issue with light leak

Here I have outlined where two stacks show differences (it might be better to look at original image without markings - markings are distracting, just take note of areas they are pointing to):

Top image is brighter in two upper circles and darker in bottom circle.

Here is what I read from this:

1. there is signal in the image that moves depending on where scope is pointing (issue with calibration files will be always in the same place - same calibration files used)

2. It is not due to high altitude clouds as gradients are too similar between first bunch of lights and second bunch of lights. Clouds tend to be blown away by wind rather quickly (they stay the same few subs at most)

My guess is that this is signal that is captured while scope was tracking and it is external to scope as it changes places in the image. Not all of it behaves this way - but it points to the light leak of sorts. If there is light leak - some of it will change as the scope moves and some will stay the same (ambient illumination vs point sources).

It has nothing to do with calibration files as these stay the same between first and second batch.

 

[BrendanC]

OK, thanks for this Vlaiv. It's very informative.

Finally (?), for @ollypenrice, here are two versions of the lum subs, stacked in Siril again, one with flats/dark flats, and one without flats (but still with darks). I'm sorry but I don't know how to do a log stretch, so please feel free to take these images and do whatever you need with them.

I was going to attach them as JPGs but thought it best to use the original FIT files, for integrity. Apologies to Olly if this is a bandwidth hog (but surely 65MB isn't a huge download?)

So situation so far is:

• There may be a light leak from an unknown source - I'm going to wrap the OTA as a test and see if that helps, and consider properly flocking the inside too
• I should try and create flats that are >1s and with >1s between each frame (I currently have 1s pause between each frame) - and I should be able to use paper or some such thing to dim the lightbox sufficiently to do this
• I should place the light source further away from the OTA - not entirely sure yet how I'm going to do this, as in getting the light source and OTA aligned, but I'll have a go
• I'm also going to add here that it also seems I need to be much more selective with my subs, particularly when there's a lack of proper astro dark in the UK

Is this about right?

And if I continue to bang my head against this particular rock, I also have in reserve the notion that I could go back to an OSC, probably a nice simple ASI533MC or some such thing.

 

without flats.fit with flats.fit

Edited June 6, 2022 by BrendanC

[vlaiv]

For Olly's benefit (and for others who don't want to download) - here is two side by side stretched the same:

And two side by side - stretched differently (with flats is stretched more) - enough to show gradients in each:

both are linearly stretched (just setting black and white point to certain value while keeping pixel values linear).

Neither has been altered in any way (no gradient removal).

[Xiga]

Could the problem be due to fogging on the sensor cover glass? I know earlier versions of this camera didn't have a heated window. When you set up Brendan, how long does it roughly take until you actually start capturing subs? 

If it is, then the problem would show as getting progressively worse throughout the night. However, this data set was started when the sky was still relatively bright, so the fogging could have already set in by the time the sky brightness had levelled out. It might be a good idea to see a different data set shot earlier in the season during Astro Dark, and see if the first few subs look ok. 

[Astro Noodles]

32 minutes ago, BrendanC said:

I'm also going to add here that it also seems I need to be much more selective with my subs, particularly when there's a lack of proper astro dark in the UK

It was mentioned earlier that your imaging session began at appx 22:00. I think that is too early - it will still be very bright. Nautical dark will be about 23:15 at the moment in SE England. There won't be much point taking images before Nautical dark even with filters.

[BrendanC]

True, except that it didn't - it started at 23:00.

[teoria_del_big_bang]

21 minutes ago, BrendanC said:

True, except that it didn't - it started at 23:00.

I was just going by the time stamp of the fits header when I said 10 (well 22:08 to be precise) but maybe your date/time is an hour out.
But also was not suggesting that was the problem with the gradients just that it might not be helping with the final image quality.

Steve

Edited June 6, 2022 by teoria_del_big_bang