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


BrendanC

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Looking at the with-without files gave me effects I've never seen before.

Without flats: the image looked like a perfectly normal vignetted image with a smooth gradient between dark corners and a bight middle, slightly offset to the left. Such offsets are common. I tried both ABE and DBE in Pixinsight and the gradient maps (effectively artificial flats) looked exactly like the kind of flat I'd have expected given the vignetting. However, it was totally useless when applied to the light using division. To my great surprise it did not flatten the image, though it looked as if it ought to do so.

With flats: the image is flatter than it was and does, still, look to me like one on which flats have over-corrected but also introduced other oddities. Again, PI's DBE and ABE failed to flatten it.

Conclusion: nothing even slightly definitive but I think the flats are trying and failing to work. They are not totally useless but they are not consistent with the lights so they don't flatten. If we have two effects, fogging on the camera and Oddsocks' faulty flats light path (panel too close) we might find ourselves with a chaotic and unpredictable set of consequences. All I can suggest is to eliminate the fogging and try changing the flats method. (Panel further away.)

The underyling data look good, frustratingly.

Olly

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2 hours ago, david_taurus83 said:

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.

 

Indirect-light-paths-with-a-flats-panel.gif.51b040785031e84933f9bbbff2fa0661.gif

My gradient issues have got a lot better (not perfect but better) since I made a long flocked dew shield and used that to take my flats with my EL panel further from the focuser. I think a lot of my issues were light from the panel directly entering the focuser 

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35 minutes ago, ollypenrice said:

The underyling data look good, frustratingly.

 

Indeed.

OK, well, thank you @ollypenrice and @vlaiv for all your help. I'm going to flock the OTA, and try to change my flats method, but this is proving an exceptionally painful experience. I did quite a lot of research and thought the ASI1600 would be ideal, but it's increasingly emerged that something isn't right somewhere along the line.

Thanks also to everyone else who's contributed. 

Edited by BrendanC
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23 minutes ago, BrendanC said:

@markse68 How far do you take your panel from the focuser? Do you place it on the dew shield, or further away than that?

I rest it on the dew shield. IIRC the dew shield is 40cm long and it’s an 8” scope.  There’s still something there if i really stretch and Siril background removal tool makes it worse but it’s definitely a lot better.

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23 minutes ago, BrendanC said:

I did quite a lot of research and thought the ASI1600 would be ideal

I doubt it’s the camera as there’s loads of great images from that camera on eg Astrobin (that’s a good place to look to get an idea of what to expect and how long integration time you need-but you probably know that already)

One other thing i’ve run into with my camera which is not the same as yours but is cooled- I have to bin(chuck) a good 15mins or so worth of shots usually as something ices up. I can literally see it thawing after a while and then the subs are ok. Not sure if it’s the actual sensor (hope not) or the cover glass.

Edited by markse68
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Thanks. The quality of the camera is in no doubt. It's just been a case of trying to figure out what on earth has been going wrong with my subs/calibs. I'll look out for fogging/freezing.

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oh yes one more thing- I used nd lighting gel and diffuser gel to dim my EL panel so i could get 2sec flats for L. My Ha flats are 30secs and should probably be longer haha. Paper is probably fine but you can get it on ebay if you want to try it:

https://www.ebay.co.uk/itm/152856137032?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=MtLvZSzfTbO&sssrc=2349624&ssuid=5B1qLm7jTQa&var=&widget_ver=artemis&media=COPY

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1 hour ago, BrendanC said:

I did quite a lot of research and thought the ASI1600 would be ideal, but it's increasingly emerged that something isn't right somewhere along the line.

To be honest, I don't really see how it can be camera's fault?

Camera is perhaps very sensitive and reveals flaws in the rest of the system, but I highly doubt it is down to it.

Perhaps only thing to look for would be dew/fog/ice on camera window, but everything else should be fine.

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Agreed - and that's why I said 'somewhere along the line'. I meant that the camera should be fine in terms of suitability with my system, regarding sampling and guiding. I'm assuming the camera is fine, it's something to do with the OTA, the environment, or my calibration/processing technique.

Again, thank you so much for your help Vlaiv. I'm learning so much through this, most of it about how I'm doing things wrong(!), but it's all learning. :)

Edited by BrendanC
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Another thought Brendan. How quickly do you cool your 1600? I think most people choose to cool over something like 5 to 10 mins. If you do it too quickly there's maybe a chance it could lead to frosting on the cover glass. 

Next time you are out, I would suggest doing another test (an easy one). Don't immediately cool your camera on connecting it. Get yourself all set up and then capture a few subs without cooling. Then turn on the cooling and just keep capturing while it cools down. See if the first few subs look ok (apart from the higher level of thermal noise and hot pixels of course). 

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Frost / dew on sensor can be recognized with meridian flip - it will rotate 180 degrees with respect to target.

I just checked Blue, Red and Green stack as well.

Pattern stays relatively the same with changing intensity - some patches are brighter and some fainter.

Either all subs were captured on same side of meridian - or it's not dew/frost/sensor window related.

image.png.ad66aa3e475843132d68f856262dff8f.png

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4 minutes ago, BrendanC said:

Thanks - I cool it at the APT default, which is something like 3 degrees every 2 minutes or something, with a pause every now and then. I'll see how that goes. 

I see that you use APT. Out of interest, what driver for ASI1600 are you using? Ascom or native?

If you are using Native / built in driver - I'd try ASCOM instead. Maybe even give NINA a go. I know it is a long shot, but sometimes drivers can cause issues.

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I'm using the native drivers. What changes would the ASCOM drivers offer?

Ideally I wouldn't need to learn a whole new package, but I know NINA is very highly regarded. Perhaps you're right - just throw everything into the air and see what lands.

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2 hours ago, Astro Noodles said:

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.

I have analysed the subs using my FITSalize and produced the following SQM-graph based on what ASTAP managed to calculate using the H18 star-database:

image.png.609c20f95d6b87206f6941b84aa5fbef.png

Obviously ASTAP has some trouble determining correct SQM-values when the blue and red filters are in use. ASTAP has been tested against an Unihedron SQM-meter by the developer and earlier tests by myself and the developer have shown that with luminance and green filter the SQM-values are pretty correct, while blue and red generally score too high.

The thing that worries me is the differences in the blue filter SQM-values. Ignoring the fact that their values are way too high, they should be closer together and form a smooth line, similar to the other lines.

Imaging started quite early in luminance, which can be deducted from the slope during the first hour. Still at 19magn/"^2 it should be dark enough. The red session suffers from the same issue: dawn is measured during all of this session. The jump from green to red indicates that the red-filter SQM-values are too high and that the last red-sub was shot with an SQM-value of about 17magn/"^2.

Interesting is the jump around 23:00, where luminance SQM-value drops slightly. This could be an indication of local light pollution (garden illumination?).

Nicolàs

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2 minutes ago, BrendanC said:

I'm using the native drivers. What changes would the ASCOM drivers offer?

Ideally I wouldn't need to learn a whole new package, but I know NINA is very highly regarded. Perhaps you're right - just throw everything into the air and see what lands.

Few years ago - common wisdom was to use native drivers for high speed capture / short exposures like planetary and stick with ASCOM driver when doing long exposure.

I did find that ASCOM drivers produced better result from long exposure, but I can't remember the details. From then on, I always advise people to use ASCOM drivers for long exposures.

It probably has something to do with fast readout modes / electronics. Maybe something is in different mode with native drivers.

Maybe you won't see any difference, and like I said - it is probably long shot, but I think in the end - its worth a try - nothing to loose really, and it might actually help

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@inFINNity Deck OK, so I like charts etc, but if you could please tell me:

  • What do you think the problem is? I don't know what an SQM value is.
  • What is the most likely solution?

That would be really helpful! :)

Thanks. Brendan

Edited by BrendanC
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13 minutes ago, BrendanC said:

@inFINNity Deck OK, so I like charts etc, but if you could please tell me:

  • What do you think the problem is?
  • What is the most likely solution?

That would be really helpful! :)

Thanks. Brendan

Hi Brendan,

If I look at the graph, I think that imaging should have ended at least half an hour earlier and perhaps started slightly later. In addition I would have added more subs for this object to reach at least 20, but preferably 40 subs per filter.

You are used to DSLR, but remember that those shoot RGB at the same time but at a lower resolution (the Bayer pattern only has two green, one red and one blue photosite per four). So shooting in LRGB with a mono camera means considerable more imaging time than when shooting in colour to achieve the same result.

The main issue here is the total integration time: IMHO one night simply is not enough to achieve a decent result, certainly not on this faint object. There is only an hour and a half of luminance in that image. On the other hand your OTA is f/5, whereas mine is f/7, so you may expect better results in a shorter period than I do (maybe @vlaivcan comment on this, I am not sure about how to calculate the difference between your 130mm aperture f/5 Newton and my 150mm diameter f/7 APO).

My advice is to try adding more time to this object and see how results improve.

Nicolàs

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Thanks - and I do have more data, as per my earlier post (see https://stargazerslounge.com/topic/395577-asi1600-and-130pds-about-to-give-up-need-help/page/3/#comment-4251560)

Regarding total integration time, my thinking was that I could actually do LESS time. The Bayer matrix determines that I would have 50% green, and 25% blue and red. So, for mono, I could therefore shoot, for the same amount of green, 75% less red and blue.

I've been shooting Luminance and then the same amount total Lum as RGB, but with individual subs for RGB at 3x Lum. So, for example 3 hours of total exposure could be 1.5 hours of, say, 60s L, and then 1.5 hours of 3x30 minutes  at 180s of R, G and B. Given the higher resolution and generally better, more sensitive response of the ASI1600, I thought this would result in a better image. But I was wrong. Again.

I think I have a plan now though. Thanks again for all your help.

 

Edited by BrendanC
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10 hours ago, vlaiv said:

 

It probably has something to do with fast readout modes / electronics. Maybe something is in different mode with native drivers.

 

We've certainly seen this before on SGL in the context of flats. Members having trouble with flats discovered that they had inadvertently shot the flats in fast readout mode and they were not working. Just a guess, but does the software choose a fast readout if the exposures are short or make some other decision for us of which we're not aware?

My spell with over-correcting flats was cured, for a while, by capturing them in a different program (AstroArt rather than Nebulosity) though we continued to shoot the lights in Nebulosity. And then, one day, this also stopped working. This kind of thing makes me despair of finding neat solutions or explanations for everything.

I still do suspect the camera but include in that the capture software. Brendan didn't have this with his DSLR. Can an increase in sensitivity fully explain the sudden appearance of this problem?

Should we be looking again at the filters? What if they behave differently under the different illumination produced by the sky and by the flats? Filters are quite batch-specific and could be faulty.

Olly

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12 hours ago, vlaiv said:

Snip

Either all subs were captured on same side of meridian - or it's not dew/frost/sensor window related.

image.png.ad66aa3e475843132d68f856262dff8f.png

The reply to this post was that there had been no flip, so doesn't this mean that dew/frost/sensor is still in the probability pot for cause of the splodges? @vlaiv

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45 minutes ago, happy-kat said:

The reply to this post was that there had been no flip, so doesn't this mean that dew/frost/sensor is still in the probability pot for cause of the splodges? @vlaiv

Indeed - flip would rule it out as pre and post flip subs would have pattern rotated by 180 degrees.

As all of the subs were taken same side of meridian - I'm guessing that we can't rule out dew/frost completely.

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