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QHY183M & Flats = Frustration

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#76 kingjamez

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Posted 18 September 2019 - 06:35 PM

When I talk about blue sky flats, that is with the sun high in the sky. I'm not talking about twilight sky flats with a diffuser. I'm talking no diffuser, open aperture, empty blue sky, DURING THE FULL ON DAYTIME. Your exposures will be sub-second, and you'll easily be able to get 30-40k ADU. Not only that, with exposures around 0.1 down to 0.01 seconds, you can yank down 36 flat frames per channel in a matter of a couple minutes.

IMO, there is nothing easier. In fact, I've programmed my setup, which I have no automated flat panel for, to take flats like this against the north pole some time after teh sun has risen, so there can be no stars in the flats. I don't need to worry about a diffuser, nor a flat panel, or anything like that. Just empty blue sky.


That’s just crazy talk... so crazy I’m going to give it a try.

-Jim

#77 Jon Rista

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Posted 18 September 2019 - 06:42 PM

That’s just crazy talk... so crazy I’m going to give it a try.

-Jim

I will say, figuring out the range of possible exposures for an approximate time of day, while the sun was still rising from low to higher in the sky, did take some work. And I sometimes see my flats go from ~10k ADU to ~45k ADU over the run. That doesn't seem to matter much once everything is integrated, though... So you will need to spend a little time figuring out a good median exposure time that will work for morning time blue sky flats while the sun is in the sky. At least, if you want them to be fully automated. If you will be sitting there through the process, you just need to grab a few sample exposures for each filter (if you are using filters) to figure out how long exposure needs to be for each, program the sequence, then let it rip. Usually takes a couple of minutes for the full set across all filters.

 

Oh, and um...beware of airplanes:

 

ytQkVN7.jpg


Edited by Jon Rista, 18 September 2019 - 06:42 PM.

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#78 Stelios

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Posted 18 September 2019 - 06:54 PM

The dispute is international.

 

https://www.tommasor...o-superbias-no/

This is an *excellent article* (thanks for pointing it out) and highly recommended. It'd be nice to know what JR and/or other gurus think of his (ingenious whether accurate or not) method of producing a usable superbias for CMOS cameras.



#79 Jon Rista

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Posted 18 September 2019 - 07:08 PM

This is an *excellent article* (thanks for pointing it out) and highly recommended. It'd be nice to know what JR and/or other gurus think of his (ingenious whether accurate or not) method of producing a usable superbias for CMOS cameras.

I honestly cannot really comment properly, because none of the example images he shows have full size versions. Only the tiny reproductions in the article. Without looking at the result of the real master minus the superbias at 100% scale, especially with an appropriate stretch applied (something on par with the kind of REAL stretch you might apply to an integrated image), you cannot really know exactly what junk may be left in your data to add noise and maybe even behave as a fixed pattern.

 

The problem with FPN is it's impact is often "silent", for lack of a better word. Most people wouldn't know that, or how, FPN was affecting their results. The only way to really SEE the impact is to progressively integrate more and more data, and then compare the results of each integration. If FPN is limiting you, this will become evident when you flip through such a progressive set of images. Initially, when flipping through such as tack, you will see that the noise pattern in each successively deeper integration continues to change. Eventually, though, you will start to notice that a particular pattern of noise, and this "pattern" may visually appear to be totally spatially random, changes less and less the more you stack. Eventually it will stop changing at all...no matter how much more you stack. THIS is what FPN does...it puts a cap on your potential SNR.

 

It's a subtle and largely invisible issue, though, because ... few people (maybe no one at all) ... ever actually try to progressively integrate their data and see what they get as they stack more and more. I've never read of any articles of anyone else doing it, so concretely I think I may be the only nutjob weird enough to even try. :p Most people...simply integrate teh data they have. So they only see the result of stacking everything. A single sample cannot tell you anything about the progressive change in noise profile as more and more data is integrated, though. Very subtle and very invisible problem.

 

So despite the article above, which doesn't really seem to properly evaluate the impact on the final result of using a superbias, my strong recommendation is still: NEVER. Use a proper master bias. They really are not that hard. A hundred frames will usually do, unless you are stacking hundreds of lights or more. You can acquire a hundred bias frames in a heartbeat. Integrating them shouldn't take more than a few minutes. Then you are done. You don't have to spend EXTRA time creating a superbias from this, you don't have to waste any braincells worrying if the superbias might be making your data worse. Just stack a hundred biases and your done.



#80 terry59

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Posted 19 September 2019 - 06:32 AM

I did the same thing this morning after reading John's suggestion. I did diffuse it with some white fabric, but I was surprised at how bright the sky had to be before I got reasonable exposure times.

 

I waited until the sun had mostly come over the horizon so I could do a run without the sky changing brightness quickly. I eventually got a good run at around 1 second exposure with the sun mostly fully risen.

 

Not sure if I like this method as it seems pretty variable. My LED panel is always the same, but more hassle to setup.

 

-Jim

 

 

When I talk about blue sky flats, that is with the sun high in the sky. I'm not talking about twilight sky flats with a diffuser. I'm talking no diffuser, open aperture, empty blue sky, DURING THE FULL ON DAYTIME. Your exposures will be sub-second, and you'll easily be able to get 30-40k ADU. Not only that, with exposures around 0.1 down to 0.01 seconds, you can yank down 36 flat frames per channel in a matter of a couple minutes. 

 

IMO, there is nothing easier. In fact, I've programmed my setup, which I have no automated flat panel for, to take flats like this against the north pole some time after teh sun has risen, so there can be no stars in the flats. I don't need to worry about a diffuser, nor a flat panel, or anything like that. Just empty blue sky. 

I just saw this as I'm in the process of shooting new flats with the Flatman. Based on yesterdays experiment  these are just under 40k

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#81 terry59

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Posted 19 September 2019 - 06:33 AM

terry, that looks like a Valentine's day flat.  Or the Heart nebula smile.gif

 

Do you see a similar pattern with other filters, or just with Ha?

Curious...

I'm not sure, this is the only filter I have at the moment



#82 terry59

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Posted 19 September 2019 - 07:54 AM

Two more attempts and the result remains the same

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#83 terry59

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Posted 19 September 2019 - 08:04 AM

This was done using my master dark and master bias only

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#84 jdupton

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Posted 19 September 2019 - 08:16 AM

Terry,

 

   Have you tried my suggestion of Integrating your Master Flat with No Normalization instead of Multiplicative Normalization? You might try that just for grins. Be sure to calibrate the raw Flat subs first with either a Master Flat-Dark or Master Bias (whichever you have available).

 

 

John



#85 terry59

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Posted 19 September 2019 - 08:26 AM

Terry,

 

   Have you tried my suggestion of Integrating your Master Flat with No Normalization instead of Multiplicative Normalization? You might try that just for grins. Be sure to calibrate the raw Flat subs first with either a Master Flat-Dark or Master Bias (whichever you have available).

 

 

John

Hi John.....not yet. That will be the next thing I try and I'll post the result 



#86 terry59

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Posted 19 September 2019 - 10:08 AM

Here is the result of John's experiment. Unfortunately the result is the same. I've tried various ADU values, diffused and not, sky flats and flats with the Flatman and it looks the same. I've never seen this with any other camera I've used

 

 

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#87 bmhjr

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Posted 19 September 2019 - 10:11 AM

Do you have any shiny adapters in there?  Just a thought.



#88 terry59

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Posted 19 September 2019 - 10:22 AM

Everything is the same as before (scope, reducer, blue fireball spacer) except the camera, filter wheel (it is the QHYFW3) and Baader 3.5 nm Ha filter. In every flat the center ADU value is ~2k higher than the rest which varies 



#89 jdupton

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Posted 19 September 2019 - 10:47 AM

Terry,

 

   Have you uploaded samples anywhere? I don't recall seeing any. Ideally, I think we might be able to help more if you could upload one to three raw flat frames and your Master Flat-Dark or Master Bias (whichever you use for calibration). In addition, seeing your Master Dark (matching the Lights) a single frame of your lights would really help. Using FITs or XISF format is most helpful. With those, I think we may be able to figure out what is going on.

 

   Putting these few files up on a file sharing site (like DropBox, Google Drive, or other) will allow us to see exactly what you are seeing.

 

 

John



#90 terry59

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Posted 19 September 2019 - 11:00 AM

John,

 

  Three flats, the two masters, master dark flat and master dark

 

Thanks!

 

https://1drv.ms/u/s!...C_yh52?e=WPOUBE



#91 jdupton

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Posted 19 September 2019 - 11:22 AM

Terry,

 

   Got those. Can you also upload one raw light frame?

 

 

John



#92 terry59

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Posted 19 September 2019 - 11:28 AM

Here you go

 

https://1drv.ms/u/s!...C_yh52?e=lfxQ4n



#93 jdupton

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Posted 19 September 2019 - 11:50 AM

Terry,

 

   I am at a loss as to what is going on. The illumination profiles of the Flat Frames show no correspondence whatsoever to the illumination profile in the Light Frame. In, fact, the Light Frame shows minimal if any vignetting at all. That is why you have better luck omitting the Master Flat Frame during processing. The Light Frames simply do not need Flat Frame correction (from a vignetting point of view). (If you start getting dust motes showing in the Light Frames, you will have to solve the Flat Frame problem.)

 

   My conclusion is that the problem does not come from any part of your processing. The problem seems to lie in the Flat Frames themselves. It must have something to do with the method or light source used to take the Flat Frames. Whatever has changed, it must have something to do with the acquisition of the Flat Frames. Can you think of anything different after changing the camera and filter wheel?

 

 

John



#94 terry59

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Posted 19 September 2019 - 12:06 PM

John, thank you for looking at this. I get the same result from sky flats as I do with flats via the Flatman yet the Flatman works just fine with the KAF8300. No other changes to equipment or processing methods. At least I'm getting great data 



#95 Jon Rista

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Posted 19 September 2019 - 02:28 PM

Two more attempts and the result remains the same

Ok, given this...I am starting to think you have some kind of diffuse reflection issue, or perhaps an issue with the filter that is only exhibiting with stronger signal. The heart shape from the flat appears to be getting introduced into the calibrated lights. I've had that problem, and every time I've encountered it, it was due to some kind of diffuse reflection off of something (or even many things) in my imaging train. I've seen similar issues when people have an issue with a filter, and it happens more often with cheaper filters. A big exemplar were the original ZWO filters for the ASI1600, which were not fully coated edge to edge. There was a slight gap between the coating and the edge of the glass, which lead to funky issues with flats (and even lights to a degree) that caused problems with calibration.

 

I think...cfoster (not certain), actually has a fairly extensive thread in the CCD/CMOS forum where he went into great depth how he evaluated and resolved similar issues with his system. Might we worth a look.


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