Author Topic: Background pattern cause?  (Read 3329 times)

Offline macnmotion

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Background pattern cause?
« on: 2018 April 18 20:09:13 »
Hi. I'm trying to wrap my head around our larger scope as we finally have some images to process. The camera is an FLI ML16200.

The following is a stack of Ha, 110 6- minute subs which have been calibrated, aligned and integrated:



Here is the same image with a very quick ABE, which makes it much easier to see the background pattern:



Note tha pattern in the background. From noise? Chip pattern? Light pollution/sky noise?

Here is a stack from the same Ha images, calibrated but NOT aligned. I did this to see if I saw any type of chip pattern, but I don't really:



Any thoughts as to the source of this pattern? Thoughts on how to eliminate it during capture or processing? Note the subs are not dithered.

Thanks. Andy
« Last Edit: 2018 April 18 20:25:02 by macnmotion »

Offline pfile

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Re: Background pattern cause?
« Reply #1 on: 2018 April 18 22:39:40 »
it could be that the sky background has essentially 0 signal and when subtracting the darks you end up with pixels clamped at 0. usually this manifests as a moire pattern but perhaps because of the lack of dithering it does not look the same as i would expect.

you can try adding an output pedestal of 100 or 200 ADU in ImageCalibration and see if you get a different result...

rob

Offline macnmotion

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Re: Background pattern cause?
« Reply #2 on: 2018 April 18 23:07:59 »
Hi Rob,

Thanks for the reply. I assume that when you say the background sky has 0 signal, you mean we'd be at a dark site? Quite the opposite, we have terrible light pollution just outside of Bangkok.

If I'm misinterpreting your comment let me know, maybe I am interpreting the opposite of what you mean.

Andy

Offline pfile

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Re: Background pattern cause?
« Reply #3 on: 2018 April 19 09:22:03 »
yeah i mean that there's so little signal in the background that it's near the levels of the dark signal. due to the noise in the dark signal, sometimes pixels in the dark exceed pixels in the light and you end up with a bunch of negative values in the calibrated result, which get clamped to 0.

anyway certainly it's possible no matter where you are depending on filter bandwidth, telescope size and sub length. i agree it would seem unlikely in your case, but you can check the stats of your calibrated subs to see if there are any 0 pixels present.  if you are using the statistics tool make sure to uncheck the option that excludes pixels with the value 0.

rob

Offline rdryfoos

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Re: Background pattern cause?
« Reply #4 on: 2018 April 20 18:29:40 »
No way this is caused by dark skies.  That would mean dark skies are bad--and the opposite is true.  I don't know what it is--but I  suspect it has to do with the calibration.  How do the raw subs look?  Stack 20 or 30 raw subs without any calibration--just alignment, and see if the pattern is present.  If it is not....look toward the calibration frames.  If it is,  go through all 110 subs and see if a few bad ones are spoiling the apple cart.  If there are no culprits, could it be dew?  I am assuming there is not something wrong with your sensor--though that certainly is possible--or a cable.  Check to see all cables are sound--and all connections are solid (no corrosion or looseness, or moisture).  Good luck--these sorts of problems are a pet peeve of mine.....terrible.

Rodd

Offline msmythers

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Re: Background pattern cause?
« Reply #5 on: 2018 April 20 19:46:27 »
It looks like the stuff I get sometimes from hot temps, high humidity and changing LP. I've found here in Florida we get this high, very thin clouds/haze layers. It's somewhere between 1/2 a mile and 3 miles up. With our constant sea breezes it can change over the course of minutes and the local heavy LP, Bortle 8 lights up this layer. Of course my optics are in focus at this distance and there is nothing I can do about that. The worst part these days is the LP is wide spectrum LED around here so filters don't help. The only solution I've seen is wide dithering and even then there is no guarantee. On the plus side the LP forces shorter exposure times so if you can take enough images in a night you can throw out the worst of it and still have a decent set...maybe.


Mike

Offline macnmotion

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Re: Background pattern cause?
« Reply #6 on: 2018 April 21 07:11:32 »
Hi Mike,

Thanks. Sounds like you have similar humidity problems as we do.

We've wanted to add dithering into the mix. We couldn't before now because the mount we had been using, a Paramount Taurus fork mount, required settle times of 45 seconds or more to get usable unguided images of only 3-6 minutes. We've now replaced that mount with an Officina Stellare direct drive fork without this type of issue, so now we will be able to dither.

I'm not sure if we will dither before the end of our season (rainy season could start any time now), but when we finally do I'll let you all know how it goes with the background.

Thanks again. Andy

Offline macnmotion

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Re: Background pattern cause?
« Reply #7 on: 2018 April 21 07:15:18 »
Rodd, thanks for your input.

These are very good suggestions for some testing. I'm away for the weekend but when I'm back I'll do the stacking tests you recommended. I'm farily confident about the sensor, didn't think of the cable. But one thing at a time, let me do the tests and consider the contribution of the calibration frames to this issue. Like you, I have a need to understand these small things from a processing perspective. My teammates are much more adept at the actual mechanics of the systems.

Andy

Offline macnmotion

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Re: Background pattern cause?
« Reply #8 on: 2018 April 21 07:17:37 »
For anyone interested, here is the final processed result of the M 1 image I included as an example to start this thread:



Andy

Offline pfile

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Re: Background pattern cause?
« Reply #9 on: 2018 April 21 09:49:11 »
No way this is caused by dark skies.  That would mean dark skies are bad--and the opposite is true.  I don't know what it is--but I  suspect it has to do with the calibration.

sorry, but you are talking through your hat. if there is no signal in the background (for whatever reason), then you will absolutely have problems with calibration as i outlined above.

rob

Offline RickS

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Re: Background pattern cause?
« Reply #10 on: 2018 April 21 14:21:45 »
if there is no signal in the background (for whatever reason), then you will absolutely have problems with calibration as i outlined above.

This is especially likely if the camera has some bias drift, and most of them do.

Offline macnmotion

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Re: Background pattern cause?
« Reply #11 on: 2018 April 21 20:03:24 »
Thanks all. I'll pursue every suggestion in this thread to determine what the cause is, and will report back. Have a nice weekend.

Andy

Offline rdryfoos

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Re: Background pattern cause?
« Reply #12 on: 2018 April 21 21:27:02 »
No way this is caused by dark skies.  That would mean dark skies are bad--and the opposite is true.  I don't know what it is--but I  suspect it has to do with the calibration.

sorry, but you are talking through your hat. if there is no signal in the background (for whatever reason), then you will absolutely have problems with calibration as i outlined above.

rob
in that case, show me a section of sky that has no signal at all.  Many people have imaged M1 without these issues, so signal or no signal can't be the issue.  I did M1 and did not have these issues.  M1 has been imaged from the atacama desert, one of the darkest sites in the world, and this pattern is not common.  So, I am having trouble knowing what you mean.  My bet is it is not related to background signal--unless there is clouds or mist that is producing too much signal.  I believe he is imaging through light poluted skies--so there CAN"T me no signal.

Offline pfile

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Re: Background pattern cause?
« Reply #13 on: 2018 April 22 09:56:02 »
hi, i dont need to spend my time proving myself to you. this has happened to me (and RickS) on our system at SRO. i dont just make things up when i post ideas.

this has nothing to do with M1. it has to do with the camera, the filter, the exposure time and the properties of the OTA. just because you have not seen this problem with your system is not proof that it can not exist.

rob

Offline rdryfoos

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Re: Background pattern cause?
« Reply #14 on: 2018 April 22 10:36:47 »
hi, i dont need to spend my time proving myself to you. this has happened to me (and RickS) on our system at SRO. i dont just make things up when i post ideas.

this has nothing to do with M1. it has to do with the camera, the filter, the exposure time and the properties of the OTA. just because you have not seen this problem with your system is not proof that it can not exist.

rob
You suggested it was due to not enough signal in the sky (background).  I don't think that is the cause.  We are all entitled to our opinions.....even me.  More over, we are all entitled to post our opinions on the forum.  The fact that you said "you are talking out of your hat" in response to my opinion, instead of saying something like " I disagree" or "but have you considered this...." suggests that you are intolerant of others ideas or suggestions.  Either that, or you are having a bad day.  Either way.....if everyone had the same opinions, there would be no need for forums.  Now....When I hypothesized, I was speaking to the generator of this thread, not to you.  Perhaps we should endeavor to refrain from quibbling over who's ideas we like and dislike, and give our thoughts to the person who asked for help.