Stacked images with strange repeating patterns

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
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RICH.TEICH
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Stacked images with strange repeating patterns

Post by RICH.TEICH » Sun Aug 26, 2018 6:00 am

Dear Robin,
I am using SharpCap (3.1.5193.0) for the first time on ZWO ASI120MM and ASI385MC cameras.
The mount is an Orion Skyview Pro with an Orion EON 80mm telescope.

The first live stacked images of both cameras had strange repeating patterns.
The single frame captures showed no such patterns.

Here is a Word file with the images and camera settings.

http://www.share-ncal.org/ZWO_TEST_with_EON_80mm.docx


Any help would be much appreciated.
Sincerely,
Rich

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admin
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Re: Stacked images with strange repeating patterns

Post by admin » Sun Aug 26, 2018 10:11 am

Hi Rich,

what you have there is hot pixels on your sensor at the exposure length you are using - these are pixels that come out brighter than they should at longer exposure times due to inherent flaws in the sensor. As your tracking is a little off, the Live Stacking is correcting for this by aligning on the stars/galaxy. This means that the hot pixels (which stay in the same place on the sensor) appear to move relative to the stacked image and eventually show as a white streak in the final stack.

The best cure for this is to use dark frame subtraction, which will remove the hot pixels before stacking. Remember to take your dark frames at exactly the same camera settings and (as much as possible) temperature as your light frames otherwise you can end up getting dark streaks due to over-corrected pixels!

cheers,

Robin

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oopfan
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Re: Stacked images with strange repeating patterns

Post by oopfan » Sun Aug 26, 2018 10:24 am

Rich,

The ASI120 really is not designed for deep-sky. Mine had/has LOTS of hot pixels. (I've demoted it to PA cam.) Also, you might want to consider upping your color space from MONO8 to MONO12.

Brian

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turfpit
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Re: Stacked images with strange repeating patterns

Post by turfpit » Sun Aug 26, 2018 12:21 pm

Rich

Have a look in the Gallery forum for my posts

ASI120MC and DSOs
ASI120MC and DSOs revisited

For some ideas on settings for this type of camera. Lower gain helped with the hot pixels.
With recent experience, I would be trying 60x30s @ 5% gain and 30x60s @ 5% gain but the latter needs a decent polar alignment.

Dave

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oopfan
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Re: Stacked images with strange repeating patterns

Post by oopfan » Sun Aug 26, 2018 5:00 pm

Rich,

Both Robin and Dave bring up great points. Unless you have a cooled camera you really need to subtract dark frames but it is imperative to match the temperature of your darks with your light frames. The more the temperature drift the less effective dark subtraction will be and the more artifacts you will see in your stack. Also as Dave mentioned lowering your gain and lengthening the exposure can dramatically improve your images, plus it nearly eliminates "raining noise" artifacts due to imprecise guiding.

Also I agree with Dave that a total integration time of 30 minutes is about the minimum you need. For me that works well for bright DSOs however soon you will run out of the bright ones. Now I go for the fainter targets but that means a total integration time of 60 minutes or more. Here is the catch: there is too much temperature fluctuation over one hour *unless* you image in the wee hours of the morning like from 2am to 5am. I proved this to be true with my Arduino-based temperature logger that I leave outside all night. I find that I get the most precipitous temperature fluctuations from sunset to about 2am, and then it settles down until sunrise.

Fortunately the ASI120 has an integrated temperature sensor. This will allow you to build a "darks library" on those cloudy nights. The only downside is that SharpCap does not temperature-match your light frames with your darks library during its dark subtraction phase, so you have to do your stacking offline with Deep Sky Stacker or the like. For me I am old-school, so this is the way I choose to do it.

Depending on your financial resources you may want to consider purchasing a cooled camera. (I would still recommend using dark frame subtraction but you won't have to worry about temperature variability.)

Brian

RICH.TEICH
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Re: Stacked images with strange repeating patterns

Post by RICH.TEICH » Mon Aug 27, 2018 12:25 am

Dear Friends,
Thanks for all your help and suggestions.
As a Canon DSLR user for years, I have much to learn about the ZWO world and the issues around Live Stacking.

The ASI385 was an experiment to compare the performance of a "new" chip with an "old" (Canon) one in terms of noise, sensitivity and performance. The software support is always an important component.

Will immediately use dark frames, longer integration times, and working with the gain.
And will have to start budgeting for a new mount with better tracking. :-)

Thanks to Robin and his support network for all their hard work.
Looking forward to 3.2 Pro.

The best from San Francisco where these days we get the ideal mix of smoke and fog.
Sincerely,
Rich

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