Creating dark frames

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descott12
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Creating dark frames

Post by descott12 » Tue Jan 22, 2019 2:28 am

I just started experimenting with creating some dark frames to see if they would help during EAA.
I am using an ASI 294 MC Pro with the cooler on.
I created one master dark (average of 10 frames, exp 10.3 secs, gain 216, full resolution, 1x1 binning, as this was a simple test, I did not record the temp).
The image is completely black. No noise, not a single hot pixel. That doesn't really make sense....but I suppose that is good thing. Or did I create the dark incorrectly?

However, when I zoomed the dark frame to 700% in an image viewer, I got the repeated pattern displayed in the attachment. What is that? Is that a real thing in the image data or is it an artifact of extreme zoom. (Note it may not display properly in the forum. You may need to download and view on your desktop with some other app).

In addition, I have noticed a similar repeated, grid like pattern in all of my png captures when zoomed. Typically images just pixelate when zoomed but my SC images all display this grid in one form or another.

Thanks in advance for any guidance.
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admin
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Re: Creating dark frames

Post by admin » Tue Jan 22, 2019 7:09 pm

Hi,

Hot pixels tend to show up or at longer exposures, and a 10.3 second exposure isn't particularly long in the grand scheme of things for Astro imaging. If you tried making the darks with 180 second exposure I'm sure you spot some hot pixels then.

The grid that you notice when you zoom in on your images is due to the Bayer pattern of the colour sensor – remember that the different coloured pixels are arranged in a pattern on the sensor like

Code: Select all

RGRGRGRG
GBGBGBGB
RGRGRGRG
GBGBGBGB
It's normal to see this in light frames as the brightness is usually different between the different colour channels so you see a grid – like a tartan pattern – when you zoom in. You will need to instruct your processing software to debayer the image to get the full colour representation. There are some good articles on the forums here that give you guidance on how to do this.

It's unusual to see this Bayer pattern appear in a dark frame and it may hint that you had some light leakage while taking your darks perhaps.

Cheers, Robin

descott12
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Re: Creating dark frames

Post by descott12 » Tue Jan 22, 2019 7:27 pm

Hi Robin,
Thanks for the very informative reply. Makes alot of sense.

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turfpit
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Re: Creating dark frames

Post by turfpit » Tue Jan 22, 2019 7:29 pm

Dave

Some background info here viewtopic.php?f=35&t=254 which you may find useful.

Dave

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Re: Creating dark frames

Post by turfpit » Tue Jan 22, 2019 9:07 pm

Dave

A demo of hot pixels on a dark frame. ZWO ASI120MC frame is 60s with gain at 75%.

Dave
dark-frame-zoom600.JPG
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dark-frame-hot-pixels.JPG
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Re: Creating dark frames

Post by turfpit » Tue Jan 22, 2019 9:11 pm

Dave

BTW using your Hyperstar at f/2, a 15s exposure on your scope is equivalent to my f/6 scope with a 135s exposure. One to keep in mind when you are looking at other people's settings to use for yourself.

Dave

descott12
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Re: Creating dark frames

Post by descott12 » Wed Jan 23, 2019 2:25 am

Hey Dave,
Thanks for the responses. Those screenshots were sort of what I was expecting to see.
Regarding the Hyperstar - should that even matter since theoretically, there should be no source of light at all? In fact, I was thinking I could just take the darks with camera not even hooked up to the scope and with the cap still on. Is that not correct?

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Re: Creating dark frames

Post by turfpit » Wed Jan 23, 2019 8:20 am

Dave
I could just take the darks with camera not even hooked up to the scope and with the cap still on. Is that not correct?
Yes. As long as you use the same exposure/gain/offset as your lights. Temperature match between dark and lights is important. A good test of the temperature match would be to take some lights and obtain a set of extremely temperature mismatched darks. Upon processing you end up with either 'raining' or an image that looks like it is melting and running down the canvas. I have achieved both (not intentionally) in my early days with imaging.

The ASI120 settings were extreme but serve to demonstrate the hot pixels - some useful material for the book ;). In reality, I would run that camera at between 5% and 15% gain. https://www.astrobin.com/332681/?nc=user

If I recall, you have a cooled camera (maybe). If that is the case then you can get about building a library of darks. What I would be doing is get on Astrobin, search for the camera in question (& Hyperstar). Look at some objects and note settings that give good images. I would then build a darks library. This needs some planning as it needs to be decided what gain/exposure combinations are going to be used (which also means offset/brightness needs to be established for the given gains). With the darks library, no need to waste valuable sky time collecting darks. Very quickly you should be able to settle on a combination of exposures/gains that work well for various types of objects.

After some reading of the Starizona website I had been meaning to mention the Hyperstar effect to you. For example, with my f/6 scope I might shoot 60s exposures. For the Hyperstar that would equate to around 5s to gather the same amount of light. That translates into being able to operate with broken cloud.

Have a look at https://www.astrobin.com/users/robinandcurtis/. Robin (@saguaro) on this forum has a C14, Hyperstar, cooled camera and uses Live Stack. Have a look at the images and capture settings. There is a wealth of info in there which would save you a lot of time.

If I had come across different websites & information earlier in my imaging days, my kit would look a lot different than it does now plus I would be able to pick it up walk with it (think get to a dark site). Instead, I have wasted a year or so with a couple of false starts. Sometimes you just have to do things to find out and take the hit on the wallet. :(

Dave

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