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Something strange in PI with debayering.

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#1 gatehealing

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Posted 16 October 2021 - 11:07 PM

I have not noticed this before. See screenshots. After Debayering I went to Blink to inspect the images. Used the Intensity Transformation "Screen Transfer Function" to fix the green color. While it did work, notice the stark difference in the background. The first image has a noisier background, which is what I am used to at this point, but the 2-31st images have a very black background. I don't know what happened. I am following the Elf's video again for this. I erased my Calibrated Lights and Cosmetic Corrected Lights and re-did them (neither had the background difference when inspected in Blink). When I finished Debayering the same thing happened.

 

I have no idea what is happening here.

 

Subs were taken at 180sec exposures, gain 120, temp -20c (this is what I already have darks for; I'll be doing new darks for either 0c temp, or -10c temp, and probably some shorter exposures just to experiment with).

 

 

Attached Thumbnails

  • Screen Shot 2021-10-16 at 10.58.18 PM.jpg
  • Screen Shot 2021-10-16 at 10.58.26 PM.jpg


#2 gatehealing

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Posted 16 October 2021 - 11:20 PM

May have just answered my own question . . . I re-watched Elf's video in the Blink and noticed that his background when zoomed in also seems grayer in the first image, and blacker in the subsequent ones. . .I'll keep going and see what happens.  . . could be that I did it more correct this time.

J



#3 Jared

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Posted 16 October 2021 - 11:39 PM

Blink gives you two choices for screen transfer function. It can either apply one stretch to all images, or apply a different stretch for each. The default, since it is quicker, is to calculate a stretch on the 1st image, then apply that stretch to all images.

It is not unusual for a first image—one taken just as the sky is becoming fully dark—to have a slightly brighter background than later images. If the stretch is optimized for that slightly lighter image, it will leave the background nearly black for darker images. That’s what you are seeing. Nothing is wrong. If the difference is too dramatic for you to judge quality of later frames, press the button that applies individual stretches to each image.
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#4 gatehealing

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Posted 16 October 2021 - 11:45 PM

Blink gives you two choices for screen transfer function. It can either apply one stretch to all images, or apply a different stretch for each. The default, since it is quicker, is to calculate a stretch on the 1st image, then apply that stretch to all images.

It is not unusual for a first image—one taken just as the sky is becoming fully dark—to have a slightly brighter background than later images. If the stretch is optimized for that slightly lighter image, it will leave the background nearly black for darker images. That’s what you are seeing. Nothing is wrong. If the difference is too dramatic for you to judge quality of later frames, press the button that applies individual stretches to each image.

Thanks! That actually made sense.

 

Here's my Master Light before moving to post-processing (Stars fuzzier in this . . . I think something is up with my dew strips . . . or the Pegasus Powerbox advanced as it uses a temp/humidity thing to control them. Guide scope strip never came on at all):

 

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  • Screen Shot 2021-10-16 at 11.40.55 PM.jpg


#5 the Elf

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Posted 17 October 2021 - 03:54 AM

My current way of using blink is to select an image from the middle of the list, hit Ctrl-A for the stretch and then go back to the start. This only came to my mind some time after I shot the video, sorry! Anyway, doing it this way shows how images get darker after sunset and brighter again in the morning. Often I sort out the first and last images. There is no point adding images with a bright background to a stack of dark images. There was a time when the city turned off the street lights at midnight. That made a big change to the background. Now they have an improved security concept as they call the light pollution and light are on all night.



#6 WadeH237

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Posted 17 October 2021 - 07:39 AM

The Blink process can apply an auto-stretch to each image independently.  This will make them all appear with similar brightness.  Just click the button that I have circled below.

 

blink-auto-stretch.jpg



#7 gatehealing

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Posted 17 October 2021 - 12:00 PM

Thanks y'all!

J




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