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Old 31-03-2016, 09:00 PM
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lazjen (Chris)
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Help with Image Problem

The attached image is an auto stretched L image, 120 sec taken tonight at 20:05. Note the strange dark areas around the edges of the image.

What is it and how can I fix/get rid of it?

Click image for larger version

Name:	L_L_2016-03-31_20-05-10_2016-03-31_20-05-10_Bin1x1_120s__-10C.jpg
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ID:	197075

More info:
* It happens for RGB as well.
* Going to 300 sec makes no real difference.
* Does NOT happen for Ha (300 sec).
* I think the images improve the further into the night and I'll be testing that again soon. I've switched to getting a few Ha images as I write this up.
* From memory, I think this affected SII and OIII as well - don't have those filters in right now to test though.

I know I've had this sort of problem for a while but haven't worked it out. My only real guess is that it's something to do with Light Pollution, but I would have thought it would be washed out over the whole image, just like the centre bits.

Camera is the Sbig ST-8XE. (Btw, it's not vignetting.)

Ideas?
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  #2  
Old 31-03-2016, 09:03 PM
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rmuhlack (Richard)
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Looks like frosting of the CCD to me. I also have a ST8XE and have seen the same thing. I'd say its time to bake your dessicant, or replace the dessicant pouch as in the case of my camera (which fixed the problem btw).
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Old 31-03-2016, 09:04 PM
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Here's another 120 sec L auto stretched:

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Name:	L_L_2016-03-31_21-00-57_2016-03-31_21-00-57_Bin1x1_120s__-11C.jpg
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Note the significant difference.
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Old 31-03-2016, 09:05 PM
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lazjen (Chris)
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Quote:
Originally Posted by rmuhlack View Post
Looks like frosting of the CCD to me. I also have a ST8XE and have seen the same thing. I'd say its time to bake your dessicant
Ok, that's interesting and I will do it. Why does Ha not get affected?
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Old 31-03-2016, 09:08 PM
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as I said, in think it is frost/ice on the sensor. what temp is the camera set to, and have you recently (or ever) recharged the dessicant? Does the camera also do the same thing when the sensor is at ambient temps (ie cooling off with time to re-equilibrate to local air temp)? this will help to isolate the problem
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Old 31-03-2016, 09:09 PM
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Quote:
Originally Posted by lazjen View Post
Ok, that's interesting and I will do it. Why does Ha not get affected?
perhaps your Ha frame was taken before ice had a chance to form? if you take another you may see it, especially if you heavily stretch the image
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Old 31-03-2016, 09:15 PM
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lazjen (Chris)
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Quote:
Originally Posted by rmuhlack View Post
as I said, in think it is frost/ice on the sensor. what temp is the camera set to, and have you recently (or ever) recharged the dessicant? Does the camera also do the same thing when the sensor is at ambient temps (ie cooling off with time to re-equilibrate to local air temp)? this will help to isolate the problem
It's -10 deg atm.
Yes, I've baked the dessicant, but I realise now that it's been a while and the humidity here has been beyond high.
I'll "warm it up" and do another image.

Also, I actually think you've got the right answer, but it's the Ha that confuses me.

To answer your other post:

The images were taken L, then Ha, L, then Ha, that is the Ha was in the middle of the session. I'll try to really stretch the image to see, but the images from the other night (all Ha) seemed fine when I was processing them.
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Old 31-03-2016, 09:28 PM
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Just warmed up the camera, did another 120 sec L - no bad edges. Time to bake, I think.

The Ha working still puzzles me, but all the evidence is pointing to frosting, so I'll try that fix first.
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Old 01-04-2016, 07:44 AM
ericwbenson (Eric)
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I have seen the same thing with my old ST8 many years ago, so it almost certainly is frosting as suggested. The background in Ha is often so dark you can't see LP gradients or vignetting structures, so it is possible that is why the frosting seemingly has no effect here.
Regards,
EB
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Old 01-04-2016, 09:51 AM
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The dessicant plug is getting baked today. Skippysky seems to suggest clear skies again tonight, so I hope to try again and see what happens. If it works, it looks like I may need to bake more frequently.
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Old 01-04-2016, 08:30 PM
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Well, it seems baking it didn't help - exactly the same problem is appearing tonight.

I presume there's a lifetime to these dessicant plugs and I need a replacement?
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Old 01-04-2016, 09:04 PM
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what temperature did you bake the dessicant at, and how long did you run the camera for before concluding it didn't work?

the manual suggests baking at 175C for 4 hours. it then says that after replacement into the camera it will take 2 hours for the camera to reach a frost free state. see here (page 71): https://www.sbig.com/site/assets/fil...sbmanrev14.pdf

i found with my ST8 that i had to refresh the dessicant twice. the first time i guess adsorbed the bulk of the moisture that was in the camera, but it wasn't fully dried out until after I had refreshed the dessicant a second time and then after waiting for a while.
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Old 01-04-2016, 09:13 PM
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OK, I only did 4 hrs at about 150-175. And I must have misread the 2 hrs - thought it was 1 hr. When I posted, it had been cooled for 1.5 hrs.

It's improving (as it always did in the past), and it seems to be getting better quicker. I'm doing an image run now, so I'll see how the images progress.

And I'll do another bake tomorrow, plus allow 2 hours cooling next time. Might need a couple or more rounds to deal with all the moisture around.
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Old 02-04-2016, 04:31 PM
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Last night things went mostly ok until unrelated issues stopped my progress for the night.

However, luminance filter was getting some strange black areas on it that moved from shot to shot (I was rotating between LRGB filters).

Today while I was baking the dessicant again, I pulled apart the filters/camera - and on the luminance filter was part of a leaf/bark floating around. I suspect removing that obstruction will help make the L data a bit better.
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