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Strange artifacts in subs

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#1 ac4lt

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Posted 18 February 2019 - 07:07 PM

On Saturday night I was trying to fine tune my guiding and was using the Rosette Nebula as my test subject. I wasn't really trying to get anything useful because the moon was so bright but I figured if I could get a night of decent guiding performance I'd be happy.
 
The next morning I processed the subs just to see what I got and it was quite strange. I was getting this very dark triangle in the lower right corner and not much real nebulosity at all. I went back and checked my flats and darks and they looked ok so I went back to the raw subs. I pulled one up and it looked ok but when I debayered it, it had these odd dark marks all over it. When I blinked the debayered subs the stars moved agains the marks. Whatever this was was in the optical path since it was fixed relative to the frame and not moving with the stars when dithering occurred.
 
Here's a screenshot showing the artifact:
40175755633_fe63d41cb6_o.png
 
When you apply an unlinked STF to that file, the dark spots and the very steep gradient on the lower right corner pop out but they weren't at all visible before debayering which is why I didn't notice when they were being acquired.
 
I've no idea what I'm seeing here. Is it condensation on the sensor perhaps?
 
The camera was an ASI294MC Pro being cooled to -15C and the ambient temperature was around -2C when this was shot. The optical path was a Stellarvue SV80-3SV (80mm F/6) with field flattener to the camera. I did some subs on the rosette the week before and they were fine. Whatever this is was new behavior that I've never seen before. 
 
Anyone got any ideas?


Edited by ac4lt, 18 February 2019 - 08:25 PM.


#2 fmeschia

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Posted 18 February 2019 - 07:42 PM

The attached picture doesn’t work, but the PI sub shows moderate light fall-off, about 25%-30% less light in the corner than in the center, depending on the channel. Looks like vignetting to me, not even too severe. But it’s not symmetrical, so that may point to an issue in your optical train.

Francesco



#3 ac4lt

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Posted 18 February 2019 - 08:27 PM

I moved the screenshot from dropbox to Flickr. Hopefully it's displaying ok now. The bottom right corner does look like vignetting but nothing has (so far as I can tell) changed in the imaging train. But the dark spots are really the puzzler for me. I've no idea what's causing them.



#4 fmeschia

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Posted 18 February 2019 - 08:32 PM

I take it that the dark spots don’t go away with flats?


Edited by fmeschia, 18 February 2019 - 08:33 PM.


#5 ac4lt

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Posted 18 February 2019 - 08:35 PM

Correct. I noticed them in the calibrated subs and when I went back to the uncalibrated subs they were ther also after debayering.

#6 fmeschia

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Posted 18 February 2019 - 08:37 PM

Are they present in all uncalibrated subs or just in some?

#7 ac4lt

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Posted 18 February 2019 - 08:38 PM

All of them

#8 fmeschia

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Posted 18 February 2019 - 08:39 PM

And not in the flats? Did you take your flats just after finishing the subs?



#9 ac4lt

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Posted 18 February 2019 - 08:42 PM

Yes, I took the flats right after. I don’t see them in the flats. But, while writing this it occurs to me that the flats don’t get debayered. Going to try that with one and see if I see it there.

#10 ac4lt

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Posted 18 February 2019 - 10:06 PM

I debayered a flat and it looked fine so far as I can tell. There are a few dust mote circles but otherwise looks like what I'd expect. No funny dark shapes.

 

The subs were pretty much horrible anyway. High clouds passed through and trashed most of them, but those that weren't clouded out all have the dark blobs in them. I'll see what happens the next clear night which looks like it's at least six days and several inches of snow away. Hopefully this is won't repeat itself but I'll check by debayering some of the subs as they come in to see what's happening.



#11 fmeschia

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Posted 18 February 2019 - 10:19 PM

Were you dithering, by any chance? If so, can you see if the dark shapes are fixed relative to the pixel matrix, or to the stars?



#12 Rudix

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Posted 19 February 2019 - 04:36 AM

My first though was that it must be something on the sensor but then you would have seen it on the flats as well. Seems like the same/similar shape repeated. If it is not something in the optical path you could maybe use different software to look at the subs in case it is software related.

 

73, Rudi de ZS6DX



#13 elmiko

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Posted 19 February 2019 - 07:17 AM

Linda , a couple of weeks ago I was getting a similar artifacts on my subs also . It turned out it was moisture on the sensor? Atleast that is what I have determined. I recharged the desiccant tablet inside my camera and it went away.

My camera is an Atik CCD the tutorial on there website says that you have to do that every couple of years or so. My camera is just over a year old. I think that debayer subs show the artifacts more than the undebayer subs. You may want to try that . The Atik site said to put the desiccant tablet on a pan place in oven 200 f for 1 hour. Then let the tablet cool down for 24 hours before you put it back in the camera. Also, leave the cap for the desiccant tablet off during that 24 hour time to let any moisture evaporate out of the camera.

Hope this helps Mike


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#14 ac4lt

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Posted 19 February 2019 - 07:19 AM

Elmiko, thanks for that info! I'll look into how to do that for my camera. It's less than a year old but it is fairly humid here in the mid-Atlantic.



#15 elmiko

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Posted 19 February 2019 - 07:35 AM

You're welcome, check out the Zwo site. There's info on doing what I suggested.

Good luck Mike



#16 elmiko

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Posted 19 February 2019 - 07:49 AM

Step8: redry the desiccant (prevent the camera from dew problem)
Usually, when you open the chamber, the air will come into it and cause dew problem. You need to
redry the desiccant or replace the desiccant to make the chamber keep dry after you seal it again.
1.Take out the desiccant from camera
2.Put them into microwave oven, heat for 2 minutes.
3. Put them back to Chamber when it is still warm.

Linda this is straight from the ZWO site, I couldn't attach the whole thing.


Edited by elmiko, 19 February 2019 - 07:50 AM.


#17 ac4lt

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Posted 19 February 2019 - 09:52 AM

I found the instructions on ZWO’s website and I’ll give that a try tonight. We’re expecting anywhere from 4-12” of snow tomorrow and days of clouds after that but hopefully I can report good news by next week.

Condensation was the only thing I could think of when I saw the images so it’s good to hear confirmation from others! I suspect the reason I don’t see it in the flats was that SGP shut down long before dawn when the target went behind the neighbor’s house. When I recoiled the camera to shoot the flats it was probably less humid or perhaps the cooler hadn’t been running long enough for condensation to form.

Thanks, everyone for your insight!
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#18 elmiko

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Posted 19 February 2019 - 09:54 AM

Good luck



#19 ac4lt

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Posted 27 February 2019 - 07:45 AM

Elmiko's suggestion was spot on! I followed ZWO's procedure to remove the desiccant tablets and microwave them for two minutes and last night was finally able to put it to the test. 

 

No more odd artifacts!


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#20 elmiko

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Posted 27 February 2019 - 02:56 PM

Happy to hear that! Good job Linda!

Clear skies Mike




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