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Troubleshooting - what is going wrong with my flats?

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#1 AussieGazer

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Posted 23 February 2020 - 03:20 AM

Hi all

 

As some of you will be aware, I've been working on Tarantula for a couple of weeks now with many challenges along the way.

 

I've got around 3.5 hours of useable lights now, which is by a long shot the most lights I have ever accumulated. It's not going brilliantly, and I might give it up, as I'm not seeing significant improvement between 1 hour and 3.5 hours, and doubt another 3.5 hours or more would improve that.

 

In any event, I've got a problem with my flats. I've done it twice now (re-took flats after I saw it the first time).

 

Apparently I double deleted my Tarantula Nebula, so I can't restore it and I don't really want to stack it again as I know it's faulty. Regardless, I had the exact same issue with my Centaurus A Galaxy, which I didn't delete. I have linked below, and have include a shot of the issue.

 

What am I doing wrong? My process is to get a white shirt nice and tight over the dew shield, put an iPad with a white screen against it, and take the flats. But I have this very odd 'banding' issue.

 

Can someone enlighten me?

 

Full file here for those who want to look at all the data:

https://www.dropbox.... issue.tif?dl=0

 

Photos of how it looks in my post below.

 

Cheers

 

AG


Edited by AussieGazer, 23 February 2020 - 04:17 AM.


#2 sharkmelley

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Posted 23 February 2020 - 03:31 AM

You won't get many answers to your question when it involves downloading a 225Mbyte file to see the problem.

 

I downloaded it and unfortunately I can't see any banding.  Is it horizontal? Is it vertical? Is it concentric rings?  It's always best to post an image or image crop that shows the issue.

 

It's surprising that you can't see significant improvement between 1 hour and 3.5 hours.  If all exposures have similar quality then the 3.5 hour image should be visibly less noisy.

 

Mark


Edited by sharkmelley, 23 February 2020 - 03:38 AM.


#3 AussieGazer

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Posted 23 February 2020 - 04:03 AM

You won't get many answers to your question when it involves downloading a 225Mbyte file to see the problem.

 

I downloaded it and unfortunately I can't see any banding.  Is it horizontal? Is it vertical? Is it concentric rings?  It's always best to post an image or image crop that shows the issue.

 

It's surprising that you can't see significant improvement between 1 hour and 3.5 hours.  If all exposures have similar quality then the 3.5 hour image should be visibly less noisy.

 

Mark

My sincere apologies, I uploaded the wrong version (with the flats from a previous night). I'll upload and link the right version with screen shots to highlight what I mean.

 


Edited by AussieGazer, 23 February 2020 - 04:24 AM.


#4 Dunc

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Posted 23 February 2020 - 04:09 AM

Hi AG

 

IMHO the banding is probably related to shutter speed and Ipad screen refresh time. I've had this sort of problem  with some light sources and my Canon 1100D and found longer exposures fixed the flats. Have you tried just the t-shirt and some sky? Or a different light source?

 

HTH 

 

Dunc



#5 AussieGazer

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Posted 23 February 2020 - 04:22 AM

I have attached a shot of the issue 'highlighted' (the first photo), and another shot of how it looks when you open it up with photoshop (the second photo).

Attached Thumbnails

  • Highlight of issues.jpg


#6 AussieGazer

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Posted 23 February 2020 - 04:23 AM

Second photo

Attached Thumbnails

  • Normal highlight of issues.jpg


#7 AussieGazer

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Posted 23 February 2020 - 04:25 AM

Hi AG

 

IMHO the banding is probably related to shutter speed and Ipad screen refresh time. I've had this sort of problem  with some light sources and my Canon 1100D and found longer exposures fixed the flats. Have you tried just the t-shirt and some sky? Or a different light source?

 

HTH 

 

Dunc

Thanks Dunc, see image below your reply. I don't think it's the same issue.

 

P.S. - I'm happy to dry dawn/dusk during the next session. And could go for a longer exposure on the iPad, will try to troubleshoot.


Edited by AussieGazer, 23 February 2020 - 05:09 AM.


#8 sharkmelley

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Posted 23 February 2020 - 04:31 AM

Thanks Dunc, see image below your reply. I don't think it's the same issue.

What length exposure did you use for the iPad flats that are causing the problem?

 

Mark



#9 AussieGazer

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Posted 23 February 2020 - 05:00 AM

1 second

#10 klaussius

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Posted 23 February 2020 - 09:01 AM

1 second with a white screen would have saturated the sensor. It would be overexposed for sure. When I do long flats, I use a gray dimmed screen, otherwise the flats are overexposed.

 

Can you post your master flats?



#11 nimitz69

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Posted 23 February 2020 - 11:59 AM

Unfortunately many so called white panels do not produce even light across r,g and b which can cause all sorts of flats issues.  Look at the histogram for one flat and wee what each individual channel looks like.  I had this issue with one of those white tracking no panels and finally just ended up buying a spike-a-flat ... problem solved as this panel was specifically designed to take AP flats ....


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#12 richorn

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Posted 23 February 2020 - 03:19 PM

Unfortunately many so called white panels do not produce even light across r,g and b which can cause all sorts of flats issues.  Look at the histogram for one flat and wee what each individual channel looks like.  I had this issue with one of those white tracking no panels and finally just ended up buying a spike-a-flat ... problem solved as this panel was specifically designed to take AP flats ....

I had similar issues, and the Alnitak Flip-Flat was my "throw money at it" solution!  ;-)



#13 klaussius

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Posted 23 February 2020 - 05:43 PM

I had similar issues, and the Alnitak Flip-Flat was my "throw money at it" solution!  ;-)

 

My solution was a bit more low-cost. I switched to a colored screen to compensate the differing sensitivity of the system.

 

Ie: if my red ADUs are low, I add more red to the screen.



#14 nimitz69

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Posted 23 February 2020 - 05:44 PM

Yep, that works too,!  Since my setup is in an observatory and doesn’t move I use one set of flats for months at a time and decided to ‘just’ get the spike-a-flat  dalek12.gif



#15 AussieGazer

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Posted 23 February 2020 - 08:28 PM

Thanks all.

I'll try again with 1/4 second, like I did the first time I 'successfully' took flats. I recall reading however that flats are supposed to be between 0.5 and 2 seconds? I had thought therefore that 1 second was a good/safe compromise.

I'll post the original master flat when I get home tonight.

#16 AussieGazer

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Posted 24 February 2020 - 05:20 AM

Hi all, thanks again for the advice.

I re-shot my flats in 1/4 second, and 1/2 second. Same issue.

 

Then I started looking at everything, hoping it wasn't my lights. To my relief, it was not. Then I looked at my bias frames - fine.

 

Then looked at my darks... I've attached an overexposed (using photoshop) example.

 

Did light manage to get in? It sure looks like it. I had the lens cap on, and covered the image train with a heavy and dark top. Maybe I should do it in a dark room next time? Maybe some light got through from underneath?

 

Does camera temperature matter?

Attached Thumbnails

  • Cloudynights dark.jpg

Edited by AussieGazer, 24 February 2020 - 05:21 AM.


#17 NightBear

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Posted 24 February 2020 - 08:47 AM

Is your viewfinder covered? Light can get in through there as well.



#18 AussieGazer

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Posted 24 February 2020 - 04:25 PM

Is your viewfinder covered? Light can get in through there as well.

No I did not.

I also realised I didn't cover where the adapter connects to the telescope. It's a tight fit.... but we're talking multi minute exposures.

I'll give it another go!

#19 AussieGazer

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Posted 26 February 2020 - 07:31 AM

Thanks for your help all.

Managed to get my darks right, by taking them with the whole image train covered.
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#20 sharkmelley

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Posted 26 February 2020 - 08:14 AM

Thanks for your help all.

Managed to get my darks right, by taking them with the whole image train covered.

I'm glad you solved the problem.



#21 Phil Sherman

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Posted 26 February 2020 - 07:24 PM

Canon supplies a cover for the viewfinder to prevent light leaks caused from light entering the camera there. You might want to try covering yours and see if that makes a difference in your darks.




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