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Newbie with light pollution question  

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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 15, 2020 19:37  

This photo is 56 minutes of 2 minute subs with flats, darks and bias using a QHY168C on a 6" Astrograph. No filters. since ALL my images start out this way I am guessing it is light pollution. Is this really light pollution? I am not sure. I am in Florida and a Bortle 4 area that is kinda dark. I have stacked these in APP using all default setting except for DRIZZLE, I did use drizzle. No other processing.

webquestionsmall

 I am able to remove most all of this milky color in step 9 remove light pollution. However I am not sure if it really is light pollution and if it is, would anyone suggest using a light pollution filter on the scope to eliminate some or most of this to start with or is this normal and have to be removed in processing like I have been doing. The other picture is what I ended with after processing in APP and small amount of PhotoShop Elements. So do you think this is light pollution and would a LP filter fix this or most of it.

Thanks

Joe

new fireworks nocropweb

 


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
July 15, 2020 20:50  

To me it looks like your flats have not had the desired effect since the corners are dark and the center is not which is the typical signature of vignetting. How do you create the flats? What exposure time?


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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 15, 2020 22:26  

Well I'm not sure if i did it properly or not , but i placed a white t shirt over the scope and made sure it was flat and no wrinkles. I shot them with the camera cooled to the same temp that I image with and same camera and focus. i did them a while back so not sure what the exposure time was. Maybe i should try some new ones or    reprocess without flats to see if there is a difference. I also made a master flat with DSS and have been using that and have not created a master flat with APP. I think maybe I should do that and test also. Anyway thanks for the info that it may be a flats issue. I can play around with that for a bit and see what I come up with. Did not think of that being an issue.

 

Thanks

Joe


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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 15, 2020 23:04  

I just tried processing again not using any flats bias or dark and got the same result. Then I tried with just using darks and again the same results. I am lost now but will keep poking around


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
July 16, 2020 10:27  

If you are using a master flat created with a different application than APP then that for sure is the issue. Please try processing with the flats so APP can create the master flat for you.


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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 16, 2020 14:26  

Someone on another forum may have given me the answer. Since creating the calibration frames I have been using, I added a coma corrector. Never even thought of that affecting the calibration frames. Also Wvreeven, thanks for that info. I also had not started using APP until after taking the calibration frames. I will be remaking them all as soon as time allows and try that. Hopefully it will work. Thanks again

 

Joe


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
July 16, 2020 16:59  

Joe,

Yes, adding a coma corrector will definitely modify the vignetting etc. of your optical train so you will need new flats. I hope you'll share the result here once you have reprocessed with the new flats!

 

Clear skies, Wouter


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(@tmyers)
Red Giant Customer
Joined: 2 years ago
Posts: 56
July 16, 2020 22:30  

Are you shooting flats during every imaging session?


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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 17, 2020 01:25  
Posted by: @tmyers

Are you shooting flats during every imaging session?

Still new at this and have already messed up my first set of calibration frames. At this point I need to get a set that works and go from there and then worry about new sets. I have read if nothing in the optical train changes then the same master can be reused. That is my plan for now if I get a good set. I usually image at the same temp on my cooled camera and same gain. Now that I have the spacing on my corrector set after some trial and error, I will not need to mess with the train. However IF I do, I now know to have to redo my calibration sets. I have everything set up and on a scopebuddy so I just unplug power and roll back into the garage for next time. Nothing changes unless I am experimenting with a new device but since I have everything I want I am good for now. Just need the weather to cooperate. When I get this sorted out I will post a new image of something. Thanks

Joe


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1226
July 17, 2020 17:09  
Posted by: @tmyers

Are you shooting flats during every imaging session?

If I don't remove the camera from the telescope then I usually do not and I reuse the flats for several days in a row up to a week or so. But I try to take flats as much as possible.

 

Wouter

 


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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 18, 2020 00:25  
Posted by: @wvreeven
Posted by: @tmyers

Are you shooting flats during every imaging session?

If I don't remove the camera from the telescope then I usually do not and I reuse the flats for several days in a row up to a week or so. But I try to take flats as much as possible.

 

Wouter

 

I will also take more, especially if the light tablet I expect to be delivered next week works well. Hope it does, I don't like the t-shirt method that much. I know there are other ways of making them but have not researched much of it yet.

 

Thanks

Joe


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(@joec)
White Dwarf Customer
Joined: 2 years ago
Posts: 10
July 23, 2020 00:25  

Today I received the what I am calling a light box which is actually a large tracing board that is led lit. I set up APT flat frame tool and put in what I believe to be correct ADU of 15000. Took 25 flat frames and calibrated all my dark and bias together with these flats. The result are promising. The image on the right is before and on the left is after. The before image also was using the flats that I had originally made and was suggested here that they  were not working correctly and it was not a light pollution issue. So this problem appears to be resolved somewhat. Not sure if there is room for improvement but I am excited with the result I did get. Thanks to all for recommending the flats problem. The bottom image is a little tweaking in Photoshop Elements. Thanks again.

good flats St
bad flats St

 

 

finished

 

 

 


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(@astrogee)
Neutron Star Customer
Joined: 3 years ago
Posts: 127
July 23, 2020 12:44  

Looks like you got it sorted out. I'd say though that now you can see the light pollution gradient from left to right on the first image. No problem, you can use the light pollution tool to remove that if you haven't already. The final image looks great. 


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(@tmyers)
Red Giant Customer
Joined: 2 years ago
Posts: 56
July 24, 2020 20:05  

Joseph,

That's looking pretty good.


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