unequal brightness in image

Discussion of using SharpCap for Deep Sky Imaging
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oopfan
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Re: unequal brightness in image

Post by oopfan » Sun Jun 21, 2020 11:34 pm

Joe,

I grew up in Hillsdale, NJ which isn't too far from you. The NYC light dome wasn't too bad in the 1970's but I could definitely see it to the south-east. I have no I idea how bad it is today.

Looking at sky charts I see that the Veil Nebula is directly east and 55 degrees above the horizon at 1am tonight. The NYC light dome should be off to the right. It is a factor but how much I can't say. The other thing you need to worry about is what lies directly across the river: Hastings, Ardsley, Elmsford, Yonkers, White Plains. So looking east is problematic for you, unfortunately. Looking west should be better, correct? Hopefully there are no trees blocking your view.

One thing you can do to minimize light domes in your images is to wait for the target to move higher in the sky. Unfortunately 1am is already late. You could wait until 2am to begin imaging. That would give you a solid 1 hour worth of data before hitting the meridian. Question: with your mount can you image through the meridian? With my scope I have to do what is called a "meridian flip".

There is another possibility, and that is to choose a different target, something like M57, the Ring Nebula. It is small but bright and colorful. It rises two hours before the Veil Nebula so you can retire earlier. If you begin imaging at say 11:45pm then you can get in a solid 1 hour. It will be high in the sky so light domes will be at a minimum.

Joe, I just re-read your earlier post. 30 degrees above the horizon is too low. You definitely need to wait for it to get to a minimum of 45 degrees. That, I think, will be your secret to success. I can't guarantee that it will completely eliminate light domes but they should be significantly less of a problem. Of course you have the option of imaging towards the west. Hopefully no trees there.

BTW, I have Bortle 5 skies. I refuse to go below 30 degrees. The atmosphere is just too thick and full of LP. I prefer to start at 40 degrees. In your case I think you should wait for at least 50 degrees.

Good luck!

Brian

psy1280
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Re: unequal brightness in image

Post by psy1280 » Wed Jun 24, 2020 3:01 pm

Brian,
I"m so grateful for your response. Unfortunately I only have a north-east and east view. Everything else is totally blocked by trees. It's the worst of all worlds, i.e., Bortle 8, east view. (well, I guess I could be in Manhattan). I shall definitely take your advice about waiting until things are closer to 50 degrees. Makes perfect sense. I'm also planning on joining the Rockland club to hopefully do some darker sky imaging.

When you say "light dome," is that just the most light polluted area around NYC?

Thanks again and congratulations for moving to a Bortle 5!

Joe

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Re: unequal brightness in image

Post by oopfan » Wed Jun 24, 2020 3:44 pm

Joe,

Are you sure you are Bortle 8? If you are 8 miles north of the GWB then that puts you in Alpine, NJ. Light pollution maps put you at Bortle 6-7.

Anyhow, light domes are everywhere. You could be a rancher in the middle of Montana and your neighbor who lives 10 miles away turns on his barn's floodlights. For me, there is a shopping mall 5 miles away on the other side of a hill. I can see its light dome when I look southeast.

Brian

psy1280
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Re: unequal brightness in image

Post by psy1280 » Sun Jun 28, 2020 7:02 pm

Brian,
Can you suggest a light pollution map that I can check out?

Thank you,
Joe

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Re: unequal brightness in image

Post by oopfan » Sun Jun 28, 2020 7:16 pm

Joe,

https://www.lightpollutionmap.info/#zoo ... TFFFFFFFFF

Then click these two buttons in the legend in order to turn on the mode you need:
lightpoluution.info.jpg
lightpoluution.info.jpg (51.46 KiB) Viewed 97 times
Left-mouse click anywhere on the screen. A pop-up window appears with information including Bortle class.

Brian

psy1280
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Re: unequal brightness in image

Post by psy1280 » Tue Jun 30, 2020 6:47 pm

Brian,
Much thanks for sending this. I was very happy to see that I'm in Bortle 7. I know I'm limited by my eastern sky and NYC, however just knowing I'm not a Bortle 8 (which I thought was the case), makes me feel less victimized by the light pollution.

Joe

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Re: unequal brightness in image

Post by oopfan » Tue Jun 30, 2020 8:00 pm

That's great news, Joe! Now only if we could get some clear nights.

I think you will be in good shape if you wait for objects to get higher above the horizon. Your Hyperstar is a "light sponge" so you should be able to get decent results in an hour. Try the brighter objects first, like M57 and M27, and then try the fainter ones. You may find that the brighter objects won't require any post-processing tweaking or removal of light pollution gradients. It is only when you start going for the objects that are at the limit of your sky conditions that you will need to learn techniques for removing light pollution.

Brian

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