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Getting Decent Looking Stars in the Background

Adobe Photoshop Processing Beginner Deep Sky
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Evan Boyle avatar
Evan Boyle avatar
DSO First Attempt
Hey guys, first post here in the forums
I recently took my first image of a deep sky object (the Lagoon Nebula) and I'm happy with how it turned out for a first try, but the thing that annoys me a lot is that while the nebula itself looks decent, the stars in the background of the image look like a mess. I was wondering if anyone has any tips on how to fix this issue? I think one of the main problems is that perfect focus wasn't quite achieved? But also in post-processing in photoshop the stars look really gross and ugly, any tips would be greatly appreciated.
Thanks!
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dkamen avatar
dkamen avatar
Hi Evan,

You are right, the problem is the focus. This will not show so so much on the DSO since it is a diffuse object, although it is still quite evident when comparing with sharper images. Do a search for Lagoon here and you'll see.  However, bad focusing WILL be much more prominent on the stars which are point sources, especially the smaller ones. Also, the stars are a little elongated which could point to a tracking error  (usually because of PA) but this is secondary compared to the focus. You'd still get slightly elongated stars if focus had been perfect but they would look sharp, and the tiny ones wouldn't even be elongated.

You can probably improve the situation somewhat with a sharpening algorithm such as regularized Richardson-Lucy used in PixInsight's deconvolution tool. But in the end there is no substitute for sharp focusing. A Bahtinov mask would help a lot and is easy to DIY. Alternatively you can use the camera's live view, try to get the tiniest stars visible and as tiny as possible . If you cannot focus properly no matter what you do, maybe you have a spacing issue with the flattener in which case you need an extension or a tilting issue with your whole optical train in which case something needs to be tightened.

PS I forgot to mention that sometimes the image will be fuzzy like that because of bad atmospheric conditions or dew on the front glass element. In those cases there really is nothing you can do. Just try another night.

Cheers,
Dimitris
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Darknytsky avatar
Darknytsky avatar
Given I too easily have the same problem, even with a bahtinov mask and 10x live view, I’m curious what your focusing method is?
Evan Boyle avatar
Evan Boyle avatar
Hi Evan,You are right, the problem is the focus. This will not show so so much on the DSO since it is a diffuse object, although it is still quite evident when comparing with sharper images. Do a search for Lagoon here and you'll see.  However, bad focusing WILL be much more prominent on the stars which are point sources, especially the smaller ones. Also, the stars are a little elongated which could point to a tracking error  (usually because of PA) but this is secondary compared to the focus. You'd still get slightly elongated stars if focus had been perfect but they would look sharp, and the tiny ones wouldn't even be elongated.

You can probably improve the situation somewhat with a sharpening algorithm such as regularized Richardson-Lucy used in PixInsight's deconvolution tool. But in the end there is no substitute for sharp focusing. A Bahtinov mask would help a lot and is easy to DIY. Alternatively you can use the camera's live view, try to get the tiniest stars visible and as tiny as possible . If you cannot focus properly no matter what you do, maybe you have a spacing issue with the flattener in which case you need an extension or a tilting issue with your whole optical train in which case something needs to be tightened.

PS I forgot to mention that sometimes the image will be fuzzy like that because of bad atmospheric conditions or dew on the front glass element. In those cases there really is nothing you can do. Just try another night.

Cheers,
Dimitris


Thanks so much for your insight! I knew the focussing would be an issue since, being the novice that I am, I spent ages on setting up and whatnot, and then didn't spend nearly as much time as I should've to focus. I certainly need to learn some of the astro-specific software like PI, and I've heard of StarTools too? At this stage, I've pretty much just been using PS. But I know no matter how good anyone is at post-processing there is no substitute for good data.
Cheers
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Evan Boyle avatar
Evan Boyle avatar
Given I too easily have the same problem, even with a bahtinov mask and 10x live view, I’m curious what your focusing method is?


I used this method on the night, though the live view was giving me some issues, eventually, I managed to use the 10x view with the Bahtinov mask and focussed as best I could, but clearly it wasn't quite good enough.
Hopefully next time I can improve a bit!
dkamen avatar
dkamen avatar
Given I too easily have the same problem, even with a bahtinov mask and 10x live view, I’m curious what your focusing method is?


Hi,

I use Ekos' focus module. You tell it to take e.g. 2 second pictures and center around a specific star. Then it starts looping pictures and every time  it measures displays the HFR. The smaller the better. I will not always reach the same value because first of all it is an approximation and second it is relative and depends on a lot of things (size of star chosen, sky clarity, ISO and exposure). I just go for minimizing. This can be quite a bummer because you don't really know when you are at the minimum until you go past it at least once, but it generally takes no more than 5 minutes to achieve focus.

It can also cooperate with an auto-focuser but I don't have one. Well actually I do but it can't be made to work with the Vixen (much to my disappointment) and I don't really need it for the small apo. I keep it in case I buy a RC in the future.

In the past, before I added the raspberry pi to the mix, I would just use tiny stars that I know are there. For example, there is one tiny star real close to Vega, practically within Vega's disk if you are not focused. So when you are able to resolve that, you know things are good.
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