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H-alpha NB filter and plate-solving (and probably focus) don't like each other


BrendanC
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I just tried using my SVBONY h-alpha filter for the first time. It's attached to my coma corrector, on my EOS1000D astro-modded DSLR.

I'm trying to plate solve in APT and it simply will not have it. Of course, this is because I now have a filter that is killing the stars. I expect I'll have the same problem when I try to focus.

I thought this might be a problem but I'm stumped now because it really is a problem. The only way around it that I can see is to get the camera in focus, and the scope pointing in the right direction... and then take the camera out, put the filter in, start shooting, and pray that it's still in focus. I don't like this approach, and it really won't work at all when accurate rotation is important.

Any ideas? Have I done something silly by getting this filter? I did it after seeing Trevor Jones's blog and video on the subject - using H-alpha in the red channel or as a luminance layer.

Thanks, Brendan

Edited by BrendanC
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One problem.. the ha won't be in focus..

I'd plate solve without the ha filter in or it's worth upping the iso very high and have a exposure length much longer.. with focus do the same but ha deffo won't be focused at the same point as broadband 

Much that I'm not a fan of Trevor Jones,  the ha data red channel can be used as a hargb.. 

Edited by newbie alert
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Thanks - I have noticed that the Ha is out of focus. I have managed to get it to plate solve once, but I'm up to 4 minutes exposures now and it's not working again.

I was thinking I could plate solve without filter, pop the filter in, focus, and off I go. But no - I focus using a brighter star such as Vega or Altair, so I'd still then need to be able to plate solve to the target. Given that my plan for tonight was to add Ha to an existing object which was at precisely 75 degrees rotation, that's something I would sort out at focus, and only then. I simply cannot take the camera out after that. So, it's plate solve with Ha filter in, or nothing.

Edited by BrendanC
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I can imagine how annoying this is. Is there an option in the platesolving software to alter the minimum number of stars extracted? Or a log option that indicates what part of the process platesolving has failed on? It might not even get to the matching stage if it is trying to find say 30 stars.

Martin

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as mentioned above , with my D600  and Ha narrowband filter , I just knock the ISO all the way up to platesolve my target

sometimes takes a fairly long exposure as well to get enough good stars for it, especially with my longer focus scopes.

Once on target then run at the more usual ISO

Guiding should keep you spot on target.

 

 

Edited by fifeskies
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OK, progress has been made.

I plate solved without the filter to Vega. Then put the filter in, popped the Bahtinov mask on and amazingly, the Bahtinov grabber tool worked even with the poor image from through the filter. So, I was able to focus with the filter in.

Then, I changed to Platesolver2 instead of ASTAP because I know it's more tolerant of bad stars. After a few goes, bumping the plate solve exposure time from 10s to 90s, it's working.

So, I think this is the way forward.

Thanks for all the help. :)

Edited by BrendanC
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1 hour ago, BrendanC said:

Thanks - I have noticed that the Ha is out of focus. I have managed to get it to plate solve once, but I'm up to 4 minutes exposures now and it's not working again.

I was thinking I could plate solve without filter, pop the filter in, focus, and off I go. But no - I focus using a brighter star such as Vega or Altair, so I'd still then need to be able to plate solve to the target. Given that my plan for tonight was to add Ha to an existing object which was at precisely 75 degrees rotation, that's something I would sort out at focus, and only then. I simply cannot take the camera out after that. So, it's plate solve with Ha filter in, or nothing.

What software are you using for plate solving? Presume you can up the iso within the software just to plate solve and focus 

As for camera rotation APT will tell you the camera angle when plate solving..  not too sure on the others 

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I only use plate solving on one setup, the other two having non-ASCOM compliant mounts, so it really isn't a necessity. GoTo, look at a test image, frame up. It's easy.

I sometimes can't focus my Astrodon 3nm Ha filter in my target either so, as you did, I just go to a bright star to focus.

Once you have your Ha, I'd suggest being careful over what you do with it. Using it as luminance is a bad idea in principle. It means you light your whole image in the light of deep red. Why would you do that? It will subdue any blue and green signal for one thing and it's entirely false for another. OK, on purely emission targets I'll use up to 15% Ha as luminance just to tighten up the structures in the gas but, if you use more than that, you'll turn the image pink and create blue stellar halos.

A nice way to use it is to add it to red in Photoshop's blend mode lighten. That way it will brighten the red channel only where the Ha is brighter than the red. It will leave the stars and the background alone.

Olly

Edit: I had a look at the Trevor Jones HaRGB tutorial and thought it was floundering. He adds the Ha to the red in blend mode normal at a random percentage like 35% or whatever appeals. He then points out that this reduces the size of the stars in red, which is why you shouldn't do it! For the same reason, you cannot just replace the red with the Ha. It also means that the contribution of your best emission detail is applied at 35% of red. What happens to the other 65%? It's lost. I stopped there but, judging by his final image, he also used Ha as luminance with the consequences I mentioned above.  For better tutorials look at for Adam Block, Warren Keller, Rob Gendler, R Jay GaBany and others.

Edited by ollypenrice
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Thanks Olly, great advice. The plate solve and focus issue is now solved.

I think the issue with Trevor Jones's stuff is that he documents as he learns, so I guess he would also do things differently now. I wasn't planning on using Ha for luminance for the exact reason you give, but that's great advice about combining it with Photoshop's blend lighten. Exactly what I'll need to know - when I finally learn how to align the Ha frames! Which is the subject of the question I'm about to post... see 

 

Edited by BrendanC
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1 hour ago, BrendanC said:

Thanks Olly, great advice. The plate solve and focus issue is now solved.

I think the issue with Trevor Jones's stuff is that he documents as he learns, so I guess he would also do things differently now. I wasn't planning on using Ha for luminance for the exact reason you give, but that's great advice about combining it with Photoshop's blend lighten. Exactly what I'll need to know - when I finally learn how to align the Ha frames! Which is the subject of the question I'm about to post... see 

 

Yes, this idea of 'Documenting as you learn' is very prevalent at the moment. It baffles me entirely but, hey-ho.

Olly

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