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Achromatic filter question

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#1 JimB1

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Posted 28 November 2021 - 07:40 AM

So I picked up an older Astrotech AT80 in the classifieds here a few months back as a grab and go portable that I can just use with a camera tripod. It’s an 80mm f/6 doublet achro so a decent amount of purple fringing in the views that I don’t really get in my 90mm f/11.
 

I have also been taking baby steps into astrophotography for a while so I stepped back even further and am going to also use this as a really basic training setup for that as well. I got the AT80 field flattener and have that connected to an older Nikon D70S mainly because it’s lighter than my D700 but may use the D700 because it’s got a better image and can work with control software better. I am using Digital SLR Astrophotography (Practical Amateur Astronomy) 2nd Edition by Michael Covington as a sourcebook and I’m taking it slow.

 

Like I said, baby steps smile.gif

 

F9B045ED-4379-4AFD-BA71-28FF13772603.jpeg

 

Anyway, I am trying to decide how to handle the CA on this scope. I am thinking this scope type is the reason for the Baader Semi-apo’s existence but anyone have any better options they can think of? 

Any help would be appreciated.

Thanks

-Jim
 

 



#2 Scott Beith

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Posted 28 November 2021 - 07:50 AM

A long time ago CN member Pollux did an excellent writeup on minus violet filters.  You might have to google it since it may be archived in CN.  

 

I found the WO VR-1 to give a neutral color view with my Stellarvue AT1010N Nighthawk I used to own.  

 

That or the Baader Semi-Apo would be where I would look.



#3 drd715

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Posted 28 November 2021 - 11:29 AM

Better to get an ED OR Better scope for AP.

But you use what you have so consider your limitations. The reason you get purple blooms with a short achro is that the focal points of different colors are not at the same physical length from the objective. Depending upon your scope design some of the colors will be at the same focal point. If you can find the data (crossings curve diagram) for your scope that might be of interest to you to determine if a Duo band filter (Ha/Olll) might be advantageous on nebulas (if Ha and Olll focus at near the same point). This may be somewhat less efficient for the Ha signal on an unmodified dslr, but some Ha will register. Mono narrow band imaging works on achros because you focus on a single wavelength, everything else is blocked (if the optical lens is figured well).

Do some research on your two cameras. The sensor used in some cameras have less noise and more sensitivity to low light. The newer generation of sensors use technology that greatly lowers electronic noise (read noise plus amp glow) and have much higher quantum efficiency. Look for "back illuminated" sensors.

Go ahead and expose some images its the only way to improve your technique. Work on your polar alignment and tracking or add on autoguiding to your astrophotography challenge.

Some image processing software can help clean up your bright stars. You can separate the stars and work on them for color correction and sharpening the edges then merge the nebulae and stares stars together. But nothing is better than good clean data to start with.

Use what you have to find out what can do with future improvements in equipment and techniques.

Sent from my SM-N950U using Tapatalk

#4 JimB1

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Posted 29 November 2021 - 06:18 AM

A long time ago CN member Pollux did an excellent writeup on minus violet filters.  You might have to google it since it may be archived in CN.  

 

I found the WO VR-1 to give a neutral color view with my Stellarvue AT1010N Nighthawk I used to own.  

 

That or the Baader Semi-Apo would be where I would look.

Doesn’t look like the WO VR-1 is still in production. I’ll keep an eye out for a used one though. Thanks for the tip…



#5 JimB1

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Posted 29 November 2021 - 06:21 AM

Better to get an ED OR Better scope for AP.

But you use what you have so consider your limitations. The reason you get purple blooms with a short achro is that the focal points of different colors are not at the same physical length from the objective. Depending upon your scope design some of the colors will be at the same focal point. If you can find the data (crossings curve diagram) for your scope that might be of interest to you to determine if a Duo band filter (Ha/Olll) might be advantageous on nebulas (if Ha and Olll focus at near the same point). This may be somewhat less efficient for the Ha signal on an unmodified dslr, but some Ha will register. Mono narrow band imaging works on achros because you focus on a single wavelength, everything else is blocked (if the optical lens is figured well).

Do some research on your two cameras. The sensor used in some cameras have less noise and more sensitivity to low light. The newer generation of sensors use technology that greatly lowers electronic noise (read noise plus amp glow) and have much higher quantum efficiency. Look for "back illuminated" sensors.

Go ahead and expose some images its the only way to improve your technique. Work on your polar alignment and tracking or add on autoguiding to your astrophotography challenge.

Some image processing software can help clean up your bright stars. You can separate the stars and work on them for color correction and sharpening the edges then merge the nebulae and stares stars together. But nothing is better than good clean data to start with.

Use what you have to find out what can do with future improvements in equipment and techniques.

Sent from my SM-N950U using Tapatalk

Well, like I said, it’s for learning. If I can get some decent photos and CA becomes the limiting factor, I’ll think about going a more photo-capable rig. If I suck at this then at least I won’t have dropped a lot on the experiment lol.gif



#6 39.1N84.5W

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Posted 29 November 2021 - 08:15 AM

I use the Semi-Apo and it's not going to clean up the CA all that much on astrophotography. Achromats are great for H-Alpha narrowband nebula, but you would need to spend another $700 on a tracking mount and $300 on narrowband filters.

#7 SeattleScott

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Posted 29 November 2021 - 03:50 PM

There are two ways to get better views than through the eyepiece. You can amplify light or you can accumulate light. Amplifier costs a few thousand for a good one. A camera can accumulate light but it requires a tracking mount because of the pesky Earth rotation thing. Experimenting with imaging on a camera tripod without tracking is like experimenting with mountain biking with a tricycle. Especially with an achro and without narrowband filters. So you can give it a whirl and see what you get but realize your equipment is really limiting you. Sort of like when someone heard I had the Rev Imager and they sent me an image showing stars drifting and asking if this is as good as it gets, and I had to explain that the Rev Imager really needs to be used on a tracking mount.

Scott

#8 TareqPhoto

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Posted 30 November 2021 - 03:47 AM

Narrowbanding filters and green filter can be ok with this kind of scopes.


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#9 JimB1

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Posted 30 November 2021 - 06:20 AM

As far as tracking goes, I can drop this onto my lx85 or buy a more portable tracker but I am trying to take smaller steps first and start simple. If I am taking single frame, wide angle shots at faster exposures (less than a couple of seconds) then I don’t think I need to worry about tracking initially. 

 

following the, keep-it-simple rule for now until I get a better handle on what I am doing then I will slowly add complexity to the mix when I have the basics…

 

I thought about going full tilt and using my sn6 with the lx85 and doing a guide scope and all that but it’s too much to learn at one shot so I decided to go the slower route and add things to the mix so I don’t overwhelm myself. It’s a hobby to me and if I get frustrated with it I know I’ll just stop and I’m trying to overcome that source of frustration by limiting the amount I need to learn at a given time. 

At the moment it’s the CA issue. Next it’ll probably be post processing images, then who knows, maybe tracking or maybe something else will come up that I need to solve for.

 

I know some people can just go full tilt and are good with studying and learning everything at once but I’m not one of those types :)

 

-Jim


Edited by JimB1, 30 November 2021 - 06:29 AM.


#10 SeattleScott

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Posted 30 November 2021 - 12:42 PM

Gotcha, so you have the LX85 when you get ready for exposures more than a few seconds, perfect.

Edited by SeattleScott, 30 November 2021 - 12:42 PM.



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