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RGB frame comparison on a doublet (longish post)


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Earlier in the year I did my first LRGB imaging with some (very) limited success (using a Megrez 72 doublet) - shooting galaxies with their fairly sparse star field exhibited problems between the RGB frames in star bloat but wasn't too bad.

Now with the summer here I've been out to shoot some reflection nebula and have found the problem is really amplified by the sheer number of stars in the frame. Here are some example stacks (screenshots of unstretched data) just using the STF tool in PI.

Care was taken to focus each filter in turn - even though they are meant to be parfocal.

Lum

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Green

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Red

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Blue

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The Lum & Green frame are pretty good, but Red & Blue pretty awful! I was expecting to struggle with the blue, being aware I am using a Megrez 72 doublet, but didn't expect the red to be so poor too!

I was able to put the image together with reasonable success but had to sacrifice almost all star colour due to the mismatch sized stars - they exhibited terrible halos.

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So where do I go from here - I don't really want to restrict myself to just narrowband targets, my sky is limited by houses and trees but is fairly decent in terms of light pollution. Narrowband only (where I suffer no such problems) would leave me with a restricted list of targets.

A scope upgrade? I can't afford the type of refractor that would solve these issues totally - but a move up in quality would make them much less of an issue?

So I guess it's new scope time? :)

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Im using a zentithstar 70 and they are pretty much the same scope, for narrowband they are really good but really quite poor for LRGB imaging. Consider an ED80, not very expensive and it handles the channels much better than the WO scopes.

Edited by shaunster
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Im using a zentithstar 70 and they are pretty much the same scope, for narrowband they are really good but really quite poor for LRGB imaging. Consider an ED80, not very expensive and it handles the channels much better than the WO scopes.

It's a real shame these scopes don't perform better on RGB - I love my Megrez :) & it's a cracker for narrowband.

The ED80 is a little long in focal length for me at 600mm, I really love the FOV I get from my Megrez & Atik 314 - something in the 480mm focal length area + a 0.8 flattener?

Edited by johnrt
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Bit more expensive theres the new WO GT81 triplet, im waiting for more reviews before I buy one though.

You could always use a reducer with the ED80.

If you want the really tight stars you get using NB filters with LRGB filters then you have to get into the very expensive stuff, think Tak FSQ etc

Edited by shaunster
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Bit more expensive theres the new WO GT81 triplet, im waiting for more reviews before I buy one though.

You could always use a reducer with the ED80.

If you want the really tight stars you get using NB filters with LRGB filters then you have to get into the very expensive stuff, think Tak FSQ etc

I'm using both my kidneys at the moment - so that rules out Tak/Borg/FSQ etc.

The WO GT81 looks interesting - would it be compatible with the WO field flattener II I use with the Megrez?

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What about the new TS 65 mm Quadruplet?

Wp

It looks like a brilliant scope on paper - but quite a few people have had their fingers burnt with bad ones which suffered from astigmatism.

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I use an FSQ so I know how good such comparisons can look. In the past when I used a WO Zenithstar 80FD I found a similar problem, eventually solving it with my wallet.

If you look at a 90% stretched image in Photoshop you can look at the individual channels. Flicking through them shows instantly which channel has the biggest/smallest stars. You can then try some careful star reduction, using different amounts on different channels to get both red and blue to match green in size. Fiddly but possible. Can you do that in PI?

Dennis

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What about the new TS 65 mm Quadruplet?

Wp

Don't go anywhere near them.....lots of them suffer from astigmatism due to pinched optics in cold weather. Several members (myself included) have had outr fingers burned with them, and Ian King sent his entire stock back.

It's a shame....it was a well built scope in most ways with a great focuser.

Rob

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Skywatcher ED80 with a x 0.8 reducer would be a 480mm f/6

WO GT81 with a x 0.8 reducer would be a 382mm f/4.7

Where is Nadeem when you need him to chip in with his thoughts on the GT81???

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I use an FSQ so I know how good such comparisons can look. In the past when I used a WO Zenithstar 80FD I found a similar problem, eventually solving it with my wallet.

If you look at a 90% stretched image in Photoshop you can look at the individual channels. Flicking through them shows instantly which channel has the biggest/smallest stars. You can then try some careful star reduction, using different amounts on different channels to get both red and blue to match green in size. Fiddly but possible. Can you do that in PI?

Dennis

There is no layering capability as such in PI so if would be a very difficult process to keep overlaying the channels for comparison.

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