Alpha Centauri and NGC6231

[TrevorW]

Just a couple of images I did on the 9th Sept through the 80ed resized in PS adjusted for levels etc

Cheers

[iceman (Mike)]

Hi Trevor
What camera are you using?
There looks to be an awful amount of CA in those shots, more than I would expect (and I see) from my ED80.

[[1ponders] (Paul)]

Ditto to what Mike said, that is an aweful lot of Chromatic Abberation for an ED80

[TrevorW]

Originally Posted by iceman

Hi Trevor
What camera are you using?
There looks to be an awful amount of CA in those shots, more than I would expect (and I see) from my ED80.

These were taken through the Pentax I did have some focusing issues and I don't think the seeing was that good> I'd be surprised if there was any CA caused through the scope itself considering my moon shots generally show no false colour on the fringes.

Cheers

[gregbradley]

An ED doublet is most likely to be out of focus in blue as that is where they are often weakest. Green is probably in focus. It is not impossible but usually the weak spot of doublets to get all colours in focus hence the desirability of high quality triplet (and more difficulty in manufacturing).

The Tak FS series is one of the best corrected doublets with its fluorite front element (I am told the FC series was slightly better corrected with the fluorite element at the back).

Per Roland Christen though, colour correction comes from the mating elements so the fact of having an ED element is somewhat of a marketing thing and the mating element's glass type is not usually mentioned. No doubt to hide the fact that a cheaper glass type is often used.

The blue halo is most likely highlighted when the scope is a bit off focus.
But blue halos around stars is somewhat a trait of doublet scopes or scopes not fully corrected.

I have demonstrated this to my own satisfaction by imaging Omega Centauri with a Tak FS scope and an RCOS 12.5 inch with no glass lens elements at all )only mirrored surfaces). The RCOS showed virtually no blue haloed stars (it did show one very large bright bluish star but then that was a blue star). The FS scope showed some minor blue halos around brighter stars.

I notice my AstroPhysics 140mm has perfect colour correction and shows no blue halos at all like the RCOS did. But that's Roland Christen's genius at work. Very few can do it. TEC can as well, TMB, Tak triplets.

More triplets seem to be coming on the market. A triplet is no guarantee of colour correction though as I believe it is a difficult process hence Roland Christen and Yuri at TEC's status.

So as usual buyer beware as even though telescopes may be considered quite a conservative item there is a lot of marketing hype to get through to get the facts from the bull.

Another alternative is a minus violet filter if it persists.

Greg.

[TrevorW]

Greg

I've read also that CA is more prevalent with bright stars as well

Thanks for the feedback

[bluescope]

Hi Trevor

Indeed there does seem to be some strong halo effect in these images but at least you're out there giving it a go mate ! Keep at it and I'm sure things will get better as time goes on ..... the idea of hobbies is to enjoy them.