ED Eyepieces and CAberration

[Regulus (Trevor)]

Is it possible to buy eye-pieces that can correct for chromatic aberration in good quality (Megrez/Explore Scientific etc) achromatic scope??
It seems that the cost of doing it this way is reduced due to the smaller quantities of quality glass required in an eye-piece as opposed to the acres of it in an 100/120/150 refractor.http://www.iceinspace.com.au/forum/....estionicon.gif

[Profiler (Profiler)]

It will be interesting to hear what others have to say but, in my view, the short answer is basically - No.

There are a host of avenues in which CA has been tackled in refractors but EP's without some sort of filter, in of themselves, do not correct the CA. Indeed, the filters don't really correct the CA as so much as simply suppress the actual visibility of CA

[Regulus (Trevor)]

Can you advance any good reason why the problem of CA cannot be resolved via eye-pieces??
Yeah I'm also looking forward to other peoples ideas on this. Who knows, could get lucky.
Cheers. iTrev

[Profiler (Profiler)]

Hi Regulus,

You will forgive or perhaps understand that I don't have time to type out a long explanation but simply look up various websites on optics and how refractors and objective lens work.

[doug mc]

The eyepiece only looks at the image at the focal plane. If the image formed by the objective has CA, then that is what it will magnify.

[brian nordstrom (As avatar)]

Yes I think its no as well .
The CA is started in the main lense and is
made worse ( longer ? ) at the point of focus , Sad but true .
But there is a thing called a 'Chromocor' made by 'Aries' in Russia and these work when set up properly but the down side is the cost , about $1000 US .
And set up right they work .
How do I know?
I have one on my Saxon 150mm f/8 achro and it holds its own against a friends 152mm f/9 Astro Physics Starfire and anothers TV 127 up to 400x or so .
Not bad considering my set up with HEQ5 mount is $2800 au and the Starfire and TV start at 6k OTA only .
Chromacors are rare as Aries manifacture small quantities at differant times with no fan fare , luck of the draw .
ps. this is the technology that fixed the " Hubble Space Telescope ".
Brian.

[Profiler (Profiler)]

Yes - What Doug stated below is a great one sentence summary - if you need more details there is plenty of information on the web which explains everything

[Regulus (Trevor)]

Thanks Folks,

What you said all makes sense. Just a sad fact of optics.
My friend is buying the Explore Scientific 127 Achromatic and is hoping the CA isn't going to disappoint him esp. when it comes to his first forays into astro-photography.
Although the limited reviews I've seen say good things they do mention CA around bright objects (eg Moon) and I don't know whether the problem becomes more noticeable in long exposures of the usual photographic targets.

[brian nordstrom (As avatar)]

The 127 will be very good , as todays achros are all very good , with the coating technology and computer designed and polished lenses.

I have a 150mm Skywatcher f/5 here in OZ ( My chromocored f/8 is still in NZ ) and even at f/5 its super sharp and shows heaps of deatail on jupiter , with some CA , not to bad , but there of course .
If its to bad for some people I use the 100mm hole in the lense cap to turn it into a 100mm f/7.5 and the CA almost dissapares .
But CA does not worry me to much , as every ones tollerance is differant.
Brian.

[Regulus (Trevor)]

Thanks for the responses. Always something new to think about.

Brian that's an encouraging comment
The 127 will be very good , as today's achro's are all very good , with the coating technology and computer designed and polished lenses.

I have a 150mm Skywatcher f/5 here in OZ
Regards this I have been steering clear of the wider apertures to get away from CA and had already thought that a 5" or 6" f8~9 achromat would probably lose a fair bit of CA if i dropped the aperture down with a mask, so I was pleased by your other comment too.
If its to bad for some people I use the 100mm hole in the lens cap to turn it into a 100mm f/7.5 and the CA almost disappears
and
But CA does not worry me to much
Yeah I think my friend and I can live with a little bit too given our budget and the quality of modern scopes.
My chromocored f/8 is still in NZ
This is what is interesting me at the moment. If one of these does what it is advertised to do then it is very attractive. Wondering whether an adapter and Camera can be attached tho.

Thanks again.
So many things to learn and so much info. Fun eh??? :-)

Trev

[clive milne]

Originally Posted by brian nordstrom

ps. this is the technology that fixed the " Hubble Space Telescope ".

Really...?
I was under the impression that the HST suffered from spherical aberration, not chromatic aberration.