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achromat into apo

Started by cysmarc, 02/14/2006 07:56AM
Posted 02/14/2006 07:56AM | Edited 02/14/2006 07:58AM Opening Post
Other than chromacors which seem to be hard to come by, is there any other cost effective way (under $700) to get an achromat to perform like an apo. I've read about violet filters and understand that they don't do the job.
I have an etx 105 now which I like although for the right price I would part with it to finance a refractor.
I don't want to go higher than a 120mm refractor because of portability issues.
What concerns me is that I don't want to spend the $ on the refractor only to have 120mm of clear aperture lose out or even equal (due to chromatic abberation) the views of a mak 105 which has a central obstruction.
Thanx so much guys. I really appreciate all your constant replies even if I don't send it to each of you directly.
Clear Skies
Posted 02/14/2006 08:21AM | Edited 02/14/2006 08:22AM #1
Itz, there's NO easy way around it...

If you want APO views you need an APO scope. Besides the Chromacorrs there's really nothing out there that will make an ACHRO become an APO...

Yes, you could use Minus Violet filters, Fringe-Killers, etc, etc... but they simply won't give you the TRUE APO performance you are looking for...

Sorry, but IMHO it is as simple as either you have an APO or you don't...

Clear skies! 8)

Ivan Gastaldo 8)
Coconut Creek, FL

Ivan's Observatory
Lat 26N 16' 48" Long 80W 10' 48"
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Posted 02/14/2006 11:32AM #2
Since a long focal length will reduce, but not entirely eliminate CA, does anyone manufacture 70mm or 80mm lenses with a long focal length? If so, how long would an f15 70mm scope be and how portable do you think it might be?

Also, because Itz is apparently looking for a mid-range priced APO, wouldn't the Orion 80mm and some of William Optics semi-apos fit the bill?
Posted 02/14/2006 11:09PM #3
One thing no one has mentioned is that you have the option of masking the 120/8.3 down to a smaller aperture to lessen the c.a. If you take a 120/8.3 and mask it down to 105 mm you'd have a 105mm f/9.5. I'm not sure if that would make a big difference or not. But then there's the central obstruction on the mak. Take the 120 down to 90mm and you'd have a 90mm f/11 and, while that would show some color, it would probably not be objectionable to most people and would probably still beat the centrally obstructed 105mm mak.

You might well prefer the extra aperture in spite of the c.a. I think I would. But masking it is an option, at least for purposes of comparing it to the mak.