Would a good quality compound beat out an Achro refractor?

Question

Would you prefer a 6 inch Mak over any 4 inch achro ? All things being equal, quality wise, would a 6 inch Mak always beat out a 6 inch achro? How might they compare if the achro were fitted with a violet minus filter? Would a good quality Mak still beat out this (modified) achrochromatic?

Answer 1

Hi Monk -

An achromatic refractor has a two - element objective lens, and is typically more prone to chromatic aberration, especially on bright objects, than it's big brother the apochromatic refractor. The apo generally has a three - element objective lens, is much more expensive, and generally delivers a much higher quality image. An achromat can get around the false color issues with a longer focal ratio, but that is impractical for an aperture bigger than 90 mm or so because the tube gets long and unwieldy. I am not aware of any filter that can correct the effects of chromatic aberration.

I think you generally get what you pay for, at least in the new scope market. I would favor a 6 inch Mak over any achromatic refractor, due to the reasoning described above. If you want top quality, I suspect a 4 inch apo would probably beat out a 6 inch Mak, although it sort of depends on what you are using it for. As you say, all things being equal, I cannot really tell the difference in image quality by just looking through each one (a Mak and an apo). If you can afford a big apo, then you probably already know what it can do. But an achromat? I don't think so. Good Luck and Clear Skies.

Answer 2

A good refractor has one thing that no scope with a central obstruction can attain: The best contrast for it's aperture. It also will have slightly better resolving ability for its aperture due the lack of extra diffraction from the obstruction. Because of these things, a good refractor can typically top the performance of slightly larger scopes that have central obstructions.

My 4" ED refractor can provide views of planets and the moon that can rival what my 11" SCT can do. The SCT will show them brighter and can handle a bit more magnification sometimes, but the 4" is very pleasing to view with. The smaller aperture actually helps a little in that it makes the scope slightly less beholden to the seeing conditions.

Answer 3

It really depends what you want to do with the scope.

Maks tend to have long focal ratios, so they're good for planets, but you may not be able to get a wide enough field of view to observe everything you'd like.  

The minus violet filter will help reduce chromatic aberration on an achromat, but bear in mind it's wasting some light.  A longer focal length reduces chromatic aberration because of increased depth of field, but frankly a 4" F15 achromat isn't nearly as portable (or as well corrected) as my 5.5" F7.5 apo.  

Longer or heavier tubes will need beefier mounts, and that's another consideration.     Maks tend to be compact and reasonably light.

The central obstruction on a Mak will reduce contrast - for a demonstration of how this works, hold your thumb up at arms length and look at something behind it using both eyes.  

But frankly, having colors coming to focus at slightly different places with an achromat will hurt contrast too.  

If it was for me, of the two choices, I'd go for the mak - but I'd also look at schmidt newtonians.  

If you can visit your local astronomical society, or attend a star party before you make your decision, it's a great idea because it'll give you a chance to look through other people's scopes, and this will help you decide what you'll enjoy the most.

Take your time - research the heck out of this, and do take any opportunities you can get to look through scopes similar to ones you're considering.