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Galaxies in APO

Started by kulginov, 01/29/2005 04:41PM
Posted 01/29/2005 04:41PM Opening Post
Do faint fuzzy things look any better in a fine apochromatic refractor than in an acrhomat of the same aperture? At low levels of brightness, the resolving power of human eye decreases and I would expect that it cannot take full advantage from a better optical quality.

Same question for premium mirrors: does one see a lot more in a 1/10 wave mirror than in 1/4 wave one when it comes to faint galaxies?

Dmitri
Posted 01/30/2005 10:41AM #1
OK, how about faint stars? Unlike with galaxies, I suspect that an APOCHRO will have an edge against an ACHRO here.

Dmitri
Posted 01/30/2005 04:27PM #2
Low contrast detail is one area in which apos excel. Given equal aperture, a scope with better contrast and less scatter will give better views of deep sky objects, especially faint ones. Whether the difference is noticeable depends on many factors, including the target object itself, the sky background, the visual acuity of the observer, the experience of the observer, etc.
Posted 02/01/2005 08:12PM #3
I read a few of the posts and many mention that the APO over the Achro won't produce much better images on deep space objects such as galaxies.

There are a few reasons why many achros of the same size may not perform as well on the deep sky. Some have poor-no baffling, poor correction or even internal aperature stops. Perhaps someone has mentioned that the APO will often have a better figure then the Achromat. Unless you have a really nice achro with a great figure you may actually get better images from the well figured APO of the same size. Many of the less expensive Achromats have sphearical abberation, pinched optics or a host of other optical defects that do effect image quality even on deep space objects. For example my 80mm f5 Achro gets very soft over 80X while my 80mm APO can go to 150X which is nice on things like the ring nebula or double stars. Also, the 80mm f5 at 11X gives poor images at the field edge even when using a nice TV 32mm Plossl while at 17X with a modified Konig the stars seem pretty good at the edge of the field in the APO.

Don't get me wrong, there are some nice Achromats out there, and for some reason the Chinese 6-inchers seem to have particualrly good figures, but in general unless you go with a specialized company producing excellent achromats you may see better images with the APO due to better figuring of the lens system and attention to detail like nice focusers.

With regards to wave fron error on a mirror, usually star testing and experience win out over stated figures, additionally you have to add up all the mirrors of the optical train. You can have the best primary mirror on the observing field but if you get the 1/2 wave surplus secondary the image may not be all that it could be.

-Chris