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Apochromatic and Achromatic... ???

Started by amstel78, 05/07/2003 01:15AM
Posted 05/07/2003 01:15AM Opening Post
OK, I'm really starting to dig this astronomy stuff. I've been reading alot about refractors as of late, and wanted to know what the differences are between achromatic and apochromatic refractors. Which is better, and why? I've been trying to figure out why so many people swear by small aperture Tak scopes, and are willing to spend thousands on them. Can anyone fill me in?

Best regards and clear skies!

James

Regards,
James
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Posted 05/07/2003 08:00AM #1
This is a good read for you. all the best

http://www.dpreview.com/learn/Glossary/Optical/Chromatic_Aberrations_01.htm
Posted 05/07/2003 02:06PM #2
I've never looked through an APO, but I own an f10 achromat. On very bright objects, such as 1st magnitude stars, Venus, Jupiter, etc., the scope will show a small "violet haze" around the edge of the object. This supposedly reduces contrast on planetary detail. It can also make seeing a faint companion of a bright double star more difficult. I don't have a minus-violet filter, but I sometimes use a yellow filter to remove this haze. This can help when viewing the companion of Rigil, or other such doubles with large magnitude contrast.

The shorter the focal length, the more severe the chromatic abberation. Very long focal lengths (f15 or f16) do not suffer as much from this problem. As I've said before on this forum, I think that the short focal length refractors (almost all Chinese) that are now saturating the market are a bad idea, and are giving achromats a worse reputation than they deserve.

Apos (Takahashi, Astro-Physics, TMB, etc) use either an additional lens element, fluorite glass, or ED glass (or a combination of these) to virtually eliminate this false color, even at short focal ratios, while preserving the crisp, high-contrast images that good refractors can give. This seems to be especially important for those who do CCD photography, thus some are willing to pay very high prices for these.

Gary
Posted 05/08/2003 01:49AM #3
>I've been trying to figure out why so many people swear by small aperture Tak scopes, and are willing to spend thousands on them

10. They'd buy a BIG aperture Tak Scopes but they cost too many more thousands?

9. A Tak refractor LOOKS like a telescope. You're neighbors won't point to your truss dob and ask if you've hit oil yet.

8. Wide flat fields in a compact sized scope which you can mount and track for really nice astrophotography... where smaller aperture size can be compensated for by longer exposure time. ( *cough* )

7. You get your own section at Star Parties. While the mobs are drinking beer and soda, you can sip champagne with the other Tak owners.

6. Run your hand along with tube and focuser. Look at that hunk o glass. FEEL the quality. High end APOs are works of art as much as functional pieces of equipment. OK, fun, avocational pieces of equipment.

5. When you get to be 60+, you'll still be able to set up the Tak on its mount. ( Without back pains the next morning ).

4. They prefer getting instructions in Japanese.

3. If something isn't right, you won't have to take the refractor back and wait for hours in the Walmart customer return line

2. Rich, high contrast, velvety black backgrounds with great viewing when SCTs and Dob owners are unhappy with the seeing conditions or waiting for hours as their scope cool down.

1. Cachet

...and I've got too much time tonight!

Karl










Posted 05/08/2003 02:29AM #4
Hello James,

There was a post last May that I think you may be interested in reading the threads. Go to search forums and search for RECIPE FOR PRODUCING A 102MM SEMI-APO

The thread covered allot about what defines the apo and achromatic refractors. I hope this helps.

Paul