Posted by Astroman on December 27, 192001 at 14:37:47:

In Reply to: Barlow Lenses posted by Mark on December 23, 192001 at 07:05:10:

Hello Mark! Thanks for visiting.
The Barlow lens is placed between the eyepiece and the main optics of a telescope in order to increase the magnification of a given eyepiece by a given amount. It effectively doubles the magnification of your eyepiece collection. This, you know already. Your question has to do with the differences in magnification and length, etc.
The main concern should always be quality-a cheap Barlow will do the same thing as a cheap telescope-lead to frustration and disappointment. A 3X Barlow can be of use if your scope has sufficient quality and focal length to support it. Say your scope is a 6" F/4. You won't be able to get much more _usable_ magnification with a 3x simply because the scope won't support it. The light cone reaching the Barlow will be so wide as to become difficult or impossible to focus, degrading the image, mostly due to chromatic aberration-color around the edges and lowered contrast. On the other hand, a 6" F/9 might work quite well because the light cone is very narrow when it reaches the Barlow/eyepiece. The chromatic aberation should decrease, too. In most cases, a 2X Barlow is sufficient.
The "Shorty" Barlows work best in scopes that have little "back focus". That is, they aren't easily able to come to focus with something attached, such as a camera or off axis guider.
A variable Barlow can work well, but it should be pricey. If it's cheap and seems too good to be true, it usually is. I've used a variable eyepiece that was very, very nice, but it cost almost as much as my entire ep collection!

I hope this answers your questions. If not, feel free to email me at the address above, after removing the "x".

I own and operate a small observatory in SW AZ where I educate the public and do variable star research.

• My web pages.

• Re: Barlow Lenses Casimir 18:10:39 12/29/2001 (1)
  • Re: Barlow Lenses Astroman 07:37:43 1/02/2002 (0)

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