From: Alan Friedman <alan@greatarrow.com>

Date: November 5, 2007 9:32:45 AM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Barlows and Mars

Hi Milton,

I have three tele-extenders that I use - a 2x powermate (I had the 4x and 5x but sold both - they are too long for my 10" f14.6), an old 2x Astro_Physics barlow from the mid 1990s and a Baader flourite flat-field converter (FFC). The A-P and Baader units both work like a traditional barlow, where the magnification is increased as you move the camera (or eyepiece) further from the lens elements. The FFC is a highly corrected 4 element lens, but very expensive (currently over $700. in US dollars). Its minimum amplification factor is about 3X. I use this mostly for the planets. The A-P is a two element lens. I've never found noticeable chromatic errir with it in my images, but I am almost always working with color filters and IR blocking. 2x yields about f30 which is good for high resolution lunar and solar imaging with my scope.

The TV Powermates are very good too and many people swear by them. They don't shift the focal position much, which is very handy for me especially in very cold conditions when my moving mirror focusing system gets stiff and balky. I don't believe that they offer the same flexibility in magnification as a traditional barlow design - I know that in certain configurations I can't reach focus with mine, but I've never taken the time to record when or why this is!

best regards,

Alan

On Nov 4, 2007, at 1:10 PM, Milton Aupperle wrote:

Hi Folks;

With Mars fast approaching it's final close approach for this decade
(max 15.9 arc seconds on Dec 18th), I was wondering if people have any
comments on the barlows they use for long focal length imaging?

I have been using an Antares three element 2x multi coated achromatic
Barlow, which seems to work okay but I do get some color fringing. So
I'm thinking about maybe replacing it with a TeleVue 2.5x Powermate.

Anyone else care to share an opinion?

Thanks in advance..

Milton Aupperle

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