From: Rodolphe Pineau <pineau@rti-zone.org>

Date: November 28, 2012 8:36:49 AM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Focussing issues .. need advice

On Wed 11/28/12, at 3:10, Jim Chung wrote:

Hi Rodolphe,

I've imaged with 3 and 4" refractors in the past with 5x Powermate and  

if the seeing is good you should be able to get better sharpness.  The  

5X Powermate is a funny creature because it actually delivers more  

than 5x magnifcation .... more like 8x so the focal length is rather  

high and producing a dim image they may be hard to see on screen to  

discern best focus.  Plus you don't want to max out your gain in  

AstroIIDC and introduce unnecessary noise.

I would think about 2000mm focal length is best for a 3-4 inch  

refractor so maybe a true 3x barlow.

regards,

Jim

Thanks Jim.

What I'm doing now is calculating the Dawes limit for my telescopes and the resolution of my CCDs captor in arcsec/pixel without any barlow, with the 3x and with the 5x to see which one are over the limits for my telescopes. This will prevent me from trying something that can't work in the future.. I should have done this a while ago.

Thanks for all the feedback.

Rodolphe

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