From: Milton Aupperle <maupperl@shaw.ca>

Date: November 28, 2012 10:13:43 AM MST

To: Astro_IIDC@yahoogroups.com

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

Hi Rodolphe;

I would not calculate the arc secs per pixel except if I was shooting prime focus (even then it's still a guess that the optics match the specs) . I would measure the pixel distance in the image using known distance between several stars for larger FOV or a known double star (i.e. Mizar Alcor system has multiple star separations for example)  if your focal length is long enough.

If the back element distance is not exactly the expected manufacturers distance, then the reduction or enlargement is now unknown. This tends to be less an issue with TeleView products, but barlows and focal reducers act differently.

As Jim suggested,  you may think your using a 5x barlow - but in reality if your distance is too far backwards from the element, you may be getting much higher.

And don't forget that the brightness of an object decreases by the square of the focal length increase. So a 2x increase in focal length require 4x the light than prime focus and an 5x is 25 times darker and 8x is 64 times.

HTH..

Milton J. Aupperle

www.members.shaw.ca/maupperl/

On 28-Nov-12, at 8:36 AM, Rodolphe Pineau wrote:

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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