From: "Milton Aupperle" <milton@outcastsoft.com>

Date: May 8, 2009 10:32:02 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: New file uploaded to Astro_IIDC

Hi Alberto;

Very Nice Shot, thanks for sharing.

Do you have a full sized version of it on your web site?

Like Allan, I'm not sure about your measurement of pixel resolution. Why is vertical and horizontal different values, as the pixels on the CCD are square?

With my C8 and at a focal length (measured) of 5,000 mm using the Televue 2.5x, I come up with 480 meters per pixel resolution by measuring multiple known features on the moon. Assuming turbulence doesn't get in the way, you should be able to 200 to 220 meters per pixel.

Also, I suggest anyone using a barlow or televue that they should actually measure a known object's dimensions to calculate out what their true focal length is.

Something like Saturn's disk (19" arc seconds) or Mizar - Zeta (14.4" ) or Albireo - Beta (34.4") make for good targets so you can calculate out how many zzz pixels cover yyy arc seconds.

And the only accurate way to figure out what your real focal length is to shoot at prime focus, then shoot with your barlow / televue in place and then calculate the arc seconds per pixel value for both. Then divide the barlow view arc seconds per pixel by the prime focus view arc secs per pixel and you known you true magnification factor. Then it's multiply the prime focus F ratio or focal length by the multiplaction factor and you know the real F ratio and real focal length.

The reason I'm bringing this up is that the further back your CCD is  from the barlow, the more magnification you get. So it's pretty easy to use an extender tube between the barlow and the camera, such that your 2x becomes 3x or even 5x. With Televue it's different and in most cases it makes either no difference or very little difference in back spacing, except for the 5x.

http://www.televue.com/engine/page.asp?ID=42

HTH..

Milton Aupperle

--- In Astro_IIDC@yahoogroups.com, Astro_IIDC@yahoogroups.com wrote:

Hello,

This email message is a notification to let you know that

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 File        : /Lunar/090501-Aristo-Eudox-V1-1100.jpg 

 Uploaded by : richter1956 <richter1956@...> 

 Description : Aristoteles-Eudoxus & Co. 

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Regards,

richter1956 <richter1956@...>