(meteorobs) IR Camera is up. -Important notes....

[Chris Peterson]

Hi Larry-

I think you're fooling yourself. A 4mm camera lens can't distinguish between 
clouds and the Moon. No telescope can distinguish between different parts of 
the Moon, DSOs, and stars. The math is simple. A 4mm lens images an object 
at true infinity exactly 4mm behind the principal plane. The formula for the 
position of the focal plane is 1/d1 + 1/d2 = 1/f. Plug in for an object 50km 
away, and you get that the new focal plane is 3.99999968 mm behind the 
principal plane. That is, it's 32 nm close to the lens than for infinity. 
1/10 the wavelength of light. You're not making that adjustment with your 
focuser, no matter how good it is! The camera isn't even thermally stable to 
that precision.

Long focal length telescopes (meters) have to adjust their focus very 
slightly from "infinity" (everything outside the Earth's atmosphere) when 
they image meteors in the upper atmosphere (something that has only been 
done occasionally). It's not an issue with camera lenses.

BTW, if your SCT has a 2000mm focal length, the focal plane shift between 
stars and the Moon is 10nm, which is also meaningless mechanically. The 
focus difference between the edge of the Moon and the center is subatomic!

Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com


----- Original Message ----- 
From: "stange" <stange34 at sbcglobal.net>
To: "Global Meteor Observing Forum" <meteorobs at meteorobs.org>
Sent: Wednesday, December 10, 2008 10:56 PM
Subject: Re: (meteorobs) IR Camera is up. -Important notes....


> Chris I think you made a "typo" :-)  Our telescopes have distinct focusser
> travel noticeable between edge of moon, center of moon, and DSO's or Stars
> for example. Especially noticeable at prime focus with an astro camera or 
> an
> eyepiece.
>
> A low power eyepiece or fast telescope tends to not distinguish stars
> compared to moon.
>
> I routinely find cloud focus is far away from moon or stars in my cameras
> even with a 4mm lens. The one in the IR camera is an 8.5mm.
>
> Also for these focussing reasons I once mounted a digital vernier caliper 
> on
> a refractor and a precision 15 turn dial on an SCT.
>
> Larry