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

mexicodoug at aim.com mexicodoug at aim.com
Thu Dec 11 01:52:51 EST 2008

Chris wrote:

"...It's not an issue with camera lenses...The focus difference between 
the edge of the Moon and the center is subatomic!"


You are right about having to focus.  Don't forget your eye is part of 
the lens system.  What may be comfortable for a point focus may not be 
the proper theoretical focus even if you believed it sharp as a tack.  
The DSO may be seen better depending on optical aberrations of both the 
optics and the eye - sometimes having nothing to do with the actual 
glass.  You may even adjust slightly the distance of your eye to take 
in a greater FOV in some cases - would that not require a focal 
adjustment for more comfortable viewing?  Yes, as your retina isn't at 
a fixed to the ocular.  I think these are the differences you are 
trying to reconcile with Chris.  That is why at a star party everyone 
is playing with the focuser even when it is "focused" on an object 
perfectly by the scope's owner.

Best wishes

-----Original Message-----
From: Chris Peterson <clp at alumni.caltech.edu>
To: Global Meteor Observing Forum <meteorobs at meteorobs.org>
Sent: Thu, 11 Dec 2008 1:30 am
Subject: Re: (meteorobs) IR Camera is up. -Important notes....

Hi Larry-

I think you're fooling yourself. A 4mm camera lens can't distinguish 
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 
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 
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 
1/10 the wavelength of light. You're not making that adjustment with 
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) 
they image meteors in the upper atmosphere (something that has only 
done occasionally). It's not an issue with camera lenses.

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


Chris L Peterson
Cloudbait Observatory

----- 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 
> travel noticeable between edge of moon, center of moon, and DSO's or 
> 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 
> 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 
> on
> a refractor and a precision 15 turn dial on an SCT.
> Larry

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