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

[stange]

Thanks Chris & Doug!

I was not aware of my eyesight creating so much subtle error. This means 
then that I must focus by finished picture images rather than judge by 
Monitor or eyesight in any way.

I wonder what other parts I have don't work well...........

Larry
YCSentinel


----- Original Message ----- 
From: <mexicodoug at aim.com>
To: <meteorobs at meteorobs.org>
Sent: 2008/12/10 22:52
Subject: Re: (meteorobs) IR Camera is up. -Important notes....


> Chris wrote:
>
>
> "...It's not an issue with camera lenses...The focus difference between
> the edge of the Moon and the center is subatomic!"
>
> Larry,
>
> 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
> Doug
>
>
> -----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
> 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
>
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