[sac-forum] Re: --[AZ-Observing]-- Re: Good Seeing Predicted for Tonight

Good grief.  Of course!  Why didn't you just say so? ;->

I was grappling internally with how a fixed light cone from a given
mirror could be altered - without thinking about the obvious "Barlowed"
effect.  

Sorry for the wasted bandwidth.  Chalk it up to another semi-novice
oversight.

(Albeit a well-intentioned one - at least I'm thinking about the
mechanics).

Thanks for the clarification (for me),

Darrell

-----Original Message-----
From: sac-forum-bounce@xxxxxxxxxxxxx
[mailto:sac-forum-bounce@xxxxxxxxxxxxx] On Behalf Of Tom Polakis
Sent: Wednesday, February 06, 2008 1:24 PM
To: sac-forum@xxxxxxxxxxxxx
Subject: [sac-forum] Re: --[AZ-Observing]-- Re: Good Seeing Predicted
for Tonight

-- "Spencer wrote: 
> Forgive my ignorance. I can, conceptually, grasp the benefits of
longer
> focal lengths for detailed imaging.  But, I cannot visualize the
> mechanics of how you're varying FL to this extent on your Newtonian.  


Darrell,

To increase focal length, I am simply placing a Barlow lens in the
system. For the Mars image, I used a 5x Powermate, which would indeed be
only 5x if I inserted an eyepiece into it.  In fact, there is a color
filter wheel downstream of the Barlow before the camera is inserted.
This increased projection distance results in the 5x increasing to about
7x in practice.  So 7 multiplied by the native focal length of 1400mm
gives you about 10 meters of focal length.  The bad news is that the
focal ratio then becomes f/38.

Tom

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