[ATM] Re:(ATM) F/10 or F/8?

[Thomas A Simmons <tasimmons@raytheon.com>]

Curtis,

  Go with the F10-12. Once you have worked this to a sphere breath quickly
on it and parabolize it. If you overshoot, with a wee bit of work you can
recover the sphere. It isn't "easy", but with a little bit of work and some
patience you can do it. Besides, how many things do you value that are/were
"easy" to come buy? For example, did you marry the "easy" girl?

  With my F12 on a Dob mount, the only "ladder" you'll need is a single
step,
and yet the planetary and double star views? Priceless. Split the
double-double
with a 25MM eyepiece while those fellas with commercial Schmidt Cass scopes
drool with envy.

  "You can do it. We can help"

Tom
Tucson, AZ

Date: Tue, 14 Jun 2005 09:50:54 -0600
From: Curtis Spivey <curtis.spivey@gmail.com>
Subject: Re: [ATM] Re:(ATM) F/10 or F/8?
To: atm@atmlist.net
Message-ID: <3ff6d87f05061408501b222476@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

On 6/14/05, jsgrmfg@gracemfg.com <jsgrmfg@gracemfg.com> wrote:
>
>  F/8 is common and often recommended because it is the longest focal
ratio
> that can be tested easily by a beginner.  An F/10 is too short to leave
> spherical but so close to a sphere that the Foucault shadows are very
> subtle and hard to read.  I read somewhere that if you are testing an
F/10
> and can see a definite Foucault "donut", you are already way
overcorrected.
> The lateral wire test might be a good solution for the subtle shadow
> problem but building the tester might be more trouble and complication
than
> most beginners want.
>
> Jim Sturtevant
>

So, would it be better to go all the way to say a F/12 and leave it
spherical for simplicities sake?  Or would image quality suffer?
Remember this will mainly be a planetary scope.  I also plan to make
it a truss-style scope on a Dob mount just to be different.  So, a
focal length of 72" (F/12) for a 6" objective would still be
practical.  The eyepiece would still be well within ground level for
viewing.

--
Curtis
'03 ST1300 (Hers)
'97 ST1100 (His)
'00 NightHawk (Daughter)
Nampa, Idaho

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