Re: ATM Alternatives to Newtonians

["Frank Ward" <thewards@mindspring.com>]

Rod,

Why did you exclude the Lovejoy secondary (by your hometown  boy)?

You potentially could have your f/5 and eat it too.  Adding the Lovejoy
secondary in front of the diagonal looks like a great application.
And you move up to f/12 or f/15 while it's installed.  You could remove
it and use the support as a baffle for the diagonal.  Screw it in and out.
(with a well designed tubular secondary holder.)



Frank Ward
Atlanta, GA



----- Original Message -----
From: Rod Brackenridge <rjb@netconnect.com.au>
To: <atm@shore.net>
Sent: Friday, June 11, 1999 9:29 PM
Subject: Re: ATM Alternatives to Newtonians


>
> Scott,
>
> I am interested in this too because I am getting the material together for
a
> Dall Kirkham.
>
> For what it's worth, here's what I think:
>
> Firstly, note that the Dall Kirkham cassegrain has more coma than a
newtonian
> of the same focal length.  Mostly it works out equivalent to an F6 to F8
> Newtonian.  Coma in a Dall Kirkham increases as the amplification of the
> secondary increases and the primary focal length decreases.  A classical
> cassegrain has the same coma as the equivalent focal length newtonian but
is
> more difficult to construct.
>
> The main advantages, to me, anyway are:
>
> 1.    A compact instrument which does not require a ladder (Perhaps not
> important for a 10 inch instrument).
>
> 2.    You already have a long focal length which you can amplify further
with a
> barlow so that long focus eyepieces can be         used to achive very
high
> magnification.
>
> 3.    In a Dall Kirkham, the primary is an elipse requiring less
correction
> than a parabola (usually around 70%).  If you have the space, it can be
null
> tested at conjugate foci.
>
> Hope this helps.
>
> Regards,
>
> Rod Brackenridge
> Daylesford
> AUSTRALIA
>