Re: ATM Alternatives to Newtonians

[Rod Brackenridge <rjb@netconnect.com.au>]

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