Re: ATM Dall Kirkham

[Jan Bentz <jbentz@inreach.com>]

Jana Jooste wrote:

> What f ratio are you looking at with such a large Dall Kirkham
> 
> I suggest f20 - f 30 with a secondary magnification of 2 if you want a
> descent planetary scope. Otherwise you are going to suffer severe off axis
> degradation of the image. Remember also that the DK  has strong filed
> curvature.  To get an off the shelf sphere of the correct diam and RC for
> the secondary is going to take longer than grinding one yourself.
> is to be supported on the baffle tube.
> 

   I think the degradation off axis in a Dall-Kirkham style design seems 
to be vastly overrated.  The D-K has 4 to 5 times as much coma as the 
Classical Cass which in turn has an amount equal to the equivalent Newt 
parabola.  If the overall focal ratio is kept above f/13 or so, a D-K 
will have about the same or less coma than a f/6 Newt.  Scott R's f/17 
D-K should be fine as long as he doesn't use 4x5 film. The relative 
curvature of field depends almost totally on the difference in radii of 
the two mirrors, no matter what brand of cassegrain it is.  It is only 
when attempting "fast" systems and wide fields that you have to consider 
the R-C form, say f/8 with 3/4 deg field or more.

Collimation sensitivity is mostly a function of the primary mirror's 
f/ratio.  The D-K's spherical secondary does make it easier to align 
than one with an optical axis, but the secondary still should be 
accurately centered.  Using a commercial precision pl-cx lens may work 
for the secondary if it was from Melles or some other expensive maker. 
These are usually 1/2 fringe guaranteed but would likely be better 
especially  if edged by 1/2" or so.

Jan Bentz