Dall Kirkham primary-cell trouble

miki63au · #1 18-04-2008, 10:03 PM

G'day all,
I hit a snag. Try to do the mirror cell for my Cass.
The DK Cass. primary has a vortex, and I think for proper collimation
need a lateral adjustment for centering.
I studied Tak's Mewlon300 DK and Takahasi using only a 4" cork pad and a
center hub for a cell. No 9 point floating cell, no lateral adjustment.
The 12.5" mirror is full thickness, just like mine.

Can I get away with it? No edge support? I assume the mirror will sag a bit and get just a bit less correction, which may effect the optimum BFL for the design.

The optics I have is the original AO 12.5" f16 DK Cass.
In that the cell was a wood plank, and the mirror was sit on 3 round bolt.....

Thanks for your help.
Mick.

Rod · #2 19-04-2008, 10:58 AM

Hi Mick,

I might be missing something but can't you use the secondary vane adjustments to centre the mirrors in respect to each other?

I have a full thickness newtonian sitting in a four point cell - very easy to build. Laterally the mirror rtests against 3 plastic poles. This seems to work fine. Full thickness mirrors are fairly forgiving.

On a cassegrain you could provide the lateral support via the baffle tube but if you do so, read this first:

http://www.atmsite.org/contrib/Sapp/Baffle/

Good Luck!

Rod.

Satchmo · #3 19-04-2008, 11:26 AM

Mick

It shouldn't be too hard to design a cell with adjustable lateral supports.

You wont get away with center mounting a 2" primary , it would need a conical shape.

A ring support at 50% would be ok though...this could be a dozen or more silicon pads. I wouldn't stress too much about the cell until you can verify the quality of the optics. With an F4 primary these Cass scopes would have to have superb optics to match a modern Newtonian with premium Powermate amplifier , for example.

miki63au · #4 19-04-2008, 05:36 PM

Rod we talking about the primary cell.
Mark, I hear a skepticism from you regard of these DK Cass. from AO.
You think there is a minimum chance to get close to the TAK Mewlons?
With the original OTA and the horrible mount, I couldn't test it properly.
So I decided to rebuild the OTA, and see what happens. If the star test is
no good, there is a option to step down the aperture to 12" like the Mewlon.
I just hope my effort is will pay off. To bench test the lot I need a 14" flat. No chance of that....
Mick.

miki63au · #5 17-05-2008, 03:47 PM

Hi Mark,
would you have a suggestion how to test this cass. without a expensive flat? I think because the primary is ellipsoid, can be null tested (I need a bit of learning for me to do this...).
But I like to test the full system with the secondary, and optimize the
distance for best spherical correction.
Any suggestion?
Thanks,
Mick.

Satchmo · #6 17-05-2008, 06:13 PM

AS the system is quite slow ( F15? ) just use a ronchi grating on a star instead of an eyepiece.This will have ample sensitivity. If you can get a picture of this with a digital camera and I can help with the diagnosis. This will not be adequate to diagnose surface roughness and astigmatism unless very bad.