Wide fast mirror and swappable focal plane

[skybadger]

Hi all

I'm thinking of a design for a mirror I have . It's a 12" f/2.7 . I was thinking of using it in Newt mode and then wondered, why bother. Why not just place two cameras at the focal plane, a large ccd and a video camera, say a lodestar. The original secondary on the donor Cass scope was 88 mm across so this should fit. 

I then started thinking about Coma and the video camera being off axis and wondering if that meant that image would be unusable. Then I though about a sliding focal pane, such that I could switch the sensors in and out of the on-axis position. What do people think? Too hard or just a bit of mechanics.?  There will need to be a mpcc in front , which is what defines the axis and nulls the coma. Filters will be hard unless screwed to the mpcc each time. I'm expecting a 60' fov at the focal plane. Focusing will need to be by adjusting the entire unit in and out.

Other than the big pro scopes, anyone seen this done anywhere else ?

Cheers 

Mike

[Tim]

Hi Mike,

Sounds like an exciting, and ambitious plan  My concerns would be with accuracy in maintaining squareness of the CCD to the focal plane, as depth of focus on a f2.7 mirror is going to be tiny. Sounds like a lot of collimation tweaking...

Would a built in paracorr work better than an MPCC I wonder?

[Peter Drew]

I almost finished a 18" F3.6 instrument along those lines. The secondary mirror was replaced by a Watec video camera fitted to a single vane spider. The main "tube" was a single strut length of 2.5" square aluminium. The top of the tube carried a shorter length of 2" square tubing which supported the spider vane, this smaller tube slid up and down inside the outer tube on 8 small ballraces arranged in similar fashion to a Crayford focuser. Focusing control was by spring loaded internal threaded rod. For unrelated reasons it's since been on the backburner.   

[skybadger]

I was thinking of a central cylinder of around 80mm diameter suspended on a traditional 4-vaned spider in the secondary cage. The cylinder has an internal sliding fit cylinder with a closing plate at one end. On the closing plate is mounted the cameras in a horizontal sliding fit sprung-loaded carriage. Essentially a baked bean can with cameras in it and a hole at one end to let the light in. 

The challenges are:

arrange the sliding fit to be the focuser and add controls - triple rack rails and big exterior gear with drive pinion ? It has to be internal really or it will stick out into the light path. Or focus using the primary...

Sliding fit sled  - it needs to move about 40mm - half the width of a lodestar and my sx 9X camera combined. 

So it sounds like its worth knocking up a prototype.. 

cheers

Mike