Drilling hole in 12 inch primary

[xelasnave]

I have been considering rebuilding my 12 inch dob in an effort to reduce weight and please my eq6 mount. Also my perceivement of the reduction of flex is appealing.

One idea was to cut the tube approx in half relace the secondary with a flat surface primary mirror parrallel to the primary of approx 6 inch dia. and drill a 2.5 inch hole in the primary so focus would occur in the camera mounted behind (outside) the primary.

The mirror needs a resurface so it has little value.

I dont fear the loss of light in using a 6inch secondary (or down to a 5 inch)as the surface area left means I would have around the equivalent a 10inch (a little more probably) which would be great for photos.

All visual tracking will be thru a separate guide scope.

Perceived advantages 1. the tube would have less flex and
2.less heavy than original.3 More easy to manage and transport.

I hopefully expect that it would be inexpensive as the real cost would be in getting the hole drilled and getting a 6 inch secondary (would like unbevelled sides but at worst would lose 1/4inch to frame a conventional secondary).

Why should I give this idea up what have I missed? Can such a hole be drilled either at home or professionally in Australia.
Will the new optical path work?

Any suggestions gratefully received.

alex

[Wavytone]

You save weight by shortening the tube.

You're adding weight in the form of a secondary mirror and supporting spider significantly larger than the diagonal of a newtonian.
You're also going to need to add baffles to stop stray light coming obliquely past the secondary and through the hole in the primary (more weight).
I can't help feeling the result will be much the same weight as before, but optically worsened by having a 50% secondary obstruction. You've going to throw away another chunk of aperture off the rim off the secondary, so your losing perhaps 30-40% of the incoming light collected by the 12" mirror.


I can't help feeling you'd do better by leaving what you have alone and spending the $ on a 20 or 25cm f/5 mirror with about the same lightgrasp, and build a smaller and much lighter OTA that won't overload the mount.

[clive milne]

Alex... this course of action is not advisable.
I would estimate that the secondary mirror required to get the focal plane out to a reasonable distance behind the primary would result in an obstruction ratio in excess of 60%.
Irrespective of how much money you threw at it, the result would be a compact 12" with the light grasp of a 10" and a really, really poor contrast transfer function.... and that's the best case scenario, if you're lucky.
For the cost of core drilling the primary, refiguring, coating the primary, buying a new secondary mirror, secondary holder & custom rear cell for the primary, you could pick up a used C11 and have change in your pocket.

[xelasnave]

Thank you for your well considered thoughts.

I find the weight of the secondary to be approx 10 lbs so the appeal lessens.

I feel maybe a new scope may be in order and that I use the 12inch (even with the less than ideal mirror surface) for visual, which of course is what they were built to do..

Thank you for helping me organise and review my ideas.

alex

[Merlin66 (Ken)]

Alex,
Treppaning a hole is relatively easy....but the design you have in mind is far from optimum!

[ZeroID (Brent)]

Rolf (SkyViking) is building a 12" to go on his EQ6 but the mirror is a conical and he is using his magnificent Serrurier truss design. My 10" is built similarly and it saves a heap of weight. Fully loaded mine comes in at about 18kg but with careful design could have come in lighter still. It also balances rather well, no added weights necessary.