Dob conversion concept - criticism required!

[badgerchap]

Hi,

My old dob is, in a word, knackered. The base is fallen apart (cheap chipboard and dew eventually take their toll!) the tube is beaten up, the collimation bolts are rusty. Basically, the old beast is dead. The optics, however, are just fine.

So seeing as I have another scope now, I don't mind hacking the old girl up and fiddling around trying to build something new.

These are the criteria:

1. Must be mountable on an EQ5 - so must be really lightweight

2. Must be dismantleable for transport

3. Must be able to incorporate a dslr and / or webcam for some planetary AP (webcam) and maybe the odd DSO. My other setup covers DSO's so it's really lunar, planetary and solar that count for this.

4. The focal length of the mirror is 1250mm, hence why I have employed this folded design - I don't want anything over a metre on the EQ5.

5. MUST BE DONE WITH THINGS COMMONLY FOUND IN THE HOME - I could spend a load of cash, but then I might as well just bin it and buy a new scope - but where's the fun in that eh?

Here is a (very) preliminary sketchup of the basic concept I have in mind. Ideally the whole thing would be shrouded, but that's for later.

Basically, I'd be very grateful if you guys could try and pick out as many problems as possible with the basic design - I know there are pleanty, I just want to make sure I have everything covered before I draw up a more final blueprint.

I am aware of the balance issue and stability of the primary, and have begun to think of ways to tackle these.

Primary is 200mm, secondary is 40mm

Connecting plates are the old mounting from the dob.

Thanks!

[badgerchap]

Another point - there doesn't need to be much concern with regards to visual use - would be solely for AP

[jimmyjamjoejoe]

That's actually a pretty cool concept, really hope you go through with this.

[badgerchap]

Well, I've begun!

Got the base / mounting plate sorted, and started on a bracket to fit the primary.

Having a bit of a head scratcher with the secondary though - need to have it at 10 degrees from the vertical, but it's naturally shaped to be used at 45 degrees.

Can you file down glass? Probably not, don't want to try....

[FraserClarke]

Your secondary will have to be a lot bigger (~100mm) if you want to fold the focus back on top of the primary, otherwise you're going to lose a lot of light. You'll also need to be very careful about the baffling the camera, as it's going to be pointing towards the target.

Re: secondary shape. Yes, you can grind glass, but I wouldn't do it with a mirror which is already coated. You also need to be careful how you do it -- it must be kept wet; you *do not* want to be breathing in glass dust!

I'd have thought a classical newtonian would be easier to do, even if it is a bit longer than 1m.

[JamesF]

I think your proposed design will be quite tricky to get right and the much enlarged secondary will block a lot of light from the primary.

What's the motivation for mounting the camera above the mirror? Just to shorten the tube? How about a Nasmyth-style design?

James

[badgerchap]

Lack of an extra mirror really! I am thinking now to turn the secondary through 90 degrees, to place the camera to the side of the assembly, rather than above, that way I can have a shorter i.e. stronger linkage with the baseplate.

I know it'll be tricky, but that's what I get for putting those constraints on it really :/