DIY 8.5 inch binoscope??

[furrysocks]

I have recently purchased two secondhand 8.5" primaries and two flats. I am intending to blog the build of a single dobs while spending as little as possible.

Because I have two primaries, I've been daydreaming about building a binoscope.

A couple of questions on my mind at the moment are:

- how close do the focal lengths of the two mirrors need to be?

- I have a 43mm and a 47mm secondary - should they really be matched?

When I get the primaries coated, I'll may request a test sheet for each.

Any thoughts or advice would be very welcome.

[furrysocks]

PS. I think the mirrors are f/7, TBC.

[Peter Drew]

Hello Furrysocks. I've built several reflecting binoscopes from 3.5" up to 12" aperture. I've not had any problems with differences in focal lengths up to 0.5", differences in eyepiece focal lengths are more likely to be a problem for a stated focal length. You do need to allow for any differences when calculating the mechanical setup so that the eyepieces are at the same level when focused. The secondary sizes are relatively unimportant provided that the smaller is large enough to pass all the light from the primary.

[furrysocks]

Many thanks, Peter.

Matt.

[furrysocks]

I've determined that one mirror is approximately f/7.57 though I haven't checked the other one. Anticipating that they're going to be a suitable pair, or even if not, I'm putting together two OTAs. Here are the wooden tubes, in their current state.

Each tube is comprised of 12 lengths of 3mm hardboard held together at either end with 3-section baffles cut from 9mm MDF laminate. They are weak - I'm making this build up as I go along.

The next step is to saturate the internal and external surfaces of each tube with polyester resin and later wrap the outside with polyester tissue with more resin. External rings in 18mm MDF, similar to the template visible in the photo, will be fitted to each tube section, pinned from the inside, soaked with polyester resin and assembled together with 1/2" aluminium channel in a double truss configuration. I anticipate the final weight of each OTA to be 10-11kg.

Despite the twists and turns, this is progress of sorts. I now just have to sneak 30m of aluminium past SWMBO when the courier arrives.

[8kids]

Looks good, have you tried ratchet straps for gluing up jobs like that ( the ones without the hooks) if your careful they'll apply even pressure to all the joints which would save you a few rolls of tape. Also good for gluing up mitred boxes and the like. I reckon it's going to be heavy! Good luck with the build.

Gary.

[furrysocks]

Cheers, Gary.

In hindsight, straps may have helped. The tape was just to assemble the hardboard into one long strip, they don't provide any clamping pressure at all. The pound shop "high strength adhesive" is just to tack glue the tubes together (a retrospective analysis). The resin and tissue will provide the rigidity (I hope).

Upper tube may be 2.5kg, primary mirror tube somewhere around 5-6kg. Given the length, it shouldn't be significantly heavier than the shorter commercial dobs 8" OTAs, proportionally speaking. But granted, I am making no efforts at all to reduce weight. And it will probably end up weaker, though hopefully aesthetically pleasing. I'll factor in time for kicking myself later.

[furrysocks]

I reckon it's going to be heavy!

For context...

Hardboard is 300g per 10cm of tube, total 30cm top, 35cm middle, 20cm bottom == 2.5kg.

Ribs/baffles are 80g per round, probably minimum 10 in total == <1kg.

External rings tbd, likely 200g each?? == 1kg-ish

Aluminium channel is 150g/m, total 7-8m == 1.2kg.

Primary mirror is 3kg, plus cell <3.5kg

Secondary, focuser, eyepiece and finder, probably somewhere around 1.5kg.

Total is therefore somewhere around 10.5kg, plus resin, threaded inserts, bolts and trunions.

Heavy? Yes.