Constructing a dob

Excellent Michael. Glad to hear it's working out for you. I'm jealous though - your's sure is purdy!

The legs on the base board are most in need of urgent attention

That's the main issue with mine also. Mine are a nightmare on grass. I also need to get rid of the Obsession style split blocks altogether, they just ain't working out for me.

I reckon, there is a point at which the poles can be cut so that they all come to focus.

I decided to cut mine so that my main EPs (Moonfish 30mm and Hyperion 8-24 zoom + powermates) reach focus with quite a bit of space to spare). The reasoning was that I may add binoviewers at some stage. At least I can always use a 2" extender if some EPs don't come to focus.

Actually, does anyone have any idea as to how much backfocus is usually required for a binoviewer without an OCA?

T

I finally got around to doing some more work on the 12" tonight. Being surrounded by truss tubes at whirlpool was more than enough motivation to get cracking again.

Anyway I varnished the cage and started work on the split blocks. I bored all of them and they are a nice tight fit for the poles.

Trevor, how come the split blocks didn't work out for you? I was hoping that they would be an easy solution for attaching the poles.

Still unsure of how to attach the poles to the cage. I saw michaels obsession style clamps for the cage at whirpool and am thinking of trying to fashion something similar from some mahogany I have. My tubes are only 22mm outside diameter so there are no precast ones available to fit.

Anyone have any ideas on easy to make / readily available solutions.

(I really dont want to compress the ends of the poles and use an L shaped bracket to attach)

Hi Trevor,

Don't compress your tube ends - very messy! Two other methods to consider (both of which involve joining two poles together at the top end):

1 - If you can get your hands on a short length of square U-section aluminium channel with an inside dimension of 22mm, then for each pair of poles cut a short length (say 50mm long) so that you can fit both pole ends inside the channel with a little space for a 6mm bolt to attach it to the top cage. Each pole is placed in the channel and drilled with a 5.5mm bit. You then thread this hole with a 6mm thread cutting tap and run a 6mm gutter bolt through, cutting and filing off the excess. I used this method on the 20" scopes I used to make and it works well. The pole-pairs and channel then attach to the top cage by sitting into a matching length of L-shaped angle aluminium and clamped with a bolt.

2 - If you can't get 22mm U-section channel you can do the same thing with just an aluminium or 1/4" or 1.2" plywood plate on just one side of the poles. This also works reasonably well, though not as well as the U-section channel.

Let me know if you want pics and I'll take some from my scope.

Phil.

PS - check out ochra.110mb.com/Home%20Page/Astronomy%20...20f4%20Newtonian.htm for some distance shots of the scopes!

Hi Trevor,

I just could not get them to grip the tubes strongly enough, they are only .75" steel tubes. The holes were made to be 1mm wider than the tubes. Could also be the wood type I used.

I have it working now though using a split block design I stole from another site, here:

from www.stathis-firstlight.de/atm/index.htm

This version works much better for me.

I used the crushed tube method to attach to the secondary.

Oh, I finally got it out on Saturday night. Quite hazy here at the time so I didn't expect much. Tried M13 and M27 amongst other things. Wow! I can't believe the difference between my old 8" SCT and this 12.5"; Dumbell was the clearest I've seen it and shape was very obvious. M13 was like a photograph, individual stars filling the EP at 200x. I guess only having a 17% central obstruction improves the image a good deal so I can't wait to try it on Mars.

Still got to fix the feet though, it wobbles like crazy for a second or two uneven surfaces.

Cheers,
Trev

Really nice looking bunch of scopes there Phil!

Hi Trev,
That's a clever method of attaching the truss poles to the scope, I see that this clamp is on the inside of the mirror box, did you have the obsession type clamps on the inside also, if not did you have to make other modifications to cater for the change?
One thing you'll find with Mars with a bigger scope like that is that it can take a pretty high magnification.

BTW Phil, that's a really nice looking website you have there, I like the way you have all the club logos circling the IFAS logo in your links section.