10inch F3.947 Dobsonian

[Saturnine (Jeff)]

Originally Posted by PJO

Jeff, your help is invaluable,

Also, what should I use to glue the secondary to HDPE (plastic)?

Adhesive Silastic is popular and seems to hold well, check online on how to set up the positioning. Also , you could make some clips from thin metal. Will post a photo later of a secondary as an example, of one that I've done .

[Saturnine (Jeff)]

A secondary holder I made a while ago.

[PJO (Philip)]

Hi everyone the scope is almost finished! Just need to finish up the focuser , shroud, and adjust balance, etc. I was recommended Sikaflex 252 by Testar for gluing my secondary, (there went 40 bucks) so Ill do a practice glue today. What shape bead should I do? As in is three small blobs the best way still, or is one big blob better, or should I just spread it everywhere?
Thanks

[Saturnine (Jeff)]

Sikaflex is good stuff but probably overkill for your application, used to use different grades of it a lot in my working days . Anyway all you need is 3 or 4 equal sized and spaced blobs but put some spacers between the mirror back and the face of the holder in between the blobs. Like this

https://stellafane.org/tm/dob/ota/mount-diag.html

If you were not aware of the Stellafane site before, it is a good resource for ATMers, lots of tips and tricks and links .

[PJO (Philip)]

Well, it was officially finished on the 5th (technically I kept working till the 8th) of August. Then we went overseas for 3.5 weeks on the 9th. Happily, though, it all fits in my carry-on suitcase so it came with us and I had first light on the other side of the world in a tiny, extra rural town of 10,000 people. First light was great. It was smoky but at the zenith (no dobsons' hole) visibility was good and the clusters around Scorpious were amazing, it's much better than my 20cm SC.
The scope works well, so where should I start? I've made a 40cm wooden tripod that also comes apart and fits flat in the suitcase since it was too short without it. The base of the mount sits on top and there are plastic blocks underneath to hold it in place.
More posts and pics to come.

[-Ephemeral- (Alex)]

Looks great! How much does the whole package weigh?

[PJO (Philip)]

well...

[PJO (Philip)]

So, Alex, here's the weights:

Upper ring: 450g
Diagonal mount and mirror: 240g
Focuser: 340g
Finder (GSO 8x50): 540g
Trusses and String: 1050g
Mirror (Skywatcher 30mm thick): 3200g
Mirror cell: 1000g
Base: 3160g
Tripod: 1410g
Covers and shroud: 400g

Whole telescope = 11.790kg.
Considering I was aiming for 5-7kg I think thats alright isn't it?
If you add in the carry-on suitcase, 3 eyepieces and the packing material it comes to around 15 kilos, but I put the trusses and A-frames in my check-in luggage to meet the carry-on requirements.

Next post: The Focuser!

[PJO (Philip)]

To start with, the focuser board is made from a piece of 1.8mm aluminium I had and attaches to the upper ring with 3 wing nuts. I drilled a 51mm hole (hole saw I bought specially) at the end and then cut a V at the bottom of the hole. A piece of aluminium angle slots into the V and rivets through the flanges hold it and a short identical piece in place. Then I mounted 3 pairs of bearings on the angle. I had a lot of trouble at first trying to get the 3 pairs of holes exactly in line but then I realised - it doesn't even matter. I can adjust everything by placing the bearings at different places on their bolts. Then since the weight of the eyepiece was making the drawtube tip, I put a piece of angle to hold the drawtube in place and hold the axle (an M3 90mm long bolt - the thinnest they had). The oversize knob is just a piece of leftover plastic, and the drawtube ended up being 3D printed at a school. Actually I then had to sand it wider on the lathe but its perfect.
Right now it's serviceable, but since it's fully adjustable I'll be able to get it perfectly smooth again, (it was perfectly smooth to start with but 92 hours of travel and at least a week of 4 wheel driving shook it up a bit).
As an afterthought if I was really thinking I would have put the V at 45 degrees so that the drawtube sits better on the bearings, but it doesn't matter.