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New Dob Build - Top Half

Astralstroll

By Astralstroll
in DIY Astronomer

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jabeoo1

jabeoo1

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I have been stalking this thread with great interest.  Fantastic work Dave, I am in awe at anyone who attempts this feat.  I am still very happy having paid someone else my hard earned cash for the fruits of their labour.  

All the best with it Dave I look forward to your updates.  If you need a pair of hands for those precarious balancing act moments where being an octopus would be an advantage, just drop me a personal message...I live in the same city and would be very interested in seeing the scope !

James

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Astralstroll

Astralstroll

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Many thanks for the encouraging words guys. It certainly helps keeping the momentum going. With the rather disappointing weather of late, the build is keeping my mind suitably distracted and not thinking about the generally cloudy skies.

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Astralstroll

Astralstroll

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Finally got to spend a couple of hours in the garage today. The Mk 1 spider is now sort of made. Not sure how many iterations it will need before it's functional. Apologies for the pics, it's harder to photograph than it looks! I'm not going to do any further modifying until I decide on a mirror size and get it delivered.

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  • 4 weeks later...
Astralstroll

Astralstroll

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Well the focusser has now arrived safe and sound. Next on the shopping list is the secondary mirror. Whilst this is on it's way, I will finalise the spider assembly. Still undecided as to the exact configuration, but have one more idea to try about before I finalise the design.

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  • 3 weeks later...
Astralstroll

Astralstroll

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The secondary mirror has now arrived, so its time to put a bit more work into the top half of the Dob. Whilst I am sorting out the spider assembly, the next question that I am pondering is how to attach the mirror to the diaginal part of the spider. This leads me to two questions.

The diagonal part of the spider is plywood, so can i simply silicone the mirror to the plywood face, or do I need to apply some surface treatment.

When siliconing the secondary, do I need to attach using say three separate blobs of silicone rather than by a single wide blob.

Dave.

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Rustysplit

Rustysplit

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Dave, if you give the ply face a coat of epoxy resin first, the silicone will stick just fine. Mine was done like that and been ok for the last year. Best to use 3 blobs, 120degs apart to fix. Also, silicone on a safety cord to the back of the mirror, that can be tied to the central boss, just in case the fixing blobs fail. Now is a good time to decide if you are gonna want a dew heater on the secondary. I made one up from some resistors, and glued that to the back of the mirror before fixing the mirror to the mount.

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mapstar

mapstar

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Dave I've made  the secondary support from 3mm plate Aluminium which I can get a wider area for fixing, as the secondary I'm using for the first scope is 120mm. It's been machined on a miller (the kind use of my friends workshop) to be an ellipse just like the secondary. I'll post some pics when the build is nearing completion as I've not got the time to put a full thread up yet.

Keep up the good work and I'll keep an eye out for your posts.

Damian 

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faulksy

faulksy

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Dave I've made  the secondary support from 3mm plate Aluminium which I can get a wider area for fixing, as the secondary I'm using for the first scope is 120mm. It's been machined on a miller (the kind use of my friends workshop) to be an ellipse just like the secondary. I'll post some pics when the build is nearing completion as I've not got the time to put a full thread up yet.

Keep up the good work and I'll keep an eye out for your posts.

Damian 

nice idea, bigger surface area = stronger bond, like it :grin:

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  • 1 month later...
Astralstroll

Astralstroll

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Finally managed to get a bit more time to work on the top half of the scope. This weekends tasks include assembling and gluing the spider assembly together, fitting the spider and fitting the focusser. Next jobs are to paint the woodwork and spray the spider assembly. Then I need a moment of bravery and sort out gluing the secondary mirror in place. I will add a few pics later.

Dave

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mapstar

mapstar

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Looking really good there Dave. How or what will you be finishing the upper optical assembly with?

I've gone for exterior varnish and the inside of the UOA in black board paint.

The moon light looks great on there too and I bet your well chuffed now its nearly complete

I may have to have a rethink on the focuser as the mirror I'm grinding is going to be F3.7-F3.8 so the addition of a coma corrector may add lots of weight. A feather touch with SIP's system may be the way to go although expensive.

What are your plans for the lower optical assembly? Are you going for a traditional kreige and berry cell?

Damian

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