Telescope project

[Stuart BridgerParticipant @stuartbridger82290]

[Stuart BridgerParticipant @stuartbridger82290]

I have been helping out a friend over the last few months on his telescope project. My main contribution was to create a set of brackets for the tube ends of the main structure

This afternoon we had a major step forward, as he came round with the rough cut tubes which were milled to length. We then did a trial assembly and it is looking pretty impressive.

One very happy Astronomer, whose design calculations all worked out

[Nathan SharpeParticipant @nathansharpe19746]

Sorry Stuart, not enough info for something that impressive ! Could your "happy astronomer" inform us please? Nathan.

[Stuart BridgerParticipant @stuartbridger82290]

I'm not too much up on the technicalities other than it has a 1500mm focal length and the mirror is 14 inch diameter Pyrex (mixing units sorry). The structural tubes are 1.5 inch in diameter. Black centre box was welded up from aluminium box section by another friend the the village. Other components were custom made by a specialist in Germany. We estimate that the assembly weights about 25kg in total. For more information on the astronomy aspects, he has a website at **LINK** Not too much information on this project yet, but lots of pictures from his current telescope.

[john carruthersParticipant @johncarruthers46255]

Nice Serrurier truss. F4 ish, could lose contrast with a large secondary mirror?
What sort of mount?

[Stuart BridgerParticipant @stuartbridger82290]

Posted by john carruthers on 12/06/2017 08:01:06:

Nice Serrurier truss. F4 ish, could lose contrast with a large secondary mirror?
What sort of mount?

As I said, I'm just the engineering dude helping out on the practical stuff, My mate has a degree in Astrophysics from Cambridge and he quickly gets me lost Not sure on the mount, I know he was using a Losmandy Titan, but I believe he may have upgraded this year.

[Neil WyattModerator @neilwyatt]

And moonlight focuser by the look of it?

Nice.

Neil

[Graeme Coates 1Participant @graemecoates1]

One happy astronomer here (Wondered where the spike in web traffic to my site came from, but all obvious now!

Stuart has done a great job in helping out in machining the brackets to hold everything together for me, and it does look the business when all assembled – much better than anything I could have done with a hacksaw and drill press!

So some further technical stuff for you all follows:

The whole aim is to rebuild to create a much more stable scope than the original 1mm Al tube that it started life in (it was an Orion SPX350 originally) – any weight hanging off the focuser threw the collimation out – critical for fast scopes.

Primary Mirror is a 350mm f4.6 from Orion Optics (actually f4.52 -= 1582mm fl). That's sitting in a 9 point cell from Orion as well. Made of Pyrex, it's about 25mm thick, and has a peak-valley error of about 1/12lambda, RMS ~1/30lambda (lambda being 500nm) – so it's a very good mirror.

The secondary is a 89mm minor axis – and quite right in that there might be a bit of a loss of contrast with size of the secondary, but the scope is designed for imaging and so the calcs are designed to give me a fully illuminated field for anything up to a KAF8300 size sensor which has a 22.5mm diagonal.

It's put together in a serrurier arrangement, though I didn't go down the full structural analysis route of working out the deformations – rather aiming for equal moments around the central brace (which is 3x50mm Al box section welded together, with 5mm equal angle corners for the supports – all expertly welded by another talented local resident . The tube is 1.5", 16swg, so the bending moment should be within tolerance.

Good spot on the Moonlite CR2 focuser! The secondary spider and mount (the bit that is hidden inside the top section) is custom made by a German chap who makes the same items for TS Optics – it is one of the best spiders I have seen and is built with secondary offset already in place. The focuser is mounted on 4mm Al plate screwed into a cut out section in the top rings. There is a large sheet of sheet plastic lining the inside of the secondary cage and this is lined with black flock to reduce light scatter – I will need to flock everything else as well once it's confirmed to be working.

The things remaining to do:

* Re-mill the tubes – design calcs were fairly good, but not spot on (it's really hard given the positioning of the primary in the mount and the secondary's thickness). The first test put the focal plane at 32mm above the focuser's lowest position – this needs to be about 60-65mm to fit a camera in, so we'll have to have to reduce the lengths a little – but a normal way of getting an exact result. (At least we didn't cut too short first time round….).

* Do the dovetail – as Stuart said, this is to mount on a Losmandy Titan, which uses a 4" wide, 60 degree dovetail mount (a D-series dovetail – fairly standard for astronomy). That'll attach to the centre brace section.

* Find a seamster/seamstress to make shrouds top and bottom to stop extraneous light entering from the side…

* Flock everything!

* Electronics/heaters/fans/focuser etc need to be mounted, wired in and so on once it's up and running.

Think that's about it – I should really write this up for my own site…!

Cheers

Graeme

[Graeme Coates 1Participant @graemecoates1]

Oh – forgot to mention the wooden rings – they are routed out of 19mm Birch Ply and are 390mm inside diameter and about 50mm wide. I varnished them with dark exterior varnish to protect against moisture. The two at the top are joined with 4 bits of the same 1.5" 16swg Al tube. All the tubes used have tube inserts from Jetpress (sold as tube connector nuts) which take an M10 bolt – these are used for connecting both the top and truss tubes.

Cheers

[Neil WyattModerator @neilwyatt]

Thanks for the extra details Graeme. I'm sure the results will justify the effort!

I've just invested in a Baader 7nm Ha filter, but with my modded DSLR all the really faint stuff is in a soup of noise – the downside of imaging in t-shirt temperatures. Now looking at cool finger designs!

Neil

[Graeme Coates 1Participant @graemecoates1]

Let's hope so!

HA imaging is always challenging – you need enough signal to drag it out of the read/thermal noise which usually means really long exposures… I go to 15min on my ccd…!

Re: cold fingers – a real challenge, especially avoiding frost/condensation on the sensor!

[Graeme Coates 1Participant @graemecoates1]

Thought I’d finally get round to writing a construction thread for the telescope here (which is pretty much up and running!).

Hope it’s ok to link to the thread on an astro forum – not sure I can face putting it back into another post!

https://stargazerslounge.com/topic/314094-14-double-truss-newtonian-build-thread

First light was very pleasing – just need more clear skies (and darkness!!)

Cheers

Graeme

[Stuart BridgerParticipant @stuartbridger82290]

My nice brackets hidden from view now

Nice write-up though. A lot of stuff that means nothing to me…
Here is the link in clickable form
**LINK**

Stuart

[Neil WyattModerator @neilwyatt]

Looks good Graeme (AKA Stub in the other place )

Neil

[john carruthersParticipant @johncarruthers46255]

Nice mount

I've been helping AWR in a small way with their PEM…

**LINK**

[Michael GilliganParticipant @michaelgilligan61133]

Posted by john carruthers on 13/05/2018 08:35:20:

I've been helping AWR in a small way with their PEM…

**LINK**

.

Looks interesting, John

I must confess l had no idea what a PEM was, until I followed your link.

Portable English Mount sounds like an oxymoron, but there it is !!

MichaelG.

[Author]

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