Ad Astra Observatory

Trixie has been sitting in the corner of the observatory, gathering dust, for almost 3 years.

I first built the scope up during the spring and summer of 2008, but the scope had a fatal “sag” problem.
I made two attempts to fix the sag, most recently in 2010. I ended up with a scope that is very “fiddly”. It takes a lot of tweaking to get the collimation right. That’s no fun.

I knew that the scope needed to be completely redesigned, because the original design idea of a 3-pole straight-tube scope is really not going to work for me. But I have already sunk lots of dough into the build, and maker or no, it’s hard to justify spending a lot more to rebuild the scope again when I’m still not convinced it will work.

The scope has two main problems.
1) I need to convert to standard triangle trusses to prevent sag, and
2) The balance point of the scope is slightly too far forward.

To solve the balance problem, I could add a 4th ring, so that I can move the saddle plate a bit. This adds complication on several levels, not the least of which is that I would need to buy more plywood in order to cut them out (I have one “spare” ring lying around, but I can’t reuse the middle ring as it currently exists).

To solve the triangle truss problem, I considered Moonlite ball-and-socket truss connectors. Figuring out all the angles required is difficult, especially if the ring attachment points are not coplanar, and besides, drilling an accurate hole at 10° from vertical is not straightforward. Using the ball-and-sockets would make figuring out all the angles a cinch, but the connectors are $25 each, and I would need a total of 12 of them (if I stuck to 3 rings), or 18! (if I went to 4 rings) That’s an extra $300 to $450 that I really could spend somewhere else…

So I was pretty stuck about what to do. Things stayed like this for a year or so.

I finally had an epiphany a couple of days ago.

The first part of the epiphany is that, if I leave the balance problem for later, and stick to a 3-ring system, I already have the third ring on-hand.
The second part is that because I now have a drill press, drilling accurate angles is no longer a problem. I can even do complex non-coplanar stuff using the tilt of the drill press table plus a wedge. Building the pole seats myself means that I can use wood that’s already on-hand, which means that the $300 expense disappears, and all I have to buy is maybe some new aluminum tubing (there will be a lot more poles now) and a handful of nuts and bolts. Bonus!

So a new plan emerged.

First, remove the non-working middle ring.
Second, carve out the “spare” ring into a new middle ring.
Third, start making pole seats and installing triangle trusses into them.
Fourth, … no, that’s it. That’s the whole plan.

So I started figuring out what the angles needed to be, using the old Telescope worksheet.
From there, I was able to figure out how long the trusses in the triangles needed to be.
That let me figure out how to take the poles I have and turn them into poles for the new configuration (and how many more I needed to buy).

So here are things that I have figured out.

1) Using the current ring spacing, the “upper” poles would have to be a little over 37″ long. That’s unfortunate, because the hardware store sells 6′ lengths… but
2) If I put in some spacer blocks around the DC17.25 mounting plates, I could force the upper poles to 36″, a much more triumphant number. So,
3) Given a length of 36″ for the uppers, I can cut the current set of 3 poles (each ~53″ long) in half and have enough length to make the 6 new lowers (which will need to be about 21″ long). And
4) That means I just need to buy 3 six-foot poles to turn into the 6 new 36″ uppers. I bought these tonight.

I took Trixie apart tonight, breaking down into the UTA, the LTA, and 3 tubes (the old middle ring is scrap).

On the todo list:
– cut the uppers to 36″
– build a pair of pole seats and test them out; confirm that the same angle will work on each of the 2 upper rings.
– once the pole seat design is finalized, cut out 6 of them (4 more).
– lay out the new middle ring; needs pole seats at each corner (18″ apart), and main hole cut out of the center (diameter = F+sin(1°)); baffle groove? (diameter?)
– build out the new upper assembly
– once the upper assembly is complete, measure the final distance between the upper rings
– BEFORE CUTTING THEM, measure the old tubes (this is the full and correct distance between the upper and lower rings)
– determine how much space needs to be between the bottom pair of rings (remember the thickness of the middle ring!). This determines:
– how thick the spacer for the mounting plate needs to be
– how long the lower truss poles need to be
– what angle to cut the lower pole seats
– once the lower measurements are taken, cut out a pair of lower pole seats and test them
– then cut out 6 of them (4 more).
– cut the lower tubes to length (start a little long…)
– build out the lower assembly
– install mounting plates.
– star test.
– cut the lower tubes to correct length
– star test.

It’s a long list, but it should be able to come together pretty quickly once I get the hang of drilling those angles.

Trixie, you deserve to see some starlight. I’m going to try to make you really work this time.