Ad Astra Observatory

As part of the work party, I put Trixie back on the mount. I got everything set up, and tried some guided exposures, and there’s definitely still flexure in the rig, even after the focuser mod.

I decided to swap out the guide scope and swap in the off-axis guider, to hopefully rule out differential flexure. If you’ll recall from my previous pain with the OAG, you might recall that in order to use the OAG, I have to swap coma correctors, from the MPCC to RCC. The RCC has a deeper backfocus, allowing the extra depth of the new gear. However, the MPCC actually moves the focal point inwards quite a bit, effectively “giving back” some focuser travel. All this adds up to needing to recollimate the mirror further up the tube when the RCC goes in.

After the 2011 rebuild of Trixie, I never really got a chance to get the scope 100% working before everything got packed up for several years. A bit of playing around in the driveway got the scope collimated, but I had to replace the collimation bolts and springs with longer ones, because the pole clamps I built added an inch or so of extra length to the OTA.

Essentially, what this meant was that I didn’t have any more bolt travel to get the RCC working. So I needed to shorten the OTA somewhat, so that I could get both the MPCC and the RCC working, and the easiest way to do that was to shorten the poles. I measured the collimation bolts, and decided that I could take a bit more than an inch of pole off, and still have some room in the mirror cell to get aligned. I was a little concerned that I’d throw the pole angles off (meaning I would need new pole clamps), which would be a Real Bummer, but I forged ahead anyway.

In short, the modification was a success. I got the poles cut to the same length, and the angles were not off by enough for concern. Best of all, when the whole rig went back together, I can now get both the MPCC and the RCC to focus without having to recollimate! Woot.

I got a little clear sky, and started getting the OAG focused and aligned and stuff. I need to learn how to use the OAG a little better, because it makes stars really, really dim, and also kidney shaped. I was floored at how dim they were — this is a 13″ scope! Anyway, I didn’t get it working. The clouds rolled in, and I’m left with another tough decision.

If Trixie is ever going to be a photo telescope, I need the frame to be much more rigid than it currently is. I am not as constrained by weight as I was when I first started building, back in 2008, which is important, because anything I do at this point is likely to make Trixie a lot heavier. However, it does mean that I can replace the 1/2″ plywood rings (which I have always felt were a little too scrawny for this work), without as much fear of overloading the mount.

I looked at carbon fiber tubes, that’s a complete redesign.
I thought about building an Obsession-style upper cage, which would solve a couple of problems, stiffening up the focuser area, and also letting me choose where the focuser points (I find it’s easiest to balance when the focuser points down the CW shaft). I would probably have to rebuild the mirror cell area, too — again, this is a huge redesign.
I could try adding a Sonotube inside the superstructure, but I don’t know how to secure it in a way that would be helpful.
Complete rebuild out of steel or aluminum will both be heavy and expensive.

I really want a fast, big scope. A C-14 just does a different job than what I need, and I think Fastar would irritate me a lot.

For now, I have decided to use Trixie as-is, but only as a visual scope. People will be impressed by the views, I think, and I get a little more time to think about it.