22 June 2020

Though I haven’t been posting here much, things are happening on the Midlana Builders’ Forum. There’s is growing excitement as several builders are reaching major milestones, including getting their cars on its wheels, first engine start, and the beginning of final assembly. So if you’d like, wander over and take a look at the build logs to share in the fun!

16 June 2020

I’m still here, no virus, though I did have a nasty 24-hr flu that shares many symptoms. Just been working from home, and with the 1.5 hours saved everyday from commuting, the time’s been spent getting the 16″ f/4.3 telescope done, and it nearly is.

The design is commonly called a “hexapod”, and consists of six tubes assembled into three truss assemblies 120­° apart. There are several reasons to go this way over the more traditional 4-truss 8-tube design:

1. The telescope is collimated (aligned) solely by adjusting tube length. The tube assemblies are just like the toe control links in Midlana, with each using a left and right-hand rod end to set its length. What’s interesting is, once fully assembled, turning just the upper ends of the truss ends adjusts the upper assembly tilt, and turning just the lower ends adjust its location relative to the main mirror.

2. #1 means that the main mirror cell and secondary holder do not have to be adjustable, which lowers weight, increase stiffness, reduces cost, and make the assemblies simpler—sound familiar?

3. To some extent, focusing range can be adjusted by altering the upper-to-lower assembly distance.

The last point has consumed the most time. Every primary mirror has a unique focal length, this one is 68.7″. This means that from the main mirror, to the secondary diagonal mirror, and out to the focuser has to be 68.7″ inches, sort of. The catch is that the value gets modified if a “coma corrector” lens is present. Anyway, if it ends up being wrong (outside the range of the focuser’s travel limits), the strut tube lengths can be adjusted if it’s slight, or different length threaded Delrin tube inserts may become necessary. I started with one set and found that focus was off by about 2″, so a second set was made that was 2″ longer. What’s left me scratching my head is that it didn’t correct the situation by the expected 2″. The catch is that I may or may not have had the coma corrector in place, and probably didn’t use the same eyepiece. That’s another issue, that different types and brands of eyepieces all have different focal points. This means that before the scope can really be considered finished, all the eyepieces have to be checked to confirm whether they all come to focus… and one eyepiece (one said to have a strong focus offset) is currently back ordered.

In the meantime, servo drivers for tracking start targets are being installed. Some people consider such a thing a waste, but I find it annoying having a planet or star under high magnification drift fast from the field of view, and then the scope is pushed to follow it, only to then lose it. Anyway, that system will take a bit to learn.

Even with composite construction for the upper assembly, carbon tubes, and low-profile lower assembly, the overall assembly is around 65 lbs, enough that I don’t want to carry it, so wheels and handles are being added as well. After it’s fully operational and checked out, it all has to come apart for paint. While part of me would like a furniture-grade varnish-like finish, nope, paint it will be. I don’t want to deal with the time, mess, and smell that beautiful wood finishes entail, especially all the sanding. Been there done that with Kimini, no more sanding!

The little voice in my head did ask though, “A scope, that’s nice, so how does someone who has to be in bed by 9:30-10pm in order to get up at 5am for work planning to spend hours out under the stars?” Good question, but this project and other going-ons in the garage are about planning ahead about how I’ll spend my time in retirement, someday. Anyway, through dumb luck, it turns out that Mars is going to be the closest in its orbit to Earth this Fall, so I’m really looking forward to that.

Stay safe!