new base

After testing the previous mount, I came to a couple of conclusions. 1) The split trunion design that served as the foundation of my "ultra-compact scope" was going to require too much maintenance to keep it from feeling very rickety. The hinges were a little loose to start with and having any sort of gap in the altitude bearing surface is bound to cause the scope to jump around as you're aiming it. That could easily lead to it being essentially unusable in the field. 2) As the date of arrival of the (expensive, fragile, etc... ) primary mirror arrived, I began to reconsider the wisdom of not having it in a very secure box at all times. These conclusions made me think that the ultra-compact base was really not practical, so I basically remade the whole thing in the "classical dobsonian" style. Here's a picture:



You can see the same mirror cell and truss mounts within two boxes. The inner box is called the "mirror" box, the outer one is called the "base". The inner box moves relative to the outer box. Instead of the split trunions I was using earlier, I decided to just fork over the cash and get a pair of cast aluminum C-arms from Obsession Telescopes, and use them instead. They will rest on bearings attached to the base. I will probably just use the track rollers I have and see if I can avoid teflon pads altogether.

The aluminum C-arms are used in the classical style Obsession Scopes made by Dave Kriege, one of the authors of the guide book I'm using. These parts would be very hard to make by one's self, bending aluminum rectangular stock was the only way I could think of to come up with something similar, but that seemed like quite a headache and prone to irregularity. So I just forked over the cash, and I'm glad that I did! I haven't mounted the arms yet. To do so, I still need to drill holes through the 1.25" of solid aluminum, and am not looking forward to that process (especially since my drill is a wireless 18 volt Hitachi that I have a feeling will need to be recharged multiple times).

The mirror box is a very simple 2' x 2' x 1' box made of imported birch 13-plywood. Most importantly, it passes the "trunk test", which is that it fits laying flat into the trunk of a '97 Nissan Altima! The base is 30" x 18" x 12" and just serves to support the trunions. I'll also use the same lazy-susan style azimuth mount that I had earlier. I'll make some variety of lid for the whole thing too, but this is what it will look like when in use. I'm feeling better about this new base already!

primary mirror fabrication underway

Well, it's been a while since I've last posted, but in the interrim, fabrication of the key part of the scope has been undertaken! I commissioned Portland's Steve Swayze to make the primary mirror and he's well underway. His mirrors have an excellent reputation in online forums and people I've talked to. His prices are very competitive relative to other mirror makers. I won't say exactly what I paid but the index number of the check that I wrote him for a 50% deposit was the same as the amount of the check. Steve has been great about keeping me in the loop about progress on the mirror. It will be 16" diameter, f/4.5, 1.6" thick, and made from pyrex that was slabbed from a previous thicker mirror. Here are some pictures of the work in progress, copyright Steve.



These are the two slabbed pieces of pyrex. I don't know the provenance of the original glass, but it looks like it must have been from a 2.5-3" thick mirror or so. Slabbing it to 1.6" reduces the weight (it will be 26 lbs total), lets it cool quicker, and is generally not felt to cause the mirror to bend to any noticeable degree given the right flotation system supporting it.



This is the glass on the grinder. The grinding tool itself seems deceptively simple to me, it's just made of store-bought square tiles arranged in a circular pattern underneath the plywood disc. The gray material is "pitch", made of various different grades of abrasives dissolved in water. Using several grinding patterns and finer and finer pitch material, the glass is first ground to a spherical shape, then refined to a parabolic shape. There are a lot more details of this process that I haven't learned, so it's great that there are mirror makers like Steve who have!



The rough surface of the mirror status post some degree of grinding. At this point the shape still needs to be confirmed to be parabolic, and of course it needs to be coated with a reflective material.

Okay, I'll post more on the primary mirror as it progresses!