Got a tube for the objective thrown together today and had first light this evening.
The tube is an aluminum tube from a scrapped 60/700mm Chinese achromat. I just wrapped the 50/540 cell with cardboard and a little tape and push-fitted it into the tube. Works like a charm and the back focus is adjustable! The objective sits deep into the tube, a full 150mm (6") from the front. Dew is hardly going to be a problem. Collimation is spot on. The focuser is a plain aluminum R&P with Vixen M36.4 threads. I use my Vixen M36.4 diagonal on it and 0.965" or 1.25" adapters.
The scope is mounted on an EQ-3-2 mount on a lightweight aluminum tripod. Notice how little counterweight is needed. The scope, as shown, with diagonal and O-16 eyepiece weighs just 1180 grams! It is incredibly lightweight and the mount just laughs at it. The mount even has a motor drive, an unimaginable luxury for the customers the 50/540 objective was originally designed for! I wasn't using the drive during my testing, though, as the batteries had run out and I didn't have time to replace them.
First light was the Moon in early twilight. I was using a 25mm Zeiss huygenian eyepiece to properly mimick what someone starting out with one of these from a Zeiss "Bastelsatz" would have seen during first light.
To my shock and horror, the Moon wasn't sharp in the 25mm or the 16mm huygenian. There were clear signs of pronounced astigmatism! After suffering a mild panick attack, I calmed down and took the scope back inside and pushed the lens cell out again with a broomstick. I inspected the cell and found that the retaining ring was quite firmly tightened. I loosened it with a screwdriver enough to give a faint rattle and pushed it back into the scope again. Now the Moon had slipped behind a tree, so I aimed at Altair instead. I couldn't see a thing. Puzzled, I aimed at the Moon again, because despite it being mostly behind the tree, I could at least be sure I was aimed at it. It was badly out of focus and I realized that in my hurry, I hadn't pushed the objective as far in as the first time. After adjusting the back focus, the scope now snapped in focus. Relief!
Aiming at Arcturus, I conducted a quick star test. Everything looked fine now! The star test was very good, with a sharp airy disk at 90x (6mm Zeiss ortho) and basically identical patterns on each side of focus.
Saturn was faint in the bright twilight and quite fuzzy from haze. Still, the ring was easily seen at all powers. I have no doubt more details can be seen under better conditions.
I picked up the scope and moved it a few meters so that I could observe the Moon. The seeing was very soft so low in the sky, but it was clear that the lens showed a very clear image with almost no false color. I removed the eyepiece and looked for signs of stray light. I was a bit startled to see none whatsoever. Upon closer inspection, it appeared that the baffle in the tube actually stopped the objective down a little. I would need to measure it when getting back inside, but for now I decided not to worry about it.
Slowly, darkness fell, though it was still twilight I changed to 1.25" eyepieces and started to observe deep-sky. In a sky so bright, I could *just* make out mag 3.9 Eta Cygni with the naked eye, the scope had no trouble showing M57 as a tiny, fuzzy glow at 22.5x with a 24mm ES68. A 14mm ES82 (39x) showed it effortlessly. In an 11mm ES82 (49x) I could begin to glimpse the darker center. M13 was also shown without any problem whatsoever at 22.5x.
Soon, the Milky Way began to slowly glow, high overhead. In a darker sky, the views the 24mm ES68 offered were nothing short of stunning. The stars are absolutely pinpoint across the entire field, from edge to edge. I checked carefully, and stars are just as pinpoint at the field stop, as they are in the middle, although I'm sure that the accomodation of my eyes have some part to play in this.
A bit worryingly, I began to detect astigmatism at high magnification again. Not much at best focus, but clearly there. I wonder if the cell is *just* barely large enough and pinches the optics slightly when it cools down? It could also be that the cemented lens has developed some stress, when the cement has aged. This is not unheard of. We'll see in the coming weeks, when I can hopefully observe with the scope under different conditions.
A layer of clouds were now moving in from the west and had already covered most of the western sky up to the zenith. I changed to objects in the eastern sky.
M31 and both companions showed up rather nicely, though the companions were faint, which is understandable, given the conditions.
The Double Cluster was stunningly sharp at 22.5x and should be a glorious sight on a really dark night.
NGC 7789 was faintly visible as a large, dim glow at 22.5x.
Gamma Andromedae was clearly resolved at 39x.
Overall, what I'd consider a very good first light.
Back inside, I measured the aperture with the laser method and was mildly shocked to find that the baffle stopped the lens down to 45mm! Once more, I pushed the cell out with the broom handle and then used the handle to push the baffle further back in the tube, towards the focuser. After a bit of trial and error, I found a good position, where it didn't stop the objective down and there was only mild vignetting along the edge of the field of a 1.25" eyepiece and yet only a very small portion of the inside of the tube could be seen from the focuser end.
Now I'm eagerly awaiting second light!
A couple of thoughts:
A 2" focuser probably isn't needed. A magnification lower than ~20x is basically as well or better covered with normal 50mm binoculars. The strength of the 50/540 is that it can use higher magnifications than the binocular and here you don't need 2" eyepieces. The views in the 24mm ES68 were quite amazing and felt very wide, actually.
A small RACI finderscope is DEFINITELY needed!!
The scope is almost TOO lightweight! Even something as modest as the 24mm ES68 throws it out of balance in both DEC and RA. A little more mass, properly invested in a more solid tube, focuser and finderscope, wouldn't hurt.