I have couple of 60mm Unitron 114 refractors on an Alt/az mounts. I really like this model and consider it one of the best 60mm refractors made. The mechanics of the mount are wonderful and the scope just has that "precision" feel when you use it. I use mine fairly often for white light solar viewing, observing Venus in the daytime and just for quick look around since it is outside in minutes and back in the house in minutes. One of the ones I own unfortunately has an objective with a fair amount of astigmatism that would require the lens to be refigured. I have plans of one day refiguring but in the mean time these Unitron 60mm objectives come up for sale on fairly regular bases and one showed up a couple of days ago being sold by a regular member of our Classic group.
So I purchased it with the hopes that it might have a better figure. It was advertised that one of the screws that hold the retainer ring in place was missing and I can could see the red paint on the two other screws had been broken so someone had been into it in the past. Not a good sign but who knows what you'll find until you get up on the test bench.
So the lens showed up yesterday in the mail. It was well packed and it good shape. So down into the basement to check it out using double pass autocollimation and see what the figure was like. It was BAD, the Ronchi lines were all distorted ! So not to panic since one of the retainer screws was missing so the air gap between the elements was nowhere near uniform. So I removed the lens from the cell and used my monochrome light source to view the interference rings between the element. The good news was they were visible, the bad news they were arcs and not rings that were centered. So the arcs meant that one of the three spacers was different in height vs the other two. The spacers are metal foil and which some type of backing. The foil looks like it was corroded in places. So I pressed on the spacer opposite the curve in the arc of the interference pattern and the arc moved inward and turned into rings. Good, that told me that, that spacer was too thick. I carefully shaver it down in thickness by scraping it with a razor blade and then checking the results.
Now I had nice round rings that were centered. So I reinstalled the lens back in the cell but I only had two screws to hold the retainer in place. I carefully tighten the two screws and was able to keep the interference rings round and centered but it didn't take much to throw them off. I re-centered them and carefully took the lens back down to the basement and back up on the test bench. Good news, nice straight Ronchi bands, the lens is well figured !
So now out to the garage to see if I could find a replacement for the missing screw that hold the retainer ring. I would need all three to keep even pressure on the lens so the air gap was uniform . The screw looks to be metric but didn't match a kit I have. Luckily I found one that matched in my collection random screws. Now I was able to install the retainer ring correctly. Before I did that I placed three taps of Mylar tape on the retainer at 120° centers so the tape would be the points of contacted and clearly defined. The lens was placed back in the cell so the spacers were next to each bolt and the retainer placed so the taps of tape where over the spacers. Now while observing the interference rings under monochrome light I adjusted the pressure of each screw so the rings were round and centered. Once that was done back down to basement to test the lens again. Straight Ronchi bands. The result is now a correctly adjusted lens that will give me excellent images and it will replace the bad one in my other Unitron 114.
Here are some pictures showing the lens fully assembled in the cell. Note that the air gap spacers are next to the bolts that hold the retainer ring. An image of the interference rings, the rings are perfectly round when view directly from above but show as slightly oval in the picture because of the camera angle. An image of the straight Ronchi bands when the lens was correctly adjusted and tested via double pass autocollimation.
Happy 2021 !