Cleaning an elderly refractor objective:

[chiltonstar]

I know you shouldn't, but "needs must".

Back in 1982, I made a 4" (102mm), f13 refractor from a Vixen achromatic doublet I acquired, which I mounted in a machined reinforced plastic cell, in the scope tube itself which was reinforced PVC and aluminium. With time (35 years of frequent use!) mould had appeared between the crown and flint lenses, as well as an oily deposit, plus a slight problem from one of the three 2mm square spacers used in the doublet. The effect was to lower image contrast a lot, and surround bright stars with a faint cloud resembling dewing. There was also slight chromatic aberration visible around eg Vega which I suspected was due to the spacer which was damaged. So, despite misgivings, a week of rainy weather prompted me to do something (there is the 2018 Mars season approaching as well of course, for which the f13 frac is ideal because of the low altitude of Mars and the poor seeing likely for some of the time) - with nights of good seeing, the 180 Mak will excel! As I thought there was little to lose, and I have had quite a lot of optics experience (day job) in the past, I thought a rescue attempt was worth a try.

I took out the lens cell and marked the two elements edges with a pencil so I could align them again, and have the pair pointing in the right direction. I removed the low-vapour plastic tape I used originally to seal the pair of lenses and the three Al spacers by soaking the doublet in acetone for 48 hours, and then ultrasonicating each element in very dilute lab detergent in deionised water, and finally drying in pure ethanol. The cleaned lenses still had a slight bloom, so I polished all four surfaces with a clean lens cloth, and then re-assembled the doublet, using thin Al tape cut to 2mm squares at 120 degrees at the edges to hold the elements apart, sealed the edge with polyester tape and put the lens cell back in to the scope. 

As luck would have it, I had to wait more than a week for a night with some stars visible, even then there was 50% cloud cover and the speed of the clouds hampered viewing and made id'ing anything difficult. First view was Polaris (x150) for a star test; surprisingly good result, with near perfect diffraction rings in and out of focus and a very clean background with no haze visible, and Polaris B as a nice, bright point of light. Epsilon Lyrae showed two textbook pairs, with Airy disks about the right diameter, and with some of the stars between the two pairs visible. Finally, Castor had just appeared above the horizon which was always going to be difficult because of the amount of atmosphere the line of sight was going to travel through, but the pair was nicely resolved, albeit with some atmospheric dispersion effects.

So, success? Apparently, although there is a bit less coating visible than when the achromat was new, although this didn't seem to affect performance, and for some reason, the slight chromatic aberration I had always had with the scope seemd to have disappeared or diminished.

Scope (before the Spring clean) in the attached image.

Chris

 

 

[Stu]

Nice job Chris, and a lovely scope. It sits well on the mount too.

Bring on Mars