As I described above, close inspection of the objective showed that there was a hazy film between the elements and the only course of action was to remove the lens for cleaning. This is not unusual for telescopes of this age and I have done this several times before with other refractors including my Unitron 150 and 155. This is a fairly simple process if the lens can be easily removed. If not, then there is a very real risk that one wrong move will chip the lens. The thought of damaging the lens of the 510 was very concerning to say the least, but there weren’t a lot of options.
Before starting the process, I checked the collimation using Turk’s centering mask and a laser collimator. As expected, the collimation was spot-on.
The lens cell of the 510 is similar to other adjustable Unitron objectives with the cell attached to the tube with 3 spring-loaded screws The lens elements are held in the cell with a retaining ring that is in turn secured with 3 pairs of push/pull screws. The first step was to mark everything so that they can be put back in exactly the same orientation. Next, the 3 collimating screws were carefully and evenly backed out while holding the cell. Once free, the cell was placed face-up on a clean surface. The retaining ring was then removed by removing the 3 pull screws and then lifting the retaining ring off and setting it aside. While holding the lens assembly between lens tissues the lens stack was placed face-down onto a cylinder padded with lens tissue. The lens cell is then lifted off of the lens stack, which hopefully comes out of the cell, leaving the front and back elements on the padded cylinder.
Good news and bad news…
The good news is that the front element easily slipped out of the cell.
The bad news is that the rear element did not; not surprising, but disappointing. I applied a little pressure to the rear element to see if I could gently nudge it out of the cell, but if you’re not careful you can easily pinch the optic and chip the lens. Since I didn’t need the rear element to be removed from the cell to clean the lenses, I decided not to push my luck and I left the rear element alone.
Close inspection showed that the hazy film was on the front face of the rear element. I cleaned both elements starting with a puffer bulb to blow off any loose dust, then a gentle cleaning with a lens tissue moistened with isopropyl alcohol (IPA) and ultra-pure water, repeat with a lens tissue moistened with just ultra-pure water, and finally the ole’ breath fog and a dry tissue.
Once the lenses passed the flashlight test the procedure was reversed and the lens stack was re-assembled. Since I didn’t touch the push screws the retaining ring should have gone right back to where it was, and no adjustment should be required. The cell was reattached to the tube with the collimation screws slowly and evening turned all Wo the way in, and then backed out ½ turn. The lens was then collimated using only adjustments that tightened the screws.
Whew! Job over. A few nights later I had a chance to check out the results and test using the 510 on my Orion Atlas EQ-G. (More on that later.) The final checkout looked great! Star shapes were perfect with sharp, compact Airy disks and sharp, tight diffraction rings. And… no hint of haze even with bright stars in the field. Woohoo!
Except…
I couldn’t stop thinking about the rear element being stuck in the cell. If it were really stuck bad things could happen. If I was lucky all that I may see is a bit of stress on the objective in the form of pinched optics. Worst case; differential thermal expansion between the lens and cell could result in catastrophic stress on the lens ending with either a chip or (heaven forbid) a crack. Now to be honest the lens has seemed to survive pretty well over the past 50+ years so it was probably okay, but that’s the problem with being an engineer, you worry too much. My biggest fear was that while I was testing to see if the rear element was really stuck or just firmly held I may have inadvertently cocked it just enough to lock it in the cell, which would be a bad thing. I just had to know for sure. So, once more into the breech, it had to come out again.
Having done it once before, this time I paid close attention to each step. Once the front element was out I took close look at both lenses and I was pleased to find that both edges of both lenses had a generous chamfer, making it unlikely that I would chip the edge unless I did something very unwise. Next, a close examination of the rear element shows that it was not stuck, just firmly held. If I gently, carefully, and evening apply a load to the back edge of the lens I could nudge it forward. So, I gently moved it back, made sure that it was evenly seated, cleaned and puffed everything off, put it back together, reset the collimation, and gave it a thorough check out (again on the Atlas) and all was well.
One thing that I should mention is that I could see the erosion of the coating on the back face of the rear element that Turk describes in his extensive refurb thread. Casual inspection of the lens doesn’t show is, but you can see it of you look very closely with a bright light. It doesn’t seem to have any notable effect on the image quality. I also noted a few scratches on both of the inner faces between the two lenses, which indicates that lens has been taken apart before.
Soooo, this little adventure has come to a successful conclusion, and I can move forward knowing that the objective lens is optically and mechanically in great shape!
Next up… some observing notes with the scope now that I am spending less time working on it and more time using it. Also, I managed to sneak in an all-night imaging session with some interesting/fun results.
The Journey Continues…
Enjoy!
A few pics...
The separated front element (left) and the rear element in the cell (right)...
That might be some of the coating erosion visible in the reflection at the top of the rear lens. The damage to the coating can only be seen in reflected light.
A close-up of the rear element showing the generous chamfer around the edge of the lens...
The lens back on the scope after the first cleaning. You can see a few dust flecks that I removed during the second cleaning. I should have taken a before picture, but I didn't...
You can see the 3 collimation screws that hold the cell and the 3 pairs of push/pull screws on the retaining ring.
Edited by jgraham, 23 July 2023 - 07:14 PM.