Forward:
While my 130mm scope had been working well, I somehow managed to catch aperture fever. I found this scope on craigslist, did my due diligence, and determined that it was a great deal if all the parts worked. I had conversations with my wife about it, and in the end, on a rainy night, we purchased this beast and the rest is detailed below. While I believe everything to have worked out well for the price paid, some might think that a bit overly positive. I’ll let you decide.
The Scope:
What I purchased was a Konus Konusky 200-motor. It’s a very basic 200mm f/5 reflector. The finder is an 8/50 non-corrected scope. The mount is a knock-off EQ5 with Dec & RA motors for tracking, but not a GoTo model. It also came with non-WFOV EPs (32mm, 26mm, 15mm, 9mm, 6mm, 4mm), 8 different filters, and a barlow. When the declination motor failed to work, the fella knocked $50 off and we picked the whole rig up for $450.
I thought it was a steal… Still do…
The issues:
Right away we had the issue of the declination motor not working. I tore it down, brought the motor assembly to work and started trying to find a replacement. Mid-way through the process I realized that a soldier joint was broke, and that only the shrink-wrap was holding the wire to the motor terminal. This made my heart sing, and indeed by fixing the soldier joint the declination motor started working strong again.
Once I reassembled the scope, I took it out to run it through its paces, and it indeed tracked well. Unfortunately, even though the OTA appeared to be the older version with the Orion Dovetail finder assembly, I think I happened into one of those rare spherical mirror model ones…
Basically, everything I looked at had a glowing halo surrounding it. Moving the focuser inward just a tad would remove the halo, but then the main object became unfocused and blurry. I tried ALL my EP’s, barlow, no barlow, and side beside compares to my 130mm f/5 reflector, which did NOT exhibit the same issue. What confused me the most, is that lower powers resulted in nice solid outlines between black space, and bright object, but out of focus. And it was terribly difficult to get any real detail out of the bright light. Increasing the power brought out the details better, but also brought with it that halo…
Confused I did all the normal stuff, by the book, as slowly and patiently as necessary. Cleaning the primary and secondary, center doting the primary, re-collimating the scope very, very precise. But in the end I still received the same visual aberration. After reading up, posting to CloudyNights.com, and racking my brain, I decided that the mirror must be a spherical mirror and not parabolic. I ran this conclusion through it’s paces and ultimately it seems to be the only cause that sticks.
In the end, I made a cardboard ring, coated it with felt, and put it right up aginst the mirror, obscuring the outer 3/8” more or less. As well as helping to correct the halo issue, it also will help eliminate any TDE / TUE issues. And FWIW it seems to have helped. Comparing equal powered visuals of Jupiter in both my 130mm (160x) and my konus (167x) I received almost the same picture. The Konus had a lot more “wavy” focus (Atmo-Distortion), but essentially they were the same, and not much, if any noticeable halo. Stars still never came into pinpoint focus, but they didn’t glow either…
The solution:
Like mentioned above, I’ve tempered the Konus by putting in a limiting ring to help with out of focus shallow reflections, and also TDE / TUE. But ultimately we have solved the problem by buying a new OTA. It should arrive on Thursday and skies willing, I’ll finally be able to put this issue aside once and for all.
My fear is that it won’t show any better visual than the konus, and that what I was seeing was what you normally see through a 200mm+ aperture scope in normal spring seeing… However, I really don’t believe that, and won’t until I see it with my own eyes. I’ll report back with my findings.
-Tobin