Telescope Reviews: First Light with a Zambuto drop-in replacement

I finally got under the stars last night with a new 200mm Zambuto replacement primary, mounted in my Orion XT8i. To summarize: The weather conditions were only intermittently decent, and I had house guests to entertain, but I did get the chance to do a little viewing. Even under the sketchy seeing conditions, I had moments of atmospheric stability that showed the best views of Saturn, more detail in galaxies, and better double-splitting abilities than I have I ever had with this scope. The most overt difference is the contrast. The background is much darker, with targets popping out with more detail.

There has been a lot of talk in another thread about this option from Zambuto Optical Company (ZOC) lately, so I’ll first describe why I decided to go this route. If you are not interested in that, you can skip the next three or four paragraphs.

Though I don't think this program is for everyone, I am very happy with it. I have had my XT8i for about seven years. I've done several of the tweaks and tricks others have outlined, and the scope has served me pretty well. However, a few years after I got it, the Chinese 80mm ED refractors became hot items and I got a smokin' deal on one. The contrast and sharp views in that little scope eventually relegated the XT8i to the closet. Actually, before it went in the closet, I did most of the tweaks like flocking, motion adjustments, and closer attention to collimation (I became proficient with the Tectron kit). I spent many nights comparing the two scopes. The dob was better able to reveal faint fuzzies, but for planets and double splitting, the little 80mm scope was still cleaner with darker background.

So, not long ago, when I was researching what I could do to bring my 8" up to the performance level that I felt it should have, I joined the Zambuto mirror group. At the time, there was a thread on that group about the viability of a drop-in replacement program. I talked to Carl Z about it, but did not make a decision right away. I strongly considered getting a completely different scope, or buying good optics and building something myself. My observing habits and preferences dictate that something in the 8-10" range would be best for me. As I continued to talk with Carl about the progress of the first mirrors he was making to get the replacement program going, he seemed quite impressed with the way he was able to smooth the BK7 glass. I finally decided to pull the trigger. Comparing all my options, the $695 Zambuto mirror came out on top. It would be the simplest to implement, satisfaction was guaranteed by Carl, and I would have spent more (even in the used market, including shipping) to replace my current scope with a premium optic tool. My drop-in is not a pyrex mirror (although that is an option now), but in the 8" range, that did not concern me too much. If I already had something bigger (particularly in pyrex with a satisfactory anneal), I may have gone for a refigure.

I am not sure if this will be the case as this ZOC offer evolves, but since I was the guinea pig for this, Carl was able to send me some info about the mirrors that I sent in. He said that my primary appeared to be well-corrected in the center, but with an outer zone that was about 1/4-wave overcorrected. He said that the surface was rough and zoney, and that there was a TDE. On the other hand, my secondary mirror was better than the replacement that he had originally suggested. Mine was 1/20th wave P-V with a smooth surface, so he changed his suggestion to keep the secondary I already had.

After coating, ZOC center-marked the primary mirror and shipped it to me. My secondary mirror was also recoated and it finally found its way to my mailbox a few days ago. I went through the simple process of siliconing my secondary back on the spider, and “dropping in” the replacement primary. Because my original mirror was a 203mm (Synta) mirror, and the replacement is 200mm, I had to make a make a minor modification to the way the mirror is held in the cell. I simply replaced the thin felt tape tabs that came in the cell to hold the mirror with thicker, adhesive-backed, 2”-long, felt strips from my hardware store. This felt comes in 1/2”-wide strips and is sold for mounting under furniture to be used on hardwood floors. It worked perfectly; the mirror dropped perfectly into the cell without any slop, and without being pushed or forced to fit. I should also mention that I re-positioned the cork tabs on which the mirror rests. From PLOP analysis, Carl Z suggested that I move the tabs to a position closer to the center of the cell.

Then, last night arrived! I got the scope collimated, and I set it up (along with my 80mm refractor for comparison) in my driveway, where I have viewed from 100’s of times before. There was some upper level turbulence and about 25% cloud cover. Using the refractor as my standard, the seeing conditions were a bit swirly, but with intermittent periods of calm. I let the Dobsonian equilibrate for about 20 minutes, dropped in a 24mm EP, and then lined it up on Regulus. I was immediately struck by the blackness of the background and pinpoint size of the stars. The high contrast actually made the spider spikes more noticeable on this bright star. However, its companion was more easily seen than ever. Both in- and out-of-focus, the rings were mostly swirly due to the atmosphere, but occasionally cleared into perfect concentric circles.

The Leo Trio was easy, and the galaxies displayed some definition, compared to the three (and often only two) blurbs I used to see with the original mirror. I had out-of-town guests visiting so I was unable to spend too much time tracking down more challenging targets. But I got them to view some highlights. They were most interested in the crowd pleaser, Saturn. Most of that viewing was through an 8mm Radian (150x) where the views blew my little refractor (also at 150x) away. The difference was significant. This XT8i had never shown better views on planets than my little refractor had shown. When the sky was calm, banding on the globe and the Cassini division were obvious. I barlowed up the Radian to 300x and got the best view of Saturn I have ever had from my house.

I put in an 18mm Radian and took a stroll through Virgo. I did not take time to ID specific galaxies, but several were evident on the black background. Other targets that we turned the scope to included M51 (where I glimpsed arm structure), M63, M81, and M82, all of which showed the most shape I have ever seen from them. Some clouds began to roll through, and my guests were ready to retire. Later, when I went back out to break down the equipment, the summer triangle was low in the east. Despite the low altitude and the ensuing dew, I got a great split on the double-double at 150x, beautiful color in Alberio, and I topped off the night with a view M57. This morning, I realized that I did not target any globulars. This gives me something to look forward to.

I can’t wait to take the scope to my dark sky location where I imagine the contrast difference should be even more pronounced. For me, the increased contrast has made this upgrade very much worth it. Even though I did not gain any aperture, I am able to detect fainter objects better now that they stand out from the blackness of the background. The sharpness of the views is as though a veil has been lifted from between the skies and my scope.

--Tad

Edited by Tad S. (04/26/11 12:25 AM)