Over my observing life I have had multiple C8s, C6s, and 4" and 5" Maks. Never liked how long they took to acclimate and with the SCTs, the coma and field curvature (FC) was a significant annoyance. But, I keep coming back to the design because it is after all quite compact. In my latest attempt I decided to try a conventional non-Edge C9.25. I've often read how many feel that it is more than any other SCT size closer to the Edge series in performance. So not willing to drop an extra $1000 for an Edge OTA, not to mention the cost for a pricey Edge reducer (when they make one for the 9.25), I decided to yet again try to like an SCT and borrowed a C9.25 from an observing friend.
On first use, I moved the C9.25 from a warm 70 degree house to a 35 degree outside yard. After 2 hours the scope actually reasonably cooled, not the experience I ever had with a C8. It was not perfectly acclimated, but what thermal activity was left was slight enough to not be a very big deal as the star points were reasonably tight and behaved. So instead of a full woolly star as I like to say, maybe they were just slightly fluffy. In any event, definitely better that I had expected.
Using a variety of XWs in the scope, the FC of the scope is easy to see. Coma not so much though. As example with the 40XW the stars at the field stop are quite bloated, but can be refocused to a nice pinpoint. So mostly all FC. Using the Celestron .63x reducer helped a little, but not significantly IMO. The FC, which IMO is severe in a C8, is at the edge of being tolerable for me in the C9.25. I also used my 40mm Titan-II ED. Similar to the XW, it showed a lot of FC in the C9.25 (note that these eyepieces show little in my 81mm through 152mm Apos, and none in the XT10 Dob with Paracorr). So this made me wonder about all the posts I've read where people say the Edge 9.25 is not really significantly better off-axis than the Edge 9.25. Either people have a lot of tolerance for FC or the extra $1000 for the Edge model is not getting one much. Which I am not sure as I have not observed critically through the Edge 9.25.
While I had the C9.25, I decided to give it a compare to my 152 Apo. In previous side-by-side of my 152 Apo against a C8 they seemed more on-par than different. The C8 did go a very slightly bit deeper on Globs but not significantly so to my eye. Going back and forth at equalized magnifications between the 152 Apo and the C9.25, brightness of the stars were readily apparent as being much brighter in the C9.25 compared to the 152 Apo. So things like the Double Cluster looked much brighter in the C9.25. When examining the Double Cluster more deeply, it was interesting as I could actually see a few of the faintest stars in the cluster just a little easier in the 152 Apo. I think this is due to the finer star point the refractor was mustering. The more fluffy star points of the SCT meant that the faintest dimmest one, which typically need a perfect focus setting to see well, were difficult in the C9.25 but were readily apparent in the Apo. Just talking 2 or 3 really dim ones so not a lot of stars. It was interesting though, as the combination of the slightly more fluffy stars and having them all appear obviously brighter in the C9.25, made the Double Cluster much more pronounced and dimensional looking in the C9.25 than in the 152 Apo. So regardless of the more precise and tight star point in the 152 Apo, the Double Cluster still looked more aesthetically pleasing in the significantly brighter and larger aperture C9.25.
As I pumped the magnification up to 200x in both scopes, it was interesting to see some of the brighter stars in the Double Cluster show a classic airy disk pattern in the 152 Apo. The C9.25 however showed none of this at 200x. Now the star points in the C9.25 were not bad as they were fairly tight, but they still had that fluffy attribute because there were still some very slight thermals going on in the OTA that I could see. I do have a CatCooler to maybe bring the thermals fully under control in the SCT, but I have yet to conduct that experiment.
Staying at 200x I then moved to the Gamma Andromedae double. Colors were nice in both scopes, but in this case the more precise and tight star points in the 152 Apo made doubles much more pleasing. Gamma Andromedae was not bad in the C9.25 actually, but in comparison to the Apo, it could not hold its own on this class of target.
Now my main purpose for borrowing this C9.25 was a desire to move from a 10" Dob to a more compact and user friendly platform. If I could achieve this, then what I feel is important is if I can keep a relatively good star point off-axis and get something more than around 1 degree TFOV. The DOb with Paracorr easily does 2 degrees TFOV and stars are pinpoint with Paracorr in the off-axis. So far with the C9.25, the off-axis loses a lot in comparison to my eye as the FC is quite significant. Again, not as much as the C8, but still at an annoying level unfortunately.
Next up I did some experimenting with the Celestron .63x reducer on the C9.25 to see if I could stretch that TFOV some. I used the reducer with the Baader BBHS 2" diagonal. This combo with the 40XW unfortunately vignetted the FOV more than I liked. The views were just not nice. So I moved to the 30XW and was much more pleased. There was some very slight darkening near the field stop, but the field stop stayed sharply defined and stars did not wink out when they approached the field stop so I thought quite acceptable. The additional TFOV was also quite nice, making observing the Double Cluster very aesthetic with lots of room to spare, and even most of M45 fit in the FOV or at least enough of it to make it an interesting and engaging observation! So I was very happy. The off-axis FC also improved a bit with the reducer in place. The off-axis still showed obvious FC with the reducer, just not as much as before, so more tolerable. Still though, off-axis stars in M45 or the Double Cluster were obviously bloated from the FC so not entirely happy.
So after all this evaluating it was looking like as much as I liked the SCT form factor, I was just not liking the off-axis performance enough to replace my 10" Dob. I was a little bummed too as the thermal behavior of the C9.25 seemed a lot better than my experiences with the many C8's I've had. Every evening I took the C9.25 out into the cold weather, after 90 minutes or so it was plenty stable for some good viewing. And what little thermal activity remained, seemed to destabilize star points much less dramatically than the C8s I had. So I was very happy with the C9.25's thermal behavior. It was just a shame that the off-axis had such noticeable FC to my eye in everything from the 40XW to the 10XW.
Before I returned the C9.25 to my friend, I decided to try one last experiment. I've never read of anyone trying a Paracorr on their SCT, but I decided to give it a shot. So with the C9.25 configured with a short 2" visual back, the 2" Baader BBHS diagonal, and a 40mm Titan-II ED, I slipped my adjustable top Paracorr I into the diagonal, then the 40mm eyepiece on that. To my delight I could actually reach focus! And guess what....star points were perfect all across the FOV. Wow...this was a nice view and what I was looking for! Now I did notice with this arrangement that there was some slight dimming near the field stop. This did not occur without the Paracorr. But it was slight and not a bother. Turing to The Double Cluster and M45 it was grand to see this conventional C9.25 performing all across the FOV supposedly like an Edge series would. Best of all, since I have these components already for my Dob, also means no need to spend the extra $1000 for the Edge too as I was getting pinpoint stars all across the FOV. Moving to the 30XW there was now no dimming near the field stop and the view was simply killer!
Finally, I was wondering could a reducer work with this multitude of added glass components to the train. From previous experiments I knew I could only come to focus with the reducer and BBHS diagonal if I removed the nose of the diagonal and screwed the diagonal directly onto the reducer. So I did this, placed the Paracorr in the diagonal, then the 40mm Titan-II ED eyepiece in the Paracorr. Unfortunately, my hopes were dashed as it was about a half a turn of the focuser knob from being able to focus. I tried the 30XW and still no joy in this experiment. Too bad as getting the little extra TFOV is really nice with the reducer and the 30XW.
Before I decided to break everything down and go back into the warmth (it was 35 degrees this last night of experimenting), I remembered that the Baader Zeiss 2" Prism diagonal has a shorter light path than any mirror or dielectric 2" diagonal than I tested or used before. So I figured I would give it a shot and removed the nose of the Zeiss prism so I could screw the diagonal directly onto the reducer, then the Paracorr in the diagonal and 40XW into the Paracorr. Note that in doing this, the diagonal tightens not straight up but at a 45 degree angle from straight so not so convenient for viewing. However, all you need to do is to put a small shim on the reducer's threads so the diagonal then tightens in the straight up position (I use an alt-az mount so no need to readjust position - would be more of a pain with a GEM I imagine).
Once the Zeiss prism diagonal was in place and the 40XW with Paracorr in it, I was delighted that to see that there was plenty of focus travel left in the C9.25 to reach focus. And better yet, once again all the star points were perfect all across the FOV. Wow was this a pretty view! The vignetting was still there using the 40XW, so I switched back to the 30XW and all was fine in the FOV with almost no dimming and of course enough TFOV to fit the majority of M45 in the FOV. And yes, perfect star points all across the FOV. I was a very happy camper on seeing this as it meant that yes, I could get a nice and portable setup compared to my XT10 and could manage maybe close to a 1.3 deg TFOV with reducer and Paracorr with perfect star points [Edit: Measured -- 40XW w/no reducer gets me 1.1deg TFOV; 30XW w/reducer gets me 1.5 deg TFOV). So looks like I can achieve a more portable setup with very close to the same aperture punch as my XT10 but in a nicely smaller package. And better yet, no need to shell out another grand for an Edge 9.25...not that I would have anyway.
Edited by BillP, 10 December 2016 - 10:27 PM.