First Light: Celestron-9.25 OTA on a Sphinx
Posted 04 April 2005 - 02:19 AM
This set up consists of a Celestron 9.25 OTA with the xlt coating on a Vixen Sphinx mount. This is pushing the Sphinx mount a lot - it's rated for a maximum payload of 22 lbs so with the C 9.25 OTA at 20lbs, it seemed borderline, but possible. My viewing location is almost always completely calm so I wasn't worried about wind. What WAS important to me was getting a scope that I could get from my office on to my back deck for observing and a scope I could realistically bring along to astronomy club gatherings. With constant back problems, I can't hoist a big weight around, so my goal was to get the largest scope possible at under 45lbs total weight -- i.e. without having to remove the tube, since hoisting a tube on to a mount is sometimes a struggle in the dark. The Celestron 9.25 normally comes as either 65lbs (tube and Forks) + tripod, or with the Celestron GEM which is considerably heavier. I thought about a Losmandy G8, but the mount and tripod are already at 36lbs before counterweights, so I decided to give the Sphinx a try. Later on, I may build a permanent pier or two for it - especially if I get into astrophotography as this mount is way too overdriven for that.
This combination achieves my weight goal although to get everything down to 45lbs, I do have to remove the two, very small counterweights (OTA = 20lbs, Sphinx mount = 15lbs, Tripod = 11 lbs). The two counterweights are only 1.9kg each and slip on very quickly (the Sphinx design manages to use much smaller counterweights than most GEM's). The tripod is a bit short, so I am going to need a chair, but I really need one anyway with my bad back!
Learning to slew around the sky with a GEM after having used a pair of 25x100mm binos on a p-mount is definitely a sobering experience - it seems that whatever button I pressed, the scope promptly slewed in the opposite direction.
Like anyone with a new scope, I couldn't wait for it to get dark, so as soon as Sirius was visible, I was trying to find it, and trying... and trying... Of course without knowing if the scope was in focus, and with an unaligned finder scope, this was quite a challenge. Collimation was a bit off, but my Bobs Knobs won't be here till Monday, so I decided I would just wait till later for the final tweaking. Contrary to what some have reported, I find that really accurate collimation is surprisingly important on an SCT.
As darkness set in, I was madly reading the Vixen manual to figure out how their controller works. It's unusual in that it has a fair-size LCD screen that displays a map of the sky. As others have noted it's ridiculously bright even at the lowest setting, but fortunately, it now ships with screen overlays to darken it considerably. Its ergonomics leave something to be desired - it's easy to hit the wrong buttons (why didn't they illuminate the buttons?!) and it seems that when slewing and looking through the eyepiece, you end up holding the controller so the bright screen is glaring right at you. It also needs a hook so it can hang from something - almost trod on it a couple of times while getting up to move my stool around. And I'm going to glue a bump on the Select button so I don't accidentally hit it while pressing the 4-way arrows (easy to do). Yes, the Sphinx mount is definitely being pushed a bit with this large OTA, but it seemed to work reasonably well (and I think it will work better if I get the OTA REALLY accurately balanced). Only problem is run-on with the slewing (probably because the weight is pushing the limit). But all things considered, I can live with the run-on to have such a light-weight mount. Settling time after being bumped was less than 2 seconds so the mount is pretty stable.
Ok, enuff on the mount. What about the 9.25? Well, even without perfect collimation, it was immediately apparent why this scope gets good reviews. First stop was at M42. What struck me right off the bat was the excellent contrast - the wisps really stood out dramatically against the Blackness - certainly the best view I've had yet on a scope under 12". I was hoping for E or F in the Trapezium, but no go - although the skies were pretty clear, there was quite a bit of turbulence, and my back deck is 10' off the ground and anything but stable. I thought I caught E one time, but after another five minutes, I decided that was more wishful thinking than a real appearance. I was using a 25mm Celestron Plossl which yields 94x, a good range for initial cruising.
Next was Saturn. I've seen this several times before through smaller scopes so it did elicit an appropriate wow! Cassini division was as clear as anything and faint banding was visible on the planet. And more ring detail was visible than I've ever seen before - since I've never seen anything MORE than the Cassini division, guess I'm now going to have to dig out the textbooks and learn more about Enke division and the crepe ring! Clearly caught Rhea, Tethys, Dione, Titan and even Iapetus right next to it. Tried for Enceladus but could not catch it (very close to Saturn this evening and probably too difficult in Saturn's glare). A 13mm plossl provided a good view at 180x, although it was cold and the eyepiece kept on fogging up. 6mm plossl didn't provide any more detail as it was pretty turbulent. I am toying with the idea of splurging for a 10mm Pentax XW which at 235x would provide both good eye relief and a good power for planetary viewing.
Then on to Jupiter. Slewing was s..l..o..w - seems like it took almost two minutes to get from Saturn to Jupiter. Of course with a GEM you have to stop and think for a moment how to orient the scope. My first 12v battery died about this time so maybe it was running low on juice. Jupiter was even more amazing - have never seen this much detail. Usually I've been happy to see the two main equatorial bands - so again, it's back to the textbooks to learn more. I definitely want to pick up some filters - the brightness makes it difficult to see everything the scope is capable of resolving. And Jupiter was positioned over the roof of my cabin so extra turbulence was thrown into the works as well.
Finally, I decided to test out the alignment procedures and they seemed to work well - after pointing the GEM North and the Tube west at startup, you pick any item on the LCD screen, press SLEW and then manually fix it and press align. Do this 3 or 4 times and each time, the subsequent slew is that more accurate (first slew is pretty rough). Of course it would have worked better if I had noticed that I put in the wrong time . Every now and then when slewing across the sky, the Vixen controller would announce that it was reversing and shift direction unexpectedly causing a frantic pushbutton frenzy to stop it - not sure why it decided to do that, but presumably I'll figure this out. GoTo feature seemed to work fine - after just a couple of alignments, items plopped into the 100x field of view pretty consistently. Next clear night, I'll have a lot more things to view!
At this point, temperature was down to 38 and still falling and since I didn't put on a jacket and my hands were freezing, I decided to call it a night and warm up. Overall, I'm pleased with this combination, even though I spent most of the time learning a new controller. It's certainly not a combination for most people, but weight was a huge issue for me - I know that I would never use a scope that was pushing 80-100lbs - I wanted a scope I could get completely set up in just a couple of minutes. And when I do need to travel to an astronomy gathering, it breaks down into three parts the heaviest of which is 20lbs. At the same time, it also made it clear why I won't part with my 25x100mm Binos. Maybe it's just a matter of getting used to an upside finder-scope and seeing everything upside down (think I'll put a red-dot finder on this as well!), but I don't think I'd like to do a Messier Marathon with an SCT on a GEM locating everything visually. There's still something very compelling about a p-mount and having binos that you can just swing around where you want them. But then again Jupiter wasn't that impressive through the binos!
Posted 04 April 2005 - 05:58 AM
The Sphinx is a mount I may purchase for travel purposes in the future.
Posted 04 April 2005 - 06:25 AM
Posted 04 April 2005 - 10:02 AM
Posted 05 April 2005 - 03:42 PM
Reports like yours really gets the Ninaquawta fever going.