Review: Celestron 9.25-inch SCT Part I

Ross Gould

Earlier this year I took delivery of the current German-mount version ('Advanced Series') of Celestron's 9.25-inch Schmidt Cassegrain telescope (SCT). This particular optical assembly has had a reputation from its inception for better than average optics for an SCT. It has a noticeably longer tube than usual, a result of the primary mirror being F/2.5 instead of the usual F/2, which results in a lower amplification secondary (4x instead of 5x) to produce the usual f/10 final focal ratio. This easing of the optical parameters may be part of the reason for the often-described better image quality.

I first looked through this design in California in 1995/6, and was impressed at the time by the high quality images, side by side with a C8, Meade 10", and some Orion Newtonians also on show. This newer version lives up to that first experience.

PickUp

The scope and mounting came in several large cardboard boxes, with sufficient padding to preserve the contents from harm. I was able to fit them into my old Ford Telstar, and once the packaging was removed the scope became obviously portable and quite compact. Part of my reason for choosing this design was a need to have good portability, as I don't expect to have a permanent building for the scope in the near future. The scope consists of three parts – optical tube, tripod, and counterweights. Each of these is approximately 10kg in weight, so I don't need to be of Sidonio strength to move it around. There is a carry handle attached to the back plate of the scope, nicely placed for moving the OTA whether with one hand or two.

The version I have has an aluminium tube, not carbon fibre; the tube is a dark olive-green with matte finish black end sections. The equatorial head and the ends of the tripod legs are in a variant dark green finish, with satin chrome on the mid-sections of the legs and the declination shaft.

Mounting

Early versions of this scope, when Celestron was owned by Tasco, had a flimsy inadequate shaky tripod. Now Celestron is owned by the Celestron managers, and this part of the engineering has been greatly improved. The current tripod has a sturdy set of 2-inch chromed steel legs, and the shake of former times has gone.

The equatorial head is a version of the ubiquitous EQ5 from China, though this one seems better made than some I've seen. The short SCT tube means the mounting is adequate for the telescope weight (indeed, I see Celestron are now offering the 11-inch OTA on this mount). I have had no problems with vibration or shakiness in visual observing. The tripod legs can be extended, so it's possible to use the scope seated or standing.

The German (Fraunhofer) mount does not have the vibration of some fork mounts in the past, and makes access to near-polar objects (such as the Magellanic Clouds) easier than with an equatorial fork. All mountings have certain awkward points on the sky, but personally I find the German style probably the best compromise in an equatorial mount.

Eyepieces

Only one eyepiece comes with the 'scope, a 25mm Plossl labelled 'E-Lux', certainly an improvement on the 'SMA' (Kellner) type eyepiece provided in the past. It is however merely 'fully coated', not 'fully multi-coated', and there is a noticeable difference when comparing it with the Plossls that come in Celestron's eyepiece and filter set.

Compared with the multi-coated eyepieces, the sky background is less dark, and image contrast is a bit lower. There is a sharp field edge. The apparent field (field-angle) is slightly less than that of the 32mm Celestron Plossl. Definition is good.

I would rate it as an adequate economy eyepiece, and an improvement on the Kellners of the past.

Eyepieces Special

With the scope I paid $200 extra to get a set of five Celestron Plossl eyepieces, and a basic Barlow, together with seven filters - colour filters for planetary observing, and a Moon filter - in an aluminium case. At the price this is a bargain, providing eyepieces of 32, 15, 9, 6 and 4mm focal length. The eyepieces sit vertically in the foam-lined case, a better arrangement than the horizontal placement of some other brands.

The 32mm eyepiece is a very nice item, providing roughly the maximum field of view possible with a 1.25-inch eyepiece barrel (about 40 arcminutes at 74x on the C9.25). The sky background is dark, contrast is very good for a low-power eyepiece, and the field edge is well-defined. It has good eye-relief, and a roll-down rubber eyeguard – this last is a feature of all the eyepieces. Touring through the LMC on a moonless night with this eyepiece was a joy.

The other eyepieces vary. The 15mm is sharp, of good contrast, and with a dark background, but lacks the definite field edge of the 32mm. It does however give very good image quality. The 9mm has good contrast and definition, a sharp field edge, but restricted eye-relief. The 6mm and 4mm eyepieces have inadequate eye-relief, and provide rather high powers with this focal length (2350mm) – nearly 400x and nearly 600x respectively. I'm inclined to regard them as of little use, given and the sheer difficulty of looking through an eyepiece where the eye has to be pressed so close to the eyepiece. One also needs very good seeing to use these high magnifications.

Overall, my feeling is that the choice of eyepiece focal lengths is more appropriate to a shorter focal length telescope – but that the shortest two eyepieces are of little point even with that. For SCTs in the 8- to 11-inch range, replacing the two shortest eyepieces with others of greater focal length would be a useful step forward in creating a good basic set.

I've recently ordered some extra eyepieces – Televue Radians – to supplement the Plossls I have, and I'll do a review of the Radians in the new year.

Optics

I ordered the version of the scope with the new XLT multi-coatings on the optics. My feeling was that a little more light might be useful, but I'd expect the main benefit to be in improved image contrast.

My impression of the optical quality of the telescope is very positive. It arrived slightly out of collimation, and a few small corrections put that right. Schmidt-Cassegrains are VERY sensitive to mis-collimation, and only a small error takes the edge off the optical performance.

I haven't done much planetary observing with the scope, but it's impressive on lunar detail, and a pleasure on deep-sky objects. Focus is definite – either in or out – without the mushy image uncertainty I've seen on some scopes of not so good optical quality. Stars are crisp sharp little pinpoints of light across the field, the image contrast is high, and there's no sense of the image being "not-quite-focussed" that I've sometimes seen elsewhere. High power gives an Airy disc with a diffraction ring, when seeing allows. I've not seen evidence of spherical aberration, which plagues some examples of the SCT design.

47 Tuc breaks up into a multitude of tiny stars, particularly impressive at medium power (156x). The Trapezium in the Orion Nebula shows stars E and F without difficulty at the same power, and mottled structure in the nebulosity. The Tarantula Nebula is a complex and detailed pattern, more extensive than I expected (without any filter) – and the nearby bright star cluster was well resolved.

Overall, I'd rate the optics as very good – reserving "excellent" for the best apochromatic refractors. However it's as good as the best SCTs I've seen up to now, and in a similar class optically to the 7-inch Zeiss Maksutov I reviewed ten years ago for Southern Sky magazine.

Continued in July 2005