Potent Scope on A Lightweight Computerized Mount [Update 07/07/2009]

Compared to their diameter, the Schmidt Cassegrain Telescope (SCT) is the most compact design yet derived. Celestron, the company who pioneered their commercial manufacture forty years ago, made a breakthrough in 1970 with the C8, which was and remains as close as anyone has come to an all purpose telescope. The NexStar 8SE takes this scope and mounts it on a computer driven mount derived from the one used with the NexStar 5i, and also used on the Nexstar 6SE. This variant is the most compact mount carrying such a large telescope and offers a lot for the money. More general information on telescopes can be found in my article on Picking a Telescope.

Background

The SCT is a multi-purpose telescope with a secondary mirror in the center of a glass corrector plate which puts the image conveniently at the back of the telescope. This design is extremely compact; no other design will pack as much aperture or focal length into less space. Tom Johnson started Celestron Pacific in the 1960s, and the product he loosed on the world was a series of SCTs sized so amateur astronomers could own unusually flexible telescopes in an era when you could either get a very long focal length refractor or a moderate focal length Newtonian, and that was it.

In 1970, Celestron introduced an 8" diameter SCT they had developed after a decade of making various models in small numbers, and the combination of mount and aperture took off, and it was 10 years before anything challenged it, and when it did, it was in the form of refractors so expensive they were destined to be high-end boutique specialties for the next twenty years.

Things are changing again, and a lot of other telescopes like Maksutovs or refined refractors get a lot of press. We are in the closest thing there has ever been to a golden age of telescopes. Superb scopes of every sort are now available at lower prices than ever. After twenty-five years as high end luxuries, Apochromatic (APO or ED) refractors have come down in price enough for the three major types; Refractors, Newtonians, and SCTs to be a buying decision based on weighing technical merit directly against cost.

This is a quick listing of design characteristics for different types to help consider the NexStar 8SE against:

Refractor:
-Best performance compared to diameter.
-Cost of a this 8" GOTO SCT equates to a 100mm ED refractor on a GOTO mount.
-Limited focus range due to draw tube.
-Large movement of eyepiece relative to ground.
-Long optical tube (focal length + 20%)
-Works well for photography
-High contrast images if false color well corrected.

Newtonian:
-Lowest cost compared to diameter. 8" GOTO SCT translates to a 12" newtonian on a dobsonian mount, or a 10" newtonian on a GOTO equatorial mount. These telescopes become so large with additional aperture, there is a bit of a dead space in being able to buy more optics above 12" diameter as the optical tubes become much larger and much more expensive than the NexStar 8SE.
-Limited focal range due to focuser on side of tube.
-Unusual image orientation.
-Physically bulky
-Continuing realignment (recollimation) maintenance required.

SCT:
-Very compact mount and scope is easily portable.
-Largest possible focus range.
-Less eyepiece travel as scope moves.
-Scope can use the Celestron Focal Reducer to move between f/10 and f/6.3.

So, what is special about the Schmidt Cassegrain is its portability, ability to bring just about anything to focus, ease to convert to radically different focal ratios, and moderate cost. In other words, lowering the cost of the high quality refractor doesn't make it an ideal general purpose telescope.

The NexStar 8SE is the most common size for Schmidt Cassegrains. The 8" variant from Celestron dates from 1970, and the smaller 5" first appeared in 1971. No in-between size has come about until 2006, when the 6" NexStar 6SE was introduced. The 8" and the 5" have a very large difference in diameter and area, and are radically different in size. The 8" scope is the size of a 2 gallon paint pail, while the 5" is the size of a coffee can. The difference in light gathering area is over 2.5:1, so the 8" is a much bigger telescope than the C5.

Looking at the three different sizes of SCT currently offered by Celestron on single arm fork mounts yields a wide range of capabilities. If the 5" NexStar is compared to the 6" NesStar, it is a price difference of $200 to get 44% more light gathering area. Between the 6" and the 8", the difference is $400 to get 78% more area. Strangely, the 6" comes on both the single arm fork mount offered as the NexStar SE, or a computer guided German equatorial at the same price. The 8" scope is $100 more on this same German Equatorial than it is on this fork mount.

Description and Usage

Even more so than other 8" SCTs, NexStar 8SE can't hide in the corner; it's metallic orange. Celestron made orange telescopes back in the 1970s when orange was kind of cool, and a lot of these old orange tube telescopes are still in use. Now, orange is cool again, and the scopes are glittering orange, which looks like an automotive paint. The rest of the assembly is black, and the tripod is in polished stainless steel. All in all, it looks sharp enough for the living-room.

The 8" optical tube is actually 9" in diameter, and makes the overall assembly larger than the other NexStar SE series telescopes. The fork mount is a development from the one used for the NexStar 5, 5i, 8, 8i, is also on the NexStar 6SE. In a welcome nod to practicality, this scope is mounted to its fork arm with a Vixen dovetail. This means you can loosen this scope and scoot it forward to balance it with a heavy Digital SLR mounted on the back (I have been using a Maxxum 7D this way).

The dovetail also means you can release the dovetail and completely remove the optical tube from the fork for convenient transport, or mount another scope, such as the Onyx 80ED Refractor or the AT-66ED So, you could take a couple scopes along to the countryside and share one mount. I have been doing this with the older C5 mount, and it works very well for small refractors.

What sets this telescope apart from the smaller NexStar 6SE is this optical tube is already the largest telescope this mount can practically carry, so any second telescope to put in its place will be there to get a lower focal length than the 2032mm C8, or its 1280mm length with the focal reducer. So, given the C8's limit of a field of view just over one degree across, obvious second optical tubes would be something with a focal length quite a bit shorter. It would appear one of the reasons the C5 series exists is their native focal length is 1250mm, which is very close to the C8. In practice, I find the C8 causes a feeling of wanting to be able to step back and see the big picture. With larger versions of the C8 such as the NexStar 8 GPS and newer CPC 800 are able to accept a second telescope mounted directly to them. However, this simply isn't a realistic option for the smaller single arm fork used on the NexStar 8SE. Another way of looking at what is going on with this telescope is it has been put on the smallest mount capable of carrying it to get mobility. The larger twin arm forks lose much of their mobility, but in exchange are able to piggyback more than one telescope to overcome the limitations of the C8 when it comes to getting low power wide fields of view.

The NexStar 8SE I used was using an auto adapter From my previous experience with the other large NexStars, the set of AA batteries was only good for one night of observing, so planning on some other source of power is necessary for any extended outing. The three star alignment this mount uses is simply by turning on the mount (there is a now a red ON light) and then putting in the location (it has a list of cities, so this is easy), then steering to any three bright stars in the sky. The computer then thinks about what they could be, and then produces an alignment solution.

This scope took the alignment on the first try, and the pointing accuracy after aligning to three stars in different parts of the sky was quite impressive. This low-care alignement was as good as the more careful alignments I have done with the NexStar 8 GPS. Higher alignment accuracy did result when using a reticle eyepiece.

The NexStar 8SE mount slews quickly and with authority to viewing targets. From previous experience with the NexStar 5, these mounts get strange on you if they start running low on power. They won't put up a low battery indicator, but instead some of their functions won't work so well- usually it has the form of getting a bit lost. I recommend getting the AC adapter to save on battery costs.

The tripod has a small tray made up of the spanner for the tripod legs, which presents one of the larger problems- where does the front lens cap go? Given it is 9" across, this is actually a large part to find a home for. I suggest getting round velcro stick-on pads and putting one on the center of the outside of the lens cap, and one on one of the tripod legs to hang the cap sideways so dew doesn't form on it and dust does not get in it. Make sure you can site the front lens cap where it is wedged against the tripod brace so it doesn't bounce in any breeze and cause mount vibration.

One of the reasons the C8 has stood the test of time so well is it does a really good job on everyday observing targets like the moon and planets. These are crowd pleasers, and showing a good view of the moon at half a degree across is effortless for this telescope since it means using a moderate 25mm eyepiece, which produces a very bright and extremely sharp image. On Jupiter, the stripes and cloud details are easily in range of this telescope. On the Orion Nebula, the scope I got to use showed four pinpoint stars in the trapezium (a good 8" scope shows 6), but I am not sure it was in true collimation. Larger objects like the Plieades don't quite fit into one field at low power, even with the focal reducer, though these blue-white stars look like diamonds on velvet. This is, in particular, one of the objects which prompts me to wish for a wider field when using a C8.

In the mechanics of the mirror system, this scope is like every other Celestron 8' SCT I have used with some mirror shift when moving from forward to reverse when focusing. So, to get true focus, you purposely drive across focus a few times to figure out where it is, then drive back to it from one direction and STOP.

It has often been said the C8 is one of those telescopes where anyone who is sincerely interested in astronomy really should own one at some point. The reason is it just does a huge amount in one package. That combined with the ability to swap in other optical tubes makes the NexStar 8SE a really neat package, especially since you can save money by getting a small wide-field refractor as a separate optical tube without needing to get a mount. You will need to get a dovetail bar from Orion (www.telescope.com- search on Dovetail), more detail is in my review of the Orion Dovetail.

Very frequently, new observers find themselves in a phase called "Aperture fever" where each telescope suggests a larger one will outperform it- and they do. The cruel side effect is the observing list becomes thinned out with the larger optical tubes necessarily having narrower fields of view and becoming more sensitive to atmospheric conditions while they quickly become more difficult to move and set up. The C8 really is a lot of telescope, but its size in the NexStar 8SE configuration avoids most of the problems a scope this size presents. It also means going any bigger than this comes with a huge growth in the size and weight of the equipment.

Single arm mounts sometimes get comments like "Is it stable?" When nudged, this mount damps out the shakes within 2 seconds, even with large taps. The NexStar mount is a single piece aluminum casting, and it is very stout as a result. The original variants are surviving use and proving to be reliable over time. The biggest limitations will be apparent if you try to migrate into astrophotography. First, the mount will need to go onto an equatorial wedge to keep the scene from turning during exposures.

Conclusion

The NexStar 8SE easily fills a role for a general purpose telescope. The NexStar control system on it is mature at this point and organic to the telescope. There simply isn't another package of mount and telescope offering this much optical capability in such a compact package at any price. The NexStar 8SE goes outside all in one lift, so it meets the requirements for a quick observing session. If you want to do astrophotography, this mount will only track well enough for short exposure planetary photography. All in all, the NexStar 8SE is a very satisfactory telescope system vor. Currently Celestron has permanently reduced the cost on this telescope, which makes the full-up cost of one of these systems $1199, currently the best price for an 8" SCT with any mount.