Pirich's Full Review: Celestron NexStar 4 SE (240 x 102mm) Telescope
Compared to their diameter, Maksutov Cassegrain Telescopes (MCTs) offer a long focal length in a short package. Today, some of the nicest telescopes made by anyone, Questars, are Maksutovs. More recently, tubes made in China are being sold as compact all purpose telescopes. The NexStar 4SE takes this scope and mounts it on a small computer driven mount related to one used with the NexStar 5i, and also used on the Nexstar 6SE. This variant is a smaller and lighter, and somewhat more compact, mount carrying a 4" diameter Maksutov. More general information on telescopes can be found in my article on Picking a Telescope.
Background
The Maksutov design came from Russia during World War II as a practical means to make a very capable long focal length telescope cheaply without sacrificing quality. Maksutov telescopes are a hybrid design which most obviously has a deeply dished in corrector plate in the front with a secondary mirror in the center. Visible in the back is a large concave mirror with a hole in the center and a black tube protruding from it. The way the telescope works is light goes through the glass corrector and is purposely distorted so the reflection from a spherical primary mirror in the back will produce a net bending to the light path as through the primary mirror had a much more complex (and difficult to figure) shape. The secondary mirror in the middle of the glass corrector plate puts the image conveniently at the back of the telescope. For the Russians, this was a breakthrough because all of the surfaces in it are spherical, and mass manufacturing of spherical optical surfaces is easier than any other shape. This design is extremely compact; the secondary acts as a focal length magnifier, so it isn't uncommon for Maksutovs less than 300mm (or 12") long to have a focal length of 1324mm (52") or more, as in the case of the NexStar 4SE. In comparison, an equivalent refractor would need to have an optical tube close to 60" long.
This is a quick listing of design characteristics for different types to help consider the NexStar 4SE against:
Refractor: -Best performance compared to diameter. -Cost of a this 4" GOTO MCT equates to a 102mm achromatic refractor on a GOTO mount. -Limited focus range due to draw tube. -Larger movement of eyepiece relative to ground. -Long optical tube (focal length + 20%). -Has False Color. -High contrast images if false color well corrected.
Newtonian: -Lowest cost compared to diameter. 4" GOTO SCT translates to an 8" newtonian on a dobsonian mount. These telescopes offer a massive amount of capability for the dollar. Limited capability exists at this price to get a computer guidance system as well. -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. -All available models today are larger than the NexStar 4SE as well as more expensive. So, what is going on here is the NexStar 4SE has an edge in its portability, ability to bring just about anything to focus, computer guidance, and moderate cost. However, I find it somewhat dubious to suggest this instrument as a general purpose telescope.
The NexStar 4SE is a moderately sized telescope. The 4" variant of a Maksutov works reasonably well on planets, since a 1324mm focal length means getting a lot of wide field objects like the Pleiades or Double Cluster into a field can only be done with the lowest power eyepieces, such as a 40mm plossl. But this still only yields a field of view 1.36 degrees across with a magnification of 33X, combined with an apparent light amplification of 6.4 times what the unaided eye would see. While this can capture a large number of astronomical objects, I am afraid it would seem less useful when brought down to see objects on the ground. Larger fields of view especially like what one is used to being able to see with binoculars, just aren't in this telescope's repertoire.
Description and Usage
The NexStar 4SE 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 NexStar 4SE's Maksutov telescope is an optical tube unique to this installation- Celestron doesn't sell it on another mount. It is made in a configuration similar to what Questar pioneered in 1950, with the back of the optical tube made to be a mirror diagonal with a built in flip mirror so a camera could be attached at the back, but an eyepiece could be installed in the top of the optical tube and you could switch between the two by toggling a knob on the side. The telescope focuses by moving the primary mirror, so the eyepiece or an installed camera do not move when focus is adjusted.
With that said, the slow focal ratio of this telescope means it is only really usable for photographing bright objects, such as the moon or planets. This may be just as well, since the included mount is not capable of the slow super high accuracy tracking needed to get images of dim nebulas and galaxies. The basic configuration on the polished stainless tripod has the telescope operating in what is called "Alt-Az" mode, where it moves with its axes aligned with the ground instead of with the earth's axis.
This means it has to move motors in both directions as it tracks objects in the sky, while objects seen this way also rotate as they cross the sky, which again would distrupt an image. The telescope attaches with a dovetail clamp, so the optical tube can quickly be removed, or shifted forward to balance if a camera is attached to the back of the telescope. But because Celestron has used an industry-standard Vixen dovetail bar, this also means you can release the 4" Maksutov and completely remove the optical tube from the fork for convenient transport, or to mount another scope, such as the potent Onyx 80ED Refractor or the AT-66ED So, you could take a couple scopes along to the countryside and share one mount between them. I have been doing this with an older C5 mount, and it works very well for small refractors. Note, the NexStar 4SE mount has a fairing for the outer diameter of the optical tube, so installing larger diameters would be difficult.
The NexStar 4SE I used was using an auto adapter to take power from a car. The three star alignment this mount uses is simply by turning on the mount 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 alignment 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 4SE mount slews quickly and with authority to viewing targets. The main problem is it does make a bit of motor noise, especially compared to the bigger NexStar mounts. From previous experience with the NexStar series, 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 since like all GOTO mounts, the NexStar 4SE is happy to gobble up a set of new alkalines in a single evening.
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 5" 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.
My first serious small and portable telescope was a 90mm Maksutov, which though it gets less use these days, still shows there is nothing wrong with it every time it does get outside. Like the little 90mm scope, the NexStar 4SE has true color in its images. However, its larger diameter and longer focal length change the image scale. The NexStar 4SE is able to split all four parts of Epsilon Lyrae, four stars in tight formation where they initially look like car headlights, and only over 100X does it start to become apparent there are more than two stars, but rather each of the two original spots is in fact, two stars.
On strength of the 4" Maksutov is it does a really good job on everyday observing targets like the moon and planets. While these generally do not need a computer to find, the tracking system makes it quick to go between them. 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. In fact, this eyepiece puts up about the best image this optical tube is capable of.
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 Larger objects like the Pleiades just barely can fit into one field at low power, and give a "Diamonds on Velvet" effect since the stars clearly have a blue tint from the high temperatures of newly formed stars.
In the mechanics of the mirror system, this scope has no obvious mirror shift when moving from forward to reverse when focusing. 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. The mount tends to shake some while focusing, and this can make getting an accurate focus a bit slow.
One nice feature of the NexStar 4SE is the ability to mount other optical tubes, 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.
And that is an entry into what I feel is a necessary subject- the NexStar 4SE is a bit of a dead end as a telescope. As far as its quality as a telescope goes, it is definitely in the OK, but not great or disastrous category. If you only do small amounts of observing, it may feel like enough scope. If you go to a star party and look through a bunch of others, it will start to feel like it has limitations as telescopes close to it in size are able to outperform it. Note all of the other options will be more expensive, and the NexStar SE's more lasting contribution may be in providing a compact computer guided mount for its successor.
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 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. At this point, the more serious fault becomes apparent- this mount is not driven by a worm gear, which means the drive error can't be directly accounted for by Periodic Error Correction. Instead, the drive runs slightly fast or slightly slow, though it synchs up to the spot where true speed correction for earth's rotation would put it every 10 minutes. In short, The NexStar 4SE's mount just isn't going to do long exposure astrophotography.
Conclusion
The NexStar 4SE easily fills a role for a beginning general purpose telescope. The NexStar control system on it is mature at this point and organic to the telescope. Few instruments package this much optical capability in such a compact package . The NexStar 4SE 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. Ideally, I would see this as a telescope for taking along for camping, apartment dwellers, or for older children.
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