Pushing Performance and Mobility to the Extremes
Written: Oct 23 '04 (Updated Aug 13 '05)
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Product Rating:
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Pros: Compact profile, high performance, packed with features, lots of accessories
Cons: High single component weight, narrow field of view, not suitable for novices
The Bottom Line: This scope is big, automated, and capable. The narrow field of view, lack of flexibility, and large heft mean it's not for beginners and isn't a general purpose telescope.
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| Pirich's Full Review: Celestron NexStar 11 GPS (660 x 279mm) Telescope |
The NexStar 11 GPS is the descendant of a line of telescopes dating from 1980 when the first Celestron C11 appeared. It is near the upper end of what can be moved by a single person, and is built for performance.
I have often thought 11" diameter, or 280 mm Schmidt Cassegrain Telescope (SCT), was a peculiar size for a telescope tube since it is significantly larger than the relatively easy to move C8, but is quite a bit smaller than the really big scopes in the 14" and bigger range where mobility is very difficult. In this case, the NexStar 11 GPS now bears the distinction of being the largest fork mounted telescope Celestron makes. And there is one very good reason for this: Anything bigger would be impossible for one person to move.
Background
The Schmidt Cassegrain optical design is intended as a hybrid to produce a flat field image in a compact optical tube. These telescopes were derived from a design Estonian optician Bernard Schmidt developed to make large photographic telescopes with flat fields of view in the era before hyperbolic mirrors could be reliably made. Hyperbolics are not cheap, so giant telescopes from the Keck to the Hubble space telescope are Ritchey-Cretiens with complex hyperbolic optics. The Ritchey-Cretien has only become available in the last few years to amateurs, and is approximately 5 times the price of an equivalent SCT (though its optical performance is better).
In the 1960s, the people behind Celestron realized the Schmidt design previously used for observatory survey cameras was very practical for making an extremely compact telescope compared to its focal length. The spherical main mirror makes producing a high quality telescope easier since figuring and optical testing can be very accurate for this shape. The trick is to correct the light path with a specially shaped corrector plate which overcomes both the spherical aberration of the main mirror and eliminates the strong coma effect Newtonian telescopes have where stars to look like little comets at the edge of the field of view. This hybrid telescope was intended to be an all-purpose instrument where it would be a mid point between a long focal length (f/15 or higher) and short focal length (f/5 or below) design.
This results in an f/10 telescope which has a flat field of view across its available image. This optical design is the most physically compact available of any design. A Schmidt Cassegrain Telescope, or SCT, typically has proportions similar to a mid-sized coffee can at any diameter. A Newtonian reflector or a refractor will be as long as or longer than their focal lengths. As a result, a moderate focal length Newtonian, a 6" diameter f/8, is around 4 feet long.
An 11" diameter SCT is about 30" long with eyepiece and diagonal attached, and far lighter than equivalent Newtonian reflectors or the Refractors, which would both be around 9' long.
The NexStar 11 GPS
This variant is the first version of the C11 to perform as a Go-To telescope, where the user only needs to select the object they wish to look at and the telescope automatically slews there. Although this technology had appeared in the C8 line as the Ultima 2000, this telescope was the first one this large offered by Celestron to have this type of automatic pointing.
The NexStar 11 GPS incorporates every innovation Celestron has developed for the telescope itself along with every development they have introduced for mounting one. The mount has the high end version of the slow motion tracking hardware, Pittman motors, and GPS location and local time finding features.
Optical Tube Assembly (the telescope itself)
(1) A carbon fiber optical tube. This saves weight, but more importantly it reduces the coefficient of thermal expansion between the front and rear of the telescope, so it stays in focus as the temperature drops at night.
(2) Fastar- The secondary mirror unscrews from the telescope and a special lens assembly goes in its place to turn the telescope into an 8" f/2.1 (yes, a 588 mm focal length) CCD imager.
(3) A large 9X50 finder- with a telescope this large, many objects visible to the telescope will be impossible to see with a smaller finder scope.
(4) Fit and finish is beautiful. The tube is a matt finish with clear resin so the carbon fiber is visible- very pretty.
Mount:
(1) Twin arms with the scope mounted between. This is a very modern mount and all of the electronics are concealed inside the base and the arms with only the detachable hand controller
(2) Twin axis variable rate fine control electric drive system. Note, the earliest Nexstar 11 GPS will make a buzzing sound when fine slewing, while ones from late 2002 and on are virtually silent in all modes of operation.
(3) NexStar control (see my other reviews for more- all of them are similar in operation). When set up, it can slew to any of 40,000 preprogrammed objects, or items you store in its memory, or accept control from a computer.
(4) The scope comes with a rugged adjustable tripod. The same tripod is used for the 8" and 9.25" Nexstar GPS telescopes. The mount comes with vibration suppression pads to put under the feet to absorb vibration when the scope is on a hard surface, but I have found there is no vibration if you set the scope down on a grass lawn.
Usage and Observations
I used this telescope for three days at a star party set up adjacent to my NexStar 8 GPS, so it was a good chance to compare the two models and their performance. The 42 pounds of the NexStar 8 means you need to have a plan before moving it; the NexStar 11 GPS's weight of 65 pounds for the fork and optical tube assembly makes methodical and careful movement a vital necessity. If you have a bad back, the NexStar GPS telescopes are off the menu unless you don't have to lift them.
The NexStar 11 GPS and NexStar 8 GPS are surprisingly similar in size, with the much larger telescope looking from a distance much like the NexStar 8 GPS. The mounts have the same drive base and similar shapes, with longer fork arms for the NexStar 11 GPS. The extra length is needed to allow the telescope to swing past the hub when looking straight up.
The NexStar 11 GPS has the same type of integrated handles as the other GPS series scopes for picking it up and moving it safely, despite its 65 lb. weight. The arm away from the hand controller has a handle parallel to the arm, and if you lift here, the tube and fork assembly will hang sideways balanced on this handle; it is difficult to lift at this weight. There is a "scoop" handle under the hand controller slot which lets you lift the fork assembly (FA) with the base down to go on the tripod. The weight is well above OSHA's 50 lb. single person carry limit, but the good handles make moving it possible. Three bolts hold the mount base to the tripod. A peg in the middle of the tripod top makes alignment easy. Here is what you do to take the scope outside:
First, unbolt the fork-tube and put the 3 bolts in your pocket or in a helper's hand.
Lift off the FA and set it on a table where no one can bump it.
Pick up the tripod and take it out to the observing area and set it where you will observe from (look up to make sure you are clear of obstructions since when this thing is loaded with the FA, it is going nowhere).
Go back and get the FA, take it out, and set it down on the tripod. There is a step in the center of the base, so getting on the peg may take a couple tries. Keep the tube against your body so you are balanced while doing this. The bolts go in from the bottom, but you need to have the base turned the right way for the holes to align. There are protruding foot-like features on the base which go midway between the tripod legs. Feel for the hole underneath and have one bolt ready. When you feel it click into the threads, only thread the bolt in far enough to hold it in place. If you tighten it down, the others will not go in. Loosely thread the second bolt, and thread the third and then tighten all of them finger tight. After the tube is on the tripod, you will need to tighten the tripod spider knob since the weight of the FA pushes the legs out.
Lock the clutches on the base and by the drive arm with the optical tube pointed straight down. Run the power cord, plug it the scope, and turn it on. The controller comes up with GPS align as the first option. The first run is longer since it is finding satellites and also has no true North declination for the internal compass. If you go a long distance, you will have the same situation. The compass needs to have a declination from true North, and this is set in the Utility menu after you have the scope precision aligned for better alignment next time.
That said, alignment for this scope appears to be a little easier than for the NexStar 8GPS. The reason is the 40% higher magnification means more accuracy when you center a star in the eyepiece than with the NexStar 8 GPS. So, you turn it on, tell it to GPS align, and wait for it to slew to the first star (may seem a little slow at 3°/second), center it in the finder, follow up by centering it in the telescope, do the same for the second star it slews to, and you are ready to observe.
The tour has a large selection of objects to look at- stars, nebulas, clusters, and galaxies. Planets and the moon only come up on the Planets menu, not the tour (since these are always the most impressive objects to see, this is a problem I have with NexStar's suitability for beginners who are less likely to know if a bright spot in the sky is a star or actually a planet). Different objects are listed different ways, and you can get to them either by tour, list of types, or a catalog ID (like NGC 1421 or M13).
The best part of the C11 optical tube is the view it puts forward is very sharp, bright, and commanding. This telescope came out of transport out of collimation, and more startling, the locking ring for removing the secondary mirror was loose, and the owner reported already having one very lucky catch when it had come out before. I assisted with locking the mirror in and collimating the telescope. Without a set of Bob's knobs, a Phillips head screwdriver is needed to collimate the secondary mirror. This was straightforward, and accomplished very quickly.
The NexStar 11, due to its longer focal length, has definite limitations. While all of the Double Cluster between Cassiopeia and Perseus pops out in beautiful detail with the visual back and the f/6.3 focal reducer in the 8" scope, the C11 forces you to choose which half of the Double Cluster you want to look at even WITH the focal reducer. Or, for photography, the NexStar 8 can photograph the whole scene, while the NexStar 11 will have to make a photo mosaic or use Fastar to get the image.
Like all SCTs, this telescope comes with the ability to attach just about anything to the back end. I tried out using a Celestron binocular viewer (#93690 ) with this telescope, which splits the image up for two eyepieces so you can look with both eyes. The results were a very deep 3-D effect. I wrote a review on this accessory separately. All I can say is it makes a huge improvement in the observing experience with better visible detail and eliminates eye strain.
One interesting note is the mirror shift did not appear to be much different from the NexStar 8 GPS, when older C11 series telescopes I have used all had more mirror shift than an 8" version. This makes focusing easier, and you can always attach other focusers if you can't accept the mirror shift.
Tips and Improvements
(1) Have a plan before you try to move it. Think through exactly what you are going to do and what order you are going to do it in. Otherwise, you can find yourself going back and and forth to open doors and such.
(2) Make sure the clutches are tight before you start alignment. The position encoders are on the motors, so if it doesn't move with the motors, it loses orientation. It can still move some with the clutches off, so check. Keep this in mind because you need to loosen the clutches to move it without risking damaging the drive.
(3) Wait for the controller to finish a move before punching in new commands. It is possible to make it lose its place if you give it a string of commands while it is moving. Look for the little spinning "/" icon in the upper right corner of the controller display to disappear before giving it new commands.
(4) The telescope doesn't have a glare shield/ dew cap with it like a refractor. At this observing event, there was no electricity available, and that turned out to be a major problem due to extremely heavy dew every evening (water was running off the scopes). I found you can defog one of these literally by hand. Point the scope slightly downwards and touch the bottom of the aluminum forward corrector cell and warm it with your hands. This is a little painful, since the corrector cell will be cold. As you warm the area, the rising air will slowly defog the corrector. The Celestron dew shield accessory was not as effective at preventing dew formation as the homebuilt one I used on the NexStar 8 GPS, but both were defeated within about 45 minutes.
The alternative is to take a big battery with you and use something like a Dew Buster heater which keeps the front of the telescope slightly warmer than the ambient air.
(5) You need a fair-sized battery to drive this telescope unless you can connect to a car or to an electrical outlet. I suggest if you are using a battery, this should not be the power for dew prevention or removal. If you blow a fuse from a heater, you are done for the evening if that is your scope power. The little battery blow dryers for truck drivers need a lot of power, also. I suspect dew shields may need to be at least 2 diameters long (quite large) to still be effective on a very heavy dew night.
(6) Get a set of Bob's Knobs (www.bobsknobs.com). They make collimating the scope easy, and the performance just isn't at its best until you get the collimation perfect.
(7) The owner had a problem with the finder scope losing alignment with the main telescope. The reason for the poor alignment turned out to be the rubber O-ring used to hold the back of the finder. The finder bracket has a set of three set screws at the front end and the back end of the little telescope is pressed into a ring with a large rubber O-Ring around the barrel of the finder. However, the finder has a couple of indented areas on the barrel, which look like they are there to hold the rubber O-ring. But, if you have the ring in one of these, it will no longer stick out enough to engage with the bracket.If the rubber ring isn't firmly engaged, the finder will be able to shift. If it is engaged, it seems to hold alignment indefinitely.
Summary
This product is one of the largest portable Go-To telescopes sold. For any task, there are accessories available from Celestron or other vendors to do it. This telescope is really big, and capable, and It is even handsome in the living room. With its 2800 mm focal length, the 11" scope is not a general purpose telescope. It guides itself, and the images are beautiful, but wide angle views are not part of this picture. It is possible to get the optical tube to photograph a wide angle with a Fastar and camera. As a result, I do not feel it would be responsible to recommend this telescope for a novice.
I found pitfalls in this design which had not come up with the NexStar 8 GPS, such as the loose secondary retainer, which could result in fatal damage to the telescope (a trip back to Celestron for a major repair is the result of breaking this part). The later NexStar GPS series telescope have obvious improvements over the first ones built, and I would recommend these over early ones if you buy one used.
Recommended:
No
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Epinions.com ID:
Pirich
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 in Electronics |
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Member: Rich W.
Location: Tucson, AZ
Reviews written: 137
Trusted by: 40 members
About Me:
Dad, Engineer, Scientist, Astronomer, Traveler; order may vary.
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