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Impressions and Opinions of My NexStar 11 GPS

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20 December 2001

Note: Celestron has upgraded the firmware in their GPS line to version 1.6,
this review is of a scope with version 1.2.  Read a review of version 1.6 here.

Following are my first impressions of the NexStar 11 GPS.  After additional use, I may post new notes from time to time.  If so, these additional notes will be in braces { }.  I won't mention too much about the basic specifications of the scope, you can easily find that information on the Celestron web site -

The viewing was done from my back patio which has three bright local lights and in the middle of a city with extreme light pollution.

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It's pretty amazing. I use 'wow' a lot around this scope. 'Wow' when I saw the huge boxes in my foyer. 'Wow' the first time I set it up on its tripod. 'Wow' the first time I looked at a celestial object through the eyepiece.

First Light - Optics
By the time I got home and finished puttering around it was 10 PM. I looked outside and while most of the sky was still cloud covered, I'd say about 15% of the sky at any given time was visible through breaks in the clouds and some of the clouds were thin enough to see Jupiter and Saturn and the brightest stars.

So, I did an alignment (took about 5 minutes because I had to wait for the holes in the clouds to correspond with the alignment stars) and set it to slew to Jupiter. At the time, Jupiter was peeping through a hole in the clouds and ..... WOW! What detail! The Great Red Spot (well, really the Light Tan Dimple) was cleanly visible as was all sorts of detail on the several bands. Even as clouds drifted by - thick enough that I couldn't see Jupiter naked eye - I was seeing loads of detail. Slewed to Saturn - WOW! Clearly defined Cassini division and all five of the big moons clearly visible. Also some detail on the planet's disk.

And finally the thing that really floored me. Slewed to M31. That area of the sky was entirely cloud covered, I was not able to see a single star naked eye. I could still see M31! In fact, I got as much detail through those clouds as I can get at a dark-site with my 80mm refractor.

It turns out that 11" of aperture and accurate goto have the effect of that 'cloud filter' we all joke about!

CCD imagers should take a close look at this scope.  The native focal ration of the N11 is f/10; with a standard barlow lens you get f/20.  The N11 optical system is capable of using the Celestron Fastar system (see of CCD imaging for a fast f/2 exposure.  You can also use a focal reducer to provide an f/6.3 ration.  These various options allow you to optimize the exposure time as well as CCD area for celestial objects of different size and brightness.

Mirror shift is minimal although noticeable at high power (200x and above).   Focusing is smooth and accurate.

So, visually, this scope is everything you should expect from a high quality 11" Celestron Schmidt Cassegrain telescope.  Definitely top of the line. {Since this initial session, I have had a little more time under the stars and again, the optics are certainly first-rate.  Collimation was a little off, but it traveled more than 7,000 miles to get here, so what would you expect!}  I won't comment more on the optical quality of this scope, there are many reviews of the C11 on the Internet and this is the same optical system.

Of, course, folks aren't buying the N11GPS just for the optics.  I also own a NexStar 80GT, so I knew how to use this scope out of the box.  In fact, if you want to know what I think of the hand controller software, read my review of the new GT controller and almost everything applies.  The main differences are: GPS Alignment, EQ Alignment, permanent Periodic Error Correction (PEC) and motor training.  I won't comment on EQ Alignment, PEC or motor training as I haven't experimented enough with them, but I can report that the GPS Alignment works wonderfully.  While it certainly isn't a necessity, it is convenient to simply turn on the scope, go about your setup and by the time you turn back around (2-3 minutes tops) the scope is waiting patiently for you to center the first of two required alignment stars.  It's also a good way to impress friends and influence enemies - the 'star-hopping only' crowd :-).

And naturally after alignment, I wanted to know how accurate goto and tracking were.   At this time, I can say that goto was generally very accurate, particularly in the area of the sky where the two alignment stars were located.  In this area, goto was within the center third of a 1/3 degree field of view (FOV) - accurate enough to leave a 12.5mm (224x) eyepiece in the scope and still get the object cleanly in view.   However, when slewing to other areas of the sky, I often had to switch to a longer focal length eyepiece (2/3 degree is the widest field you get with 1.25" eyepieces) to get the object in the FOV. 

In the first sessions I have had with the scope (about 3 hours total), I have not had time to completely determine if this can be improved.  I expect it can and that after an accurate leveling of the tripod and an accurate alignment, one should perform the calibration of the internal level - the manual clearly states this is required.  This should then improve the GPS alignment's accuracy on goto for reasons discussed in the NexStar Alignment Guide.  As stated in the Guide, it is required that the scope start with the axes orthogonal in order to optimize the calculated model of the sky.  The GPS mode of alignment performs the leveling of the optical tube automatically and on my scope it currently misses by about 5 degrees.  After I perform the level calibration, I will report the results here.

Tracking is great anywhere the scope is pointed.  Even in Alt-Az mode, objects stay in the FOV for very long periods with very little drift.  In EQ mode after performing the PEC recording, I would expect it to be pretty much dead on.  It should certainly be sufficient to perform unguided short to moderate length CCD exposures in the EQ mode with an optional equatorial wedge.

Some owners report motor-induced vibration problems that show up as fuzzy views in the eyepiece.  My scope does have a small amount of vibration that I can feel if I lightly touch the diagonal, but even at high magnifications I can see no difference in image quality with the motors on or off.  In any case, Celestron has acknowledged that this is a problem and has updated their design to pulse the motors at a higher frequency.  This eliminates the harmonic vibrations that cause the problem.   Newer N11GPS scopes have this upgrade installed at the factory; if you have one of the original GPSs, you should contact Celestron about the upgrade.  Currently it is expected that upgrading will not require a return to the factory as simple hand tools are all that is required to replace the required circuit board.   You can tell if you have the upgrade by the sound the motors make when tracking.   The upgraded model makes a sound that has been described as a 'bee in a soda can' while the older model makes more of a purring sound.  It should also be noted that all N8GPS scopes already include the upgraded board.

Regarding stability of the mount, the tripod is the bare minimum required for the extreme weight of this scope (60+ pounds for the fork and optical tube), but surprisingly vibrations quiet down quickly.  I would say this is partially due to the vibration suppression pads that Celestron includes as standard equipment and partially to the fact that it is so massive, it doesn't vibrate much when you adjust focus.  It does suffer though when the wind is strong.  The large profile of the 11" tube catches a lot of wind.  Some have suggested that if you intend to use the EQ wedge, you should upgrade to a larger tripod or use a pier.  One of the tripods frequently mentioned is the Meade giant field tripod.  The treaded rod in the center of the Celestron tripod is then used on the Meade tripod to aid in mounting the N11GPS.

Is This the Scope for You?
Needless to say, I'm now spoiled. But there are some immediately obvious concerns. If you have back problems, this scope is NOT for you. If you weigh less than 150 lbs. of bone and muscle, this scope is NOT for you (otherwise you WILL have back problems). (I would add that we know of one rather petite female that does haul this monster around, so I may be off on this one.)  If you want to take a scope to a dark site every chance you get, this scope is probably not for you (depending on your vehicle). Notable exceptions to the first two issues would be if you can leave it setup on wheely bars and roll it out when you use it OR you have an observatory with a roll-off roof.  If you want similar mechanical performance at less weight, the N8GPS might be the ticket, but compare it to an LX90 to be certain you need the additional features and performance of the N8GPS.

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Michael Swanson
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