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NexStar 11 GPS Problem

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Posted by Bruce Wood on August 24, 2001 10:24:12 UTC

I acquired a Celestron NexStar 11 GPS telescope recently. Following is my correspondence with Celestron about what I thought was a problem, but turned out to be a result of misleading advertising.

The first night I set it up, I used the GPS Align mode to get it started. It went first to geomagnetic north - and aimed in the direction of the pointer stars of Ursa Major, approximately 12 degrees to the west (left) of Polaris! It then quickly located enough GPS satellites to locate my longitude and latitude to within one second of my
stand-alone GPS receiver. From there it moved to Arcturus and asked me to align that star in the finder scope and telescope. It aimed the same
12 degrees left of Arcturus. I slewed to Arcturus and it went to find Altair - and again was 12 degrees left of Altair. After I slewed to Altair, it found all objects and coordinates I requested very closely to the center of the telescope.

This magnitude of the deviation is most disappointing. I was expecting the alignment stars to be within the field of the finder scope and they were not. This is because, even though the telescope knows exactly where on earth it is sitting, it does not take avantage of that
information to adjust to True North.

My recommendations are:

Most preferable: have the telescope go to geomagnetic north and locate the GPS satellites. Before it finishes slewing to the first alignment
star, have it calculate the deviation between geomagnetic north and True North and adjust its aim at the first alignment star based on that
deviation calculation. This should put the first alignment star within the finder scope field.

Less preferable: have the telescope do what it does and expect the user to adjust the alignment to the first star, which Celestron expects now. At that point, have the telescope computer remember that adjustment for that location. The next time the telescope is set up a question would come up asking if the computer has moved more than x miles, say 300 miles, east or west of the last viewing location. If not, it would use the last adjustment to calculate True North and the alignment stars would be close. If so, it would start over from scratch.

Least preferable: allow the user to aim the telescope to True North and then let the GPS take over to initialize the other required fields.

Following is Celestron's reply:

"I have forwarded your comments to our engineers for review. We will take these suggestions under advisement, and may work something like this into
future versions of the telescope, but at the present time the scope functions as advertised. You are required to center the two alignment stars
and hit "Enter" in order to perform the alignment. Once manually centered, the scope will put objects within the field of view of the eyepiece provided.

There are no scheduled upgrades planned at this time. When they are, they will be made available through our website."

My reply was:

Following are reasons I expected closer alignment from the start than I find the NexStar 11 GPS delivers, all taken from Celestron advertising:

>From a Celestron press release of January 19, 2001: "It's so advanced that as you turn it on, the integrated GPS and compass system automatically pinpoints your exact location and points to your first alignment star."

>From the Celestron advertisement on the back of the September 2001 Sky & Telescope magazine: "Just turn on your telescope and the NexStar GPS will pinpoint your coordinates, the time and the date, the integrated electronic compass will find north and level the telescope, an the vast GOTO database will seek out whatever it is you want to see. It's actually that simple."

>From the Celestron NexStar 11 GPS Instruction Manual, page 15: "...slews to an alignment star and asks you to confirm that the star is in the center of the eyepiece."

Do you still really think it "functions as advertised?"

His reply to this was:

"The scope does in fact work as advertised. The issue is with your interpretation of what you are reading. Where, in these examples, does it say that the alignment stars will be within the field of view of the finderscope?

The first example simply states the scope will point to an alignment star. It states that the GPS and compass will pinpoint your exact location, but does not state that the scope will point exactly to the alignment star. As I previously explained, this will depend on your exact location on Earth and the exact declination offset of magnetic north, which can vary for many different reasons (geological formations, power lines, or anything else that can disrupt the local magnetic field).

The second example is 100% true. The GPS pinpoints your longitude & latitude and receives the time signals from the GPS satellites. The integrated compass finds north and levels the tube. The goto database will seek out whatever you want to see, once the computer is aligned. Again, I see no reference to the fact that the alignment stars should be in the field of view.

The third example is also true. The scope asks you to confirm that the alignment star is centered in the field of view. If it is not, you are required to place it in the center of the field of view.

Once aligned, the scope has reliably placed items within the field of view of the eyepiece. It works as advertised."

He also advised me, by telephone, that the NexStar 8 will have a magnetic compass starting in October.

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