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SkyScout Accuracy and Orientation

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#1 MikeBu

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Posted 23 January 2011 - 03:25 PM

I ordered a SkyScout and received it last weekend. I was happy when it would get a GPS lock in the house and the next night took it out. It seemed the accuracy of locate and identify was pretty far off, needless to say I was pretty disappointed. Today I thought I’d try to measure how far off it was. I put the SkyScout on a camera tripod and set up my good Lensetic compass about 3 feet away. I aligned the viewfinder with the compass pointed North and hit identify, then looked at the azimuth coordinate. The SkyScout measured a little over 20 degrees, and since the magnetic declination (i.e. the difference between magnetic North and true North) here is -3.3 degrees; it was about 23.3 degrees off. Very disappointing! As I was taking it down I was thinking I had to send it back, then I wondered just how sophisticated the system was. Did the measure Earth’s magnetic field in only 2 or all 3 dimensions?

Let me explain for the benefit of those unfamiliar. To visualize hold you left hand out palm up, fingers facing forward and thumb out to the side. This represents the surface of Earth with the fingers pointing North. Now hold a pencil level over your hand pointing in the same direction as your fingers; this is Earth’s magnetic field (pointing true North). If you rotate your hand so your thumb point up the pencil still points in the same direction as your fingers, North. Now hold you palm face up again, this time tilt the pencil down toward your hand a little. The pencil still points in the same direction as your fingers when you consider only the directions of your fingers and your thumb (only 2 dimensions). Now rotate your hand so your thumb points up again. The pencil no longer points in the same direction as your fingers when you consider only the directions of your fingers and your thumb. Rotating your hand is the same as rotating the SkyScout.

If the SkyScout only measures Earth’s magnetic field in 2 dimensions, then rotating it on its side will affect its accuracy. I saw no information in the manual specifying an orientation, and I had been holding it just as it was pictured. To test the theory I rotated the SkyScout 90 degrees on the tripod and aligned with the compass pointing North once again. I measured the azimuth several times and it was always between 356.5 and 357.0 degrees. Accounting for my magnetic declination of -3.3 degrees that put it right on (360-3.3=356.7 degrees)! Next I rotated it upside down from this new position. If my theory was correct it should add about 3.3 degrees to its North measurement instead of subtract. Sure enough the azimuth read about 4 degrees. The inclinometer measured altitude correctly in all 3 positions, so it does measure altitude in 3 dimensions. The inclinometer is, of course dependent on gravity and not Earth's magnetic field. This information, along with a 3 dimensional measurement of Earth’s magnetic field is sufficient to allow the SkyScout to function in any orientation, assume it performs the calculations. If the SkyScout is sensitive to orientation it seems like this would be important information to put in the manual. Mine seems very accurate (according to these results) only when the display is pointing up. (My tripod didn’t allow me to rotate it upside down from the original position where it was off 23.3 degrees.)

I’m anxious to try it again, but it is typically so cloudy here and I don’t know when I’ll get another chance. But I’d be interested in what others find. Does your accuracy change when you rotate the SkyScout, display face up, to one side, face down, …? The change in accuracy will vary depending on both your magnetic declination, and how parallel the flux lines of the magnetic field are to Earth’s surface where you are.

#2 Jawaid I. Abbasi

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Posted 24 January 2011 - 02:14 PM

SkyScout is very sensitive to magnatic things so make sure it is away 2 to 3 meters from it. Also check the batteries.

I bought it last year and it has been working perfect. I mainly used as a DSC system with my binocular.

#3 DavidD

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Posted 24 January 2011 - 08:00 PM

Interesting.....

I like the test you set up, checking the direction the SkyScout thinks it's pointed rather than trying to sync it up with a star that keeps moving.

I haven't tried mine with the display up. I mount it with the screw hole down. Mine is off a bit - the object I'm trying to locate is usually right on the edge of the display circle. But, I also have issues with magnetic interference (though the camper next door is gone for a while, so maybe I should try it again!).

So, I don't find mine 100% accurate, but it definitely isn't off by as much as yours.

One other note....I've found in using the SkyScout and various other GPS and electronic compasses....sometimes it helps to rotate the device around, both horizontally and vertically, slowly a couple of times, which causes it to have to do the calculations more or less continuously, which sometimes corrects things.

David

#4 Don Trinko

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Posted 25 January 2011 - 08:36 AM

I have had 2. They both acted the same.
They even got the same wrong GPS reading when near my truck out side and both got the correct reading when in the middle of my back yard. I found the "locate" to be inaccurate but the " identify" to be fairly accurate. Don T.

#5 Jawaid I. Abbasi

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Posted 26 January 2011 - 12:08 AM

I have had 2. They both acted the same.
They even got the same wrong GPS reading when near my truck out side and both got the correct reading when in the middle of my back yard. I found the "locate" to be inaccurate but the " identify" to be fairly accurate. Don T.

That is I call it very sensitive. I usully use at my back yard or front yard but 15 feet away from my car and it accuracy is pretty good.

#6 Phil Wheeler

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Posted 29 January 2011 - 12:41 PM

I put the SkyScout on a camera tripod ...


Many tripods have metal in them, or in their fittings. Seems like that metal could affect the magnetic field measurements of the SkyScout.

#7 MikeBu

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Posted 29 January 2011 - 05:24 PM

Thanks for the replies. When testing I do not get a magnetic field interference symbol, and my tripod is aluminum. Even if the field flux vector was disturbed, unless it was extremely localized (as from the batteries) I would expect its altered direction should affect the compass the same as the SkyScout. Obviously the SkyScout cannot know the magnetic field is pointing in the wrong direction when it displays the interference symbol, since it would first need to know the correct direction (and if it knows which way is North already, why bother). It must either be measuring either a static or dynamic variation in field strength. If objects like a car affect it then it must detect a static field strength. The total field strength here is 54609.6nT.

I checked and my magnetic inclination is 69.725 degrees, which means if the SkyScout is accurate but is sensitive to orientation it should have been of by a lot more than 20 degrees when on its side. It's been continuously cloudy and I haven't had a chance to try it outside again. But thinking about this a little more makes me believe the SkyScout shouldn't be sensitive to orientation.

#8 Phil Wheeler

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Posted 29 January 2011 - 06:48 PM

(and if it knows which way is North already, why bother).


How does it determine North if the magnetic field direction is distorted or disturbed?

#9 MikeBu

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Posted 29 January 2011 - 11:33 PM

It can't. At any location Earth's magnetic field is a vector, which means it has 2 properties - direction and strength. Magnetic sensors, like a compass, use the direction of the field to tell which way is North. It really doesn't matter how strong the field is as long as the sensor can sense it. Because Earth's magnetic field is 3 dimensional a magnetic sensor can not only tell which way is North, but also the angle relative to the ground. (A compass needle will actually tilt up or down a little but the strength of the field is so small compared to gravity and the weight of the needle, that it usually isn't that noticable. When determining direction, all any sensor can do is determine which direction the field is pointing.
If something around the sensor causes the field to point in a different direction it will give wrong information.

If all a sensor knows is what direction the field is pointing, how can it tell when it's pointing in the wrong direction? One way would to determine the field strength. If there is a magnet next to the sensor, it could tell the magnetic field is much stronger than it should be. But there is potentially another problem. Just like electricity prefers to flow through conductors, a magnetic field finds it easier to flow through some materials depending on their relative permeability. The field will bend toward and flow through ferrous materials, concentrating the field in the ferrous material and weakening the field just outside the material. A sensor capable of measuring field strength can determine if the field is being affected in such a way. Another way to determine if the field is being distorted is to use 2 sensors spaced some distance apart. Unless you are right on top of a magnetic pole the direction North for 2 sensors separated by a small distance is virtually unchanged. If two sensors give different results it is because the field lines through them are distorted; they are being bent by a nearby magnetic or ferrous material. But there is no way to determine which sensor, if either, is correct.

A 3 dimensional sensor has 3 orthogonal sensors, each simultaneously measuring the field strength in 1 direction. By summing the strength of all 3 sensors and determining what fraction each individual sensor is detecting a "unit vector" is calculated. That unit vector tells the direction of the magnetic flux lines of the field. The sum tells the field strength.

#10 MikeBu

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Posted 29 January 2011 - 11:42 PM

I have had 2. They both acted the same.
They even got the same wrong GPS reading when near my truck out side and both got the correct reading when in the middle of my back yard. I found the "locate" to be inaccurate but the " identify" to be fairly accurate. Don T.


This seems odd. It would seem pointless to have separate sensors to determine direction for locate and identify since you only use one at any time. Unless it was calculating altitude and azimuth differently for each why would one function be any more or less accurate than the other???

#11 Don Trinko

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Posted 30 January 2011 - 06:06 AM

Yes it is odd. When I "Located" the moon it was abought 5 degrees to the left, when I "identified" the moon it was correct. I'll eventualy get another and we will see if it's the same. Don T.

#12 Jawaid I. Abbasi

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Posted 30 January 2011 - 02:59 PM

Yes it is odd. When I "Located" the moon it was abought 5 degrees to the left, when I "identified" the moon it was correct. I'll eventualy get another and we will see if it's the same. Don T.

Identify is most accurate then locate and the reason is that the inner two rings must be overlap when the object is located. The trick is to look through at least 12" away

#13 Jawaid I. Abbasi

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Posted 30 January 2011 - 03:01 PM

Another thing when hand hold or tripod mounted is to move slow this is my trick :)

#14 Phil Wheeler

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Posted 01 February 2011 - 12:33 AM

My SkyScout arrived today. Acquired GPS in about one minute and so far has IDd stars and such perfectly. Only needs a right-angle viewing adapter to save my stiff neck :)

#15 Jawaid I. Abbasi

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Posted 01 February 2011 - 01:36 AM

My SkyScout arrived today. Acquired GPS in about one minute and so far has IDd stars and such perfectly. Only needs a right-angle viewing adapter to save my stiff neck :)

Glad to hear it. Where can I find a right-angle viewing adapter?

#16 Phil Wheeler

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Posted 02 February 2011 - 01:34 PM

Where can I find a right-angle viewing adapter?


Beats me! Likely a "roll your own" project :)

BTW -- the GPS receiver in this thing is pretty good. Acquired 10 satellites while sitting in my living room this morning in less than 60 seconds.

#17 Jawaid I. Abbasi

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Posted 02 February 2011 - 03:38 PM

Oh no!
I am not good that kind of project :)

#18 Phil Wheeler

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Posted 02 February 2011 - 03:52 PM

Nor I!

#19 MikeBu

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Posted 02 February 2011 - 05:20 PM

Glad to hear your SkyScout functions well.

#20 Phil Wheeler

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Posted 03 February 2011 - 01:41 AM

Thanks, Mike: Me, too. Bought it on a bit of a lark, since I know the sky pretty well (I'm 74 and have been viewing seriously off and on since 1954). It is really a fun thing to have .. and my wife really likes it (it's hard to not like an astro gadget in that category!)

#21 donnie3

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Posted 23 February 2011 - 03:40 PM

just wonder if a person could use one of these things with a lasor pointer attached to it on a tripod, locate a object, then point your scope in that location. at least it would get you close. don

#22 Phil Wheeler

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Posted 23 February 2011 - 09:00 PM

At one time you could buy a mount to attach a GLP to the SkyScout. Not sure that is true now.

#23 Edward Swaim

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Posted 22 March 2011 - 02:53 AM

I put my SkyScout on a wooden 1 x 2 screwed to the side of my homemade Dobsonian mount for my 130mm f5 reflector. To adjust, I put the ball-joint head from a dollar store miniature tripod on the end of the board. The board is about 18 inches long to get it away from the telescope tube and to give me room to sight through the SkyScout. No magnetic interference indicated so far. (It does cause balance problems with the mount, but I can move the tube back and forth to compensate.)

Alignment was simple: I put my telescope's red-dot BB gun sight on Sirius, then adjusted the SkyScout so the star appeared in the sighting circles.

With a 25mm eyepiece,I succeeded in getting the desired object within the field of view. This is helpful, because I live right in the middle of a big city, and it is difficult to star hop.

My last test was to have the SkyScout point to the Beehive Cluster. The full moon was up, and, with the moon and light pollution, the sky was blue, rather than black. Bingo! The SkyScout put me close enough I just had to nudge the mount a bit.

The best part is that my "star finder" gets my 9-year-old away from the TV and into the front yard with me. The telescope, alone, was too boring...

#24 Edward Swaim

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Posted 27 March 2011 - 02:03 PM

Here's a photo:

Attached Files



#25 wncRanger

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Posted 06 April 2011 - 03:26 PM

I was planning on doing the same thing, Edward! Sort of an "intelliscope" upgrade, lol.






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