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03-02-2019, 10:01 AM   #1

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Astrotracer weirdness.

I've attached a knob/dial with a couple of small magnets inside on top of the camera (K1). By turning this knob i can manipulate the internal compass and get it to show whatever heading I want. The idea being that the built in compass isn't always that accurate and heavily dependent on the calibration routine, and if I know the correct heading I might as well dial it in myself.

Anyway, with a 135mm lens I centered m42 in the viewfinder, and then used Skysafari on my phone to get the current heading to m42. Then entered this heading into the camera by turning the magnets. Yet the tracking was still off somewhat.

But if I tuned in about 7-8 degrees less then Skysafari reported, for instance 192 degrees instead of 200, then the tracking seemed ok. That's kind of weird considering that (I assume) Skysafari reports heading in relation to true north, and the built in compass is supposed to also report against true north.

This is kind confusing as I figured that I had eliminated any local magnetism variances by entering the correct heading directly.

03-02-2019, 01:20 PM   #2
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Would the magnets manipulating the internal compass be affecting the cameras built in compass by reporting the wrong true North?
03-02-2019, 02:43 PM   #3

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You can introduce problems with external magnets. First off, you have a chance of magnetizing metal parts in the camera, and then it will never calibrate accurately.

A good calibration should make your camera pretty accurate, but it has to be done carefully and with no outside magnetic interference. Cars or metal buildings can throw it off. It's hard to guess what having a strong magnet nearby would do. It will override the earth's magnetic field so your camera can't use it to get a good indication of directions.

I'm not sure I follow what you did, but couldn't you point your camera in the opposite direction from M42 and use the magnets to get a proper reading (for M42)? Of course that wouldn't allow the camera to track M42 accurately. Unless your camera is pointed at exactly the same coordinates Skysfari used, it won't read the same (even then Skysfari might have used a slightly different lookup table) and making it read those values artificially, won't get it to work correctly.

I would try a careful camera calibration without any magnetic interference (sometimes it has to be done more than once), and see what you get. Forcing the system to work using an external bias isn't exactly the way it was designed.
03-02-2019, 06:35 PM   #4
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As Bob notes
QuoteOriginally posted by Bob 256 Quote
You can introduce problems with external magnets
Those magnets might appear to correct things when your camera is pointed north, but when you rotate it, their "corrections" may have nothing to do with reality - the field orientation has changed. And, as Bob also noted, you run the risk of permanently magnetizing stuff in the camera.

So, get rid of the magnets, and start astrotracer calibrations from scratch.

If you have magnetized something in the camera, you should demagnetize it - maybe hard to do with a big bulky item unless you have access to an old-fashioned magnetic tape bulk eraser - something that creates a 60 Hz varying field in which you immerse your object (camera) and slowly withdraw it while the demagnetizer is on. (We used to do this to reels of magnetic tape which were 2" thick and 12-14 inches in diameter.)

03-02-2019, 06:53 PM   #5
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That 7-8 degree discrepancy sounds suspiciously like a declination issue -- that one of your devices is showing magnetic north and the other is showing geographic north.

The other issue is that the magnetic north pole has been moving quite quickly of late (about 50 km/year). If you aren't that far from the north pole (e.g. Sweden) and one of your devices has a declination map or file that's some year's old, the shift in the pole would would create this kind of error. (A quickie calculation suggests that a 5-year-old declination map might create about a 7° error for someone on the Arctic Circle in Sweden.)

P.S. The other open issue might be whether the magnetic inclination of the field (how much the magnetic field dips toward the earth) is causing some issues. I can't see how it would have an effect but it is another variable in the system, is quite high in high latitudes and is probably something that the little adjustment magnet could be messing with.

Last edited by photoptimist; 03-02-2019 at 06:59 PM.
03-03-2019, 03:07 AM   #6

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I'm using a couple of tiny magnets (something like 2x3x3mm in size) so the magnetic field is weak unless you are very close to them. So unless I'm actually attaching them directly to metal I don't think there is much chans of magnetizing anything. But they are strong enough too override earths magnetic field within in a few centimeters, so I can fool the cameras compass sensor to show any heading I want.

I do have access to an industriell demagnetizing apparatus but I don't think it would be wise to pass a camera through it. The camera is full of magnets and coils and I'm not sure the camera would be the same afterwards. A coworker placed his laptop on a large electromagnetic chuck once, and the chuck accidentally got activated. That was the end of that laptop.

As for calibrating the compass normally (without any magnets) I have tried pretty much any metod I could think of and still the results are not repeatable. Sometimes the result is great, but most of the times they are only partially great as in spot on in most directions, but with larger errors in other directions. I also have a O-GPS1 which I used with a K5 long before the K1 was available. And the compass in the O-GPS1 is more accurate. I guess that is because the magnetic senor is further away from the camera body itself.

The declination SHOULD be automatically added to the displayed heading on the camera according to this link Electric Compass | GPS UNIT O-GPS1 | RICOH IMAGING at least on the O-GPS1, so I guess the same thing is true for the built in version in the K1.

But maybe it isn't.. Maybe the displayed value is magnetic north and the declination is added afterwards and used in the astrotracer calculations. That would explain the results I got (assuming Skysafari reports the right heading). Skysafari says 200 degrees, I enter 192 degrees by manipulating the magnetic field, the camera adds 7-8 degrees declination after getting my GPS position and checking some internal database. And then it all adds upp to a "perfect" Astrotrace solution.

Hm, I need to do some more test.
03-03-2019, 03:49 AM   #7
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My guess was also the rapid movement of the magnetic pole.

Following this thread with interest.
03-03-2019, 12:54 PM   #8

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So I did a little experiment, unfortunately the result just adds to the confusion.

The idea was that if I aim due south (180 degrees) at the horizon, the stars would appear to travel horizontally, and as a consequence the astrotracer would also move horizontally. If I aim to the right of due south, for instance 190 degrees the stars would appear to move slightly downwards. And thus slightly upwards if I aim to the left (170 degrees).

Since it is raining I just stayed home and placed the camera on a tripod (near a window to receive GPS signals) then aimed at a LED on the other side of the room. I made sure the camera was horizontal according to its own sensors. Since the LED is standing still I expected a straight horizontal line from the astrotracer when aiming due south.

Thanks to the dial with magnets I can fool the camera to believe it is aimed 180 degrees, and then start a trace. (20 sec with 600mm, it was actually 135mm but to get a little action I entered 600mm). The result was slightly downwards.

I adjusted the magnets to 172 degrees, which was to far and the line pointed slightly upwards. 175 degrees was the sweet spot with a perfectly horizontal line. So 175 degrees on the cameras compass results in a perfectly horizontal trace.

Now to the really confusing part, I adjusted to 355 degrees and expected a horizontal line again, but as in due north. But no, I had to adjust to 5 degrees to get a perfect horizontal line.

So when aiming south I had to remove 5 degrees, if I want 180 I enter 175. But when aiming north I had to add 5 degrees as if I want 0 I had to enter 5.


03-04-2019, 02:04 PM   #9
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I managed to totally crash K-1 when I tried to calibrate the compass here:

I had to pull out battery to get it running again. Stuffing some magnets right on the camera sounds like a recipe for disaster as well.
04-09-2019, 05:43 AM   #10
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Magnetic North recently has been moving faster away from Canada towards Russia. I think it was around 50-60km per year. Not sure if sky Safari takes that into account as well.

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