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04-05-2020, 05:10 PM   #1
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GPS Connection - Astrotracer

Hey guys,

Thought I'd like to start pointing my K-1 upwards a bit more. I get the feeling that with school etc I have more time at night than during daytime..: confused:
In reality I'm simply more interested in doing more astro stuff.

With our current lockdown situation I thought it'd be quite nice to shoot from the comforts of the balcony. The GPS signal is predictably all over the place.
Is there any way to keep the precise calibration from a "different" location? If I do the precise calibration sticking the camera out of the balcony as far as I can I get decent tracking (50s at 135mm), but on the next few photos the camera seems to try updating it's position - I shot 33x50s frames this evening. 6 were out of focus (user error clearly), 9 okay-ish, 1 tracked perfectly (as were the 6 that were out of focus...), 17 showed star trailing in different directions between shots.

Does the camera update its position after every photo? Is there anything I can do to trick it, or feed a position through a laptop or something? - It currently doesn't seem like the right time to sit out in the local park for hours on end but I've got a really bad astro-itch!


04-05-2020, 05:19 PM - 1 Like   #2
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First consideration is that the constellation of GPS satellites is constantly changing as the satellites move in their orbits. So, yes, the Astrotracer needs to update its ephemeris. However, once it has done this once, it should be able to update it without doing another calibration since the satellite orbits are part of the ephemeris. But there is also the Precise Calibration. This attempts to sense the local magnetic field with the intent to correct the magnetic variation for any local deviation. So if you move your camera position, you should do another calibration routine. There are some locations which have such a strong local magnetic field (such as iron-rich soils) that attaining a good calibration is impossible.
04-05-2020, 05:52 PM - 1 Like   #3
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Astrotracer does not need a super-precise GPS lock to work. The coordinates could be off by some miles/km and it really would not have a big effect. The locations and movements of the stars really don't vary much from place to place -- it takes a shift of some 60 miles or 100 km to change the sky's motion pattern by even 1.

As jbinpg said the Precise Calibration of the magnetic compass is actually the more sensitive part of the astrotracer. Iron in the balcony railing, building, tripod, lens, etc. can distort the Earth's magnet field and throw off the compass. If the compass is off by just 1, it has the same effect as a GPS location error of 60 miles or 100 km! Precise Calibration can correct for very local anomalies but if the north-south alignment of the magnetic field is turned over a larger distance by larger iron objects (big beams in a building, iron soils, etc.), then it might struggle.

That you got some good results suggests that your magnetic environment isn't too terrible. Doing a careful Precise Calibration in chosen location of the shot and keeping the camera away from steel/iron things might help.
04-05-2020, 11:20 PM   #4

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Agreed. Accurate magnetic compass reading is the key to effective tracking.

04-05-2020, 11:40 PM   #5
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The OM clearly describes the importance and sensivity of the compass calibration:
Changing the lens or the angle of the monitor after performing
a calibration may have some effect on the magnetic field environment. In this case, perform a calibration again.
04-07-2020, 04:27 PM   #6
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Thank you very much, this all makes perfect sense!
I somehow totally forgot that the precise calibration isn't about GPS - but I had wondered why such a precise GPS calibration would be needed. Just didn't go along the right lanes with my thoughts afterwards haha.

I've tried calibrating the camera, looking at the compass and moving around a bit. South sometimes was off by as much as 60! But I'm pretty sure I can (hopefully) find a good spot again. Wondering if I could do some magnetic trickery.. Looking forward to experimenting and will update you all.
05-20-2020, 04:08 PM   #7
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Sounds silly, but could it be that it's actually the lens messing with it?!

I just had a SMC Takumar 150/4 on it and everything was fine at 40s, but it seems like it doesn't quite reach infinity so I exchanged it for my Super Varexon 135/2.8, recalibrated.. And nope. Streaks in the wrong direction at 40s. I can't solve it by recalibrating. 30s seems absolutely fine though?.. Didn't move anything else.

Ways to re-ignite LBA #132. I now feel like I need more lenses in the 135-250mm region.
05-20-2020, 06:58 PM   #8
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QuoteOriginally posted by Topsy Quote
I now feel like I need more lenses in the 135-250mm region.
Rokinon 135 f/2 will be your best friend.

One other thing worth mentioning - astrotracer is most effective the closer the camera is pointing toward the horizon. The more vertically you're pointing, the easier it is for the camera to get it wrong.

05-21-2020, 10:14 AM   #9
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I've been eyeing the William Optics RedCat. But I'm not sure about the price, I'm usually very much a budget lens buyer. Ideally I'd like a tracking mount too. Still frustrating if I can't get the astrotracer to work properly anymore though!

Funnily enough it seems the least effective when the target is closer to the horizon for me!

I shot the region around Antares and the region around Polis (both ~17 last night). The first three 40s shots with the Takumar 150/4 are perfectly tracked. Swapped lens and out of the next 106 shots at30s I have 2 (!!) that are really tracked properly. about 20 are usable-ish and the rest is really only fit for the bin I fear.
The streaks I'm getting are horizontal, so it seems like the sensor simply is moving in the wrong directly. Interestingly enough the good/best shots always are the first ones after realigning the camera every so slightly.

{Note: I'll feel extremely stupid if it's tripod creep!!}

Out of the 1 shots I took of Lyra I've got 10 usable ones, and that's at 60 and a perfectly tracked 60s exposure of.. erm.. somewhere near Ophiuchus (35)

{Jogged my brain a little, will try and get hold of a different tripod for this evening!}

Will report back, cheers

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