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11-14-2020, 07:52 PM   #1
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O-GPS1 + K3 + Dobsonian ???

Was wondering if the gps star tracker would work in a Dobsonian telescope. K3 with a T ring adapter. This would position camera 90 degrees from what your aiming at. Not sure how that effects compass and sensor movement.

Thanks

11-14-2020, 08:23 PM   #2
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QuoteOriginally posted by no694terry Quote
O-GPS1 + K3 + Dobsonian ???
It should work, in practice is like putting it on a tripod, what you must do carefully is the calibration and make sure there are no magnetic fields,
even one or two spare batteries are useful because of the energy consumption is increased.

Ciao M.
11-14-2020, 11:39 PM - 1 Like   #3
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I don't think it will track correctly with a Dobsonian (aka Newtonian).

Taking the most extreme case, when the scope is aimed near north star you'd want the astrotracker to barely move, but the camera will be pointed towards the celestial equator and try to maximize astrotracker movement. The mirrors flip the image (compared to a camera lens) and can cause the tracker to move in the wrong direction.
11-15-2020, 06:29 AM   #4
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I'm with DeadJohn on this one. It won't work.

Unless you can mount your camera so it looks along the telescope axis.

And, then - what is your focal length? 1000mm or more? Your tracking time would be very short before objects move too far for the sensor to compensate.

11-15-2020, 07:34 AM   #5
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Agreed. It’s hard enough to get it tracking when the camera is pointed straight at the target, let alone mounted at a 90 degree angle to said target.
11-15-2020, 03:12 PM   #6
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Maybe better to get a computerized tracking scope then.
11-15-2020, 04:15 PM   #7
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Having run some fairly long glass (560mm max) with astrotracer I would expect getting everything to play nice with a real scope would be an exercise in disappointment. Especially considering that the camera won't be mounted parallel to the the scope. That is the key part as having it pointing perpendicular to it will just result in tracking that is off. One of the assumptions astrotracer makes is that the camera is pointed the same direction that the lens is.

Assuming you get thing setup so that the camera points the same direction as the scope then you get to deal with the very short tracking ability offered by it at long lengths. Pointing it the celestial pole you in theory can get the 5min max tracking asusming calibration is good since all the movement is rotational. However moving away from the celestial pole there will be an ever increasing linear component to the movement and at long lengths you will end up with very short lengths.

My best advise is to instead look into getting a proper equatorial that can carry everything (be under 50% max capacity).
11-15-2020, 04:52 PM   #8
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Pretty much beyond 500mm forget it.

11-15-2020, 09:12 PM   #9
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Pretty sure it won't track correctly since the body and GPS are facing 90 degrees to the field of view and tracking for that "view" instead.
11-16-2020, 05:07 AM   #10
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It won't work with a dobsonian but would work fine with a cassegrain or similar arrangement having the camera facing the same direction as the telescope. The exposure time would be quite limited due to the long focal length but stillnot too bad, about 30 seconds for a 1500mm focal length. Also, the maximum focal length it can accommodate is 2000mm, at least on the K1-ii, other models may have different limits.
11-17-2020, 01:47 PM   #11
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QuoteOriginally posted by lister6520 Quote
about 30 seconds for a 1500mm focal length
Not for anything along the celestial equator. If you are pointed close to one of the celestial poles maybe assuming a good calibration but calibration issues will show very quickly at 1500mm. Here is the specifications for O-GPS1 which show the max theoretical tracking time for various cameras. Declination is where you are pointed in respect to the celectial equator and celestial pole. So a declination of 0 is not pointing straight out at the horizon but is pointing along the celestial equator. Also a declination of 90 degrees does not mean pointing straight up at the zenith but instead means pointing at the celestial pole. Pointing at a declination of 90 degrees means that you are pointing at the celestial pole so movement in the frame is all rotational so tracking length is only determined by the amount the sensor can rotate. Focal length doesn't matter since the angular speed of the stars are the same. However when pointing at the celestial equator all of the movement in the frame is linear so tracking time is dependent on how much linear movement the sensor can provide and how long of a focal length you have. Focal length matters since the linear speed of the stars across the sensor is higher with longer lenses. At declination between 0 and 90 degrees there is some combination of linear and rotational movement in the frame so you end up varying tracking times. To get the best out of it the camera has to know where it is located on earth (north/south) and how it is oriented in space so getting a good calibration is necessary.

QuoteOriginally posted by lister6520 Quote
Also, the maximum focal length it can accommodate is 2000mm, at least on the K1-ii, other models may have different limits.
On the K-3, K-3ii, and K-500 there is the same max length.
11-20-2020, 06:51 AM   #12
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QuoteOriginally posted by MossyRocks Quote
............. assuming a good calibration but calibration issues will show very quickly at 1500mm. ............
Something I found with regards to the tracking accuracy is that it is not the only thing that matters. It is also important to have a magnetically 'clean' environment. The calibration can only compensated for the local magnetic variations caused by the camera itself and anything directly attached to it, such as the lens and the quick release plate. Any other local variations cannot be accounted for and will result in an incorrect North.

The worst causes of such uncorrectable variations I have encountered are rebar in concrete structures, especially if standing on them such as on a building or even a concrete pavement and nearby cars. By nearby I mean within less than about 10 meters (30 feet) or so. Other hidden things such as buried steel pipes, metal fences and such can all through off the compass enough to cause significant trails.

Something that may also make a big difference is the tripod itself, or the telescope, as those obviously cannot participate in the calibration dance. Sometimes it could be easily fixable such as replacing some steel screws with brass ones if they are the only magnetic material but if there are other structural parts that are ferromagnetic it might not be possible to sort out.

Finally make sure to keep away any flashlights, cellphone, tablet or other electrical/electronic devices as even the small currents flowing through them can cause a detectable shift in the magnetic field around them. Some devices even have magnetic catches on their cover.

When I first used Astrotracer I struggled to get trail-less stars at long focal lengths until I started discovering what was interfering with it. Shooting from home is still impossible due to the rebar in the roof which i quickly figured out but out in the open it had taken me quite a while to discover first the bad influence the tripod was having and after fixing that (by using another tripod) I found that moving the car a bit further away from where I stood sorted it out and I could get one minute exposures without trails using my Sigma 150-500. I think by extrapolation one would be able to get minimal trails or with some luck not at all even at 2000mm if one is extra careful in avoiding any magnetic interference.
11-20-2020, 09:08 AM   #13
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QuoteOriginally posted by lister6520 Quote
Something I found with regards to the tracking accuracy is that it is not the only thing that matters. It is also important to have a magnetically 'clean' environment. The calibration can only compensated for the local magnetic variations caused by the camera itself and anything directly attached to it, such as the lens and the quick release plate. Any other local variations cannot be accounted for and will result in an incorrect North.
That is a huge problem and a topic unto itself. I once setup directly atop buried power lines one night. For the life of me I could not get a good calibration that would work with any focal length. It was only once the sun came up that I saw that I was right inline with 2 power distribution boxes.
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