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04-20-2015, 05:53 PM   #31
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QuoteOriginally posted by philbaum Quote
So my question is: is my gps device working correctly because of the small tails in the 1:1 crop. The blue light was on all the time when this was taken. Afterwards, i realize i did a lot of things wrong. My car was only about 10 feet away and the neighbor's house was about 20 ft away. But so far, i'm always getting these little tails at the 5 min duration. I could have reduced the tail by a shorter exposure, etc. I think the blurred foreground does indicate the device working. (by the way - crop is from the upper right corner of the DA-21, i think its a pretty good performance from that lens :-))

What do you think - working or not???
I think that it is working - more than it isn't. Let me explain. To me it has appeared that Pentax has tried to make this fully automagic. Perhaps a bit too automagic. I wish that they would publish some numbers or at least a suggested table (focal lengths vs. suggested maximum exposure times). I have been able to go the full 5 minutes with the 10-17 @ 10 or the 8-16 @ pretty much the full range. Even at these wide focal lengths, at the full time frame, I started to see trailing in the corners, but not really in the center. As the focal length gets longer, the exposure time is suppose to be automatically recalculated to become shorter. At 300mm the computed exposure time is pretty short, something like 20-30 seconds. I have gone to some longer focal lengths and the system has let me dial in exposure times longer than what I thought was possible (or even practical). The second time or so, I tried with the focal length, the system came up with shorter exposure times.

At 21mm, normally you would see this type of trailing after about 30 seconds. So, at 5 minutes this is excellent. I just think that there is something of a bug in the unit in terms of how it calculates exposure times. I can't see, but I am also going to bet that the trailing is off into the corners, much more that what you will find in the center of the frame. I am also betting that if you set everything up once at 21mm take a shot at the units suggested maximum. Turn everything off, and then try again, it will probably come up with a shorter maximum time frame (second or third time around). That is why, I wish that they would publish an focal length / exposure table, to serve as an overall guide to folks. Automagic is great - until you have questions. I would give it a try at 3 minutes and 800 or 1600 ISO and see what you get. I am also remembering that the days are getting quite a bit longer up there, so it's probably going to be 10pm before it really starts to get dark.

Personally, it looks like you have some pretty dark skies there. You should be able to easily increase the ISO to say 800 and shorten the exposure to a couple of minutes. Essentially exposing to the right. Then through post processing you should be able to bring out a lot more stars.

20 feet from the house is plenty. 10 feet from the car I think is also good. The unit has an electronic compass that is coupled with the GPS unit. In order to track the stars, it needs to know the physical location (lat, long), time, direction and elevation angle. The lat and long position will provide the magnetic correction to the compass, so that it can calculate the true north vs magnetic north for your physical position. A big hunk of metal (car) close by, doesn't help with the compass' directions.



04-20-2015, 08:04 PM   #32
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QuoteOriginally posted by interested_observer Quote
... I wish that they would publish some numbers or at least a suggested table (focal lengths vs. suggested maximum exposure times)....
Here's a table of marketing materials that gives the times on a K-5 and K-r (the K-r is shorter because it has a less accurate DR system) Specifications | GPS UNIT O-GPS1 | RICOH IMAGING. The estimated times are overly generous and I found that halving them gives more reliable results.
04-20-2015, 09:11 PM   #33
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I have yet to figure out why it works sometimes and other times doesn't. I've shot 300mm without trailing, and I've shot 35mm with trailing coming out of the gate. I was mulling it over yesterday, in fact, and I realized that maybe I need tio insure the camea is level before shooting. If the unit isn't tied to the in-camera leveling then if the camera is cockeyed, maybe the unit will slide the sensor at a cockeyed angle as well.

Next clear night when I can get out and play I'm going to test that theory.
04-20-2015, 09:44 PM   #34
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QuoteOriginally posted by DeadJohn Quote
Here's a table of marketing materials that gives the times on a K-5 and K-r (the K-r is shorter because it has a less accurate DR system) Specifications | GPS UNIT O-GPS1 | RICOH IMAGING. The estimated times are overly generous and I found that halving them gives more reliable results.
Thanks!! I had gone looking for something like this several times, but always came up empty. This is the first I had seen this information. I am somewhat amazed that at up through 100mm you are suppose to still be able to get the full 300 seconds of tracking. I was seeing 300 seconds up through 50mm (thinking in part that something was not right), and then I seem to remember doing something and getting something less. Apparently, I was pointing it in a different direction perhaps. I have essentially gone to a max of 4 minutes in order to not have any trailing in the corners. That seems to work pretty well.

At 0 declination, yes I can believe that the time could be shorter, since the object under observations' angular velocity across the sky would be at its greatest. Also at 90 declination, I can see that it would be pretty slow. It makes sense, I just was thinking of it is a much simpler sense.

QuoteOriginally posted by Sagitta Quote
I have yet to figure out why it works sometimes and other times doesn't. I've shot 300mm without trailing, and I've shot 35mm with trailing coming out of the gate. I was mulling it over yesterday, in fact, and I realized that maybe I need tio insure the camea is level before shooting. If the unit isn't tied to the in-camera leveling then if the camera is cockeyed, maybe the unit will slide the sensor at a cockeyed angle as well.

Next clear night when I can get out and play I'm going to test that theory.
I don't know that being level should have a lot to do with it.

In my younger day, I would pull out the math/celestial navigation books (from sailing the ocean blue) and figure it out. In my present geezer state, all I want to do is take pictures. Just call me intellectually lazy.

I am going to bed and perhaps think myself to sleep on this......



04-22-2015, 07:53 PM   #35
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QuoteOriginally posted by interested_observer Quote
Thanks!! I had gone looking for something like this several times, but always came up empty. This is the first I had seen this information. I am somewhat amazed that at up through 100mm you are suppose to still be able to get the full 300 seconds of tracking. I was seeing 300 seconds up through 50mm (thinking in part that something was not right), and then I seem to remember doing something and getting something less. Apparently, I was pointing it in a different direction perhaps. I have essentially gone to a max of 4 minutes in order to not have any trailing in the corners. That seems to work pretty well.

At 0 declination, yes I can believe that the time could be shorter, since the object under observations' angular velocity across the sky would be at its greatest. Also at 90 declination, I can see that it would be pretty slow. It makes sense, I just was thinking of it is a much simpler sense.


I don't know that being level should have a lot to do with it.

In my younger day, I would pull out the math/celestial navigation books (from sailing the ocean blue) and figure it out. In my present geezer state, all I want to do is take pictures. Just call me intellectually lazy.

I am going to bed and perhaps think myself to sleep on this......

My theory on the camera needing to be level would be if the camera is level, then the GPS known it needs to shift the stars from (say) point -10, -10 to +15, +35 on the theoretical 'grid' the stars are plotted out on. Rotate that grid 45 degrees, but keep the movement along that same path, and you'll have a difference between the two end points. I'm not sure how to describe it other than to say get a piece of graph paper and draw a line. Now, rotate the paper a bit keeping one end of that line in the same spot. There wll be a marked difference (ie, trailing) between where the end point was before and where it now lays.

Of course since I don't have a clue how the unit actually plots its shifting to track, I could be completely off in my theory as well.
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