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06-11-2014, 12:44 AM   #1
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Questions about the O-GPS1 - 16mm/f2 and calibration

Calibration: If I'm doing a bit of Astrophotography I know have to do the precise calibration, but do I have to do the general calibration first or can I skip straight to precise calibration?

Samyang 16mm: When using this lens I have to select a focal length (as it is an 'A' style lens without all the AF & lens data) and 16mm isn't an option so I would select 15mm, but does anyone know how much difference this makes to the results? I haven't had the opportunity to try it myself yet as there have been no clear nights since I bought it

06-11-2014, 01:41 AM - 1 Like   #2
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Good Morning,

Normally entering a focal length for the lens will be used by the camera body for "Shake Reduction" or the image stabilization capability. For the AstroTracking capability/mode with the GPS unit mounted on the camera, the focal length is used to determine the amount of sky or the maximum amount of tracking time that is available. SR or image stabilization is not used at all (since its assumed that you are on a tripod) is it is used to track the stars (by moving the sensor). For instance I have used my Sigma 8-16 lens and at both extremes, the maximum tracking time is 5 minutes. This is the same amount of tracking time for the Pentax 10-17 and a Voightlander 20mm with is also an "A" lens. So what I have found is that from 8mm to 31mm the amount of AstroTracking time is 5 minutes. The times will get progressively shorter the longer the focal length - 300mm is quite short (I have never tried this length yet).

In terms of calibration, you need to turn on the O-GPS unit and the first thing that it does is to find and lock on or track up to 4 GPS satellites - you need 1 for time, and a 2nd and 3rd to calculate longitude and latitude, with a 4th - altitude is determined. This can take from a few seconds to a couple of minutes to achieve. You really do not need to do anything other than wait for the unit to lock on your position. This time is determined by how long the unit has been off and how far from the last positional fix your new location is. When you power it up for the first time, the initial fix was done in Japan, so upon power up it will think that its in Japan and start locking on the birds and see that things are a bit different, so it takes longer to sort things out.

After it locks on to your current position, it will tell you your current latitude, longitude, altitude and time. You can also use the compass and it will tell you the direction that the camera/O-GPS combination is pointing towards. Now, this is the point at which it (the O-GPS unit) wants you to do the fine calculation - where you rotate the unit around each of the three axis - Yaw, Pitch and Roll. I have found that if you do this away from large metal objects (truck or car) it works better. My experience is that this is pretty fast.

So for AstroTracking you need to 1) let the unit determine the location after power on; followed by 2) the fine calibration - the 180 degree rotation around the 3 axis of Yaw, Pitch and Roll. I have found that again if the unit has been turned off for a while, then you may need to do the 3 axis rotation at least once or a couple of times. I have done AstroTracking at one location successfully, turned off the camera and GPS, driven about 5 miles, set everything up again, and started to do the 3 axis calibration and gotten part way through the first axis and the unit gave me the "OK Calibration Complete". So sometimes you need to do all three axis a couple of times all the way to just maybe doing one or two axis once and the system is good to go.

When you switch lenses, you do have to make sure that if your are going TO an "A" lens or a K or M42 to re enter the focal length. This will make a larger difference when there is a big difference in focal lengths - say switching from a 20mm to a 300mm.

I started a thread as sort of log for my experiences with using the O-GPS unit - what has worked for me, etc. It sort of addresses everything from finding a good site (dark) to take an image from - with no moon, to setting up, calibrating, taking images - how long, what ISOs that work well, and all the way through post processing. I have been using it for note keeping across anything related to GPS. There are also pictures - some kind of ok and some not so ok....

Last edited by interested_observer; 06-11-2014 at 01:59 AM.
06-11-2014, 02:16 AM - 1 Like   #3
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QuoteOriginally posted by LennyBloke Quote
Samyang 16mm: When using this lens I have to select a focal length (as it is an 'A' style lens without all the AF & lens data) and 16mm isn't an option so I would select 15mm, but does anyone know how much difference this makes to the results?
You can actually calculate the difference for stars with a given declination. Take a star on the celestial equator (where stars move the fastest) and expose for 5 minutes. In that time span a star will have moved (360/24/60)*5 = 1.25 degrees.

With a 16mm lens the image of the star on and near the centre of the sensor would move 16*2*tan(1.25/2) = 0,35 milimeter if Astrotracer wasn't there to compensate for this movement.

Now, Astrotracer "believes" the FL of the lens is only 15mm and will compensate by an amount of 15*2*tan(1.25/2) = 0,33 milimeter

That error of 0,02 milimeter will correspond to 0.02/0.004 = 5 pixel widths.

Now, that was for stars on the equator and an exposure time of 5 minutes and that is about the maximum error you will encounter. Surely, if you are a die-hard pixel peeper, you may be able to see that star images are slightly elongated, but with a 16mm lens you will be doing wide-field panoramas, so no real need for concerns here.
06-11-2014, 04:38 AM   #4
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Excellent responses - thank you for such a good level of detail and for the additional info. It's really appreciated

06-12-2014, 01:18 AM   #5
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I was able to take a few photos last night when there were a few breaks in the cloud - the info helped - Thanks

I did have a few issues with focussing the Samyang 16/2 - as has been noted on other posts the infinity focus position is way off, so it wasn't easy. Although what I did discover was how good Live-view is for Astrophotography - it is really good at picking up stars that I could barely see, so composition was aided greatly. I'm not sure how good it was for focussing as I haven't had a chance to review the results (on a proper monitor) yet!

By the way...

...while I was shooting the International Space Station decided to do a fly past - so I got a shot of it (a bright white trail across the sky)

Last edited by LennyBloke; 06-12-2014 at 01:23 AM. Reason: forgoet to add I spotted the ISS
06-12-2014, 06:55 AM   #6
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QuoteOriginally posted by LennyBloke Quote
I was able to take a few photos
Congratulations! Looking forward to seeing some pictures from you soon. (I am contemplating the Samyang 14mm or 16mm for myself).

QuoteOriginally posted by LennyBloke Quote
I'm not sure how good it was for focussing as I haven't had a chance to review the results (on a proper monitor) yet!
My experiences is that live view is actually a great help with lenses that focus past infinity or have no focus stop - if you pick a reasonably bright star and use 8-10X magnification. After critical focusing, one can then always re-compose.
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