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09-16-2014, 01:19 AM   #1
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Astrotracer O-GPS1 strange behaviour

After reading the posts concerning troubleshooting when using the GPS unit for astrotracing, I still have no answer for my O-GPS1 behaving very silly:

The first photos are very good, no startrails, everything fine, and from one moment to another, the following pictures show startrails as if I would have turned off the GPS unit - basically no or only limited tracing.

I checked what I could: firmware (K5 II) is the latest, batteries fresh, unit is working and displaying 3D (indicating more then 4 satellites found), it's not mirror slap or shutter blur, just plain startrails from one moment to another. General calibration and astrotracing fine calibration were successfully executed. Exposure time was 40 seconds, the unit showed up to 4:40 mins as possible. First exposures were closer to horizon, but better traced.

In addition, I find that the compass likes to change altitude very unpredictably: if you take a look after calibrating, it shows 492 m over sea level, 10 minutes later we are up at 502 m, being rooted to the spot, mind you!

Any similar oddities among the Pentax astro photographers? Any help?

09-16-2014, 02:37 AM   #2
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Help with astrotracer problems!

Hi,
Can you please state the exposure details for the images without the trails and the exposure details where you were experiencing the trails!
09-16-2014, 03:23 AM   #3
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I usually calibrate it twice, just for the sake of it. I try not to shoot near or on anything made out of iron, like a bridge or next to my car. If I got my smartphone on me I trigger the exposure with my remote and keep my distance until the rig has finished it's business.
09-16-2014, 04:50 AM   #4
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QuoteOriginally posted by Vranx Quote
Any similar oddities among the Pentax astro photographers? Any help?
There are several things that can go wrong, so it is difficult to tell. Could we have some more details, for example:

1. Are you always shooting from the same site - there could be intermittent electromagnetic interference. Have you tried different sites?
2. Does your exif data show GPS coordinates and operational mode as it should? For all pictures?
2. What lenses were you using? Do you see the same issue with different lenses? - zoom creep could be an issue.
3. If you are using FA- or DA-lenses: Is the communication between lens and camera OK? If the camera (sometimes) gets the wrong information about FL, you will get trails.

09-17-2014, 12:23 PM   #5
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Thanks for these helpful answers! Here some more details:

- the lens was fixed focal lens SMC-M 2.8/120 in manual focus mode, correct fl put in, cable release to initiate exposure
- all exposures were 40 sec @ ISO 800, identical for blurred an correct photos, no wind
- GPS information is recorded for all photos, showing only minor changes, like N 48°32.764' to N 48°32.762', which is caused (as I suppose, by moving the tripod head to a new position), all EXIF data makes sense
- I've indeed shot the bunch were I noticed it first ca 3m away from my car, so I tried again in the garden, carefully stepping back from all magnetic sources - no difference
- after the blur started to show in the pictrues, I recalibrated GPS and Astrotracer calibration again
- I never even tried long exposures over 50 secs at all, because there is a certain amount of blur
- tripod is a solid 3kg Manfrotto, not fully extended, no central column in use

Does that help?
09-18-2014, 12:28 AM   #6
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QuoteOriginally posted by Vranx Quote
Does that help?
Well, it does seem that you have ruled out most of the more "natural" possible causes.

Next in line for scrutiny would be the O-GPS1's compass then. Something in your description indicates, that the magnetic compass might stop working properly at higher altitudes (: pitch angles).

It is easy to check the compass heading recorded against the true geographical heading (checking against prominent landmarks). I recommend that you use the 'Geosetter' software (freeware) for that: Is the heading reasonably close, i.e. within + 10 degrees? Is it reasonably steady - i.e.: Does it flicker a lot from picture to picture, (more than a few degrees)?

Also check the behaviour of the compass 'needle' on the LCD screen to see, if it is indeed steady. When I photograph from my city site, I once in a (rare) while see the compass needle rotating madly, as if I was close to for example a windmill power generator generating airborne magnetic disturbances.

If the compass seems to be working allright you should check Exif-data in respect of pitch- and roll angeles to see if there are indications of something suspicious about the accelerometers/inclinometers in your camera. For this check you will need access to all EXIF-data by means of a software such as "ExifTool" by Phil Harvey (also freeware and also used in 'GeoSetter')

If you cannot find anything here either, the last thing that I can think of would be some flaw in the actuators that steer the camera sensor.........

I do hope you will find a solution. I have taken hundreds of astrophotos with Astrotracer (well thousands if you count the subframes in) and for me, it is one of my best investments in camera gear ever.
09-18-2014, 12:31 AM   #7
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Well all I can say at this point is I have avoided using lenses at that focal length of 120mm, the magnification at that FL is likely to cause blurring. To check that the Astrotracer is functioning correctly is to try with a maximum of 50mm and check the results. I've found the AT has limited use, 120 secs or more and longer FLs show up it's limitations. It certainly can't match a real celestial tracker!
09-19-2014, 04:20 PM   #8
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QuoteOriginally posted by Dave's clichés Quote
Well all I can say at this point is I have avoided using lenses at that focal length of 120mm, the magnification at that FL is likely to cause blurring.
Not necessarily so. My main interest is deep-sky objects and my favorite focal lengths with the Astrotracer are 50mm (FA 50/1.4), 200mm (DA*200) and 300mm (Tamron Adaptall-2 Model 54B) and 350mm (Tamron Adaptall-2 Model 06B).

At 200mm and above, I rarely exceed 30s of exposure time, and for some reason I have never really used my M 100mm/2.8 for astrophotography since I used it for trying out the Astrotracer when I got my O-GPS1 about two years ago. But inspired by this thread I went out on my 4th ffloor balcony (in the midst of a light polluted city) this night and tried a couple of shots at various altitudes and azimuths. I havent developed all sets yet, but here are just two results.

First a 100% non-resized crop with Globular Cluster M13 in Hercules:


Pentax K-3 with smc Pentax-M 100mm f/2.8 and Astrotracer. 15 images stacked in Deep Sky Stacker. 7 images af f/5.6 and
8 images at f/2.8;both sets at ISO400 and 40s exposure time.

Next an unprocessed, single JPEG straight out of the camera with bright stars HR7162 and Beta Lyra/Sheliak near the diagolnal - and yes, the grey dot midway between these stars is the Ring Nebula, M57. Again, I show a 100% non-resized crop:


Pentax K-3 with smc Pentax-M 100mm f/2.8 and Astrotracer. Single exposure at f/2.8, ISO400 and 40s exposure time.

I don't say that all O-GPS1- and camera units are equal (although they ought to be!), but at least for me: It can be done!

09-22-2014, 01:00 AM   #9
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Hi Stone G.,

but from the unprocessed last picture I conclude that a minor amount of startrail blur is normal even for 100 mm and 40 secs? My impression of other Astrotracer picture results was that it should indeed be more dot-like (imagine this blur sum up in a 3 minute exposure)

What I've found out so far: sometimes the GPS device turns off and on again, probably due to battery compartment contacts contracting in cold atmosphere, which means loss of calibration. And I have to find a non-magnetic tripod head (the Manfrotto 3 way is rock solid, but all-metal).

Another no-go is changing the lens. Even if the new focal length is manually put in, calibration is gone, no matter whether old K and M or new FA or DA lenses.

Tricky little thing it is.

But thanks so far for all your assistance, guys! I'll try more and let you know.
09-22-2014, 07:53 AM   #10
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QuoteOriginally posted by Vranx Quote
but from the unprocessed last picture I conclude that a minor amount of startrail blur is normal even for 100 mm and 40 secs?
Yes - remember this is a 100% view, so you are truly 'pixel-peeping' here. With a portable equatrorial mount you may well have to struggle a bit with polar alignment to get an equally good result. And in my case, I am surrounded by all sorts of magentic materials, so all in all, I'm quite happy......
09-23-2014, 04:47 PM   #11
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my experience with the Astrotracer: about 50% of the images show some minor star trails. Further, the behavior gets worse with wider lenses (kind of counter-intuitive until you think hard about exactly how Pentax engineers pull off this magic). My take is that this device is optimized for 50mm lenses and higher. Given the "randomness" regarding star trails, my recommendation for anyone using this device is to always take at least three images with the same settings -- in most cases at least one of the three will be "spot on",

YMMV

Michael
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