Ok, This is a bit of a conundrum to me.
In almost any mode My K3 will ask what focal length I am using when I install a non auto, very old manual M lens.
However in Bulb mode, it does not, nor is there a way to input this manually.
As I have seen in action, the camera, when paired with the 0-gps1 determines the length of time you can expose without star trails
based on the focal length of the lens.
This makes sense when I install a newer lens that reports this to the camera automatically.
So how does it determine the exposure time for an older type M, non auto lens in Bulb mode when neither the lens, nor the user can input the focal length ?
If the exposure time is determined by focal length and not FOV how can this change ?

Puzzled.

DeWolf

[slartibartfast01]

Ok, This is a bit of a conundrum to me.
In almost any mode My K3 will ask what focal length I am using when I install a non auto, very old manual M lens.
However in Bulb mode, it does not, nor is there a way to input this manually.
As I have seen in action, the camera, when paired with the 0-gps1 determines the length of time you can expose without star trails
based on the focal length of the lens.
This makes sense when I install a newer lens that reports this to the camera automatically.
So how does it determine the exposure time for an older type M, non auto lens in Bulb mode when neither the lens, nor the user can input the focal length ?
If the exposure time is determined by focal length and not FOV how can this change ?

Puzzled.

DeWolf

If it really doesn't allow the focal length to be input in bulb mode change to another mode, input the focal length and then return to bulb mode.

[stevebrot]

In almost any mode My K3 will ask what focal length I am using when I install a non auto, very old manual M lens.
However in Bulb mode, it does not, nor is there a way to input this manually.

Works for me in B and all other modes, though I don't have the O-GPS1 mounted. Therein might be your clue. I might also add that the camera will only prompt on start-up and never when merely changing lenses, though you should still be able to change the setting in menu --> capture3 --> Input Focal Length.


Steve

Works for me in B and all other modes, though I don't have the O-GPS1 mounted. Therein might be your clue. I might also add that the camera will only prompt on start-up and never when merely changing lenses, though you should still be able to change the setting in menu --> capture3 --> Input Focal Length.


Steve

Well yes I can change it in any other mode.
However the menu option for focal lenght is greyed out in Bulb mode, even if the GPS is dismounted and the camera restarted.
How do it know ? lol

---------- Post added 03-07-20 at 09:18 PM ----------

Works for me in B and all other modes, though I don't have the O-GPS1 mounted. Therein might be your clue. I might also add that the camera will only prompt on start-up and never when merely changing lenses, though you should still be able to change the setting in menu --> capture3 --> Input Focal Length.


Steve

also the old M lens I am using, well one of them, ia an Asahi Smc Pentax-M 28Mm F2.8 . and also a 50mm with the screw adapter. no contacts on either

[stevebrot]

Well yes I can change it in any other mode.
However the menu option for focal lenght is greyed out in Bulb mode, even if the GPS is dismounted and the camera restarted.
How do it know ? lol

---------- Post added 03-07-20 at 09:18 PM ----------


also the old M lens I am using, well one of them, ia an Asahi Smc Pentax-M 28Mm F2.8 . and also a 50mm with the screw adapter. no contacts on either

Well, you got me! I have been shooting non-data contact* glass (M42, K, and KA) on my K-3 since I got it in 2014, including B mode, and have never had an issue with setting the focal length. The expected behavior is on page 82 of the user guide (LINK) and the general rule is that if the camera does not detect focal length when turned on, it asks. If the currently mounted lens does not have a data contact, the focal length is always settable in the capture menu.

There is an outside chance that cleaning the mount contacts on the body might help as might updating firmware.


Steve

* KAF and newer auto-focus lenses as well as KA2 non-AF lenses have the extra "data" pin that flags the body that focal length and other information is available through the mount contacts.

[ramseybuckeye]

I had though that possibly since shake reduction is off in Bulb mode it would not ask for the focal length, but my KP does ask. There must be another setting that is causing this.

Well, you got me! I have been shooting non-data contact* glass (M42, K, and KA) on my K-3 since I got it in 2014, including B mode, and have never had an issue with setting the focal length. The expected behavior is on page 82 of the user guide (LINK) and the general rule is that if the camera does not detect focal length when turned on, it asks. If the currently mounted lens does not have a data contact, the focal length is always settable in the capture menu.

There is an outside chance that cleaning the mount contacts on the body might help as might updating firmware.


Steve

* KAF and newer auto-focus lenses as well as KA2 non-AF lenses have the extra "data" pin that flags the body that focal length and other information is available through the mount contacts.

ok. cleaned the body and lens surface areas where they connect, upgraded firmware.
Bulb does not ask for focal length but does allow me to input one now, even with the GPS-1 installed and on.
Very odd, but one never knows all the stuff they do with firmware.
Next question would be, If I am using a 28mm on a 1.5 crop sensor, does the 0-gps-1 take that into account ? or should I set the focal length to the 35mm equivalent, say 42mm ?

I know a lot of the star trailing issue is location and how many satellites you can receive. I usually get 10 satellites on precision calibration. But I still get considerable trailing with a 300mm at 10 seconds, while others are getting no trails at 20 seconds.
Perhaps my geographic location is the issue. I understand the unit was not meant for more than a 200mm lens.
Still, this unit is a gas to play with, and with the wider lenses, say the 100mm macro, I have gotten some great shots of the Orion Nebula.
I get no calibration facing East, rarely South, but get one after a minute facing north and facing west I get a calibration OK before I cn even finish 1 full maneuver set.
Thanks for the cleaning/update hint. I clean regularly, but then I change lenses a lot as well.
So likely grime accumulated.

https://www.pentaxforums.com/forums/members/64086-dewolf/albums/13550-astro/picture129522.jpg

[SteveinSLC]

I don't think the GPS or astrotracer are influenced by the focal length. Possibly the calibration by a larger lens with more metal, but that's about it.

The focal length does affect tracking time possible, but that's more due to the optics than the GPS or astrotracer.

[jbinpg]

The calibration is for determining local magnetic north so an offset can be applied to your magnetic variation based on what it should be for your lat and long determined from the GPS. For a given focal length, the relative velocity of the starfield will determine exposure limits for the O-GPS unit. The celestial equator cuts through Orion so it will have the least relative rotation across the frame but the fastest star speed. Likewise at the celestial poles, rotational speed will be highest and transverse speed the lowest. Since the O-GPS has limited ability to track rotational movement, high celestial elevations will be difficult for it to track. Thus, there is a sweet spot at about 45 degrees where transverse and rotational speed tradeoffs are at a minimum.

I don't think the GPS or astrotracer are influenced by the focal length. Possibly the calibration by a larger lens with more metal, but that's about it.

The focal length does affect tracking time possible, but that's more due to the optics than the GPS or astrotracer.

My thought would have been, that since the apparent movement of the stars against the earths rotation seems accelerated with higher powered lenses, then the gps would take that into account and accelerate the movement of the sensor.
But now I see it's the FOV that matters, and that for the camera, the sensor is always the same size and the same number of pixels. The only difference is as the subject gets magnified each movement is captured on a greater number of pixels and that the sensors movement would remain the same, but the time it took for it to transverse that smaller FOV would be less, resulting in a shorter exposure time for a 300mm as apposed to a 50mm, but I am curious as to how it comes up with the exposure time in advance before the shutter is pressed if it does not know what lens you are using.
somehow it can, as after calibration with a 28mm, I changed lenses and it knew somehow and shortened the exposure time.
Yup theres a mystery in that little addon lol.

---------- Post added 03-08-20 at 12:38 AM ----------

The calibration is for determining local magnetic north so an offset can be applied to your magnetic variation based on what it should be for your lat and long determined from the GPS. For a given focal length, the relative velocity of the starfield will determine exposure limits for the O-GPS unit. The celestial equator cuts through Orion so it will have the least relative rotation across the frame but the fastest star speed. Likewise at the celestial poles, rotational speed will be highest and transverse speed the lowest. Since the O-GPS has limited ability to track rotational movement, high celestial elevations will be difficult for it to track. Thus, there is a sweet spot at about 45 degrees where transverse and rotational speed tradeoffs are at a minimum.

Interesting.
The manual says the compass determines or displays true north, not magnetic. so apparently the satellites report magnetic, the firmware translates that for the compass, but uses the original magnetic data for the actual photographic tracking.
The 45 degrees thing makes sense as I have found some objects leave little to no trail, while others seem to leave one regardless even after the calibration was done.

[jbinpg]

I don't think the GPS constellation satellites report local magnetic readings at all. They just enable location reporting and then the local magnetic variation calculation is done via a lookup table on the O-GPS unit itself. The calibration is an attempt to fine tune the local magnetic variation so that a better true north can be displayed via the Compass function. There are satellites in orbit that carry sensors that measure the Earth's magnetic flux but GPS satellites are not so-equipped.

[SteveinSLC]

But now I see it's the FOV that matters, and that for the camera, the sensor is always the same size and the same number of pixels. The only difference is as the subject gets magnified each movement is captured on a greater number of pixels and that the sensors movement would remain the same, but the time it took for it to transverse that smaller FOV would be less, resulting in a shorter exposure time for a 300mm as apposed to a 50mm, but I am curious as to how it comes up with the exposure time in advance before the shutter is pressed if it does not know what lens you are using.
somehow it can, as after calibration with a 28mm, I changed lenses and it knew somehow and shortened the exposure time.
Yup theres a mystery in that little addon lol.

As far as I know, astrotracer doesn't do this...Exposure time works the same way as it does with or without it. It's controlled by the shutter release, which you handle manually in M or B mode. You are free to expose as long as you want no matter the focal length of your lens. Because of the limits in what astrotracer does, that is a shorter time for longer lenses, but that time is not something that is internally controlled. And whether or not you get trails for a given time length does vary based on the elevation and how good your calibration is. That's largely unrelated to the lens, and in fact will vary between sessions using the same lens.

As far as I know, astrotracer doesn't do this...Exposure time works the same way as it does with or without it. It's controlled by the shutter release, which you handle manually in M or B mode. You are free to expose as long as you want no matter the focal length of your lens. Because of the limits in what astrotracer does, that is a shorter time for longer lenses, but that time is not something that is internally controlled. And whether or not you get trails for a given time length does vary based on the elevation and how good your calibration is. That's largely unrelated to the lens, and in fact will vary between sessions using the same lens.

well during a session it will change. If I start seeing trails and re-calibrate and they go away.
There is an acceptable drift in satellite orbit over their lifetime. It's usually small but it is there.
However that shouldn't affect the gps unit.
Atmospheric conditions in Florida alone could cause calibration issues. A signal passing through 90% humidity has to suffer a little I would think.
I wonder if the antenna on the unit could be modified to half wave from quarter wave or something lol, just me thinking.
I am always up for a good challenge but I would certainly need the papers on it and that is not likely
Eventually I will get a telescope mount for this purpose.

[AstroDave]

There is an acceptable drift in satellite orbit over their lifetime. It's usually small but it is there.
However that shouldn't affect the gps unit.
Atmospheric conditions in Florida alone could cause calibration issues. A signal passing through 90% humidity has to suffer a little I would think.

The satellite orbits are continuously monitored and accounted for in the GPS broadcasts.

At the level we are concerned with, humidity has NO effect on astrotracer operations. It might cause your apparent camera position to be off by a few centimeters but that will have no effect on astrotracer operation. You would have to be mislocated by 10 to hundreds of meters before the sensor motion would not be adequate to stop star trails (assuming everything else was set up correctly - which seems to be the achilles heel of astrotracer operation!).

The satellite orbits are continuously monitored and accounted for in the GPS broadcasts.

At the level we are concerned with, humidity has NO effect on astrotracer operations. It might cause your apparent camera position to be off by a few centimeters but that will have no effect on astrotracer operation. You would have to be mislocated by 10 to hundreds of meters before the sensor motion would not be adequate to stop star trails (assuming everything else was set up correctly - which seems to be the achilles heel of astrotracer operation!).

true that, it is a fussy thing, but sometimes the challenge is rewarding

Thanks for all the info mate