[wizofoz]

Wow great stuff here! I've been taking wide images using my K-5 and DA 15 of the night sky trying to catch meteors. I got a few Orionids on Sunday morning and here's the best one (2 image composite - 1 for the meteor and 1 for the foreground which I light painted in one of the exposures).



I used the camera's built-in timer to take 100 exposures at 13s, f/4, ISO 3200.
The single exposures without meteors aren't all that interesting but if I stack them all it's pretty cool looking.



These posts have me intrigued about getting some kind of device to correct for the movement like that O-GPS1 but I'm a little confused at exactly what it does. I know it can geo tag photos but without a special robotic mount how can it compensate for movement when your camera is just mounted on a tripod?

Matt, I have yet to venture out in the dark with it, but I recently got the O-GPS1 - inspired mainly by this thread. I understand that it moves the sensor in the camera. I understand it uses GPS data to work out where the camera is located with respect to what it is pointed at, and then moves the sensor to compensate for the earths' rotation.

No doubt some with more experience can give us more information. I'm keen to learn as well.

[Stone G.]

Matt, I have yet to venture out in the dark with it, but I recently got the O-GPS1 - inspired mainly by this thread. I understand that it moves the sensor in the camera. I understand it uses GPS data to work out where the camera is located with respect to what it is pointed at, and then moves the sensor to compensate for the earths' rotation.

No doubt some with more experience can give us more information. I'm keen to learn as well.

Well, you have actually mentioned the essentials yourself, but here's a bit more elaborate explanation:

If you know

your 1)compass heading (Azimuth) and 2) the height above the horizon (Altitude) at which your lens is pointing and 3) your geographical co-ordinates plus 4) the date and time,


then

it is straightforward to calculate the declination and right ascension (R.A.) of the point/star at which you lens is pointing.


Further

Knowing the declination and R.A. of a star, you can also calculate the angular speed and direction at which the star will be moving across the sky at a given date and time.


Finally

Knowing the focal length of your lens and the inclination of your caméra, you can convert that to the linear speed and direction at which the image of your star will be moving across the camera sensor plane.

All of these data sets are compiled and processed in the camera firmware.


The ingenious trick

of the Astrotracer function is then to use the shake reduction actuators in the camera body to move the sensor to counteract the travelling of the star's image during exposure. You might say that the sensor is moved in such a manner that it works as if the camera was mounted on an equatorial mount. A very clever exploitation of the Pentax in-body Shake Reduction mechanism indeed!

Obviously there are limits as to how much the sensor may be moved and thus, how long exposure times one can have at a given declination and a given focal length. Also, the accuracy of the (true geographical - not magnetic) compass reading is quite critical for the calculations which further limits the accuracy that one may reasonably expect.

[mattb123]

So how long of an exposure does this give you then? I imagine it varies by all those variables (FL, location, orientation) but how much of an advantage does it give? Anyone got an example? I typically don't go over 15s with my 15mm to keep the stars sharp. How much more time could I expect using the O-GPS1 while keeping my subject sharp?

[ErZi]

There is this Pentax O-GPS1 - Astrotrac time table. | Flickr - Photo Sharing!

But you need to have a good calibration to achieve those times with longer lenses

[stephp]

Wow great stuff here! I've been taking wide images using my K-5 and DA 15 of the night sky trying to catch meteors. I got a few Orionids on Sunday morning and here's the best one (2 image composite - 1 for the meteor and 1 for the foreground which I light painted in one of the exposures).

These posts have me intrigued about getting some kind of device to correct for the movement like that O-GPS1 but I'm a little confused at exactly what it does. I know it can geo tag photos but without a special robotic mount how can it compensate for movement when your camera is just mounted on a tripod?

Very nice pictures mattb
The way the O-GPS1 works is very ingenious: it use the anti-shaking mechanism that allow movements (translation & rotation) of the sensor. The exposure time is therefore limited by the focal length of the lens you use: the longer is the focal length, shorter is the exposure time.
Actually, with a 200mm lens, you are able to shot in a range from 30sec to 1 minute depending of the declination.

[stephp]

But you need to have a good calibration to achieve those times with longer lenses

The astrotracer time table is a bit optimistic for longer lens, from my experience (see my previous post).
However with a 50mm lens the table is right.

[mattb123]

Cool, thanks!
Does anyone know if it works with A or M lenses that don't transmit info to the camera? Seems like it might have all the info it needs if you shoot with a A or M prime and enter the FL.

[wizofoz]

Thanks for the info Stone G

[stephp]

Cool, thanks!
Does anyone know if it works with A or M lenses that don't transmit info to the camera? Seems like it might have all the info it needs if you shoot with a A or M prime and enter the FL.

I'm pretty sure it works with A lens as well as with M lens. With M lens you have to manually enter the focal length, that is sufficient for the astrotracer to compute sensor movements. The autofocus system is deactivated when you use the astrotracer function. You have to focus manually in live view mode.

[mattb123]

Great. Thanks for all the info!

[Stone G.]

Cool, thanks!
Does anyone know if it works with A or M lenses that don't transmit info to the camera? Seems like it might have all the info it needs if you shoot with a A or M prime and enter the FL.

It surely does work with M and A lenses. Here is an image with the fully manual Tamron SP 350mm f/5.6 lens (Adaptall-2 system, Model 06B):



Globular cluster M13 in Hercules: Combined stacks in DSS of five 45 second exposures, ten 25 second exposures
and ten 15 seconds exposures. All taken at ISO 1600. Resulting image post-processed in PhotoImpact. Click to see
non-resized image (crop).

To my experience, a 350mm lens is about the maximum long FL lens that one can sensibly use together with Astrotracer. Exposure times beyond 45 secs. to one minute are unrealistic and often one will have to restrict oneself to some 20 seconds. Calibration (the outcome of which you don't fully control yourself) plus local magnetic disturbances are crtical. One should expect a discard rtatio of about 70% if the goal is reasonably detailed crops like the one above at such long focal lengths.

[stephp]

Here is an image with the fully manual Tamron SP 350mm f/5.6 lens (Adaptall-2 system, Model 06B):

To my experience, a 350mm lens is about the maximum long FL lens that one can sensibly use together with Astrotracer. Exposure times beyond 45 secs. to one minute are unrealistic and often one will have to restrict oneself to some 20 seconds. Calibration (the outcome of which you don't fully control yourself) plus local magnetic disturbances are crtical. One should expect a discard rtatio of about 70% if the goal is reasonably detailed crops like the one above at
such long focal lengths.

Very beautiful picture Stone G.

Now I've to try my old A*300/4 on the deep sky !

[mattb123]

Very beautiful picture Stone G.

Now I've to try my old A*300/4 on the deep sky !

That's the lens I'm thinking of too.

[Stone G.]

Very beautiful picture Stone G.

Now I've to try my old A*300/4 on the deep sky !

Thank you very much stephp - and I look forward to see your results with the A*300/4.

If you can get about the same hit rate as I do, I am sure we are in for some more great photos from you - - - - - although I must say that I visited a photo exhibition this Sunday and was allowed to take some test shots with the DA* 200/2.8 on my own K-5 body. That lens of yours will be hard to beat!!!

[ibbe]

Hi m8,after bougth the gps1 and download the firmware I have managed to take great astropicture, But it seem that SR wont work again then gps is off, so that to do to get a working SR back?