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pentax astro
Thanks for that, Joe! Yes, the Pentax KP (which is destined to become their flagship model, by the sound of things) does not come with an inbuilt GPS unit, The unit I bought was an optional extra and is sold apparently mainly for people who want to have info on the Exif files of where the photo was taken. It also just happens to have this ASTROTRACER function that uses the movement of the sensor to follow the stars. They reckon that you can shoot a max exposure up to 5 minutes depending what lens you have on it.
To get the ASTROTRACER working you have to turn the camera through 360 degrees on all 3 axes - thus ensuring that you get the strap all muddled up and have to be careful you dont drop it! Apart from that, EASY PEASY
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The attached unit is just a GPS nothing else. ASTROTRACER is a function in the firmware and in the camera. Always there whether you have the GPS or not. But GPS input is essential to the operation. The GPS provides the camera position info. The 3 axis turns calibrate the in camera accelerometer, gyroscope and magnetic sensor, so that the camera is 3D calibrated so the processor knows which part of the sky the camera is pointing at. The processor and firmware ASTROtrace then calculates what movement of the sensor will minimize the movement of stars in the field. If you look at the image on the 2nd URL I gave you :
"Another image about 3/4 of the way down this page
http://joe-cali.com/eclipses/PAST/ASE2013/index.html "
It's taken with a K5, 300s at ISO800 with a 12mm f4. The approximate equivalent of 19s with your lens at ISO1600. The minimization routine results in streaking of the stars top right and bottom left, but everything else is relatively stationary.
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One question, though........All astro photography tutorials say how you have to do long exposures to capture the light - on all cameras - so how is it that last night I took a photo of the Southern Cross with my Pentax, ISO 1600 and F1.4 and 60 sec and it came out looking quite good. I then cranked up the exposure to 200 sec and it came out horribly white - presumably overexposed.... SO What's the go with long exposures of many minutes? How come those pictures aren't washed out??? I must be missing something... HELP
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You are overexposing the milky way....
Exposure is the product of intensity and time.
Your f1.4 lens is much much faster than the lenses and scopes most astrophotographers use.
So the light intensity reaching the sensor is much much higher.
An f1.4 lens collects light : -
2 times faster than an f2 lens
4 times faster than an f2.8 lens
8 times faster than an f4 lens
16 times faster than an f5.6 lens
32 times faster than an f8 lens
Modern sensors are more efficient at collecting light than older cameras which may have been used when those articles were written. I'm not talking about ISO. This quantity is called QE or Quantum Efficiency. Many astrophotographers shoot at ISO800 f5.6
So your ISO1600 @f1.4
at 60s is the equivalent exposure as a 26 min exposure at ISO 800 @ f5.6
at 200s is the equivalent exposure as a 104 min exposure at ISO 800 @ f5.6
Modern cameras sensors are capable of using high ISO but the lower the ISO you use, the better the dynamic range. Dynamic range is the ability of the sensor to rerd large differences between bright and dim light. In astro photos, the presents as white saturated stars, burnt out bright cores of objects such as galaxies, bright nebulae, globular clusters.
My advice : if you are going to continue using the lens at f1.4, then experiment with reducing the ISO and then use longer exposure times with ASTROTRACER.
Joe
......All astro photography tutorials say how you have to do long exposures to capture the light - on all cameras - so how is it that last night I took a photo of the Southern Cross with my Pentax, ISO 1600 and F1.4 and 60 sec and it came out looking quite good. I then cranked up the exposure to 200 sec and it came out horribly white - presumably overexposed....
SO What's the go with long exposures of many minutes? How come those pictures aren't washed out??? I must be missing something... HELP
Thanks so much. You explained much more than all those tutorials did, and in understandable language! I have read about F numbers and shutter speeds, but it just didn't sink in until now! I was starting to wonder how at F1.4 and quick speeds, the image looked better than the long exposures some tutorials insist on! I was reading my Burnhams celestial handbook last night and this bloke in the 1890's took a photo of 2 HOURS! I wonder what his F No was! 
