Originally posted by Digitalis just a question to the 8-16mm usergroup - has anyone used this particular lens with the astrotracer function on the Pentax O-GPS?
I just ran some quick tests (nothing special) in my back yard.
The Astrotracer function worked with Sigma 8-16 @8mm. The orientation (heading) is 203deg (SSW). South celestial pole is about 1/3 from the left and halfway vertically up the frame.
Processing in LR4 to show brightest stars Highlights +100, Blacks -40. This did hide some of the fainter stars and nebula which could only just seen in the image. I hoped to bring out the brightest stars more clearly against a darker sky.
8mm 300sec Astrotracer ON
8mm 300sec GPS OFF
Notes: The "lines" on the right side is my TV antenna (not a UFO
).
There's a huge gum tree obscuring the sky on the left of the shot.
An aircraft flew through the Astrotracer shot on the right side.
In the Astrotracer shot the stars in the centre of the frame are rendered as points pretty much as expected, but in the corners trails are apparent. It could be difficult to see in 1024pixels wide but the trails are "radiating" out from the centre and are longer, the further they are from the centre of the image.
In the standard shot the star trails are apparent as expected in the image and "rotating" around the pole ( with the longest trails on the right side of the image which is furthest from the southern pole)
Testing at 11mm which the K-5IIs can read directly in the exif gave a similar result except 2 planes when right thru the middle of the shot within the 5minute exposure.
I also tried orientations directly west (away from the south pole rotation) and got similar results with accurate point stars in the centre of the frame and trails lengtening out to the corners of the wider field of view. The length of the light trails varies with the distance from the centre of the frame and the orientation of the camera which determines in which direction the sensor moves to get the points in the centre so the trails are different lengths from one side of the frame to the other.
Would i bother with using the 8-16 with astrotracer again? Well yes, but not with the maximum 5minutes exposure as the trails are distracting and noticable. This means it (or any other UWA) wouldn't work for photographing dimmer objects, nebula or more distant clusters. It's a pretty slooooowww lens anyway.
I have done 5 minute exposures with an FA43 and had no issue with trails.
FA43 300sec Astrotracer ON
With longer FL of course there is limit to the exposure time as the sensor will move more quickly and reach the end of its "travel limit" before a 5minute exposure
DFA100mm 90sec Astrotracer ON.
Last edited by steve1307; 09-10-2013 at 06:10 AM.