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Forum: Photographic Technique
06-28-2017, 07:14 PM
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Just got my astrotracer module for a K-5 ( posted my own thread recently), but have been doing unguided experimental astro shots for over a year.
BTW, the astro forum on DPReview is a great source for advise from some very serious photographers who are willing to share.
I take issue with a few of your settings/points of view:
foreground features pose the problem of the astrotracer moving the sensor, so static items in foreground get blurred
with a WA lens you can get away with about 30 sec before the foreground appears blurred, otherwise take advise of previous poster
who explained taking one shots w/o tracer of sharp foreground
crisp stars immediately requires stopping down the lens, especially since WA lens designs are prone to distortions like coma (batwings), chromatic (color fringes) and astigmastism (diagonal lines)
luckily, the sensor tracking supposedly will let you get up to 5min, but realistically, more like 2min (only tracks x-y, but stars move in a great circle), so it is easy to stop down to f4 or f8 ( probably optimum sharpness w/o diffraction)
little noise requires ISO not to be any higher than 1600 but lower is better
The only way to get noise down is by taking multiple images of same subject and "stacking"...more is better, but foreground will move
For every 4x exposure, the noise goes down by 2x ...for scenery, 4 or 16 shots is probably decent
Key setting for any scene depends upon how clear the sky is ie the more atmospheric pollution (city lights, sun glow, moon glow, etc), the noisier it gets.
The histogram will tell you where to set f stop and ISO for a particular exposure time, by simply having the peak intensity located at the 1/3 of left side of screen
the raw out of camera image will look like crap w/o detail/color sharpness
Fortunately, on DPReview, a really sharp guy wrote a program named Sequator v1.3 which is free for downloading and allows single click stacking and correction.
Sequator
I gave an example in my thread ...35mm,120sec,f4,ISO1600, single vs stack of 4 images
Astrotracer O-GPS1 first experience - PentaxForums.com
 Quote: 2: On Camera prompt to enter Focal Length. (The proper focal lenth of 14MM is not an available menu choice, so choose 15MM) yeah, this adds a geometric error which may reduce actual exposure time ...you'll have to experiment
you omitted having to put the camera into bulb mode first, but here you need to do a coarse cal, first, then the precise mode ..two operations
This is WRONG ....turn it OFF
Already discussed by other poster, buy I will amplify ....you set LV to 10x looking at the brightest star near center field of view (easier done w/ long FL).
On a zoom lens infinity will vary with FL, so focus at FL you are shooting ...temperature matters and may not be same in mid winter as summer temps
Some lenses will work at the infinity mark ...test it!
Get an RF remote ( which can be used as wired remote w/o batteries) ...I saw camera shake w/ 2min exposures when clicking by finger and also use mirror lockup.
One click lifts up mirror, second click within a minute releases shutter ....and heaviest tripod you can get your hands on.
https://www.amazon.com/Pixel-Digital-Wireless-Hassleblad-Replaces/dp/B01CJ5T...+remote+pentax
here are two links detailing (more than you might want to know
Nightscape Photography with Digital Cameras, Clarkvision.com
Astrophotography 101: A Lesson Series on Photographing the Milky Way – Lonely Speck
Now have some fun and go out and take some pics!
Stew
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