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05-21-2016, 12:45 PM   #1
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Astrotracer in K-1 and telephoto lenses

My astrophotography practice was based on fast wide and ultrawide lenses until now.
K-1 will be my first Pentax DSLR with astrotracer functionality.
With K-1 I would like to try astrophotography with telephoto lenses if it is possible.
So, is it possible to mount 100,135,200mm f/2.8 lens and get well exposed images without star trails?
If it is possible, what are typical exposure parameters for milkyway with those lenses and astrotracer functionality?
I assume that maximum ISO with K1 will be 12800 (or 3200 if you have similar expiriences with K3II).

05-21-2016, 12:50 PM   #2
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That is possible, but you cannot count on super long exposures, but they will of course be a lot longer than without the astro tracer.
For best result you will have to use stacking of images.
05-21-2016, 02:31 PM   #3
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QuoteOriginally posted by Gimbal Quote
That is possible, but you cannot count on super long exposures, but they will of course be a lot longer than without the astro tracer.
For best result you will have to use stacking of images.
Based on f/2.8 aperture and ISO 12800 I think that 20-30s will be enough for a good exposure.
05-21-2016, 02:47 PM   #4
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There are lots of images and discussions up in the Astrophotography group with focal lengths beyond 100mmYou may want to stack using something less than 12800 - but it all comes down to some experimentation since few have shot with the K1.


05-21-2016, 03:08 PM - 1 Like   #5
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QuoteOriginally posted by banep Quote
With K-1 I would like to try astrophotography with telephoto lenses if it is possible.
So, is it possible to mount 100,135,200mm f/2.8 lens and get well exposed images without star trails?
Yes, it is. Astrotracer works very well, even as you increase the focal length, you just have to compensate by reducing the exposure time. Using the K-3II, anything below 100mm (so 150mm on the K-1) will handle >1 minute exposures so long as you are not aiming for 100% crops. By 200mm you will want to be at or under 30 seconds, and over that you want to keep it short. Playing around I have been able to acceptable shots at 300mm (450mm on the K-1!) when pointing direct overhead and setting it to a 10 second exposure (the fastest I can without manually timing it), but that was just a quick experiment, shooting with that sort of focal length is risky and is vulnerable to slight breezes as well as the earth's rotation! May as well use a proper scope by that point.

Experimentation is key to all of this, have a play when you are in the field, but expect to trash a lot of shots. Such is the life of any astrophotographer!

Now there are some caveats, the astrotracer is not an EQ mount, so it doesn't compensate for field rotation. You need to make sure you do a decent job of your precise calibration at each shooting site, the less precise, the more error there will be. Finally you will always get better results by using a shorter exposure time and taking many shots to combine in decent astro stacking software - the astrotracer gives you an advantage with this, especially with a telephoto lens, but it isn't a replacement (hell, stacking is advised even when using top end computerised EQ mounts!).
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