[zoolander]

I have luckily scored a nice 400mm lens a few weeks back, and I would like to use this for astrophotography soon.

I do not have the GPS unit for Pentax yet, nor an equatorial mount, and I was wondering which would be better with this 400mm lens ? Or are then any drawbacks to using a Sigma 400mm f/5.6 tele macro for astrophotography ?

If you have any experience with this lens for this task, I'd like to hear of your experience.

If you know of any good links to sources for using the Pentax GPS, like turorials etc that could really help.

Thanks in advance !

[jawsy]

I'm planning a few astro images with the K-3ii when it arrives myself but at a shorter focal length.

From what I know thus far, the 400mm lens will be at the lower limit of usability with the tracer unit. At 400mm the star points will try to 'travel' across the sensor a further distance than for say a 15mm shot. For this reason, the sensor SR servos will run out of available movement pretty quickly.

There are a few threads in PF such as the tracer accessory reviews that go into some more detail. Type in 'Pentax GPS' into YouTube and there's some there too.

An equatorial mount would be pricy but would obviously give better results.

All that aside, you will get some cracking moon shots with a 400mm lens without tracing or equatorial mounts.

Others with first hand experience will be able to give you a better answer than I can.

[Stone G.]

Further to the above: An f/5.6 system is a bit slow for use with the O-GPS1 due to the fairly long exposure times required. I have had some success with a 350mm/5.6 lens, but you will be quite constrained.


So, a better solution would be an eqúatorial tracking platform. See for example here:


Astrophotography Part 2 of 6: Equipment Selection - Articles and Tips | PentaxForums.com

[Kendigitize]

Without tracking, it will be tricky. I've used my Auto Sears (Tokina) 300mm 5.6 and K-30 to get the Orion nebula. I have no tracking unit, so a 3sec exp was about the longest I could go without star trails. I used F5.6, 3Sec and a HIGH ISO of 10,000 & LightRoom. I think if I had taken several shots and stacked them, I could have had better results and more nebula glow. I did build a barn door tracker that worked well with my big lenses. Good Luck!

[zoolander]

So, a better solution would be an eqúatorial tracking platform. See for example here:


Astrophotography Part 2 of 6: Equipment Selection - Articles and Tips | PentaxForums.com

Thanks Stone G. I've already read this article, and it claims you can only get a 20 second exposure at best with the Pentax GPS and a 400mm lens. So I'm wondering whether I should invest in one of the equatorial mounts for the longer duration of exposure.

I'm just wondering if that really is it, 20 seconds max with a 400mm.

---------- Post added 06-18-15 at 11:24 AM ----------

I did build a barn door tracker that worked well with my big lenses. Good Luck!

That sounds cool, I'll look into it also.

[Stone G.]

just wondering if that really is it, 20 seconds max with a 400mm.

With the O-GPS1/Astrotracer, based upon my experiece I should say "yes it is". On a good day, you may go a bit further - up to some 40 seconds:



K-5+350mm f/5.6 image of globular cluster M13: 5 images at 40s plus 10 at 25s plus 10 at 15s.
All at ISO 1600 and stacked in Deep Sky Stacker.


Contrary to polar alignment - which can in theory be made "infinitely accurate" at any location - 'Precise Calibration' of the O-GPS1 will always depend on a number of external factors, notably localized variances and disturbances in the magnetic field, and even small errors will become evident with long lenses, especially if you crop your images rather tight as in the example above. And isn't this the type of objects, one would use the longer focal lengths for?


More O-GPS1/Astrotracer examples here: DSLR Astophotography: Astrotracer Images with Pentax GPS Unit O-GPS1