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08-02-2015, 07:35 PM   #1
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Astrophotog, DSLR Noob

Hi all!

I just got my first DSLR, a Pentax K-3 with the 50mm kit lens. I used to do professional astrophotography work with big scopes and ccd's, and now I'm just a backyard amateur. I've been wanting to get back into astrophotography as I loved staying up to get the shots and then playing around with them in post. It's really neat to capture an image of something so far away. It's especially cool that the same thing can look so different with different equipment and processing techniques. So I finally got my K-3 this week (all you astronomers know what that means... it'll be cloudy for a month now)! I want to do nightscapes and Milky Way shots as well, so I'm saving up for a good quality wide angle lens (probably the samyang 10mm f2.8). I've got a 6" f8 newtonian telescope and a 4.5" newt. I'll be using my DSLR with the 4.5" until I can get the focuser on my 6" fixed. I'm looking forward to seeing everyone's photos and eventually sharing mine, as well as learning from the rest of the community. Clear skies!

08-02-2015, 08:41 PM - 1 Like   #2
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Good Evening and Welcome to the Forum!!

Here is a quick list of some of the main threads. There is the Astrophotography Group also. There are some very accomplished folks here. It might be easier to start from the most recent posts and go backwards, since the camera sensors are so much better now.Probably nothing that you don't already know, but take a look at LonelySpeck's article on the topic. He has a scoring criteria for comparing various lenses, based on their focus length, aperture and the sensor size along with the shutter speed... The title says Canon, but there are a number of lenses that are available in the K mount.He has reduced it to a spreadsheet...The two best are the Rokinon 24mm F/1.4 and the Sigma 18-35/f1.8


Last edited by interested_observer; 08-02-2015 at 08:46 PM.
08-02-2015, 08:41 PM - 1 Like   #3
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kewl beans! welcome aboard! don't have no telescope but really like my cheap 28/2.8 or bower 14/2.8 (which I paid some money for but still cheaper than most) for night landscape, trails or milkyway
08-03-2015, 03:58 AM - 1 Like   #4
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Again, a word of warning over the Sigma 18-35mm f/1.8 - infinity focus on that lens shifts slightly with focal length. The infinity mark and on any lens is never going to be perfectly accurate.Manual focusing is essential for this kind of photography as there isn't a single AF system in the world that is up to the task of focusing on the night sky accurately. Prevailing weather conditions can effect focus shift to a surprising extent.

Personally I wouldn't reccomend the Sigma 18-35mm f/1.8 to a newbie.It is best to do this kind of wide field astrophotography with a prime lens of f/2.8 or faster and with a focal length shorter than 50mm as there are fewer variables to get in the way of producing a satisfying image.

I use the Pentax O-GPS1's astrotracer feature to shoot night sky images without trails.



Pentax K5IIS - Sigma 18-35mm f/1.8 @ f/1.8 18mm 30s ISO 800.

This image was made with the above pictures set-up.


Sigma 18-35mm f/1.8 @ f/1.8 35mm 30s ISO 800

However at the "long" end of the Sigma 18-35mm f/1.8 vignetting is quite problematic, Coma is also beginning to become an issue as well.


Last edited by Digitalis; 08-03-2015 at 04:10 AM.
08-03-2015, 08:48 AM   #5
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Thank you all for the information! I definitely want a prime for nightscapes as a prime will be sharper, less bulky, and less complicated. I had read something about the calibration issues with Rokinon and Samyang wide angle lenses, so I'll keep that in mind when I start using one. The reviewers on the Samyang 10mm tout it's low chromatic aberration and coma, so that's appealing as well. Is there any reason why I should choose Rokinon over Samyang? From what I understand, Samyang makes the Rokinons and it's just the branding that's different. Is the FOV or are the optics better with the 14mm (or rather is the 10mm just too wide or have a little more coma)? I've also been debating between getting that GPS astrotracer or getting an ioptron tracking mount. My astro club friends love their ioptrons. But if I could get away with hauling around less equipment (ie: just the little astrotracer and regular tripod instead of a bigger tracking rig), then that might be better.
08-03-2015, 04:02 PM - 1 Like   #6
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The pentax O-GPS is a useful tool not only for the astrotracer - that is merely a bonus, the GPS tagging ability is very useful when sorting photos.

Compared to a classic barndoor star tracker the O-GPS has its limitations. With a properly made and aligned barndoor it is possible to track a star for considerably longer than the astrotracer can*, the longer you can track a star the more night sky objects you can capture. For the astrotracer to work properly you need to Calibrate it to local geomagnetic fields. This process can sometimes be fiddly and is best done without a lens attached for ease of camera movement. The battery only lasts for about 7hrs of continuous use - in cold weather the operating time will be shortened.

As for your choice in lenses - I would go for the 14mm f/2.8 - that is the limit of usability for wide field Astro imaging as the corners of the image will be smeared with anything wider than this.

For example:

Pentax K5IIs Sigma 8-16mm f/4.5-5.6 @11mm 120s f/8 ISO 3200 Pentax O-GPS1 Astrotracer on.


Upper right corner 100% crop

*
Astrotracer has an upper limit of 5mins with a standard 50mm lens, though this is strongly dependent on where you geographically.

Last edited by Digitalis; 08-03-2015 at 10:59 PM.
08-03-2015, 06:00 PM   #7
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I see. That makes sense. Thanks!
08-03-2015, 10:20 PM   #8
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QuoteOriginally posted by starrhunter Quote
My astro club friends love their ioptrons
that may be the better route since yer part of a club......you will benefit the same tracking time and accuracy they do.....the o-gps will sorta just be a novelty in comparison...especially with longer focal lengths......that's just my assumption though......yes it will be way more convenient hands down.......I have no experience with an equatorial mount but I do with the o-gps

08-04-2015, 04:15 AM   #9
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QuoteOriginally posted by starrhunter Quote
t'll be cloudy for a month now
Just a month eh, it's like that most of the year in Scotland and that's in the day time too.

I'll just say welcome here, as many others are better informed to discuss night sky stuff.
08-04-2015, 07:34 AM   #10
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QuoteOriginally posted by Kerrowdown Quote
Just a month eh, it's like that most of the year in Scotland and that's in the day time too.

I'll just say welcome here, as many others are better informed to discuss night sky stuff.
Thanks! There's a joke among astronomers that whenever you get new gear, it'll be cloudy for a while, or someone will say, "So I have you to blame for this bad seeing!"
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