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09-30-2014, 12:18 PM   #1
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Lenses for Astrophotography for Pentax K-5 II with Pentax O-GPS1

... finally went through and bought a Pentax K-5 II with a kit lens (the lens was for free actually). After almost 4 years of mingling with Micro Four Thirds I decided that I need to get back to my love of Pentax and Astrophotography. I was so tempted to go for Fujifilm for there unbeatable High ISO Performance but even with the sensor advancements of Micro Four Thirds and Samsung (NX1 is so good specked) and Sony's low pricing of A7 (even 1000 Euros in some places) they still lack one thing: Pentax O-GPS1. And how many small, portable, lightweight, good IQ, and under 300 Euros tracking telescopes do you know?

Now before I get the O-GPS1 I have to decide on a few lenses to use for astrophotography, and the general rule would be to go as fast as possible but that is one of the problems of Pentax K mount, there aren't many such lenses unfortunately.

At wide angle you only have the Samyang very good prices and IQ lenses (like the 10mm f 2.8, 14mm f 2.8, 16mm f 2) and one very expensive Pentax DA 14mm f 2.8. I would like to sacrifice 3 thirds if a stop of light for the Tamron 10-24mm f 3.5-4.5 / Sigma 10-20mm f 3.5 / Pentax 10-17mm f 3.5-4.5 or even more then one stop for the marvelous IQ Sigma 8-16mm f 4.5-5.6 (in thinking that sensors get better performance, lenses you own don't get change often enough to get wider or faster). Can't find a solution for the wide angle problem yet.

Now to a fast normal (for more panoramic shots deep space objects) I was thinking that 50mm f 1.4 would be enough but I do love the idea and the IQ of Sigma 18-35mm f 1.8 not only for speed but also I DO use the camera and lenses for OTHER stuff, like daylight shooting ? So maybe I will reserv that spot of an legacy lens (but I fear of bad coma optimization for there are not many legacy lenses with Aspherical elements to correct it, and it it where they are to close to modern and corrected lenses, like Sigma 50mm f 1.4).

... and to the biggest question so far. Deep space objects need FL ... and lot's of it (M33, M42, M43, Orion's Sword, Flame Nebula, Horsehead Nebula. etc). But speed would be needed to, to help to get longer shutter speeds or lower ISO usage. For cheap solution I was thinking of Tamron SP 70-200mm f 2.8, maybe use a teleconverter to get a bit more reach but not lose to much speed. I do find Sigma 70-200mm f 2.8 very well corrected for coma and not bad performance but a bit more heavy and more expensive. Pentax DA* 200mm f 2.8 is not very lightweight nor is it cheaper then Tamron to get a better solution, plus I would lose the 70 to 199mm part to. Would a standard zoom like the Pentax DA 55-300mm f 4-5.8 WR or Tamron / Sigma 70-300mm f 4-5.6 handle at there speed deep space objects (with the always recommended and needed image stacking).

I might try some "unorthodox" lenses, mostly for lunar imaging or even photographing some sun spots, like the Samyang (or equivelent on the market) 500mm f 8, 500mm f 6.3 Mirror, 800mm f 8 Mirror, 600-1300mm f 11 for there dirt cheap price and, possible, fun element.

I'm looking for some help some ideas, opinions. recommendation to get a few lenses for Wide Field Astrophotography and Deep Space Astrophotography within a 2500 Euro's budget (I will spend almost two year's worth of saving). I do feel a little concerned about the weight of the lenses (the reason I used Micro Four Thirds before) but if I have to sacrifice that to get from this:



... to this:



... then I will take it. Maybe one day I can afford two systems (still dreaming at Panasonic GX7 + Panasonic 12-35mm f 2.8 + Panasonic 35-100mm f 2.8 = 1KG of goodness).


Last edited by Blue; 09-30-2014 at 06:48 PM.
09-30-2014, 01:03 PM - 1 Like   #2
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Perhaps some unusual suggestions but:

FA 31mm f1.8 great lens for anything

DA 20-40mm f2.8 - 4 I'm intrigued as to how this might perform for night/astro
09-30-2014, 01:20 PM - 1 Like   #3
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The deeper your target in light years and the longer the focal length of your lens/scope, the more you will need a tracking mount. The O-GPS1 will do fine for wide field exposures of up to a couple of minutes. Stacking is your friend so investigate the various stacking programs out there. DeepSkyStacker is probably the most user-friendly. As for lenses for wide-field, look for ones with as little coma as possible. Coma produces teardrop shaped stars at the edges of the frame. Primes are much easier to correct for abberations than zooms.

Jack
09-30-2014, 01:24 PM   #4
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QuoteOriginally posted by Conqueror Quote
Perhaps some unusual suggestions but:

FA 31mm f1.8 great lens for anything

DA 20-40mm f2.8 - 4 I'm intrigued as to how this might perform for night/astro
The FA 31mm isn't very fast and I don' think it's very optimized for point light sources (have to check that one) but the price is very high for limited speed and versatility (1300 Euros where I love), and for DA 20-40mm isn't very fast to and high priced again (850 Euros, where Sigma 18-35mm f 1.8 is 700 Euros). If I could, if I had the money I would own only Pentax lenses and all Pentax lenses. But they got very expensive over the years (only third party ones can be found cheaper) and the Limited lenses got very expensive compared to there Mirroless counterparts (where they are faster, smaller, lighter, faster focusing, silent focusing) and if it wheren't for there excellent IQ I would have found little place for them in this world.

... I was afraid that if I would go back to Pentax system I would have to resort to more third party lenses (for the lack of fast lenses and there high prices). But the combo of 18-135mm and 55-300mm still haunts me for there WR and usefulness when hiking.

---------- Post added 09-30-14 at 10:30 PM ----------

QuoteOriginally posted by jbinpg Quote
The deeper your target in light years and the longer the focal length of your lens/scope, the more you will need a tracking mount. The O-GPS1 will do fine for wide field exposures of up to a couple of minutes. Stacking is your friend so investigate the various stacking programs out there. DeepSkyStacker is probably the most user-friendly. As for lenses for wide-field, look for ones with as little coma as possible. Coma produces teardrop shaped stars at the edges of the frame. Primes are much easier to correct for abberations than zooms.

Jack
Yes ... I'm very aware and afraid of that characteristic in lenses (specially when it effects so strongly with point light sources like stars). Samyang lenses have a very good performance in coma correction, like there wide angle lenses (8mm f 3.5, 10mm f 2.8, 14mm f 2.8, 16mm f 2, 24mm f 1.4). And also in this is my thinking that the smaller the object the more magnification thus longer focal length thus longer tracking thus faster lens to help coupe with that. I think 200mm f 2.8 would me the maximum of my budget. Of course there are 300mm f 2.8, 400mm f 4, 600mm f 5.6 but that would so expensive and so big that it might just be as buying a telescope. It's not that I don't like the idea of telescope but I'm not yet convinced that I CAN handle it. Maybe when I get a more taste of what can be achieved then maybe.

09-30-2014, 01:50 PM - 2 Likes   #5
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L0n3Gr3yW0lf what country are you in?

This might be worth a look depending on where you are... http://www.microglobe.co.uk/pentax-m-16.html?sort=5a&page=1&filter_id=1015 here you will find some decent prices on Pentax lenses... some are imports but clearly marked as such "UK sourced stock"

I got my DA* 55mm here for a very good price and was happy etc..

------------------

The 16mm f2 Samyang seems a good option

The 10mm f2.8 Samyang seems quite a specialist focal length

------------------

Regarding the FA 31mm ...

These are fantastic shots by a forum member

Night Passage - Pentax User Photo Gallery

Under the Milky Way - Pentax User Photo Gallery
09-30-2014, 02:08 PM - 1 Like   #6
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If fisheye is good for you, the Sigma 15mm f/2.8 is a nice choice. I needed to let mine go and settled for a k-mount SMC17 fish-eye, also nice but a bit slower at f/4.
09-30-2014, 02:21 PM   #7
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QuoteOriginally posted by Conqueror Quote
L0n3Gr3yW0lf what country are you in?

This might be worth a look depending on where you are... Pentax - Microglobe.co.uk here you will find some decent prices on Pentax lenses... some are imports but clearly marked as such "UK sourced stock"

I got my DA* 55mm here for a very good price and was happy etc..

------------------

The 16mm f2 Samyang seems a good option

The 10mm f2.8 Samyang seems quite a specialist focal length

------------------

Regarding the FA 31mm ...

These are fantastic shots by a forum member

Night Passage - Pentax User Photo Gallery

Under the Milky Way - Pentax User Photo Gallery
I currently live in Italy but I have a few job opportunity in UK and Germany next year (depends on which one comes first but I do hope is the UK one). Thanks for the link and I did looked at it at it still looks very steep prices to me (because it's equivalent in numbers to the Euro prices here but not equivalent in value, meaning 1 Pound equals to 1 Euro by those prices).

I do root for Samy's 10mm and 16mm (or maybe 14mm) and that would be about 900 Euro's out of the budget thus far.

... a bit of off-topic, one o the things that I don't like about Pentax's prices is that for example they ask about 500-600 Euros for each Limited lens (15mm f 4, 21mm f 3.2, 35mm f 2.8 for this scope of discussion) when we have Samsung 10mm f 3.5, Olympus 12mm f 2, Samsung 16mm f 2.4, Sony 16mm f 2.4, Panasonic 14mm f 2.5, Fujifilm 18mm f 2, Sigma 19mm f 2.8, Panasonic 15mm f 1.8, Samsung 20mm f 2.8, Sony 20mm f 2.8, Olympus 17mm f 1.8, Fujifilm 27mm f 2.8, Panasonic 20mm f 1.7, Olymous 25mm f 1.8, Panasonic 25mm f 1.4, Fujifilm 35mm f 1.4, Sigma 30mm f 2.8, Sigma 60mm f 2.8, Fujifilm 60mm f 2.4 Macro that are similar in size or smaller, they are similar in weight or less, they are similar in speed or faster, they are similar in price or cheaper, they are similar in performance or better or worse (the last one is very dependent on the lens), I don't want to bash Pentax when they have awesome cameras and awesome lenses but just piss poor prices (even for there IQ) compared to the competition and I find very hard to justify ( FOR ME ) the prices of the Limited lenses (be it FA, DA or HD).

---------- Post added 09-30-14 at 11:23 PM ----------

QuoteOriginally posted by jimr-pdx Quote
If fisheye is good for you, the Sigma 15mm f/2.8 is a nice choice. I needed to let mine go and settled for a k-mount SMC17 fish-eye, also nice but a bit slower at f/4.
But what about Samyang 8mm f 3.5, a bit slower and manual focus but who needs it with 99% of everything in focus. And a fisheye would give me not only most of the sky and Milky Way in the field of view but also 60 sec unguided shutter speed for no star trails.
09-30-2014, 02:46 PM - 3 Likes   #8
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The short version:
Start slowly and don't buy all the gear at once. Astrophotography is as much about processing as it is about data capture. I suggest starting on the Samyang 14mm, with or without the O-GPS1, and use it for a while to learn. Try DeepSkyStacker software to practice stacking. After you feel comfortable with processing you'll know more about what gear to get next.

Coma Distortion:
Many lenses have too much coma distortion for great astrophotography; stars away from the center of frame get distorted. Lenstip.com is one of the few places that includes coma testing.

Pentax smc DA 15 mm f/4 ED AL Limited review - Coma, astigmatism and bokeh - Lenstip.com shows a lot of coma in their test while Samyang 14 mm f/2.8 ED AS IF UMC review - Coma and astigmatism - Lenstip.com is good. I can personally vouch for the Samyang for astrophotography.

O-GPS1:
The O-GPS1 is good for wide field Milky Way images and basic deep sky objects. When I try to get "great" (subjective) images of narrower, dimmer deep sky objects that need more magnification and stacking, the O-GPS1 comes up short. Objects need to be manually recentered after every O-GPS1 frame at longer focal lengths. It's not an O-GPS1 bug just a case of the sky having moved too far to keep a target on the sensor due to Earth's rotation.

The O-GPS1 is IMO good for single images up to 100mm, marginal to 200mm, and once you get longer or want to stack then you need a motorized mount. The O-GPS1 also can't be beat for portability.

Other Tracking Options:
Trackers that move the camera rather than the sensor make it easier to stack images because every image will be centered on the same part of the sky. Options include
iOptron SkyTracker Camera Mount with Polar Scope, White 3302W
AstroTrac TT320X-AG Tracking Mount for Digital SLR TT320XAG
iOptron SmartEQ Portable GOTO German Equatorial Mount 3100

Good things with "bad" gear:


The image below was taken with an FA43; an excellent lens for daytime usage but in astro usage I found horrible coma. My O-GPS1 battery died so I put my K-5 on a fixed tripod, pointed it at Sagittarius (home of many nebulas and star clusters), then set the intervalometer to take 64 2-second images at ISO 12800. Single frame were noisy and dim, The short exposure minimized star trailing and stacking controlled noise.

The full-res TIFF looks much better than the compressed jpeg below, and printed okay on 16"x20" metallic paper. It's just to show that gear isn't everything, and 2 years of practice was more important than the equipment used.




Last edited by DeadJohn; 09-30-2014 at 02:48 PM. Reason: fixed a link
09-30-2014, 03:13 PM   #9
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QuoteOriginally posted by DeadJohn Quote
The short version:
Start slowly and don't buy all the gear at once. Astrophotography is as much about processing as it is about data capture. I suggest starting on the Samyang 14mm, with or without the O-GPS1, and use it for a while to learn. Try DeepSkyStacker software to practice stacking. After you feel comfortable with processing you'll know more about what gear to get next.

Coma Distortion:
Many lenses have too much coma distortion for great astrophotography; stars away from the center of frame get distorted. Lenstip.com is one of the few places that includes coma testing.

Pentax smc DA 15 mm f/4 ED AL Limited review - Coma, astigmatism and bokeh - Lenstip.com shows a lot of coma in their test while Samyang 14 mm f/2.8 ED AS IF UMC review - Coma and astigmatism - Lenstip.com is good. I can personally vouch for the Samyang for astrophotography.

O-GPS1:
The O-GPS1 is good for wide field Milky Way images and basic deep sky objects. When I try to get "great" (subjective) images of narrower, dimmer deep sky objects that need more magnification and stacking, the O-GPS1 comes up short. Objects need to be manually recentered after every O-GPS1 frame at longer focal lengths. It's not an O-GPS1 bug just a case of the sky having moved too far to keep a target on the sensor due to Earth's rotation.

The O-GPS1 is IMO good for single images up to 100mm, marginal to 200mm, and once you get longer or want to stack then you need a motorized mount. The O-GPS1 also can't be beat for portability.

Other Tracking Options:
Trackers that move the camera rather than the sensor make it easier to stack images because every image will be centered on the same part of the sky. Options include
iOptron SkyTracker Camera Mount with Polar Scope, White 3302W
AstroTrac TT320X-AG Tracking Mount for Digital SLR TT320XAG
iOptron SmartEQ Portable GOTO German Equatorial Mount 3100

Good things with "bad" gear:


The image below was taken with an FA43; an excellent lens for daytime usage but in astro usage I found horrible coma. My O-GPS1 battery died so I put my K-5 on a fixed tripod, pointed it at Sagittarius (home of many nebulas and star clusters), then set the intervalometer to take 64 2-second images at ISO 12800. Single frame were noisy and dim, The short exposure minimized star trailing and stacking controlled noise.

The full-res TIFF looks much better than the compressed jpeg below, and printed okay on 16"x20" metallic paper. It's just to show that gear isn't everything, and 2 years of practice was more important than the equipment used.

Thanks so much for reply. I do use Lenstip.com for all information about lenses and Photozone.de to. I do look for lenses with as much coma correction as possible (that's why I like Samyang's lenses ... and there price's). Would you recommend Samyang's 10mm f 2.8 over 14mm f 2.8, or even 16mm f 2 ? I've seen a lot of 10-20 stacked images at 200-300mm f 2.8-3.5 of deep space objects with good IQ (not Hubble-like quality but I'm not complaining), would Tamron 70-200mm f 2.8 suffice (or Sigma version might be better ?), "As you see from the crops below, presenting the diode picture in the frame centre and in the corner, the coma level in the Tamron is low. The distortions, connected with that aberration were a bit bigger for the Sigma, tested not so long ago. Once again, the Tamron is able to reach the level of the good L lenses and the Nikkor 70-200 mm VR." and Sigma's version appears to have about the same performance.
09-30-2014, 07:18 PM - 1 Like   #10
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QuoteOriginally posted by L0n3Gr3yW0lf Quote
Thanks so much for reply. I do use Lenstip.com for all information about lenses and Photozone.de to. I do look for lenses with as much coma correction as possible (that's why I like Samyang's lenses ... and there price's). Would you recommend Samyang's 10mm f 2.8 over 14mm f 2.8, or even 16mm f 2 ? I've seen a lot of 10-20 stacked images at 200-300mm f 2.8-3.5 of deep space objects with good IQ (not Hubble-like quality but I'm not complaining), would Tamron 70-200mm f 2.8 suffice (or Sigma version might be better ?), "As you see from the crops below, presenting the diode picture in the frame centre and in the corner, the coma level in the Tamron is low. The distortions, connected with that aberration were a bit bigger for the Sigma, tested not so long ago. Once again, the Tamron is able to reach the level of the good L lenses and the Nikkor 70-200 mm VR." and Sigma's version appears to have about the same performance.
I like the Samyang 14/2.8. I bought it before the 16/2 was made and haven't considered changing, but the extra stop is nice if it's sharp wide open.

The Samyang 10 might be good, too. Note that wider angles show more of the sky and more opportunities for gradients from light pollution. The 10 might require some extra processing work if you have to deal with light domes. Then again, sometimes light domes can be used in creative ways.

I have no experience with the Tamron or Sigma 70-200. That focal length might be too long for the O-GPS1. It can track for a single image but for stacking you'll need to manually reaim the camera between shots. For 200mm I would use an Astrotrac or a German Equatorial Mount.
09-30-2014, 07:19 PM - 1 Like   #11
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Here is an example of where the 300mm focal length excels - big galaxies. M31 fits perfectly in the frame. But the O-GPS1 would not be able to handle this since the individual shots in this stack of 14 ISO3200 frames were around 40 seconds. This was taken with a tracking mount. The Pentax-tweaked Sony sensors used in the recent K series cameras are beautiful for astro work since the dark noise is very low. You can almost eliminate noise if you use a few dark frames. I find astro work very rewarding and I am sure you will, too.

Jack
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09-30-2014, 07:24 PM - 2 Likes   #12
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QuoteOriginally posted by L0n3Gr3yW0lf Quote
Lenses for Astrophotography for Pentax K-5 II with Pentax O-GPS1
Just adding to what Dead john said about O-GPS1.
I have just started to try out mine for astrophotography after having it sit in the cupboard for a couple of years.
The longest shutter speed the O-GPS1 gives is 5 minutes. This maximum available shutter speed gets shorter as the focal length increases because objects move across the sensor faster as focal length increases.
I ran through some focal lengths as follows
20 - 75mm = 5mins
100mm = 4:30
200mm = 2.00
300mm = 1.20
400mm = 1.00

I leave it to others with the experience to comment on what sort of shutter speeds one needs for astrophotography for the focal lengths mentioned.

Last edited by ak_kiwi; 09-30-2014 at 07:29 PM. Reason: ....."maximum" available shutter speed....
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