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05-14-2017, 01:44 PM - 1 Like   #1
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Astrophotography settings K-1 with astrotracer

Hi all,

I am getting ready for a trip to Tenerife next week and will be taking the K1 with me for some Milky Way shooting. I bought the Samyang 20 mm f/1.8 for the purpose and will probably be using the lens at around f/2.8 to dim the aberrations somewhat.
My question is this to all of you who have used the astrotracer function for the Milky Way. What exposure time and ISO combinations seem to give the cleanest results? Did you use any in camera noise reduction or non at all ( don't know what the status of the white dot issue is).
I was thinking that maybe 2 to 3 minutes would be a good starting point at around ISO 800 to 1600.
I would really appreciate your comments and advise, and maybe some good samples, to get me going.

Thanks,

Peter

05-14-2017, 02:01 PM   #2
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I'll be interested in the responses to this post, as I haven't used the K-3II astrotracer yet. Going to the Scilly Isles in June so should get away from light pollution.
05-14-2017, 02:07 PM   #3
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I do not have astrotracer, but I have visited Tenerife! It is amazing landscape, great for ultra wide angle lenses. And up on the volcanic mountains you can get good view of the stars.

You can check here:
Astrophotography - Pictures - PentaxForums.com
Pentax K-1 Review - Astrotracer | PentaxForums.com Reviews

Take some test shots before you travel

If you have a smartphone, I recommend you get a pdf version of the manual to have it with you:
http://www.ricoh-imaging.co.jp/english/support/man-pdf/k-1.pdf
05-14-2017, 02:31 PM   #4
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I would turn on dark frame subtraction (slow shutter me) unless you're comfortable denoisong in post. A 60-90 second exposure will probably work best. I'd personally try to use the widest possible aperture and lowest possible ISO. Nailing the focus will be the most critical component, however. Good luck!


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05-14-2017, 02:34 PM - 1 Like   #5
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Dark frame subtraction really eats up time IMO. Location of moon in the sky is a big consideration for when /where to shoot. May 25th is New Moon.

I have had some success at 210" @ f4 ISO 1600. I am situated near cities so your Tenerife dark sky should yield much better results provided weather is cooperative.

Good luck - be sure to share
05-14-2017, 07:02 PM   #6
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It just so happens that earlier last week I started to consider what I should do next for a body. Stay with a cropped body (K3II replacement) or go full frame - K1. I was thinking about making a post here, but I was also doing some online research on the topic. Using the astrotracking is one of my main concerns, especially in terms of exposure time (without the stars beginning to trail). Here is one of the best posts on the topic that I found - and it really comes down to just how sharp you want the stars to be.

* K1 and astrotracker--a bit disapointed!!

In the post above - it's observed that 2 minutes is too much - and he provides examples. I too posted questions a year or so ago here about if the K1 used the full capabilities of the new 5 axis stabilization for tracking but was assured that it only tracks in plane (only in 2 dimensions and not 3).

So, from my brief research, 60 to perhaps 90 seconds would be the limit if you want pin point stars. I would also go out a bit early and make some test shots at 60, 90 and 120 seconds, then use the review capability to check the stars in a corner and see how they are trailing (or not).

05-14-2017, 10:47 PM   #7
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QuoteOriginally posted by interested_observer Quote
So, from my brief research, 60 to perhaps 90 seconds would be the limit if you want pin point stars. I would also go out a bit early and make some test shots at 60, 90 and 120 seconds, then use the review capability to check the stars in a corner and see how they are trailing (or not).
Star aren't trailing in the center of the frame , even with 2 minutes exposure. The problem is at the edge of the frame with a wide angle lens. From what I've seen, tracking would work best with fast 50mm lens and stitching of the resulting frames.

05-15-2017, 01:01 AM   #8
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QuoteOriginally posted by biz-engineer Quote
Star aren't trailing in the center of the frame , even with 2 minutes exposure. The problem is at the edge of the frame with a wide angle lens. From what I've seen, tracking would work best with fast 50mm lens and stitching of the resulting frames.
Correct. The problem is the loose of image quality at the borders of the frame, due to distortion and aberrations (particularly coma and astigmatism) of the wide angle lens.
05-15-2017, 03:06 AM   #9
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Sounds like I will have to experiment a bit then. So many different reports and experiences. We'll see. Thanks for all your replies and suggestions, I will report back after I used it.
05-15-2017, 04:15 AM   #10
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QuoteOriginally posted by biz-engineer Quote
Star aren't trailing in the center of the frame , even with 2 minutes exposure. The problem is at the edge of the frame with a wide angle lens. From what I've seen, tracking would work best with fast 50mm lens and stitching of the resulting frames.
I do agree with you up to a point. I shoot with a Sigma 18-35/f1.8 lens which has no coma, and relative to anything in the 10 to 15mm range little distortion. Yes, the problem lies along the edges and corners - I did suggest doing some test shots and checking in the corners for problems.

That said, even shooting at 18mm (cropped), I still stitch in order to get the full span of the Milky Way. Depending on its height during the night, I usually shoot anything from 6 to 12 frames (with about a 50% overlap) for a single row, to double that for two rows. Using a longer lens, you are going to be shooting substantially more frames and more rows. It all depends on the overall view that you are intending to capture.

There is also an advantage in using longer focal lengths. The aperture in the lens is larger, therefore it's able to capture substantially more light for a given exposure. But the tradeoff is collecting more light vs the number of shots in order to capture the intended field of view. In using a full frame sensor you are collecting a larger view, and by virtue of just the sensor size you have a 1 stop advantage in terms of collecting additional light.

05-15-2017, 04:56 AM   #11
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I now usually close down one stop from widest aperture; NR is turned off; ISO 800 to 1600; exposure time generally 60 to 90 seconds. I have a collection of images here using K50 through to K1. Most images still have exif data. All shots taken in southern hemisphere.
05-15-2017, 10:05 AM - 1 Like   #12
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QuoteOriginally posted by interested_observer Quote
It just so happens that earlier last week I started to consider what I should do next for a body. Stay with a cropped body (K3II replacement) or go full frame - K1. I was thinking about making a post here, but I was also doing some online research on the topic. Using the astrotracking is one of my main concerns, especially in terms of exposure time (without the stars beginning to trail). Here is one of the best posts on the topic that I found - and it really comes down to just how sharp you want the stars to be.

* K1 and astrotracker--a bit disapointed!!

In the post above - it's observed that 2 minutes is too much - and he provides examples. I too posted questions a year or so ago here about if the K1 used the full capabilities of the new 5 axis stabilization for tracking but was assured that it only tracks in plane (only in 2 dimensions and not 3).

So, from my brief research, 60 to perhaps 90 seconds would be the limit if you want pin point stars. I would also go out a bit early and make some test shots at 60, 90 and 120 seconds, then use the review capability to check the stars in a corner and see how they are trailing (or not).

Astrotracer performance is a complicated issue. The first big issue is the calibration which some people have trouble doing for various reasons.

But even with perfect calibration, astrotracer has time limits that depend what part of the sky you are looking at and on focal length.

First is the issue that stars on the celestial equator appear to move faster across the sky and the sensor than do stars at the north or south pole. Thus, photos that include the celestial equator somewhere in the image have shorter time limits than photos pointed exactly at the poles.

Second is a more complex issue with focal length in which both very long and very short focal lengths have shorter time limits. For long focal lengths the problem is obvious -- a longer focal length magnifies motion so the sensor hits the limit sooner. But a different problem happens with very short focal length lenses that is caused by discrepancies between spherical geometry (the rotation of the celestial sphere) and rectilinear geometry (the way most high-quality lenses image the world). The same effect that makes people and objects seem weirdly stretched at the edges and corners of UWA images affects astro images, too. The net result is that stars at the edge of the frame move faster than stars in the middle of the frame and the astrotracer correction cannot simultaneously remove all the motions. The exact time limit isn't easy to estimate because it very sensitive to exactly where the camera is pointed, the lens focal length, lens distortion, lens sharpness, and the photographer's subjective standards for residual trailing.


P.S. Technically speaking, the astrotracer does use data from all five axes to make it's corrections. It measures the X-Y location of the camera on Earth with GPS as well as the 3 rotation angles of the camera orientation relative to the Earth (using the compass and accelerometers). It then uses all that data to correct for the effect of the rotation of the Earth by moving the sensor in 3-axes (sensor X, Y, and rotation). (If astrotracer did not correct for rotation, then every astrotracer image would have some swirl of star trailing.)
05-15-2017, 12:36 PM - 2 Likes   #13
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QuoteOriginally posted by photoptimist Quote
Astrotracer performance is a complicated issue. The first big issue is the calibration which some people have trouble doing for various reasons.

But even with perfect calibration, astrotracer has time limits that depend what part of the sky you are looking at and on focal length.

First is the issue that stars on the celestial equator appear to move faster across the sky and the sensor than do stars at the north or south pole. Thus, photos that include the celestial equator somewhere in the image have shorter time limits than photos pointed exactly at the poles.

Second is a more complex issue with focal length in which both very long and very short focal lengths have shorter time limits. For long focal lengths the problem is obvious -- a longer focal length magnifies motion so the sensor hits the limit sooner. But a different problem happens with very short focal length lenses that is caused by discrepancies between spherical geometry (the rotation of the celestial sphere) and rectilinear geometry (the way most high-quality lenses image the world). The same effect that makes people and objects seem weirdly stretched at the edges and corners of UWA images affects astro images, too. The net result is that stars at the edge of the frame move faster than stars in the middle of the frame and the astrotracer correction cannot simultaneously remove all the motions. The exact time limit isn't easy to estimate because it very sensitive to exactly where the camera is pointed, the lens focal length, lens distortion, lens sharpness, and the photographer's subjective standards for residual trailing.


P.S. Technically speaking, the astrotracer does use data from all five axes to make it's corrections. It measures the X-Y location of the camera on Earth with GPS as well as the 3 rotation angles of the camera orientation relative to the Earth (using the compass and accelerometers). It then uses all that data to correct for the effect of the rotation of the Earth by moving the sensor in 3-axes (sensor X, Y, and rotation). (If astrotracer did not correct for rotation, then every astrotracer image would have some swirl of star trailing.)
I actually agree with everything posted, however there are a couple of additions.

First, the astrotracer is a consumer item, it's not going to be absolutely perfect. For what Pentax implemented, it is pretty good. For my purposes, untracked I can get about 13 sec of exposure time (maintaining a star in a 4 pixel box). With the aid of the astrotracer, that time can be increased to 60 seconds. Actually not bad - that is a 4.6x improvement, at the cost of about 2oz in weight and $175 in cost. I just think that it could be a bit better - and that comes from trying to understand Pentax's implementation, and how it could be improved.

Sources of error - the compass and accelerometers - I do think that the accelerometers are pretty good. That said, the compass appears to have up to a 10 degree error +/- at times (after calibration) - which has been touched on by others here in the forum in other threads. Again consumer electronics - but for all essential purposes the astrotracer is a computationally based simulated equatorial tracker. As in any tracker, polar alignment (pointing North) is critical, and a compass error is not going to help matters any.

I see and read (here on PF) all the time about wide angle lenses with their inherent distortion across a wider than normal field of view - and due to the WA and UWA focal lengths, you can't track properly. However, if you consider a camera and lens set mounted on an equatorial tracker (or even using a barn door implementation) that is properly aligned - you will obtain excellent tracking for quite some minutes (greater than 1 minute). Then, taking the exact same camera and lens set and using it in the astrotracer environment, for longer than 1 minute up to 5 minutes - you will encounter the star trailing (the longer the exposure, the more trailing). So what is the difference? With the equatorial tracking mount, you are tracking in a environment that physically moves with the spherical geometry - you are moving the entire camera/lens as a monolithic unit. In the case of the astrotracer - only the sensor is moved while the surrounding camera/lens remains stationary. The implementation as I understand it and others have stated, the sensor only moves in the plane of the sensor (X and Y and rotation within the defined XY plane - essentially yaw - no pitch and roll). There are limits to the physical movement of the sensor, but again as I understand it, with the new 5 axis design, there can be additional rotational movements implemented (pitch and roll to a very small degree). I do believe that if implemented, the astrotracer could be improved, and this star trailing problem could be mitigated to some extent. It all boils down to an accumulation of error, that manifests itself as the tracking time is increased. Basically, Pentax has done itself no favors by "advertising up to 5 minutes". It's an accurate statement, but everyone just reads "5 minutes" and comes away a bit disappointed.

I think I do understand the problem pretty well. I helped design and implement a little telescope down in Texas - over 20 years ago. I did the star tracker. The design has also been used over on a second telescope in Africa.

05-15-2017, 01:05 PM   #14
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Oh, one thing I forgot to mention - make sure the Samyang's focusing is calibrated! Samyang is infamous for sending out lenses with the distance scales on the focusing ring being wrong. They can even focus past infinity. So its best to make sure you know where infinity is on a bright sunny day, and then mark it. That way, when you are out in the field, you won't get bogged down by finding the right focus.
On my 14mm, the infinity is actually just over the 3.2m mark
05-15-2017, 01:59 PM   #15
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QuoteOriginally posted by Na Horuk Quote
Oh, one thing I forgot to mention - make sure the Samyang's focusing is calibrated! Samyang is infamous for sending out lenses with the distance scales on the focusing ring being wrong. They can even focus past infinity. So its best to make sure you know where infinity is on a bright sunny day, and then mark it. That way, when you are out in the field, you won't get bogged down by finding the right focus.
On my 14mm, the infinity is actually just over the 3.2m mark
Or another trick with the K-1 is to center a bright sky object like Jupiter or Venus, and use the zoom feature in live-view to magnify the image to X10 or X16, and then adjust the focus (leave the K-1 in manual so the focus won't change) and then move to the section of the sky you want to shoot. This is more involved than using a focus mark but it will get you spot on the infinity focus with most any lens if done carefully.
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