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10-04-2015, 07:53 AM   #1
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Astrotracer question

I just used the astrotracer on K-3II yesterday for the first time. I made a fine calibration, pointed in the direction of milky way and made a test shot. I did not put any more time in that because I was at a party in the mountains.

I noticed a problem. Does astrotracer rotate the sensor? In my opinion it does not. And depending on what focal lenght and part of the sky you point, you will get trails. Usually the center of the image has no trails but edges do. I shot with sigma 17-50 set to 17mm. Reducing exposure time helps a lot but still, am I doing something wrong? Also, take a look at the last image named 3, I got some weird trail.

First and second images are the same 100% crop, first is at the center, second upper left. Exp: 1:27, ISO500 f/5.6

third and fourth is the next image also the 100% crops of center and upper left corner. Exp: 5mins (limit), f/4 ISO800

fifth one is something weird. exp: 1:05, f/4 ISO1600

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Last edited by zvon; 10-04-2015 at 07:59 AM.
10-04-2015, 08:26 AM - 1 Like   #2
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Shooting the max is likely the problem. We've been down this road in a few conversations on the board specifically talking about the OGP-1. This thread, and specifically the linked post may be of use.

Some examples can be found here.
10-04-2015, 09:29 AM - 1 Like   #3
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I too found out that shooting the max (5 minutes) you will get trailing around the edges of the frame. I shoot at 18mm a lot and have learned to not exceed 120 seconds (and I am considering going to just 100 seconds, for a bit of extra margin).

I also use the 200 rule (rather than the 500 rule) when not using the AstroTracker (200/focal length = max tracking time) in order to keep the stars from streaking out of about a pixel. So,the difference is essentially about a factor of 10 (i.e., even using the 100 second max, it's 10 longer than what I can shoot with out it).

I ran across this the other day.... It is based on a Canon t5i body (18MP). The author is RM Clark who has an excellent website on all things astro...
QuoteQuote:
First, the 500 rule is an idea for low quality lenses, web sized images, or for people who don't car about short star trails. Here is the real story:

Your t5i has 4.3 micron pixels. With an 18 mm lens, the 500 rule is 500/18 (or 500/(18*1.6)) giving 28 or 17 seconds.
Stars on the celestial equator move 15 arc-seconds per time second. The pixel spacing in arc-seconds is:
  • plate scale = 206265 * pixel size in mm / focal length in mm
  • so plate scale for your camera with an 18-mm lens is:
  • plate scale = 206265 * 0.0043 / 18 = 49.3 arc-seconds.
  • A star will cross one pixel every 49.3 / 15 = 3.29 seconds.
The 500 rule above (28 or 17 seconds) would give star trailing of 28 / 3.29 = 8.5 pixels or 17 / 3.29 = 5.2 pixels.
A couple of pixel trail will be noticeable with a sharp lens. So a 200 rule is needed for actual round images with good lenses.

The key to astrophotography is collecting light. The f/ratio tells light density, but not total light gathered from the subject. Larger physical aperture diameters collect more light. Recommendations like 11 mm f/2.8 means an aperture diameter of only 3.9 mm, smaller than dark adapted human eyes.

An 18 mm f/1.8 lens (as in the Sigma 18-35 f/1.8 art lens) has an aperture of 10 mm, and collects ( 10 / 3.9 )2 = 6.6 times more light than an 11 mm f/2.8 lens from the subject. A 24 mm f/1.4 lens has an aperture of 17 mm; a 35 mm f/1.4 has an aperture of 25 mm and would collect ( 25 / 3.9 )2 = 41 times more light than an 11 mm f/2.8.

So get the largest aperture diameter lens that fits your subject in your field of view (or do a mosaic). The other way to get more light is longer exposure times. For night sky, that means a tracker. Something like an iOptron Skytracker (about $400) can track lenses 100 mm and shorter for long periods.

I would recommend the Sigma 18-35 f/1.8 Art lens for crop cameras, or Sigma art 24 f/1.4. If you use a tracker and will consider mosaics, the Sigma art 35 f/1.4 is in my opinion the best lens for nightscapes under $1000.
I thought a fair amount about the problem. Decided 1) the O-GPS was more than enough for me, and 2) a better lens and 18mm was reasonable for what I had been shooting - Astro Landscapes. So, splurged on a Sigma 18-35/f1.8, about a year ago, and the GPS unit 2 years ago. I had designed a star tracker for a little telescope down in Texas (HET). Between using the GPS for longer exposures and the substantially shorter exposure time with out GPS tends to provide all that I really want to do. I am to the point that I would rather go out shooting, than sit down figuring out all the math again. Also, I did not want to haul around a tracking unit and set it up and align it all the time.
_______

The reason for the streaking around the edges, is that the GPS tracks the center, in an X - Y space (discounting any movement about the z axis - i.e., rotation). The longer the duration of the track, an angular component starts to creep in, that is really not taken into account by the GPS tracker. Thus, the longer duration tracks will start to record this angular movement around the center.


Last edited by interested_observer; 10-04-2015 at 09:41 AM.
10-04-2015, 10:33 AM   #4
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excellent. it seem that I have some reading and learning to do. thank you very much on these links.

10-04-2015, 11:13 AM   #5
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QuoteOriginally posted by interested_observer Quote
The reason for the streaking around the edges, is that the GPS tracks the center, in an X - Y space (discounting any movement about the z axis - i.e., rotation). The longer the duration of the track, an angular component starts to creep in, that is really not taken into account by the GPS tracker. Thus, the longer duration tracks will start to record this angular movement around the center.
I've come to the conclusion (based on first hand experience) that the AstroTracker was designed to "fill the gap" between what you can do with super-wide-angle lenses + high ISO and dedicated, motorized star trackers. As such, the ideal lens for AstroTracer is going to be 50mm or longer on a crop sensor. I've taken some decent shots @ 250mm and it works pretty well all things considered.

Michael
10-04-2015, 12:27 PM   #6
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QuoteOriginally posted by MJSfoto1956 Quote
I've come to the conclusion (based on first hand experience) that the AstroTracker was designed to "fill the gap" between what you can do with super-wide-angle lenses + high ISO and dedicated, motorized star trackers. As such, the ideal lens for AstroTracer is going to be 50mm or longer on a crop sensor. I've taken some decent shots @ 250mm and it works pretty well all things considered.

Michael
Afternoon Michael,

I do agree that the O-GPS is designed to be a "fill the gap" device. It's not a true tracker in the absolute sense, but for me - it's more than good enough, plus I wanted the geo tagging of my shooting locations. 2x to 10x of the non tracked time does provide some excellent results. For really high quality results the 5 minutes is really marketing (at least for wide angle use). I really have not gone over 31mm with the GPS in earnest. I have been thinking that I should give 135mm a try (with an old f2.5 lens), but then I would really need to position, locate and aim. That's why I like wide field as opposed to deep sky astro. You are probably correct that 50m and beyond is the device's sweet spot.

10-04-2015, 12:29 PM   #7
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The sensor stabilization motors move the sensor in an XY plane, that's it. It's physically impossible to rotate the sensor in order to move sidereally.

You will usually do best with targets closer to the Polestar than to the celestial equator due to the lesser apparent movement of the stars across the sky , and with a wide angle, you will always do better closer to the center of the sensor where the mapping of the light from the sidereal sphere onto the sensor is closest to perpendicular to the sensor plane and where the stars appear to move in the closest approximation of a straight line with respect to each other.

The wider the angle, the worse the field curvature, and the more difficult it becomes for an X-Y movement of the sensor to track all the stars in the frame, which will be moving in different directions relative to each other.

The trail in shot three is probably a shooting star that crossed your shot during exposure. If i were an airplane, it would likely be a "dashed" line, since the lights used on planes tend to blink. If you look closely, there appears to be a similar meteor trail in #4.
10-04-2015, 01:57 PM   #8
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If you have good tracking arround the center of an image and the corners looks like it gets trails. This may be a phenomenon called coma. Or yet another optical phenomenon due to wide angle lenses.

I recommend reading the following thread about the inner workings of the astrotracer. Depending on your location, angel and direction there will be problems getting a good track...

Also give the O-GPS1 a search i.e Astrotracer for K3, K5 and 645Z for alot more info.

https://www.pentaxforums.com/forums/groups/135-astrophotography/1802-astrotra...ge2.html?pp=15

10-04-2015, 08:31 PM   #9
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@zvon, Take a close look at all 4 corners. Do they all trail in a parallel direction to each other, or do they all trail from the center of the image? The former would generally be a tracking error, the latter would be a lens aberration. A shaky tripod or drooping ball head can also cause trails.

I don't know about the Sigma 17-50 specifically, but astrophotography pushes lenses in ways that may not have been expected by the designer. Some lenses that are excellent for everything else fail to work well on stars. Many zooms don't look good if you pixel peep although the full image looks okay at normal viewing sizes.

The last image shows bright stars at one end with a diminishing trail off each. That looks like the tracker worked for a bit then had a glitch (lost GPS signal, weak batteries?).
10-04-2015, 10:01 PM   #10
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This "feature" ain't gonna work well enough to get acceptable "stars" w/o trailing..it's a waste of time and effort. The ONLY way is with a polar aligned mount, w/guiding via a seperate guide cam and guide scope, connected to a field lap top/tablet, etc....OR ..a specific sidreal tracking device independent of the main imaging camera.
10-04-2015, 10:18 PM   #11
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Eh it depends on what you are using for and with what lens. Sure, it's not ideal for taking highly technical long exposures of deep sky objects with a long lens, but for landscapes with an added night sky, the Astrotracer can help a lot to clean things up without a lot of tweaking. Likewise, for short to mid focal length stacks of constellations and various objects, it's pretty useful and requires a lot less setup and babysitting than a true tracking mount. You just have to know what its limitations are and work within them.

QuoteOriginally posted by SKYGZR Quote
This "feature" ain't gonna work well enough to get acceptable "stars" w/o trailing..it's a waste of time and effort. The ONLY way is with a polar aligned mount, w/guiding via a seperate guide cam and guide scope, connected to a field lap top/tablet, etc....OR ..a specific sidreal tracking device independent of the main imaging camera.
10-05-2015, 12:52 AM - 1 Like   #12
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QuoteOriginally posted by SKYGZR Quote
..it's a waste of time and effort.


So, this isn't worth the time (26 x 20 seconds) and effort (stacked in DSS):


Pentax K-3 with O-GPS1/Astrotracer, DA*200/2.8 lens and DA AF 1.4X Teleconverter. 16 images at ISO 3200 and 10 at ISO 1600.

Well, to some maybe not. To me it was worth the time and effort (including the three minutes to set up and calibrate my gear).

QuoteOriginally posted by SKYGZR Quote
The ONLY way is with a polar aligned mount, w/guiding via a seperate guide cam and guide scope, connected to a field lap top/tablet, etc....OR ..
Many astro photographers have started out with a camera on a fixed tripod and nothing else. As already said my MJSfoto 1956, interested_observer and dcshooter: The key thing about the O-GPS1 is to undertand the way it works and its limitations. MJSfoto 1956 calls it a gap-filler and I usually say that it is a great (and highly portable) supplement but not a replacement to any quality Equatorial Mount.
10-05-2015, 02:22 AM   #13
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QuoteOriginally posted by Stone G. Quote
So, this isn't worth the time (26 x 20 seconds) and effort (stacked in DSS):


Pentax K-3 with O-GPS1/Astrotracer, DA*200/2.8 lens and DA AF 1.4X Teleconverter. 16 images at ISO 3200 and 10 at ISO 1600.

Well, to some maybe not. To me it was worth the time and effort (including the three minutes to set up and calibrate my gear).


Many astro photographers have started out with a camera on a fixed tripod and nothing else. As already said my MJSfoto 1956, interested_observer and dcshooter: The key thing about the O-GPS1 is to undertand the way it works and its limitations. MJSfoto 1956 calls it a gap-filler and I usually say that it is a great (and highly portable) supplement but not a replacement to any quality Equatorial Mount.
I could not have said it better myself. I used the DA*300 for this same shot.
10-18-2015, 08:41 AM   #14
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QuoteOriginally posted by Stone G. Quote
So, this isn't worth the time (26 x 20 seconds) and effort (stacked in DSS):


Pentax K-3 with O-GPS1/Astrotracer, DA*200/2.8 lens and DA AF 1.4X Teleconverter. 16 images at ISO 3200 and 10 at ISO 1600.

Well, to some maybe not. To me it was worth the time and effort (including the three minutes to set up and calibrate my gear).


Many astro photographers have started out with a camera on a fixed tripod and nothing else. As already said my MJSfoto 1956, interested_observer and dcshooter: The key thing about the O-GPS1 is to undertand the way it works and its limitations. MJSfoto 1956 calls it a gap-filler and I usually say that it is a great (and highly portable) supplement but not a replacement to any quality Equatorial Mount.
How you used O-GPS1 with K-3 ? heard that there are some compatbility issues. You need some adoptors + cable ?

I've AF360 FGZ + Pentax K-3 and interested in buying O-GPS1. Can you please tell me how many items like extra adoptor, cable etc want to use Flash + GPS unit at a time ?
10-18-2015, 09:16 AM   #15
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QuoteOriginally posted by Fotorix Quote
How you used O-GPS1 with K-3 ? heard that there are some compatbility issues. You need some adoptors + cable ?

I've AF360 FGZ + Pentax K-3 and interested in buying O-GPS1. Can you please tell me how many items like extra adoptor, cable etc want to use Flash + GPS unit at a time ?
The O-GPS1 works fine on the K-3. No issues. Where did your hear that?

Using flash with the O-GPS1... Why would you want that? The earth will become blurry when tracking so if you want to light paint or trigger a flash you can do so manually with the flash in your hand.

If the reason is geotagging and use flash then thats another story for which I have no answer.
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