[osbourne one-nil]

I love the Astrotracer feature of my K1 and find it very simple to set-up and use. Not needing to align is wonderful and generally I'm getting good results. However, the trailing of stars at the edge of the image is an issue to me and one which I think would be overcome by using a simple tracking mount such as the Vixen Polarie. There are times when I would be happy to add a little bit of extra effort into the alignment of such a tracking mount to overcome this (unavoidable) shortcoming of Astrotracer. Does anyone agree or have experience of using both systems or would the benefits be so marginal as to make it pretty-much pointless?

Also, I know there's a table out there which shows the recommended tracking times for a given focal length, but I can't find it anywhere...can anyone point me in the right direction?

Thanks

[cdw2000]

I think you are referring to the "500 Rule". This website explains it: Use the 500 Rule for Astrophotography | Useful Chart to Avoid Star Trails

There is also a more complex rule that is said to apply better to digital cameras called the NPF Rule. This is also mentioned in the above article.


I have only used the Astrotracer so can't comment on your other question other to say that, even with the Astrotracer, its best to keep the exposure time as short as possible. I typically use a 60 second exposure with the Astrotracer.

[osbourne one-nil]

Thanks - I found the chart...it was actually one stating the maximum tracking time for any given focal length using astrotracer, so not the 500 rule. They suggest there's no advantage in tracking time over a specific tracking mount but one would still overcome the trailing at the edges and allow easier staking of multiple images as I wouldn't need to re-frame any shots.

I think 60 seconds is about the limit I've found too.

[jgnfld]

ANY x-y mount (whether internal as with the Astrotracer or external as a mount) will rotate w.r.t. the sky on a long exposure--think Foucault pendulum here which is the same basic principle at heart. There exist alt-az mounts that compensate by counter-rotating the sensor various ways, but a motorized equatorial mount is a much simpler solution. At lower latitudes, a simple barn door mount (easily googled) is often a cheaper alternative--though motorizing them still adds bucks.

What stacking Astrotracer photos does is essentially provide the counter-rotation necessary to prevent edge trailing in software through some extra work on your part.

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Last edited by jgnfld; 08-20-2020 at 06:42 AM.

[osbourne one-nil]

Thanks - but with the astrotracer I'm finding it faffy to have to realign every few shots to keep things in the centre but I'm thinking a tracker would prevent that. I'm probably answering my own question! I used to have something called an Astrotrac which was fantastic and one of those things I wish I'd never sold! It would give me 10mins solid at 200mm without batting an eyelid.

The trailing in the corners is probably due to lens coma.

[DeadJohn]

I've used the K-1 astrotracer, iOptron Skytracker (similar to Vixen Polarie), and larger telescope mounts.

The main limitation of the Pentax astrotracer for me is inconsistent calibration. The process is sometimes finicky. I can do a precise calibration, get bad tracking, redo the calibration the same way, and it mysteriously works better. The K-1 has been more reliable than the original K-5 plus hotshoe astrotracer accessory.

The main limitation of a Skytracker (or Polarie) is an extra item to buy, carry, and make sure batteries are charged. Aligning it has not been a problem; I can aim it near Polaris in the same amount of time as it would take to calibrate the astrotracer.

IMO the biggest advantage of any physical tracking mount over the astrotracer is for stacking sky images. The tracker keeps the camera aimed at the same target for every exposure. Stacking for widefield Milky Way shots, though, has become less important as camera sensors improved in low light. I rarely stack. (stacking is still needed with deep sky photos of tiny objects through telescopes, such as a single galaxy or nebula, but neither the astrotracer nor Polarie are designed for that)

[osbourne one-nil]

Thanks - it's probably a bit of a luxury having both a K-1 AND a dedicated tracker, but it would give me another option so I might give it a whirl. It's not the end of the world in terms of cost and could be great fun.

[Bob 256]

The issue of stars not tracking in the corners is due to the fact that moving the flat sensor can only accommodate an image that is mapped to a flat plane (and at that, only with a perfectly rectilinear lens). Stars basically map to a sphere and while tracking at the center of the field can be pretty good, the further away a star is from center field, the more error there will be in tracking. This is particularly seen with wide angle lenses where those errors greatly increase towards the edges. A tracking mount works differently and the field of the entire lens changes over time staying on the same sky area which means stars all across that field will be tracked. Even the widest angle lenses will work to the edges with a tracking mount (though there are still lens aberrations in play as previously mentioned but a good lens stopped down a bit will perform well). As you alluded, there is a certain amount of work involved in setting up a tracking mount in order to achieve its advantages. A polar scope helps in this regard and familiarity and experience makes the setup easier. Some trackers specifically designed for cameras are reasonably priced nowadays such as the Sky-Watcher or the iOptron.

[kypfer]

Just a quick note regarding Astrotracer calibration accuracy ... from experience with the external O-GPS1 on my K-70, the results a lot more reliable if I calibrate and set up at least 10ft/3m away from my car and/or other adjacent magnetic metalwork such as garden furniture etc. I've no (known) experience with power-lines, overhead or otherwise, but would anticipate problems from them as well. A handheld compass, checked against the North Star, will give a good indication of any major influences in the immediate vicinity

[MossyRocks]

The trailing in the corners is probably due to lens coma.

Probably not coma but geometric distortion and poor tracking with wide and ultra wide lenses is the likely cause.

Also, I know there's a table out there which shows the recommended tracking times for a given focal length, but I can't find it anywhere...can anyone point me in the right direction?

The table you are looking for is this one that gives tracking time for the O-GPS1 which should match the inbuilt ability of the K-1. However for going after deep sky objects I find the provided tables to be on the optimistic side of things by a lot and often find that I can get good result at 1/4 the time suggested.

The astrotracer ability in the cameras that support it functions as an alt-az mount that also does field rotation. It is dependent on knowing your latitude fairly precisely (a +/- a few hundred meters north south is good enough), what cardinal direction the camera is pointed, and how it is oriented in relation to the center of the earth. It gets this info from the compass, level sensors, and GPS and then does a bunch of math to figure out how much to move the sensor. If you look at the tables for the O-GPS1 it gives times depending on declination however most people don't understand declination correctly. A declination of 90 degrees is not looking straight up into the sky at the zeniuth and a declination of 0 is not looking straight out at the horizon. Instead a declination of 90 degrees means that camera is point at the celestial pole and if you are in the norther hemisphere you would be very close to having polaris centered in the frame. A declination of 0 degrees means you would have the camera pointed along the celestial equator and a good object basically on it is the upper most star in Orion's belt which is really close. If you are pointed directly at the celestial pole all of the movement is rotational, and if you look at the tables you will find that point at a declinatio nof 90 always gives the longest tracking time and is independent of focal length. this makes sens as the movement in the frame is all angular speed and the sensor can only rotate so much so the time it takes the stars to rotate the most the sensor can is about 5 minutes. However if you are pointed along the celestial equator the movement is entirely linear so the linear speed of the stars is all that matters. Here focal length matters since with longer focal length lens things move across the frame faster which is why you see decreased tracking time with longer lenses. Depending on where you have the camera pointed between the celestial pole and celestial equator there will be varying amounts of linear and rotational movement needed for accurate tracking.

Now on to quirks of astrotracer beyond it being rather finicky to get calibrated well. The biggest one that comes to mind is that with wide and ultra wide lenses it will almost always produce some trailing in the corners. The only time when it wouldn't is when you have it pointed exactly at the celestial pole in which case it it is functioning as a equatorial mount with a very short tracking ability. The reason for the trailing is caused by 2 things. The first is the geometric distortion that these lenses have that tends to be worse in the corners and along the edge. This distortion causes the tracking to not be as accurate as you move from the center of the frame out to the edges and corners. The other issue is that these lenses cover such as huge area that the amount of rotational movement vs linear movement varies substantially across the frame such that as you move from the center of the frame the tracking gets worse. As mentioned before this isn't a problem when point at the celestial pole since all the movement in the frame is rotational, but if you point it at the celestial equator the movement of the sensor will be entirely linear but as you move away from the celestial equator there will be more of a rotational component to the star movement.


The nice thing is that with wide and ultra wide lenses the need for tracking is greatly diminished as you can get very good results using the rule of 200 (a better variation of the rule of 500) which basically says to use as your maximum untracked exposure time 200/(focal length). So with my 12mm lens I can use a 15 second exposure and get really good results, but in reality since the stars are so small in the frame found I can push it to 20s and still get pretty good results but at 30 I don't like the trails. With a 50mm lens I can do 4 seconds untracked, but with my 400mm I could only do .5s untracked. The reason to use 200 instead of 500 is that 500 was from the film era where reciprocity failure was a thing so it worked well enough, but 200 works better with modern digital sensors that don't suffer from reciprocity failure. However this still takes advantage of the fact that our sensors have a bayer mask on them and interpolate pixels and to get truly sub 1 pixel movement with things along the celestial equator you would have to use 50/(focal lenght) and I have done some shots like that with some really long glass just to see if it was possiable. I did some moon shots at f/20, 2000mm, ISO 100, and 1/40s that way because I could and the f/20 wasn't by choice but a results of the f/10 mirror lens and 2x convert in the mix so I could get it to mount and still achieve infinity focus.

I recently got a small equatorial and the 2 biggest benefits I've found with it are first that I don't have to recompose all the time. Once I get an object framed up with a short lens I just put the camera in continuous low shooting mode and filp the switch on one of my release cables and let it take pictures until I decide to pack up. The second benefit is that with playing around with getting a good polar alignment does take longer than calibrating astrotracer it doesnt' get thrown off by things like astrotracer does so I can get good alignment up in places like the Iron Range of northern MN where astrotracer never seems to get a good calibration or any other place with magnetic items or magnetic fields. With wider lenses like a 50mm one doesn't need even a very good polar alignment to get good results at 30s per shot as I found out the other day with my first real attempt. I do need to get an intervalometer so that I can go beyond 30 second exposures with it.

I still use astro tracer as my little equatorial can't support my big lenses by themselves so I either shoot untracked or using astrotracer with them. I use astrotracer all the time with my SMC A* 400/2.8 and will even throw the 1.4X-L converter in the mix to have a 560mm f/4 and have had astro tracer work great. Granted for deep sky objects I still am stacking and the tracking times to get good results are still pretty short but I can consistently get 20s shots at 400mm and 10s shots at 560mm all the time that don't shot any trailing. Eventually I will move up to also having a big equatorial that can support a 16lbs lens and the camera and other gear you hang off of it but for now I am still using astrotracer for the big glass. I often am running multiple cameras and the last time I was out I had the 12mm lens not tracking at all but continuously shooting 30 second shots so I could do a star trails image with Polaris and the milky way, a 50mm lens on the equatorial following Cassiopeia through the sky shooting continuous 30 second shots, and I was using astrotracer with my 400mm on Andromeda.

[osbourne one-nil]

A handheld compass, checked against the North Star, will give a good indication of any major influences in the immediate vicinity

That's one of those really simple but very clever little tips that I wish I'd thought of. Genius!

---------- Post added 08-20-20 at 02:20 PM ----------

The table you are looking for is this one that gives tracking time for the O-GPS1 which should match the inbuilt ability of the K-1.

That's the one, although I notice it doesn't give times for the K-1 specifically. Does the full frame sensor change things significantly?

A fantastic reply by the way - should be pinned somewhere as it sums everything up very clearly.

When you say equatorial mount, do you mean something like the Polarie or an actual German equatorial mount as used with telescopes?

[Bob 256]

Tracking times for the Astrotracer are determined by three factors; magnification, the camera sensor's ability to slew (move), and the area of the sky being tracked.

When longer focal length lenses are used (higher magnification), edge tracking becomes almost as good as center tracking but the stars will move faster across the field so the sensor must slew faster. This uses up its range quicker so decreases the time allowed for tracking. Any tracking errors will also be magnified so tracking can worsen. Shorter focal length lenses don't require the sensor to slew as fast and for a given slew range, this results in the ability to track for longer times. Tracking errors are "de-magnified" so to speak and become less serious although now edge tracking can suffer more leading to poor tracking towards image edges (at the sensor, star images move at different rates towards the edges compared to the center field so it's impossible to accommodate all the star images and get good center and edge tracking).

Stars near the equator move faster than stars nearer the poles (whose fields also have rotation) so that has to be taken into account when determining how long a tracked exposure can last.

All this makes a table for a given camera a guide and not a spot-on reference.

With a tracking mount, tracking errors depend on how good the mount is, and how well it's set up. The best tracking comes with a high power guide scope and manual (or auto-guided) tracking corrections using a tracking mount but that's for the serious sky photographer using long lenses. Exposures using the latter technique can be hours long with good results.

[MossyRocks]

Does the full frame sensor change things significantly?

No they are all in the same general ballpark, the K-1 might do a bit better at tracking but not much. There is only so much movement that can be accomplished and I think the amount of movement of the sensor, either rotational or linear, is the same between the K-1 and K-3.

When you say equatorial mount, do you mean something like the Polarie or an actual German equatorial mount as used with telescopes?

More the real German Equatorial with an actual polar scope. I looked at the little Polarie and I think I would end up stripping that thing out in short order if I stuck the K-3+battery grip and even something like my 300mm on it. I doubt it would offer good tracking as that setup would probably be over it's capacity. What I have for a small equatorial is the Sky-Watcher Star Adventurer which has a capacity of 5kg/11lbs so for good tracking for astrophotography really has a usable load of around 6lbs which the camera+battery grip+300mm is probably getting close to. It has a polar scope and various drive speeds as well as a counter weight. It is bigger than the little Polarie but would be a better place to start from unless you want to jump up to big telescopes or monster camera lenses with it right away in which case expect to have problems. If I were to fully load it it would really only be good for viewing with my eye as the tracking and vibrations start to cause problems for astrophotography as one approaches the capacity which is why the general rule is to limit yourself to 1/2 the capacity. If you go for the Star Adventurer I suggest getting the pro pack bundle as it comes with everything you will need like the wedge, scope illuminator, and all the other accessories you will end up getting in short order. Also here is a much better description on how to get a proper polar alignment and get it set up correctly than what is included in the instruction manual.

[DeadJohn]

I looked at the little Polarie and I think I would end up stripping that thing out in short order if I stuck the K-3+battery grip and even something like my 300mm on it.

Here's my "too much camera, too little mount". K-1 and Sigma 50-500. I was HDR stacking eclipse photos. Longest exposure was 2 seconds so not as demanding as typical deep sky astro. I had bad frames due to motion but also many keepers.

(My better mount plus counterweights plus tripod is 60 pounds, needs 2 cases, and I was seeing the eclipse during a 3 week roadtrip. I didn't have the space to bring that.)