Shoot-move-shoot does what it sounds like. It moves the platform, allows time for the movement to settle, snaps the shot, then moves again. Astrotrackers like this are constantly moving at the same rate that the earth spins on its axis. This allows for the subject to remain in the same poistion in the frame during super long exposures. Depending on the accuracy of the polar alignment and the focal length of the lens, tracking allows for shots to go from <20 seconds to multiple minutes long for one exposure.
You need a long exposure to let in enough light for stars to become clearly visible. If you expose for longer than ~ 30 seconds, then the earth will rotate enough to start causing star trails. This will physically rotate the camera to counteract earths rotation.
The is a common question, but there are certain things that are just better done in camera than post. Then there are situations where a combination of in camera + post provides optimum results. Astrophotography is one that can and does use both. The longer you can expose, the more information obtained. For ultimate long shots, you need to compensate for the earth's rotation. You continue to snap away all night, and then take those images into post where you stack the images. The stacking software adds the data together resulting in a single image that is the equivalent of a single image with an exposure time equaling the sum of each shot.
Why do this? If you do single long exposure, there is a very good chance your image will have an airplane fly through, a satellite streak by, or something more terrestrial like a fellow stargazer shining a laser through your frame, car headlights, etc. Taking a series of short exposures limits those issues, and if it happens, you throw out that shot and only loose 45-60 seconds instead of 15 minutes.
Stacking can also use shots you take when the lens is covered called dark frames. You take this for the same exposure time as the normal shots. This provides a noise pattern that builds up in any electronic sensor as the heat builds up from staying energized for so long (as well as natural summertime heat). That noise pattern can then be subtracted by the stacking software. The stacking software can also realign the images so you can use shots from multiple shoot days (er, nights).
It can do both. They seem to use the product's name ("move-shoot-move") to refer to a mode which both tracks the stars and moves independently, creating a panning timelapse.
Interesting. I didn't realize there was a product/company with that name. Shoot-move-shoot is a common saying in motion controlled photography, and that was what I described.
I love the age we're in where anybody with a little know how can cheaply/affordably build something that was only available to the very well funded before. Each one of these are attempting the same thing, but trying to avoid the pain points each dev has seen from other products.
They show all the other trackers with an Alt-Az wedge, but show their tracker without that piece, to make the other trackers look big.
With the specific 2 other trackers pictured, you can detach that wedge and use them exactly like the MSM tracker without their Alt-Az mounts and they would be a lot smaller.
Personally I own a SkyGuider Pro and use it with a pan-tilt head tripod and no wedge.
Haha, I noticed the same thing. I've started watching some of their tutorial videos, and have some other nit picking comments as well. However, these will probably get someone that's never done astrophotography up and running with the ability to get some great shots totally unachievable 15 years ago in this price range.
One benefit of shoot-move-shoot is that when combining/compositing the images together in post, you can remove and adjust for temporary artifacts like satellites or terrestrial lighting that would otherwise ruin a long exposure. You can also compensate for motion platform inaccuracies by shifting the location of each layer to keep the object you're trying to image at the same location in the frame. Minutes-long exposures mean you're just ORing all the photons together, and you get what you get.
For sufficiently dim targets, you absolutely need minutes long exposure. If you go for shorter bursts, you face two main challenges:
1) Not enough photons reach the sensor to distinguish the object you are trying to photograph from the noise floor of the sensor. This is by far the biggest reason to go for a tracking mount.
2) You bog your computer down in post processing. 50x1 minute images are far simpler to align than 500x6 seconds images.
Combine (1) and (2), and there's barely any reason to go for untracked shots. As for satellites, planes, clouds, or other kind of transient phenomena that can potentially ruin a shot, that's where you apply some kind of clipping over data (usually sigma clipping or some variation of it) and problem solved.
Almost everything is cheaper to buy than to DIY. This is a hacker forum and people like to DIY for other reasons than cost, including customization, the learning experience of building something yourself, being fully FOSS, not waiting for long lead times in some locations, and other reasons.
Here's another approach to tracking, employed by Pentax and maybe others: Use 5-axis sensor movement to track stars (i.e. use the motors which move the sensor for in-body stabilization)
Given GPS coords, software can do this to the limits of sensor movement afforded by the camera body. I don't have a Pentax but modern models can apparently track for at least 4 minutes.[1] (!)
In theory, official or unofficial firmware for other models of camera might be able to do the same.
I think this requires not just a GPS fix but also fairly accurate heading information from an electronic compass and tilt information from the camera's accelerometer, because shifting the sensor in the right direction requires knowing which part of the sky the camera is pointed at. (I haven't bought the astrotracer add-on for my Pentax because it's crazy expensive for what it is, but it should be at least a bit more than just a GPS receiver.)
Surely in camera, it is just a matter of taking a shot, wait an interval, take another shot, then shift/rotate the two images until X number of bright spots line up and then form
a new composite, rinse and repeat. If you have a motor in the camera for stabilising you can use that shift/rotate parameter to move the sensor for the next shot.
> Taking one long exposure I imagine would better pick up on faint sources of light compared to multiple short exposures.
You definitely need an exposure long enough for the light sources to actually show up on the image. But beyond that point, sometimes you can do better by taking lots of separate exposures: https://en.wikipedia.org/wiki/Lucky_imaging Really depends on what the limiting factor is in your imaging system. For cheap consumer cameras, sensor sensitivity is more of an issue than atmospheric fluctuations, so longer exposures are usually better.
The motors: the working speed of the motor is identical to the speeds required to keep the scope aligned. For one of the axes, that means it causes the system to do a full 360 rotatation every 24h. Steppers (both the small kind and the big kind) can work at 1500RPM but this is irrelevant- we care much more about precise small movements.
Thank you very much; I was hoping for some more direct answers, e.g. diameter of the ring is 400 mm and speed of rotation is up to 15 RPM. I'd like to do something significantly different with this design (outside of astro-photography, so different properties and ranges are relevant) and was interested in existing capabilities.
I saw the video and knew approximate values, I just wanted to see explicit specs - like blueprints for parts with sizes, or capabilities of motor-belt-ring coupling.
you don't want this design for non-astrophotography. Much of the design is centered around rotating the imaging device in sync with the rotation of the earth so you can take exdtremely long exposures.
There aren't "blueprints" but the STL files for the printed parts and the assembly provide the same information.
Motor belt couplings like this are pretty well understood, mainly within the context of moving XY stages for 3D printers, but also in the larger context of industrial movement. Unfortunately, I don't think all of that is really documented in a way that is useful for people who aren't willing to invest significant effort into learning how to work with 3D printers, etc (or get a degree in mechanical engineering).
Let’s say that I know precisely where the AstroTracker is - X,Y,Z all within ~ 1 centimeter, and my goal is to point a laser in a specific vector from the base, how accurately could I do that?
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[ 2.9 ms ] story [ 66.6 ms ] threadhttps://en.wikipedia.org/wiki/Barn_door_tracker
https://hackaday.com/2013/08/08/building-a-barn-door-tracker...
Yep, only 17 days ago.
And I already posted link to its GitHub repo.[0]
[0] https://news.ycombinator.com/item?id=26931253
Is it simply image quality? Or are there more trade offs?
Why do this? If you do single long exposure, there is a very good chance your image will have an airplane fly through, a satellite streak by, or something more terrestrial like a fellow stargazer shining a laser through your frame, car headlights, etc. Taking a series of short exposures limits those issues, and if it happens, you throw out that shot and only loose 45-60 seconds instead of 15 minutes.
Stacking can also use shots you take when the lens is covered called dark frames. You take this for the same exposure time as the normal shots. This provides a noise pattern that builds up in any electronic sensor as the heat builds up from staying energized for so long (as well as natural summertime heat). That noise pattern can then be subtracted by the stacking software. The stacking software can also realign the images so you can use shots from multiple shoot days (er, nights).
I love the age we're in where anybody with a little know how can cheaply/affordably build something that was only available to the very well funded before. Each one of these are attempting the same thing, but trying to avoid the pain points each dev has seen from other products.
https://i.imgur.com/QD32T7r.png
They show all the other trackers with an Alt-Az wedge, but show their tracker without that piece, to make the other trackers look big.
With the specific 2 other trackers pictured, you can detach that wedge and use them exactly like the MSM tracker without their Alt-Az mounts and they would be a lot smaller.
Personally I own a SkyGuider Pro and use it with a pan-tilt head tripod and no wedge.
1) Not enough photons reach the sensor to distinguish the object you are trying to photograph from the noise floor of the sensor. This is by far the biggest reason to go for a tracking mount.
2) You bog your computer down in post processing. 50x1 minute images are far simpler to align than 500x6 seconds images.
Combine (1) and (2), and there's barely any reason to go for untracked shots. As for satellites, planes, clouds, or other kind of transient phenomena that can potentially ruin a shot, that's where you apply some kind of clipping over data (usually sigma clipping or some variation of it) and problem solved.
Here's another approach to tracking, employed by Pentax and maybe others: Use 5-axis sensor movement to track stars (i.e. use the motors which move the sensor for in-body stabilization)
Given GPS coords, software can do this to the limits of sensor movement afforded by the camera body. I don't have a Pentax but modern models can apparently track for at least 4 minutes.[1] (!)
In theory, official or unofficial firmware for other models of camera might be able to do the same.
[1] https://milkywayphotographers.com/article/2021/01/21/pentax-...
Taking one long exposure I imagine would better pick up on faint sources of light compared to multiple short exposures.
You definitely need an exposure long enough for the light sources to actually show up on the image. But beyond that point, sometimes you can do better by taking lots of separate exposures: https://en.wikipedia.org/wiki/Lucky_imaging Really depends on what the limiting factor is in your imaging system. For cheap consumer cameras, sensor sensitivity is more of an issue than atmospheric fluctuations, so longer exposures are usually better.
It is this big: https://wiki.openastrotech.com/alu-oat.jpg but if you need more accurate dimensions: https://wiki.openastrotech.com/en/OpenAstroTracker/Printing (select the appropriate parts aand look at their dimensions in a 3D STL viewer. Those are in millimeters) and then look at the youtube video where you can see it in context.
The motors: the working speed of the motor is identical to the speeds required to keep the scope aligned. For one of the axes, that means it causes the system to do a full 360 rotatation every 24h. Steppers (both the small kind and the big kind) can work at 1500RPM but this is irrelevant- we care much more about precise small movements.
I saw the video and knew approximate values, I just wanted to see explicit specs - like blueprints for parts with sizes, or capabilities of motor-belt-ring coupling.
Instead, you probably want something like this: https://www.bhphotovideo.com/c/product/689699-REG/Giga_Pan_E...
There aren't "blueprints" but the STL files for the printed parts and the assembly provide the same information.
Motor belt couplings like this are pretty well understood, mainly within the context of moving XY stages for 3D printers, but also in the larger context of industrial movement. Unfortunately, I don't think all of that is really documented in a way that is useful for people who aren't willing to invest significant effort into learning how to work with 3D printers, etc (or get a degree in mechanical engineering).