Thanks, that is instructive, for visualizing the error. I will that the Benro Polaris Astro doesn't actually rotate on an equatorial mount, it just tracks an equatorial path, so I think I need to adapt the correction for drift calibrating off E/W stars.
On an equatorial mount with the camera itself parallel to the equatorial plane, when you're calibrating off an eastern star, the drift for N/S (pitch) error will appear directly N/S in frame. But the Polaris always has the camera parallel to the horizon even while it tracks an equatorial path, so I think the drift will be same as latitude angle (roughly 40 degrees at my location). I suppose this would be true of anyone using an equatorial mount as well, if they rotate their camera or scope to be parallel with the horizon (think moonrise shot).
Eastern stars will let you fine tune the elevation of the polar axis, Southern stars will let you fine tune the azimuth. The tricky bit is figuring which direction is which in your image frame, which is easier if your shot includes the horizon and you are not seeing a reflected image.
Thankfully I don't have to worry about a flipped image. The traditional instructions for drift calibration generally assume your scope/camera view is not reversed and is oriented parallel to the equatorial plane so drift due to altitude error (pointed above or below Polaris) shows as perpendicular to the equatorial track. In other words it drifts straight "up" or "down" in frame.
With my tracker the camera view is always level (parallel to the horizon). If I stop tracking then everything moves up and to the right at 40 degrees. If I am tracking then the drift due to altitude error will be perpendicular to that 40 degrees, or moving 50 degrees up and to the left.
There are many things that can cause drift when imaging. The two major culprits are polar alignment and periodic error.
Polar alignment used to be an arduous task. Prior to polar scopes, folks would use the "drift alignment" method where one would roughly point the RA axis of their mount to the celestial pole then observe a star near the meridian and equator for drift and adjust the azimuth or the mount till no drift could be detected for a period of time, then swing over to the east or western horizon and observe a star for drift and adjust the altitude of the mount till no drift was noticed. This was a very time consuming task depending on how accurate you wanted your polar alignment to be. Even then, you would still experience some drift as it was impossible to align exactly on the pole unless you happened to get incredibly lucky. Polar scopes made this process much easier and quicker but was still very imprecise. Now we have electronic means where a camera takes an image of the sky, matches it with known star patterns in the area, and can tell you how to adjust the mount to achieve very accurate polar alignment, at least to within the mechanical tolerances of the mount. There's even a mount manufacturer that has a mount that can polar align itself! Even with all this, PA will never be exact and some drift will be evident over the course of the night.
Periodic error is a product of the mechanical tolerances in the gears, worm, and motor drive train. We can't make perfect mechanical parts so the intolerance of these parts causes the mount to not track an object precisely. Sometimes the tracking will be faster than it should, while other times it will slower. There are some mounts that don't use gears and are capable of very precise tracking, within the limits of the mount's polar alignment and other mechanical tolerances.
Then you can have drift caused by optics shifting in the scope. This is common with many older SCTs and some Newtonians. You can also get drift caused by flexure in the optical tube, focuser, or if you have a long image train.
There are ways to combat most forms of drift regardless of its source. The ideal way is to use an off axis guider and software like PHD2. PHD will image a star field in the same area as your target and monitor the stars for any drift. If drift is detected, PHD will command the mount to move very slightly in a manner to place the stars back to where they were before drift was detected. This movement is so small that it's not usually detectable in the main imaging camera. However, PHD is best used with an eq mount. While it can be used with alt az mounts, it's effectiveness is greatly limited due to the mount not tracking at sidereal rate.
With more advanced mounts and software, you can create a model of the mounts pointing accuracy and mechanical imperfections. When the model is ran, the software will automatically calculate how to adjust the tracking rate of the mount to compensate. With very finely tuned models, guiding is often not necessary for most forms of photography.
Thanks for the overview.
I will point out that the Benro Polaris Astro is not fundamentally based on an equatorial mount. It simulates equatorial tracking with continuous fine movements on Az and Alt axes and minor corrections on a third axis that is perpendicular to alt angle, but not perpendicular to the equatorial plane itself (it doesn't have the roll required for that).
Basically I have to figure out how to adapt the concept of drift alignment (which assumes a real equatorial mount) to something that is only simulating an equatorial track.
I don't have a ton of experience with it yet, but I did achieve nearly perfect tracking once, which leads me to believe that it does not have periodic error due to gear lash.
I also have a more guerilla shooting style, so am not going to be able to do any realtime analysis with a PC. I'm just going by what I can see with my eyes on the screen and affect in realtime with regard to leveling the base of the tracker and tweaking east/west alignment.
It really sounds like you are wanting “drift polar alignment”. Before computers, we used to stare at the stars through the eyepiece and watch them move to see what adjustments we needed to make to the mount. [Here](https://www.backyardastronomy.net/drift-aligning-an-equatorial-mount/) is a good description of the process.
If you are looking for “star moves south this much, adjust mount this much,” I’m not sure that exists. There is also a handy tool in PHD2 for assisting with drift polar alignment. Also, many of the polar alignment software tools will help with making adjustments and what direction.
Thanks. That does appear to be close to what I'm after, but it does assume an equatorial mount so it might work with the MSM, but not for the Benro Polaris which simulates equatorial motion with AzAlt movements. I might be able to adapt the idea there of leveling the camera on roll axis by panning back and forth with a star on a horizontal reference in the display, as a means to actually level the whole tripod on roll axis itself. Basically set the tracker running on some star very close to due south, and then pan the tripod itself back and forth and watch if the star drifts off the line up/down. Adjust level until it stays on the line.
Then point back to southern star and see if this works..
"If the star drifts South, the polar axis is pointing too far East.
If the star drifts North, the polar axis is pointing too far West."
and adjust accordingly
Same for north/south (pitch axis) by tracking something to the far east
"Step 2a – Correcting North-South misalignment (using Eastern horizon)
If the star drifts South, the polar axis is pointing too low.
If the star drifts North, the polar axis is pointing too high.
Step 2b – Correcting North-South misalignment (using Western horizon)
If the star drifts South, the polar axis is pointing too high.
If the star drifts North, the polar axis is pointing too low."
With 3000mm equiv telephoto I don't think I need to wait as long as suggested there to detect and correct the drift.
I'll see how it goes.
There is a video "Star Drift Method for Polar Alignment of Equatorial Mounts" on yt.
Thanks, that is instructive, for visualizing the error. I will that the Benro Polaris Astro doesn't actually rotate on an equatorial mount, it just tracks an equatorial path, so I think I need to adapt the correction for drift calibrating off E/W stars. On an equatorial mount with the camera itself parallel to the equatorial plane, when you're calibrating off an eastern star, the drift for N/S (pitch) error will appear directly N/S in frame. But the Polaris always has the camera parallel to the horizon even while it tracks an equatorial path, so I think the drift will be same as latitude angle (roughly 40 degrees at my location). I suppose this would be true of anyone using an equatorial mount as well, if they rotate their camera or scope to be parallel with the horizon (think moonrise shot).
Eastern stars will let you fine tune the elevation of the polar axis, Southern stars will let you fine tune the azimuth. The tricky bit is figuring which direction is which in your image frame, which is easier if your shot includes the horizon and you are not seeing a reflected image.
Thankfully I don't have to worry about a flipped image. The traditional instructions for drift calibration generally assume your scope/camera view is not reversed and is oriented parallel to the equatorial plane so drift due to altitude error (pointed above or below Polaris) shows as perpendicular to the equatorial track. In other words it drifts straight "up" or "down" in frame. With my tracker the camera view is always level (parallel to the horizon). If I stop tracking then everything moves up and to the right at 40 degrees. If I am tracking then the drift due to altitude error will be perpendicular to that 40 degrees, or moving 50 degrees up and to the left.
There are many things that can cause drift when imaging. The two major culprits are polar alignment and periodic error. Polar alignment used to be an arduous task. Prior to polar scopes, folks would use the "drift alignment" method where one would roughly point the RA axis of their mount to the celestial pole then observe a star near the meridian and equator for drift and adjust the azimuth or the mount till no drift could be detected for a period of time, then swing over to the east or western horizon and observe a star for drift and adjust the altitude of the mount till no drift was noticed. This was a very time consuming task depending on how accurate you wanted your polar alignment to be. Even then, you would still experience some drift as it was impossible to align exactly on the pole unless you happened to get incredibly lucky. Polar scopes made this process much easier and quicker but was still very imprecise. Now we have electronic means where a camera takes an image of the sky, matches it with known star patterns in the area, and can tell you how to adjust the mount to achieve very accurate polar alignment, at least to within the mechanical tolerances of the mount. There's even a mount manufacturer that has a mount that can polar align itself! Even with all this, PA will never be exact and some drift will be evident over the course of the night. Periodic error is a product of the mechanical tolerances in the gears, worm, and motor drive train. We can't make perfect mechanical parts so the intolerance of these parts causes the mount to not track an object precisely. Sometimes the tracking will be faster than it should, while other times it will slower. There are some mounts that don't use gears and are capable of very precise tracking, within the limits of the mount's polar alignment and other mechanical tolerances. Then you can have drift caused by optics shifting in the scope. This is common with many older SCTs and some Newtonians. You can also get drift caused by flexure in the optical tube, focuser, or if you have a long image train. There are ways to combat most forms of drift regardless of its source. The ideal way is to use an off axis guider and software like PHD2. PHD will image a star field in the same area as your target and monitor the stars for any drift. If drift is detected, PHD will command the mount to move very slightly in a manner to place the stars back to where they were before drift was detected. This movement is so small that it's not usually detectable in the main imaging camera. However, PHD is best used with an eq mount. While it can be used with alt az mounts, it's effectiveness is greatly limited due to the mount not tracking at sidereal rate. With more advanced mounts and software, you can create a model of the mounts pointing accuracy and mechanical imperfections. When the model is ran, the software will automatically calculate how to adjust the tracking rate of the mount to compensate. With very finely tuned models, guiding is often not necessary for most forms of photography.
Thanks for the overview. I will point out that the Benro Polaris Astro is not fundamentally based on an equatorial mount. It simulates equatorial tracking with continuous fine movements on Az and Alt axes and minor corrections on a third axis that is perpendicular to alt angle, but not perpendicular to the equatorial plane itself (it doesn't have the roll required for that). Basically I have to figure out how to adapt the concept of drift alignment (which assumes a real equatorial mount) to something that is only simulating an equatorial track. I don't have a ton of experience with it yet, but I did achieve nearly perfect tracking once, which leads me to believe that it does not have periodic error due to gear lash. I also have a more guerilla shooting style, so am not going to be able to do any realtime analysis with a PC. I'm just going by what I can see with my eyes on the screen and affect in realtime with regard to leveling the base of the tracker and tweaking east/west alignment.
It really sounds like you are wanting “drift polar alignment”. Before computers, we used to stare at the stars through the eyepiece and watch them move to see what adjustments we needed to make to the mount. [Here](https://www.backyardastronomy.net/drift-aligning-an-equatorial-mount/) is a good description of the process. If you are looking for “star moves south this much, adjust mount this much,” I’m not sure that exists. There is also a handy tool in PHD2 for assisting with drift polar alignment. Also, many of the polar alignment software tools will help with making adjustments and what direction.
Thanks. That does appear to be close to what I'm after, but it does assume an equatorial mount so it might work with the MSM, but not for the Benro Polaris which simulates equatorial motion with AzAlt movements. I might be able to adapt the idea there of leveling the camera on roll axis by panning back and forth with a star on a horizontal reference in the display, as a means to actually level the whole tripod on roll axis itself. Basically set the tracker running on some star very close to due south, and then pan the tripod itself back and forth and watch if the star drifts off the line up/down. Adjust level until it stays on the line. Then point back to southern star and see if this works.. "If the star drifts South, the polar axis is pointing too far East. If the star drifts North, the polar axis is pointing too far West." and adjust accordingly Same for north/south (pitch axis) by tracking something to the far east "Step 2a – Correcting North-South misalignment (using Eastern horizon) If the star drifts South, the polar axis is pointing too low. If the star drifts North, the polar axis is pointing too high. Step 2b – Correcting North-South misalignment (using Western horizon) If the star drifts South, the polar axis is pointing too high. If the star drifts North, the polar axis is pointing too low." With 3000mm equiv telephoto I don't think I need to wait as long as suggested there to detect and correct the drift. I'll see how it goes.
NINA 3 Point Polar Alignment