Guess I'm late to the party, but stumbled across a YT video on how to polar align EQ platforms using SharpCap Pro's Polar Align Tool (enabling "use smaller rotation').
Mind blown!!
Works great on my 8" Dobsonian and its platform. But, the 8" being relatively lightweight is "easy" to manually rotate from "starting position" to "end of run" position.
My 12" Dobsian on its platform? Different story -- mainly due to the greater weight (80lbs vs. 50lbs for the 8"). This makes manual rotation from "starting position" to "end of run" postion for the rotation part of PA a challenge.
My homemade builds used simple, single-axis RA motors and there is no super fast way to "rewind" the platform with them (takes about 30 minutes at full throttle to rewind). So, I'm pretty much stuck with manual rewinding/resetting.
I should also note that I used sandpaper on the bearing surfaces of my VNS to prevent slipping on the rollers...so there is added friction when manually rotating the platforms.
My question boils down to this: Is Polar Align tool "continuously" plate solving for PA during the rotation OR is it waiting until the rotation comes to a stop and comparing point A (prior to rotation) to point B (after rotation has stopped)?
I ask because one way that I make the manual rotation (and reset) easier on the 12" is to use an air bladder (like cabinet installers use to level cabinets) to slightly lift/unweight the vertical north segments (VNS). This way, I'm not having to both lift *and* drag the top table....it's mostly dragging. But, of course, lifting the top table with the air bladder changes the altitude where the camera is pointing.
I'm concerned that PA tool will fail if the program feels the mount/platform has moved - even though the air bladder is removed and the VNS are put back fully on the rollers.
I hope all of the above makes sense.
IF the camera is continuously in use during rotation, could a feature be added to WAIT to take a picture and solve until the user manually says "ok, I'm done rotating the mount - please compare point A to point B and calculate"???
Thanks in advance!!
Polar Aligning Large Dob on EQ Platform
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Re: Polar Aligning Large Dob on EQ Platform
Hi,
glad that you are having success already with the polar alignment tool on the smaller 8" Dob
Once you move to the rotation stage of the polar alignment tool, it will be trying to solve every frame from the camera and will calculate a rotation from every frame that does solve (badly trailed stars would cause a frame to fail to solve at all for instance). However, I don't think that will be a problem - each time a new frame arrives and is successfully solved, the calculation is updated based on that new frame and the previous ones are discarded. As long as the calculated rotation exceeds the minimum required then you can move on to the final stage.
That means that as long as the final frame is in the right position, the calculation should be right. You can move the telescope all over the place during the movement and it shouldn't affect the final result.
What you will possibly see is extra crosses drawn on screen for the interim calculated positions of the center of rotation. You'd normally expect these all to be grouped closely together for a smooth rotation and having them spread out would be an indication that something has slipped or shifted during the rotation. These are only displayed as a guide to the end user to diagnose that sort of problem - the algorithm doesn't rely on them, so you can ignore them being all over the place.
cheers,
Robin
glad that you are having success already with the polar alignment tool on the smaller 8" Dob
Once you move to the rotation stage of the polar alignment tool, it will be trying to solve every frame from the camera and will calculate a rotation from every frame that does solve (badly trailed stars would cause a frame to fail to solve at all for instance). However, I don't think that will be a problem - each time a new frame arrives and is successfully solved, the calculation is updated based on that new frame and the previous ones are discarded. As long as the calculated rotation exceeds the minimum required then you can move on to the final stage.
That means that as long as the final frame is in the right position, the calculation should be right. You can move the telescope all over the place during the movement and it shouldn't affect the final result.
What you will possibly see is extra crosses drawn on screen for the interim calculated positions of the center of rotation. You'd normally expect these all to be grouped closely together for a smooth rotation and having them spread out would be an indication that something has slipped or shifted during the rotation. These are only displayed as a guide to the end user to diagnose that sort of problem - the algorithm doesn't rely on them, so you can ignore them being all over the place.
cheers,
Robin
Re: Polar Aligning Large Dob on EQ Platform
Stl2como,
I use a 12” dob on a Ed jones platform with the synchronus motors myself. I found the problem with pa is once you get aligned resetting platform causes shifting of base board position slightly. I image from the same location on my property so I installed 2” furniture levelers as the north bearing feet. I then use cups for the feet to set in which are anchored to the cement I am observing on. Then once you get you pa set the cups stay attached to floor and you just place the ground board on cups and your polar aligned. The cups are on tinkercad website search for “dob foot cup”.
I agree the sc alignment tool is great but I find using drift alignment is eaisier for dob platforms. That way you start at platform begin position. Using reticle in sc program you start with platform off and turn camera in focuser until stars follow the reticle line. Turn on mount and watch drift and follow normal drift alignment process. Robin could automate this process in sc but there aren’t many of us using this type of equipment anymore. If I can help let me know.
George
I use a 12” dob on a Ed jones platform with the synchronus motors myself. I found the problem with pa is once you get aligned resetting platform causes shifting of base board position slightly. I image from the same location on my property so I installed 2” furniture levelers as the north bearing feet. I then use cups for the feet to set in which are anchored to the cement I am observing on. Then once you get you pa set the cups stay attached to floor and you just place the ground board on cups and your polar aligned. The cups are on tinkercad website search for “dob foot cup”.
I agree the sc alignment tool is great but I find using drift alignment is eaisier for dob platforms. That way you start at platform begin position. Using reticle in sc program you start with platform off and turn camera in focuser until stars follow the reticle line. Turn on mount and watch drift and follow normal drift alignment process. Robin could automate this process in sc but there aren’t many of us using this type of equipment anymore. If I can help let me know.
George
Re: Polar Aligning Large Dob on EQ Platform
George, thanks. I'm very interested in your drift alignment method, particularly because you mention that it lets you start with the platform at its normal beginning position.Gfischer wrote: Fri Aug 07, 2026 6:04 pm Stl2como,
I use a 12” dob on a Ed jones platform with the synchronus motors myself. I found the problem with pa is once you get aligned resetting platform causes shifting of base board position slightly. I image from the same location on my property so I installed 2” furniture levelers as the north bearing feet. I then use cups for the feet to set in which are anchored to the cement I am observing on. Then once you get you pa set the cups stay attached to floor and you just place the ground board on cups and your polar aligned. The cups are on tinkercad website search for “dob foot cup”.
I agree the sc alignment tool is great but I find using drift alignment is eaisier for dob platforms. That way you start at platform begin position. Using reticle in sc program you start with platform off and turn camera in focuser until stars follow the reticle line. Turn on mount and watch drift and follow normal drift alignment process. Robin could automate this process in sc but there aren’t many of us using this type of equipment anymore. If I can help let me know.
George
Could you describe your procedure in a little more detail? For example, what part of the sky do you point the Dob toward, how do you determine whether the correction needed is platform azimuth versus altitude, and approximately how long do you watch the star drift before making an adjustment?
Also, are you using SharpCap's reticle only as a visual reference, or is SharpCap doing any measurement/calculation of the drift for you?
My situation is a bit different...having relocated for a job and not where I plan to stay after I retire in (hopefully) a few short years. So, I'm currently just leasing a single-family detached home. My dobs rest in the yard on pavers where I do my best to put reference marks with painter's tape.
Last edited by STL2COMO on Sat Aug 08, 2026 3:24 pm, edited 1 time in total.
Re: Polar Aligning Large Dob on EQ Platform
Robin, thanks.admin wrote: Fri Aug 07, 2026 10:17 am Hi,
glad that you are having success already with the polar alignment tool on the smaller 8" Dob
Once you move to the rotation stage of the polar alignment tool, it will be trying to solve every frame from the camera and will calculate a rotation from every frame that does solve (badly trailed stars would cause a frame to fail to solve at all for instance). However, I don't think that will be a problem - each time a new frame arrives and is successfully solved, the calculation is updated based on that new frame and the previous ones are discarded. As long as the calculated rotation exceeds the minimum required then you can move on to the final stage.
That means that as long as the final frame is in the right position, the calculation should be right. You can move the telescope all over the place during the movement and it shouldn't affect the final result.
What you will possibly see is extra crosses drawn on screen for the interim calculated positions of the center of rotation. You'd normally expect these all to be grouped closely together for a smooth rotation and having them spread out would be an indication that something has slipped or shifted during the rotation. These are only displayed as a guide to the end user to diagnose that sort of problem - the algorithm doesn't rely on them, so you can ignore them being all over the place.
cheers,
Robin
Given your response....it may be that I can just turn the motors upto full speed during the rotation part....which would advance the platform the full range in about 30 minutes; BUT, error calculation and correction directions might occur earlier (say at 7 degrees rotation, not the full 15 degree run).
If I'm understanding you correctly, SC doesn't really care how slowly or quickly the rotation occurs (?) - it'll keep taking images and trying to solve them and will just report no solution (warning x failed to solve) until it does eventually?
The issue would more operator impatience than the program looping/locking into some "cranky state"?
Re: Polar Aligning Large Dob on EQ Platform
Stl2como,
the drift method has many great web pages available on line so i'll just give you a "dob" version.
Since you already have your approximate alignment marked on pavers your part way there.
Place your dob base on your marks and setup equipment. Set camera to a higher gain and maybe 1 sec exposure. Move platform to start position and aim scope at the intersection of the meridian and celestial equator. Close is good enough (within 5 degrees). with the drive off watch the stars track across field. Turn on the cross reticle in Sharpcap. Rotate camera and observe stars movement until stars follow reticle line in a parallel path. The drift you are going to observe will be at right angles to the stars path. Turn on drive and move the scope to put a star on the reticle center point. Watch the stars position for a few minutes. Ignore movement along the axis the stars followed with motor off which is the east west direction. When you see definite drift in the north south direction slightly rotate platform in azimuth ( slow moves) the direction of the rotation is determined by the north or south drift. Sorry I forget which way (you can look it up online). I just try one way and recenter star on reticle center turn off drive to make sure stars follow line (if not rotate camera again). Start drive again and watch drift again. If drift is worse than rotate the base in the other direction. Repeat this until drift is very small. With dobs platforms totally eliminating drift isn't possible due to inaccuracies of mount.
Now for altitude setting bring platform to start position again and locate a star close to the celestial equator and 20 degrees above east or west horizon. Now follow the same procedure for the azimuth adjustment only adjust in altitude only. For dob platforms this adjustment is sensitive take it slow. Once you stop drift in this part you are polar aligned. A good thing about your setup because you are placing your ground board in the same spot the altitude adjustment shouldn't need to be adjusted again as long as you put base board on platform on same spot.
Depending how accurately you can place platform base board back into the exact position you had it in when doing polar alignment you might not need to polar align again. The problem with platforms where you have to use considerable force to reset to start position the base has a tendency to slide when resetting. This is why I use the "cups" to hold the position of the base solid. The screw you would need to hold the cups to the pavers would need a 1/8 inch hole for a masonry drywall type screw to hold cup in place. Just a thought.
Sharpcap's polar alignment tool works great but drift alignment doesn't require a view of the polar area. Either system will get you the alignment you need but do your best to avoid moving platform when resetting. I really enjoy my 12" dob and due to the large aperture I get great shots with 30 min total integration times.
Good Luck
George
the drift method has many great web pages available on line so i'll just give you a "dob" version.
Since you already have your approximate alignment marked on pavers your part way there.
Place your dob base on your marks and setup equipment. Set camera to a higher gain and maybe 1 sec exposure. Move platform to start position and aim scope at the intersection of the meridian and celestial equator. Close is good enough (within 5 degrees). with the drive off watch the stars track across field. Turn on the cross reticle in Sharpcap. Rotate camera and observe stars movement until stars follow reticle line in a parallel path. The drift you are going to observe will be at right angles to the stars path. Turn on drive and move the scope to put a star on the reticle center point. Watch the stars position for a few minutes. Ignore movement along the axis the stars followed with motor off which is the east west direction. When you see definite drift in the north south direction slightly rotate platform in azimuth ( slow moves) the direction of the rotation is determined by the north or south drift. Sorry I forget which way (you can look it up online). I just try one way and recenter star on reticle center turn off drive to make sure stars follow line (if not rotate camera again). Start drive again and watch drift again. If drift is worse than rotate the base in the other direction. Repeat this until drift is very small. With dobs platforms totally eliminating drift isn't possible due to inaccuracies of mount.
Now for altitude setting bring platform to start position again and locate a star close to the celestial equator and 20 degrees above east or west horizon. Now follow the same procedure for the azimuth adjustment only adjust in altitude only. For dob platforms this adjustment is sensitive take it slow. Once you stop drift in this part you are polar aligned. A good thing about your setup because you are placing your ground board in the same spot the altitude adjustment shouldn't need to be adjusted again as long as you put base board on platform on same spot.
Depending how accurately you can place platform base board back into the exact position you had it in when doing polar alignment you might not need to polar align again. The problem with platforms where you have to use considerable force to reset to start position the base has a tendency to slide when resetting. This is why I use the "cups" to hold the position of the base solid. The screw you would need to hold the cups to the pavers would need a 1/8 inch hole for a masonry drywall type screw to hold cup in place. Just a thought.
Sharpcap's polar alignment tool works great but drift alignment doesn't require a view of the polar area. Either system will get you the alignment you need but do your best to avoid moving platform when resetting. I really enjoy my 12" dob and due to the large aperture I get great shots with 30 min total integration times.
Good Luck
George
Re: Polar Aligning Large Dob on EQ Platform
Well, I went a different direction on polar aligning my 12" Dob on its EQ platform using SharpCap Pro's Polar Align tool.
I've documented the whole process (with photos) over on Cloudy Nights: https://www.cloudynights.com/forums/top ... -platform/ I hope that's ok Robin.
Briefly, in lieu of trying to polar align the 12" Dob while it's on the EQ platform, I'm aligning the EQ PLATFORM using a small tripod and a small QHY camera. The tripod/camera is rigidly mounted to the top plate of the EQ platform "substituting" for the 80lb OTA/base.
The tripod/camera is connected to a laptop and SharpCap's Polar Align tool is launched. Checking the "use smaller angle" box. The platform is then adjusted based on the plate-solves from the tripod/camera.
The key point is: once the EQ PLATFORM is polar aligned using the tripod/camera, those are removed from the EQ platform and "replaced" with the full 12" Dob and base.
Yes, I recognize that there is some possibility that the putting the full 12" Dob and base may cause flex, etc. that MIGHT cause the polar alignment to shift, but - so far - I've had very good results with the actual tracking of my EQ platform using this procedure.
I've documented the whole process (with photos) over on Cloudy Nights: https://www.cloudynights.com/forums/top ... -platform/ I hope that's ok Robin.
Briefly, in lieu of trying to polar align the 12" Dob while it's on the EQ platform, I'm aligning the EQ PLATFORM using a small tripod and a small QHY camera. The tripod/camera is rigidly mounted to the top plate of the EQ platform "substituting" for the 80lb OTA/base.
The tripod/camera is connected to a laptop and SharpCap's Polar Align tool is launched. Checking the "use smaller angle" box. The platform is then adjusted based on the plate-solves from the tripod/camera.
The key point is: once the EQ PLATFORM is polar aligned using the tripod/camera, those are removed from the EQ platform and "replaced" with the full 12" Dob and base.
Yes, I recognize that there is some possibility that the putting the full 12" Dob and base may cause flex, etc. that MIGHT cause the polar alignment to shift, but - so far - I've had very good results with the actual tracking of my EQ platform using this procedure.
Re: Polar Aligning Large Dob on EQ Platform
I read your cn post. Very ingenious solution.
I broke one of the cups I had mentioned in previous post and had to 3d print another. After replacing I used sc to polar align. I did find that to use sc tool if you start with platform at end of travel solve then rotate to start of travel you have an eaisier time to make position correction. You don’t have to hurry to avoid running out of platform travel. Sc aligned me so I’m back to place on cups and ready to go now.
It’s one of the things I like about dobs you get to tinker
I broke one of the cups I had mentioned in previous post and had to 3d print another. After replacing I used sc to polar align. I did find that to use sc tool if you start with platform at end of travel solve then rotate to start of travel you have an eaisier time to make position correction. You don’t have to hurry to avoid running out of platform travel. Sc aligned me so I’m back to place on cups and ready to go now.
It’s one of the things I like about dobs you get to tinker
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Re: Polar Aligning Large Dob on EQ Platform
Hi,
thanks for sharing your neat idea for polar aligning on a dob platform - glad it works well.
I should also point out that there is no need to leave the tracking running during a SharpCap polar align procedure (you can leave it running, but it can equally be turned off). That's because you are viewing the sky right at the pole, so the drift of stars inside the field of view is tiny, even over a few minutes.
cheers,
Robin
thanks for sharing your neat idea for polar aligning on a dob platform - glad it works well.
I should also point out that there is no need to leave the tracking running during a SharpCap polar align procedure (you can leave it running, but it can equally be turned off). That's because you are viewing the sky right at the pole, so the drift of stars inside the field of view is tiny, even over a few minutes.
cheers,
Robin
Re: Polar Aligning Large Dob on EQ Platform
Thanks Robin:
The "aha" moment for me was understanding this: the tripod/camera did NOT have to be physically located on or optically aligned with the EQ platform's virtual polar axis. The tripod/camera only needed to be rigidly mounted to the platform and move along with it during the rotation stage of the procedure (well, ok, "only" along with all of the other requirements listed in the manual for successful PA).
From that rotation, then, SharpCap can determine the platform's effective RA axis. So, if during the rotational stage, SharpCap determines the platform's RA axis actually points to Dec +87.2 degrees, SharpCap knows that the North Celestial Pole is Dec +90 degrees. It can, therefore, determine the difference between the platform's mechanical axis and where the NCP is.
I think you say as much in the SharpCap manual (i.e., the guidscope doesn't have to be aligned with OTA)....I just had to grasp what that meant for an EQ platform where the polar axis is a virtual one, not a mechanical one (like on a GEM). Before that "aha" moment I spent a lot of mental energy trying to get the tripod/camera exatly on the same line as the virtual polar axis.
I now think that using the tripod and tilting the camera upwards fo 39 degrees (my approx. latitude) is useful for being in the right sky region (within 5 degrees of the NCP) and star field, but it doesn't have to be aligned with the EQ platform's virtual polar axis.
Hey, I'm an attorney by day job....and, I think, you know how dense we can be
In any case, I think, based on his comment, George is running his platform during the PA process; but I do not. Most of the time, I'm running it on 9v batteries so I try to conserve them when possible -lol!!
The "aha" moment for me was understanding this: the tripod/camera did NOT have to be physically located on or optically aligned with the EQ platform's virtual polar axis. The tripod/camera only needed to be rigidly mounted to the platform and move along with it during the rotation stage of the procedure (well, ok, "only" along with all of the other requirements listed in the manual for successful PA).
From that rotation, then, SharpCap can determine the platform's effective RA axis. So, if during the rotational stage, SharpCap determines the platform's RA axis actually points to Dec +87.2 degrees, SharpCap knows that the North Celestial Pole is Dec +90 degrees. It can, therefore, determine the difference between the platform's mechanical axis and where the NCP is.
I think you say as much in the SharpCap manual (i.e., the guidscope doesn't have to be aligned with OTA)....I just had to grasp what that meant for an EQ platform where the polar axis is a virtual one, not a mechanical one (like on a GEM). Before that "aha" moment I spent a lot of mental energy trying to get the tripod/camera exatly on the same line as the virtual polar axis.
I now think that using the tripod and tilting the camera upwards fo 39 degrees (my approx. latitude) is useful for being in the right sky region (within 5 degrees of the NCP) and star field, but it doesn't have to be aligned with the EQ platform's virtual polar axis.
Hey, I'm an attorney by day job....and, I think, you know how dense we can be
In any case, I think, based on his comment, George is running his platform during the PA process; but I do not. Most of the time, I'm running it on 9v batteries so I try to conserve them when possible -lol!!