Hi,
In my recent imaging session, I encountered a phenomenon where the plate solve was successful, but the alignment was slightly off each time. While pondering the cause, a question arose. Here is my thought process at that time:
・Looking back…on that day,GOTO accuracy was already a bit off even before the plate solve.
⇒Usually, when performing alignment, the first star requires some adjustment, but the second and subsequent stars are almost centered
at the GOTO stage. However, this time, even the second and subsequent stars required adjustment.
(The polar alignment is adjusted until it becomes excellent continuously . My Mount:Celestron AVX)
⇒Was there a problem with the introduction of the first star?
The change point I noticed was that for the first star, I selected “Sync mount and re-center target after plate solve” for the plate solve.
(Previously, I selected “Offset the mounts position to center the target after plate solve” or manually centered the target.)
⇒Wait wait wait… Isn’t it strange to “sync the mount” while still in the middle of alignment?? This is probably the cause.
⇒But… in this case, what kind of processing is done within SharpCap, and why does it result in a slightly off plate solve?
This last sentence is my question. What is the internal decision and processing in SharpCap when selecting “Sync mount and recenter target after plate solve” during star alignment?
Alternatively, if you think the cause might be different, I would appreciate your insights on that as well.
I’m sorry for the basic question. I’m still a beginner and don’t have a deep understanding of SharpCap yet…
Best Regards,
Takuya
Q:About Plate Solving During Star Alignment
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Re: Q:About Plate Solving During Star Alignment
Hi Takuya,
when you use the 'Sync mount and recenter target', SharpCap will send a 'sync' command to the mount telling it that it is currently pointing at the co-ordinates that came from the plate solving result. If your mount uses a simple 'sync' implementation where it just updatest its own co-ordinates to match the values that SharpCap sends it then everything works well - the co-ordinates that the mount thinks it is pointing at are an exact match for the co-ordinates it is *really* pointing at, and SharpCap's next command to GOTO the target again should center the target.
However, some mounts have the option to use a 'cleverer' approach to pointing and synchronization, where they try to maintain a complex conversion mapping between the physical co-ordinates of the mount axes and the place it is pointing to in the sky. This sort of conversion *may* be an option in the ASCOM driver. In this case, the mount may see the sync command from SharpCap as just another data point to help refine its mapping, so that after the sync, the mount co-ordinates may not match exactly the co-ordinates that SharpCap sent, meaning that the subsequent second GOTO movement will not go to the right place.
If your mount is working in this sort of mode, then you can either try to turn it off in the ASCOM driver settings (while it can help make initial GOTOs more accurate, it is not really needed if you are using plate solving), or you can switch back to the 'offset GOTO' mode in SharpCap which works around this issue.
Hope this helps,
Robin
when you use the 'Sync mount and recenter target', SharpCap will send a 'sync' command to the mount telling it that it is currently pointing at the co-ordinates that came from the plate solving result. If your mount uses a simple 'sync' implementation where it just updatest its own co-ordinates to match the values that SharpCap sends it then everything works well - the co-ordinates that the mount thinks it is pointing at are an exact match for the co-ordinates it is *really* pointing at, and SharpCap's next command to GOTO the target again should center the target.
However, some mounts have the option to use a 'cleverer' approach to pointing and synchronization, where they try to maintain a complex conversion mapping between the physical co-ordinates of the mount axes and the place it is pointing to in the sky. This sort of conversion *may* be an option in the ASCOM driver. In this case, the mount may see the sync command from SharpCap as just another data point to help refine its mapping, so that after the sync, the mount co-ordinates may not match exactly the co-ordinates that SharpCap sent, meaning that the subsequent second GOTO movement will not go to the right place.
If your mount is working in this sort of mode, then you can either try to turn it off in the ASCOM driver settings (while it can help make initial GOTOs more accurate, it is not really needed if you are using plate solving), or you can switch back to the 'offset GOTO' mode in SharpCap which works around this issue.
Hope this helps,
Robin
Re: Q:About Plate Solving During Star Alignment
Hi Robin,
Thank you very much for your prompt reply and kind support.
I always use the “Perform Alignment” function of CPWI linked with ASCOM for star alignment, but it seems that unexpected sync commands after plate solving before completing with “Perform Alignment” might cause confusion. I will try using “Offset & Center” during the Perform Alignment and “Sync & Recenter” after completion to avoid this confusion.
However, I think I might have used “Sync & Recenter” during “Perform Alignment” before, although I’m not certain. It seems I need to try further to understand what conditions cause this slight misalignment. When shooting with a wide FOV, it’s not much of an issue, but with a narrow FOV, plate solving misalignment can be a bit problematic.
By the way, when performing plate solving with a long focal length, the success rate decreases. What methods can be considered to increase the success rate? I’ve tried various things, but with my method, the practical limit is around a combined focal length of 10,000mm. Currently, I am taking the following measures. If there are any other effective methods, please let me know:
・Adjusting gain and exposure to shift the histogram to the right (enhancing it)
・Increasing binning ×2
・Trying ASPS with a fully installed INDEX (though it’s slow…)
Sorry for deviating from the main topic.
Best Regards,
Takuya
Thank you very much for your prompt reply and kind support.
I always use the “Perform Alignment” function of CPWI linked with ASCOM for star alignment, but it seems that unexpected sync commands after plate solving before completing with “Perform Alignment” might cause confusion. I will try using “Offset & Center” during the Perform Alignment and “Sync & Recenter” after completion to avoid this confusion.
However, I think I might have used “Sync & Recenter” during “Perform Alignment” before, although I’m not certain. It seems I need to try further to understand what conditions cause this slight misalignment. When shooting with a wide FOV, it’s not much of an issue, but with a narrow FOV, plate solving misalignment can be a bit problematic.
By the way, when performing plate solving with a long focal length, the success rate decreases. What methods can be considered to increase the success rate? I’ve tried various things, but with my method, the practical limit is around a combined focal length of 10,000mm. Currently, I am taking the following measures. If there are any other effective methods, please let me know:
・Adjusting gain and exposure to shift the histogram to the right (enhancing it)
・Increasing binning ×2
・Trying ASPS with a fully installed INDEX (though it’s slow…)
Sorry for deviating from the main topic.
Best Regards,
Takuya
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Re: Q:About Plate Solving During Star Alignment
Hi,
hope that you work out the exact causes leading to the plate solving offset issue.
Plate solving success is really a function of the field of view of the camera, which obviously reduces at longer focal lengths (a really large sensor helps though). In order to get a successful plate solve, you need to detect enough stars in the camera image *and* have enough stars in the plate solving database for that area of sky to allow a match to be found and verified.
I am actually quite impressed that you are getting solving to work at all at 10000mm focal length - even with a large sensor like the IMX571, the FOV is only about 0.13 by 0.09 degrees at that FL, and at that scale, the amount of data needed to give a decent plate solve index becomes enormous. As an example, SharpSolve - SharpCap's built in plate solver - uses about 40,000,000 stars in the larger star database which goes down to about 0.25 by 0.25 degree FOV. For a smaller scale like yours, hundreds of millions (or maybe even a billion) stars would be needed. Actually, I know from building the SharpSolve database that there are a few areas in the sky where there are less stars in an area than I would like for the database, even working down to 0.25 degrees - this is basing the data from the GAIA star database.
Anyway, I think you have done what you can in terms of getting more stars detected in the image (longer exposures, binning, etc) and in using larger databases, but are still hitting problems. My suggestion then would be to try to sidestep the problem by using a 'guide' scope with a shorter focal length to do the plate solving and pointing - if you can get it properly aligned with the main scope then you could plate solve on the guide scope each time and hopefully have the main scope on target based on that.
cheers,
Robin
hope that you work out the exact causes leading to the plate solving offset issue.
Plate solving success is really a function of the field of view of the camera, which obviously reduces at longer focal lengths (a really large sensor helps though). In order to get a successful plate solve, you need to detect enough stars in the camera image *and* have enough stars in the plate solving database for that area of sky to allow a match to be found and verified.
I am actually quite impressed that you are getting solving to work at all at 10000mm focal length - even with a large sensor like the IMX571, the FOV is only about 0.13 by 0.09 degrees at that FL, and at that scale, the amount of data needed to give a decent plate solve index becomes enormous. As an example, SharpSolve - SharpCap's built in plate solver - uses about 40,000,000 stars in the larger star database which goes down to about 0.25 by 0.25 degree FOV. For a smaller scale like yours, hundreds of millions (or maybe even a billion) stars would be needed. Actually, I know from building the SharpSolve database that there are a few areas in the sky where there are less stars in an area than I would like for the database, even working down to 0.25 degrees - this is basing the data from the GAIA star database.
Anyway, I think you have done what you can in terms of getting more stars detected in the image (longer exposures, binning, etc) and in using larger databases, but are still hitting problems. My suggestion then would be to try to sidestep the problem by using a 'guide' scope with a shorter focal length to do the plate solving and pointing - if you can get it properly aligned with the main scope then you could plate solve on the guide scope each time and hopefully have the main scope on target based on that.
cheers,
Robin
Re: Q:About Plate Solving During Star Alignment
Hi Robin,
Thank you for always providing such detailed responses.
When SharpSolve doesn’t work, I use ASPS. It took several hours to download the full index data… I don’t remember exactly, but I think it was about 20GB
. Even so, plate solving at long focal lengths only works properly in star-rich areas of the southern sky (from Japan). Often, I wait for over a minute only to get a “failed” message. It seems that this is the current limit of plate solving.
I try to align the views of the main scope and the guide scope accurately, but since my setup is not permanent, slight misalignments occur each time. If the 35mm equivalent focal length, including the camera’s field of view, is around 6000mm (for example, 1500mm telescope with 1/1.8-inch sensor camera), it works almost without issue (even if it’s not perfectly centered, it will be within the main scope’s field of view). However, at around 10,000mm (for example, 2000mm telescope with 1/1.8-inch sensor), it becomes challenging.
Well, of course I can center it at around 6000mm once, and then extend the focal length again. But I would prefer not to touch the OTA as much as possible, so I’ll keep trying different approaches.
Deeply appreciate kind support, again.
Takuya
Thank you for always providing such detailed responses.
When SharpSolve doesn’t work, I use ASPS. It took several hours to download the full index data… I don’t remember exactly, but I think it was about 20GB
. Even so, plate solving at long focal lengths only works properly in star-rich areas of the southern sky (from Japan). Often, I wait for over a minute only to get a “failed” message. It seems that this is the current limit of plate solving.I try to align the views of the main scope and the guide scope accurately, but since my setup is not permanent, slight misalignments occur each time. If the 35mm equivalent focal length, including the camera’s field of view, is around 6000mm (for example, 1500mm telescope with 1/1.8-inch sensor camera), it works almost without issue (even if it’s not perfectly centered, it will be within the main scope’s field of view). However, at around 10,000mm (for example, 2000mm telescope with 1/1.8-inch sensor), it becomes challenging.
Well, of course I can center it at around 6000mm once, and then extend the focal length again. But I would prefer not to touch the OTA as much as possible, so I’ll keep trying different approaches.
Deeply appreciate kind support, again.
Takuya
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Jean-Francois
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Re: Q:About Plate Solving During Star Alignment
Hello Takuya,
Do you know about a flip mirror ?
I have a Vixen flip mirror. It is very useful in a situation similar to yours.
I use it with 2 camera or one camera and one eyepiece.
The straight path is with a Barlow for the resulting long focal (with planetary camera).
The reflected path is with a larger camera or an eyepiece.
The larger camera or the eyepiece has a several times larger field of view as the camera with the Barlow.
For the centring of the target ... simple, visual or with the larger camera.
After the centring, then "flip" the mirror and you have the target in the planetary camera.
With the new feature in SharpCap, you can perform a Plate Solve and Recentre to any point on the detector.
Regards,
Jean-Francois
Do you know about a flip mirror ?
I have a Vixen flip mirror. It is very useful in a situation similar to yours.
I use it with 2 camera or one camera and one eyepiece.
The straight path is with a Barlow for the resulting long focal (with planetary camera).
The reflected path is with a larger camera or an eyepiece.
The larger camera or the eyepiece has a several times larger field of view as the camera with the Barlow.
For the centring of the target ... simple, visual or with the larger camera.
After the centring, then "flip" the mirror and you have the target in the planetary camera.
With the new feature in SharpCap, you can perform a Plate Solve and Recentre to any point on the detector.
Regards,
Jean-Francois
Re: Q:About Plate Solving During Star Alignment
Hi Jean-Francois
Thank you so much for your reply.
This sounds great! It seems like it will be much easier to locate planets using this device.
I had no idea such a tool existed.
However, since the optical path length is significantly extended, I need to check if it can achieve focus.
My telescope is a MAK127, and I believe its back focus is around 140-150mm.
According to Vixen's site, the optical path length of this flip mirror is 119mm,
and considering the back focus of the camera... it might be a bit tight.
Using a Barlow lens would shift the focus position backward, so maybe it'll work. I’ll think about it.
If you have information on this matter, I’d appreciate your help.
I apologize for going off-topic from SharpCap's features.
Best Regard,
Takuya
Thank you so much for your reply.
This sounds great! It seems like it will be much easier to locate planets using this device.
I had no idea such a tool existed.
However, since the optical path length is significantly extended, I need to check if it can achieve focus.
My telescope is a MAK127, and I believe its back focus is around 140-150mm.
According to Vixen's site, the optical path length of this flip mirror is 119mm,
and considering the back focus of the camera... it might be a bit tight.
Using a Barlow lens would shift the focus position backward, so maybe it'll work. I’ll think about it.
If you have information on this matter, I’d appreciate your help.
I apologize for going off-topic from SharpCap's features.
Best Regard,
Takuya
-
Jean-Francois
- Posts: 847
- Joined: Sun Oct 13, 2019 10:52 am
- Location: Germany
Re: Q:About Plate Solving During Star Alignment
Hello Takuya,
If you search in Google "Vixen flip mirror", you will find all what you want.
Other manufacturers propose similar hardware.
Most of the flip mirror have a M42 interface.
So in the case of the Vixen flip mirror, you can unscrew the tube and mount something different.
For example: https://www.teleskop-express.de/en/ts-a ... -short-209
Alternative with integrated focusing: https://www.teleskop-express.de/en/adap ... 5-inch-584
The other advantage of the Vixen flip mirror is that the tubes have a thread on the full length of the tube.
So if you have a fix setup, you can shorten one tube easily with a lathe machine or with a saw and a file.
One important point ... if you have a Barlow with a long tube or camera like a QHY178, take care to not move the Barlow/camera too much in the tube. The flip mirror shall move free in both positions.
Regards,
Jean-Francois
If you search in Google "Vixen flip mirror", you will find all what you want.
Other manufacturers propose similar hardware.
Most of the flip mirror have a M42 interface.
So in the case of the Vixen flip mirror, you can unscrew the tube and mount something different.
For example: https://www.teleskop-express.de/en/ts-a ... -short-209
Alternative with integrated focusing: https://www.teleskop-express.de/en/adap ... 5-inch-584
The other advantage of the Vixen flip mirror is that the tubes have a thread on the full length of the tube.
So if you have a fix setup, you can shorten one tube easily with a lathe machine or with a saw and a file.
One important point ... if you have a Barlow with a long tube or camera like a QHY178, take care to not move the Barlow/camera too much in the tube. The flip mirror shall move free in both positions.
Regards,
Jean-Francois
Re: Q:About Plate Solving During Star Alignment
Hi Jean-Francois,
Thank you for the additional information.
It’s great that the tube has threading along its full length!
I’ll consider it.
Best Regards,
Takuya
Thank you for the additional information.
It’s great that the tube has threading along its full length!
I’ll consider it.
Best Regards,
Takuya
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Re: Q:About Plate Solving During Star Alignment
Hi,
on the subject of flip mirrors, it's worth making sure that the one you get (if you get one) has 'collimation' adjustments that let you adjust the tilt of the mirror when in the 45 degree position and also make small adjustments to the rotation of the mirror. These adjustments are usually small hex bolts on the body of the mirror. By adjusting these, you can make sure that the two ports of the mirror show the same target centered, rather than having to deal with a potential offset.
I wrote a review of a 'BS Astro' one years ago (basically the same product as sold by many different brands including Orion US) - https://www.sharpcap.co.uk/other-stuff/ ... ror-review - I must admit that with a permanent setup I don't use it much these days.
cheers,
Robin
on the subject of flip mirrors, it's worth making sure that the one you get (if you get one) has 'collimation' adjustments that let you adjust the tilt of the mirror when in the 45 degree position and also make small adjustments to the rotation of the mirror. These adjustments are usually small hex bolts on the body of the mirror. By adjusting these, you can make sure that the two ports of the mirror show the same target centered, rather than having to deal with a potential offset.
I wrote a review of a 'BS Astro' one years ago (basically the same product as sold by many different brands including Orion US) - https://www.sharpcap.co.uk/other-stuff/ ... ror-review - I must admit that with a permanent setup I don't use it much these days.
cheers,
Robin