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Optimising gear ratio : alt ratio correction calculation
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Dear all,
I have used servocat+AN to follow ISS.
As ISS is moving at high speed you have to have have very good ratio adjustment (and also good pointing to have the maths done at good location => I implemented a TPAS)
To improve accuracy of tracking I started to improve ratio using the "track improvment" method described in the Sercocat quick V3 file.
To adjust Az ratio, it describes to use a star located at south.
I understand because at south it's a pure Az movement, no alt movement.
?
Then for Alt ratio, it describes to use a star located at east.
For ex you strack an object for 1/2 hour, the sky moved 7.5¡ã (=360/24/2)and you divide your offset by this 7.5¡ã value to calculate a correction factor of your Alt ratio.
What I don't understand is that for a star located at east, it's not a pure Alt movement so I don't understand why you divide your offset by 7.5¡ã?
For example I live at 45¡ã latitude.
In this case the star move only sin(45)*7.5= 5.3 in Alt (and 5.3 in Az).
After 1/2h let say I see in AN an offset for the star of 0.1¡ã in Az and 0.2¡ã in alt => I would have divided the alt offset 0.2 by 5.3 to get the correction ratio, not 0.2/7.5?
I'm certainly wrong, but could someone explain my mistake ?
Regards
Rapha?l |
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Raphael, Its easiest to use the "Simple Track Improvements Test" document found on the CD. If you don't have the CD write to me direct and I'll send you the doc. Its a simple txt doc.? Gary? ------ Original Message ------
From "Raphael Guinamard via groups.io" <rguinamard@...>
Date 4/27/2025 10:36:32 AM
Subject [ServoCAT] Optimising gear ratio : alt ratio correction calculation
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Dear Gary,
I did it already one and corrected ratio. But there is still a remaining of correction of ratio needed.?
I mean yesterday night, the scope was pointing accurately because I had just calculated the IE of TPAS (with other persistant terms already computed from previous test)so I had a computed TPAS leading to RMS of 6'. OK
Then I centered a star and after 35 min it was 20' offcenter (0.33¡ã). Frankly this is OK for visual follow up but still not accurate enough to follow ISS that move 200 times quicker than sideral.
?
But I?can't I achieve anymore the "Simple Track Improvements Test" because :
In the "Simple Track Improvements Test" it is said :? ?
"Push the GOTO ¨C it will start to move ¨C now watch carefully the GUIDE value for this axis (left for AZ, right for
ALT). The scope will move a distance and then come to a stop. It will will then quickly pick up and start moving again. You need to get the GUIDE value when it first stops" The problem is since I'm already close to perfect ratio I can't notice any stop even if I put 170¡ã instead of 100¡ã in the procedure.
Maybe this is because my scope is massive, so I need to put? a 5 seconds deceleration => could this make it more difficult to notice the "stop" if it occurs during the deceleration ? I don't know... but I can't notice any stop.
?
So this is why I wanted then to use? the "track improvment" method described in the Sercocat quick V3 file since this one is unsensitive to deceleration.
But then I have the calculation issue I indicated in my first message
I hope this clarify my original question
Regards
Rapha?l |
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Raphael, The "stop" might be with only 0.1+ degrees left. It would then just go to the second move and you'd not notice the stop. That is extremely good for the purpose for which it was designed - visual astronomy. Keep in mind - on the vast majority of 'hand made' scopes, there's enough variability in the dimensions and mounting of the drive surfaces on either the ALT or the AZ (ground board or bearings) that it isn't going to be one value.? If your alignment is accurate and especially if you are using TPAS to correct for mount errors then it should track extremely well since it is closed loop (based on the user centering the object at the start). If you want to take the time since you are dialed in so well the only way to do so is indeed to do the drift test you did over a longer period of time. Alternatively you can do the drift method for shorter period but just tweak the ratio (on the axis you're testing for - do one at a time!) by a count of say 5. Then repeat the test and see what that gives you. You can then ratio the results on 2 tests to get it (better) corrected.? The other thing is there could well be?issues like friction and slippage and other mechanical things that the system is not designed to account for. Even torque induced bending might well be enough to move the object.? My suspicion is you might be asking too much from such a large scope.? g. ?
------ Original Message ------
From "Raphael Guinamard via groups.io" <rguinamard@...>
Date 4/27/2025 11:33:22 AM
Subject Re: [ServoCAT] Optimising gear ratio : alt ratio correction calculation
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Gary,
"My suspicion is you might be asking too much from such a large scope"
Yes, for sure !!
In fact trying to get the maximum from my scope is really a target in itself.
With my small field of view, it is necessary to have pointing and ratio as accurates as possible to follow as perfectly as possible ISS.
You know it's like Gabriel Wiklund that is optimising again and again the autoguiding with wonderfull results at the end. Great to obtain the very best from a scope !
I want to go as deep as possible for optimising the scope. Maybe I will be blocked by mechanical problem, but at least I want to go to this point.
?
I will continue using drifting test, either by tweaking the ratio 5 by 5 as you suggest, either using the "track improvment" method but with the correction I was mentionning in my initial message for the alt correction. Even if I'm wrong with the correction I mentioned in my initial message, it will just take more iterations to achieve the best ratio I could achieve before mechanicalo limitation.
I will post the results after the iterations
Thanks for your help
Rapha?l
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Raphael, I'm all for 'getting the best out of the system' .... kudos to you and the best in your work.? g. ------ Original Message ------
From "Raphael Guinamard via groups.io" <rguinamard@...>
Date 4/27/2025 4:17:38 PM
Subject Re: [ServoCAT] Optimising gear ratio : alt ratio correction calculation
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TPAS is not flaky! It was proven over years of development work by WCI including but not limited to using the publicly available Keck Telescope data where TPAS achieved the same results. It is, however, by its very nature complicated. The vast majority of issues are either user misunderstandings or user error.
?
Please reframe from derogatory comments.?
?
Gary Myers
StellarCAT |
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I hope this is not "inappropriate" for posting to the group, but here it goes:
Raphael, have you considered perhaps a different type of setup for tracking the ISS? For example, there are Astro-Physics mounts out there that when paired up with software like APCC [Astro-Physics Command Center] it helps a lot when tracking satellites, ISS, Hubble Space Telescope "HST", etc. Not sure, but I think also the Software Bisque Paramounts can be programmed to do the same thing <The Sky X software, T-Point, etc>. I figure the extreme CNC precision machining that goes into these mounts would make the task/goal easier (however, with a smaller aperture scope). I mention this because many astronomers have multiple setups (mounts: alt-az & German Equatorials, push to or electronic drive systems, solar or nighttime telescopes, etc) I know not everyone is into German Equatorial Mounts "GEM's". They are a different animal, or beast, versus a big dob light bucket. I mentioned it here as "just a thought" or "friendly suggestion". Perfectly understandable if your preference is to stay committed to your project.? I hope this doesn't get me banned/ kicked out of the group. I don't think it hurts or is harmful to look at alternative means of doing things.? Fernando |
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Fernando,? Good day sir - good to hear from you. I hope all is well.? No worries on this post. Indeed I'd guess that some not insignificant percentage of dob owners are also owners of other "forms" of scopes. I myself have always been a "photon in the eye" guy... but since retiring I began looking around and realized that the equipment has come a long long way - most definitely the one piece of kit that has pushed it are indeed the CMOS cameras that are now available. They are extremely sensitive with extremely low read noise.? So - I've 'gone to the dark side' myself getting an 8" f4 imaging newt, an APS-C camera and a harmonic EQ drive. Just testing it out now.? That stated my guess is Raphael is just wanting to do all he can with that BIG scope of his... understandable and commendable.? Gary Myers ps: the local club that I belong to has a 25" Obsession w/ServoCAT/AN that was donated to the club... we had a club 'dark sky night' last night and had some decent views. I DO however miss the skies of SE AZ.? ------ Original Message ------
From "fernandorivera3 via groups.io" <fernandorivera3@...>
Date 4/28/2025 11:08:59 AM
Subject Re: [ServoCAT] Optimising gear ratio : alt ratio correction calculation I hope this is not "inappropriate" for posting to the group, but here it goes: |
¿ªÔÆÌåÓýHi Fernando
I believe your point is not inappropriate but I¡¯m not owner of group
In fact I was discussing about ISS but I m more into small galaxies : I look at ISS because the scope has just been put out, of observatory waiting for night?
But I like the challenge to be able to follow ISS and see some details in the big dob?
I say some detail but not that much as for the moment i ve only seen ISS at beginning of night so with mirrors not in thermal equilibriums?
I hope I could see in morning this Thursday morning and it may be wonderful?
I know also that fine tune ratio will be a good starting point if I decide to take pictures?
Anyway it seems so far it¡¯s ok with my setting as ISS is only seen by opportunity?
Anyway maybe one day I will buy one a the refractor or mount you indicate so thanks for giving advices?
Last thing : argonavis fully compute the trajectory of ISS but I believe you knew this
Thanks?
Raphael?
De : [email protected] <[email protected]> de la part de fernandorivera3 via groups.io <fernandorivera3@...>
Envoy¨¦ : Monday, April 28, 2025 5:08:59 PM ? : [email protected] <[email protected]> Objet : Re: [ServoCAT] Optimising gear ratio : alt ratio correction calculation ?
I hope this is not "inappropriate" for posting to the group, but here it goes:
Raphael, have you considered perhaps a different type of setup for tracking the ISS? For example, there are Astro-Physics mounts out there that when paired up with software like APCC [Astro-Physics Command Center] it helps a lot when tracking satellites,
ISS, Hubble Space Telescope "HST", etc. Not sure, but I think also the Software Bisque Paramounts can be programmed to do the same thing <The Sky X software, T-Point, etc>. I figure the extreme CNC precision machining that goes into these mounts would make
the task/goal easier (however, with a smaller aperture scope).
I mention this because many astronomers have multiple setups (mounts: alt-az & German Equatorials, push to or electronic drive systems, solar or nighttime telescopes, etc)
I know not everyone is into German Equatorial Mounts "GEM's". They are a different animal, or beast, versus a big dob light bucket. I mentioned it here as "just a thought" or "friendly suggestion".
Perfectly understandable if your preference is to stay committed to your project.?
I hope this doesn't get me banned/ kicked out of the group. I don't think it hurts or is harmful to look at alternative means of doing things.?
Fernando
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GEMs are the worst choice for tracking satellites.? It's not the equatorial mounting, per se, but rather, the need for a meridian flip with GEMs.? A fork mounted EQ mount on a wedge, for example, has no need for a meridian flip.? When polar aligned, as most EQ mounts are, a meridian flip occurs ... near meridian.? Meridian has nothing to do with a general satellite track, particularly not those that show details in our scopes.? They culminate where they culminate, and that's rarely meridian.? It's not the better build either, but rather, the ability to get a good alignment because of that better build, much more easily.
?
For OP, use the "Simple Track Improvements Test" that Gary recommended, and round down rather than rounding up.? That way, it will be slower and lagging, so all your corrections will be to "speed up", while maintaining the proper direction.? The alternative, rounding up, will have you sending command to "slow down".? Rarely, except around culmination, will simply "slowing down" switch directions (it's moving too fast for that, except in alt around culmination), but there are practical benefits of keeping the satellite slightly behind the center of the fov.? That benefit is reacquisition should it slip out of the fov.? When it's slightly behind center, there is typically more room in the eyepiece when it does drift out, so you are better able to know where it went.
?
Back to the "better alignment" issue, AN + SC has some odd features that make that more cumbersome that it need to be.? Typically, satellites that show details are low, so they have passes shortly after sunset, or shortly before sunrise.? If the pass is before sunrise, there is plenty of time to gather a large run of data to run TPAS, resolve issues, get comfortable, ... .? When the pass is shortly after sunset, there is hardly any time at all to run a new TPAS model.? Unfortunately, ArgoNavis will JUNK your alignment any time you update a TLE, so you have to do another TPAS run to get those non-constant terms.? Because ArgoNavis JUNKS the alignment, it is best to avoid having to rely on TPAS at all.? That is unfortunate, and no fix to this bug is likely coming soon.? Because TLEs need to be updated often, it is even better to not rely on ArgoNavis at all.
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SkyTrack is seemingly an alternative, but ServoCAT is not ready for that yet.? SkyTrack loads TLEs into its own software, and has the ability to plate solve along the projected path to make its own model.? It works incredibly well with my other commercial mounts from Celestron and SkyWatcher.? Those are not incredibly well built mounts, but the alignment is what overcomes that.? One can build an excellent alignment using those mounts respective control software, through plate solving, without user intervention.? One can then further refine those models using SkyTrack itself, through plate solving, without user intervention.? SkyTrack then has the ability to fine tune the tracking rates with a gamepad or the like, adjusting the corrections based on the present tracking speeds.
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What keeps SkyTrack from working with ServoCAT at the moment ServoCAT's inability to accept custom rates through ServoCAT's USB port.? One is limited to the preprogrammed rates of the handcontroller.? Because the satellites' rates change along the pass, that is now a limitation of ServoCAT itself.? ArgoNavis need not be involved anywhere, and any other DSC (or none at all) would do otherwise fine, preferably one that does not junk your alignment when one you update a TLE.? As soon as ServoCAT's USB port is "up to snuff", SkyTrack will be my preferred solution for satellite tracking with my dob.
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For OP, for now, ArgoNavis is the only workable solution.? But again, it JUNKS your alignment whenever you update a TLE, which is all the time.? In practice, that means getting by with as few stars as your are able to do so, when the pass is shortly after sunset, to get the best alignment you can.? Adjust your mount itself to make those non-perpendicularity terms as close to zero you can get them, use classical bi-directional offset so that your mirror points where your mount points, and level your mount as best as you can.? Structural rigidity will keep flexure terms at bay, and they are the hardest to deal with.? Once you can get as best an alignment as you can, with as few stars as are available, then, and only then, will AutoLock work properly.? Getting the ratios adjusted in ServoCAT as best as you can at that point simply means that the tracking rate adjustments because of AutoLock are now minimal.
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Bear in mind that you will still need to make tracking corrections.? TLEs are never exact, and their utility decays as one gets farther from their epoch.? Your time and location need to be as precise and accurate as you can get them.? Your alignment needs to be as precise and accurate as you can get it.? Even then, you cannot account for variables such as refraction, without actually modelling them first.? All of your efforts will lead you to having to make smaller corrections to the tracking speed.
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Even with all of these challenges with the present ArgoNavis + ServoCAT, I can still keep ISS and CSS in the FOV of my 15" with a 6 Ethos, yielding about 300x in a 100 degree AFOV.? The TFOV is about twenty arcminutes from one side to other, so the "available room" for error is about half of that, at only about ten arcminutes.? That's rather good!? But, there is always room for improvement, and a better protocol of ServoCAT's USB port is the way forward, for things such as updating TLEs, doing alignments through plate solving (which can align closer to sunset than my eyeball), getting the most precise and accurate times and locations as one can, and having the ability to make better models on the fly.
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With respect to the zenith problem of an alt/az mount, I can simply tip my dob to change the mount's zenith with respect to the sky's zenith.? That avoids a pass straight through the pole of the mount.? With a computer, making the adjustments for having "changed" one's latitude and longitude because of that tipping is rather straightforward.? It's much harder with a fixed single purpose computer that expects you to enter those values yourself.? Clearly, with any sized telescope, there is a limit to how far your structure can handle such tipping (the az bearing is the weakest point), but that limit increases rather quickly with larger sized scopes.? The solution that specifically designed satellite trackers employ is going to alt-alt-az, rather than simply alt-az, or a full gimbal.? That's not an option for us, so tipping in the poor man's solution to getting alt-alt-az, then simply telling the alignment software our "new" latitude and longitude (preferably through plate solving at the mount's pole). |
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