Encoders...

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WilliamPaolini
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Encoders...

#1

Post by WilliamPaolini »

What is the consensus for what the accuracy should be for "good" encoders for a mount? +/- what fraction of a degree?
-Bill

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Re: Encoders...

#2

Post by OzEclipse »

Hi Bill,
There is no consensus nor any one answer. It really depends what you want to do with the mount. For example, if you just want to have an object in the 1 degree field of a low power eyepiece, then 1/2 degree(720 steps) should always have it in field, 1/4 degree(1440 steps) definitely. Most encoders are much better than this. It gets more difficult if for example you want to place a particular body near the centre of field of a small 5x3mm sensor at the 5 metre focus of an SCT with a Powermate. Such an instrument only has a full field of a few minutes and if you want it near centre, then you probably need ~ 1 arc minute pointing. But there is more to it than just the accuracy of the encoders. To achieve that sort of pointing, you'll need to build a multi-star pointing model to get that accuracy over the whole sky to take account of instrument flexure, and altitude refraction.

From your signature, it looks like your longest FL instruments are around 1000mm-1200mm. With this focal length your low power field should be > 1 degree. Pretty much any encoders > 1000 step will be good enough for a field that wide. I don't recall ever reading about scope encoders less than about 4000 step resolution.

Cheers

Joe
"34 South: The Hilltops Observatory"
Central West NSW
AUSTRALIA
Joe Cali

Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV 31, 17, 12, 7 Naglers; D21, D14 Denkmeier’s; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
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Re: Encoders...

#3

Post by SkyHiker »

Determine the required resolution in arc seconds, say x. If the encoder has n positions and the gear reduction is g then its resolution will be 360/(n*g) degrees or 360*3600/(n*g) arc seconds. So, n=360*3600/(x*g). The gear reduction is the reduction between the encoders and the axes. They can be internal to the servo motors. For a Dob they are direct so g=1. For a G11, g=360 so 3600 internal to the motors would be enough for x=1 arc second but if you add 3:1 timing belts you would get to 1/3 arc second. Here's a thread for the G11 with useful comments, https://groups.io/g/Losmandy_users/topic/85354177#73530 . Like Joe says it depends on your situation but this is the general formula.
... Henk. :D Telescopes: GSO 12" Astrograph, "Comet Hunter" MN152, ES ED127CF, ES ED80, WO Redcat51, Z12, AT6RC, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S with OnStep, AVX, Tiltall, Cameras: ASI2600MC, ASI2600MM, ASI120 mini, Fuji X-a1, Canon XSi, T6, ELPH 100HS, DIY: OnStep controller, Barndoor trackers for 10" Dob and camera, Afocal adapter, Foldable Dob base, Az/Alt Dob setting circles, Accessories: ZWO 36 mm filter wheel, TV Paracorr 2, Baader MPCC Mk III, ES FF, SSAG, QHY OAG-M, EAF electronic focuser, Plossls, Barlows, Telrad, Laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: KStars-Ekos, DSS, PHD2, Nebulosity, Photo Gallery, Gimp, CHDK, Computers:Pi4b, 2x Pi2b all running Astroberry, Toshiba Satellite 17"
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Re: Encoders...

#4

Post by Piet Le Roux »

Something else to consider is that not all encoders are mounted equal : the older Meade mounts had encoders measuring the movement of the Ra and Dec axis directly, my old LX6 had rubber belts driving digital encoders. This ads a lot of extra hardware and expenses, in their LX90 and Lx200 models they opted for a very high resolution encoder directly on the motor before the gearbox, this works well if the necessary drive training have been done to electronically remove any gear slop.
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Re: Encoders...

#5

Post by WilliamPaolini »

Thanks for the feedback. Thinking about it, I guess the resolution can all be for naught if the mount has a lot of slop in its movement. So accuracy is really going to be a function of the encoders and mount working together. Weakest link of the two (plus the alignment accuracy by the user) will determine final outcomes.

PS - my question was related to visual only...sorry I forgot that critical point :twitch:
-Bill

U.S.A.F. Veteran - Visual Amateur Astronomer since 1966 - Fully Retired since 2019
8" f/5 Newt - Lunt 152 f/7.9 - TSA 102 f/8 - Vixen 81S f/7.7 - P.S.T. - Pentax 65ED II - Nikon 12x50 AE
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Re: Encoders...

#6

Post by OzEclipse »

WilliamPaolini wrote: Sat Oct 09, 2021 4:00 pm Thanks for the feedback. Thinking about it, I guess the resolution can all be for naught if the mount has a lot of slop in its movement. So accuracy is really going to be a function of the encoders and mount working together. Weakest link of the two (plus the alignment accuracy by the user) will determine final outcomes.

PS - my question was related to visual only...sorry I forgot that critical point :twitch:
Hi Bill,
Yes backlash can come into it.

A friend who has an uncooperative mount chooses a bright star in the vicinity of the object he is looking for. he slews to that first, using the finder and scope he centres the bright star, synchronises the goto to that star, then does a small slew to his target object. This is mainly a problem on his GEM after flipping E to W hemispheres.

Visual and at those focal lengths, you should not need to worry about resolution.

Joe
"34 South: The Hilltops Observatory"
Central West NSW
AUSTRALIA
Joe Cali

Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV 31, 17, 12, 7 Naglers; D21, D14 Denkmeier’s; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
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Re: Encoders...

#7

Post by Piet Le Roux »

Good day Joe,
With the Meade Autostar/Audiostar controller this is referred to as "high precision" mode and can easily be de-selected in the menu. I personally don't use it because it is very time consuming to first go to a bright star every time you do a goto, you can get the same accuracy by doing a manual sync every now and then when you have centered the object you have done a goto to. This is done by pressing "enter" for more than 2 seconds, the controller will then ask you to center and then press "enter" again. This will insure that your mount stay synced to its star data throughout your viewing session.The amount of error that will result during your viewing session due to gear slop will be minimised if you have done accurate drive training beforehand.
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Re: Encoders...

#8

Post by Don Pensack »

WilliamPaolini wrote: Sat Oct 09, 2021 1:01 am What is the consensus for what the accuracy should be for "good" encoders for a mount? +/- what fraction of a degree?
Depends. To achieve high accuracy with the accuracy of a poor quality mount may be beyond the capabilities of 1.3 million tick encoders, while a perfectly accurate mount
might put every target in the center of the field with 2000 tick encoders.
1' resolution requires 21600 tick encoders, so I put that at the base considering the average mount accuracy and average telescope true field size.
You won't need that with a 4.5° true field on a short focal length refractor.
I find that 32K tick encoders work adequately in most dobs.

Most DSC screens have GUIs yielding 0.1° resolution, or 6' resolution. If the mount is perfect, that is achievable with 2160 tick encoders, In practice, however, mount inaccuracies often achieve poorer resolution
with 10K tick encoders.

Of course, you can make nearly every mount more accurate, even with low resolution encoders, by simply moving to a named star near the object and synchronizing the scope on that star.
When I was hunting very faint and small galaxies in Corona Borealis, I first did a search for Alpha Coronae Borealis (Gemma, or Alphecca) with my DSC, centered and synched on the star, and every other object in the constellation
was dead center. Meade used to call this "High Precision Pointing" in their older LX200s, and it worked very well, placing every object dead center. With DSCs, you have to pick a star in the DSC database
to sync on, but it essentially works the same way.

Optical encoders seem to max out around 80K resolution. Just about everything above 10K will require constant reading from the encoders, which doesn't happen in Tangent DSCs or Sky Commander DSCs, so they are not usable with
truly high resolution encoders. Magnetic encoders use much less power and have resolutions of 192K to 1300K ticks. As far as I know, only the Nexus DSC boxes are compatible with such high resolution encoders.
The advantages of the magnetic encoders are they are waterproof, and don't have to be placed at the center of the axes to work (though most are on most mounts).
Astonomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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Re: Encoders...

#9

Post by OzEclipse »

As a practical guide, I purchased an ATM dob built by another member on this site.
It's an 18" f5.5, 2500mm focal length.
1 degree field in a Nagler 31,
38 mins in a Nagler 17,
26 mins in a Denkmeier 14.

The Nexus DSC is connected to an 8K alt az encoder pair.

I do a two star + zenith alignment at the start of the night just eyeballing the stars to the centre of field in the 31mm Nagler. I can usually put any object in the field but not centre of the 14mm for the rest of the night. 17mm & 31mm are easy. If I took the trouble to use a crosshair eyepiece I could probably get it more accurate. The Nexus has ALT AZ guidance readouts to 0.1 deg. Sometimes it drifts a little by the small hours of the morning. I could re-align, takes a couple of minutes. But I usually just memorise the two offsets in my head and keep going.

Joe
"34 South: The Hilltops Observatory"
Central West NSW
AUSTRALIA
Joe Cali

Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV 31, 17, 12, 7 Naglers; D21, D14 Denkmeier’s; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
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Re: Encoders...

#10

Post by SkyHiker »

I found this link enlightening, https://www.cloudynights.com/topic/5813 ... -encoders/ .

For AP accurate tracking is important, much more so than accurate goto positioning. The big issue is if the encoders are on the mount axes or on the motor axes. If they are on the motor axes you will have PE from the RA gear box (usually 5" to 10" amplitude) and backlash in DEC, but you don't need hi res encoders and you will need to guide out PE and/or apply PEC. If they are on the mount axes you need expensive hi res encoders but you may not have to use an autoguider if the PA and the mechanical quality are good.
... Henk. :D Telescopes: GSO 12" Astrograph, "Comet Hunter" MN152, ES ED127CF, ES ED80, WO Redcat51, Z12, AT6RC, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S with OnStep, AVX, Tiltall, Cameras: ASI2600MC, ASI2600MM, ASI120 mini, Fuji X-a1, Canon XSi, T6, ELPH 100HS, DIY: OnStep controller, Barndoor trackers for 10" Dob and camera, Afocal adapter, Foldable Dob base, Az/Alt Dob setting circles, Accessories: ZWO 36 mm filter wheel, TV Paracorr 2, Baader MPCC Mk III, ES FF, SSAG, QHY OAG-M, EAF electronic focuser, Plossls, Barlows, Telrad, Laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: KStars-Ekos, DSS, PHD2, Nebulosity, Photo Gallery, Gimp, CHDK, Computers:Pi4b, 2x Pi2b all running Astroberry, Toshiba Satellite 17"
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Re: Encoders...

#11

Post by Don Pensack »

OzEclipse wrote: Wed Oct 13, 2021 2:12 am As a practical guide, I purchased an ATM dob built by another member on this site.
It's an 18" f5.5, 2500mm focal length.
1 degree field in a Nagler 31,
38 mins in a Nagler 17,
26 mins in a Denkmeier 14.

The Nexus DSC is connected to an 8K alt az encoder pair.

I do a two star + zenith alignment at the start of the night just eyeballing the stars to the centre of field in the 31mm Nagler. I can usually put any object in the field but not centre of the 14mm for the rest of the night. 17mm & 31mm are easy. If I took the trouble to use a crosshair eyepiece I could probably get it more accurate. The Nexus has ALT AZ guidance readouts to 0.1 deg. Sometimes it drifts a little by the small hours of the morning. I could re-align, takes a couple of minutes. But I usually just memorise the two offsets in my head and keep going.

Joe

The Nexus DSC has a guidance readout that can be selected for 0.1, 0.01, and 0.001 degree. I prefer the 0.1 degree choice myself as then the numbers aren't continually changing and because it is possible to achieve 0.0/0.0 pointing, but impossible to achieve 0.00/0.00 pointing.
But if the scope tracks, the higher level readout might help attain a higher level of accuracy. Not with 8K encoders, though, which have 2.7' resolution. You're better sticking to 0.1 degree readout guidance. Perhaps with the 1.3 million tick encoders that are also compatible with the Nexus DSC.
Astonomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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Re: Encoders...

#12

Post by OzEclipse »

Don Pensack wrote: Sat Oct 23, 2021 4:45 pm
OzEclipse wrote: Wed Oct 13, 2021 2:12 am As a practical guide, I purchased an ATM dob built by another member on this site.
It's an 18" f5.5, 2500mm focal length.
1 degree field in a Nagler 31,
38 mins in a Nagler 17,
26 mins in a Denkmeier 14.

The Nexus DSC is connected to an 8K alt az encoder pair.

I do a two star + zenith alignment at the start of the night just eyeballing the stars to the centre of field in the 31mm Nagler. I can usually put any object in the field but not centre of the 14mm for the rest of the night. 17mm & 31mm are easy. If I took the trouble to use a crosshair eyepiece I could probably get it more accurate. The Nexus has ALT AZ guidance readouts to 0.1 deg. Sometimes it drifts a little by the small hours of the morning. I could re-align, takes a couple of minutes. But I usually just memorise the two offsets in my head and keep going.

Joe

The Nexus DSC has a guidance readout that can be selected for 0.1, 0.01, and 0.001 degree. I prefer the 0.1 degree choice myself as then the numbers aren't continually changing and because it is possible to achieve 0.0/0.0 pointing, but impossible to achieve 0.00/0.00 pointing.
But if the scope tracks, the higher level readout might help attain a higher level of accuracy. Not with 8K encoders, though, which have 2.7' resolution. You're better sticking to 0.1 degree readout guidance. Perhaps with the 1.3 million tick encoders that are also compatible with the Nexus DSC.
Thanks Don,
I have it set for 0.1. It’s a push to dob used visually so even with a 12mm, 200 power eyepiece the field is 0.3 deg and the 0.1 read out works fine.
Cheers
Joe
"34 South: The Hilltops Observatory"
Central West NSW
AUSTRALIA
Joe Cali

Bortle 1-2 skies, 148 E, 34 S

Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops

web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80, M70, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
Eyepieces: TV 31, 17, 12, 7 Naglers; D21, D14 Denkmeier’s; Pentax XW10, XW5, Unitron 40mm Kellner, Meade Or 25,12
Cameras : Pentax K1, K5, K01, K10D / VIDEO CAMS : TacosBD, Lihmsec.
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