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Encoders

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#1 JerryX

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Posted 23 February 2021 - 11:56 AM

How important are encoders? Do they really bring anything to the party when you're using active guiding, or are they mostly useful when using unguided tracking? At what focal length does having encoders start to make a difference?

 

Some mounts that offer encoders have a version that has an encoder only on the RA axis and another version with encoders on both axes. Is there any benefit to having encoders on both axes, or is RA enough?

 

Bottom line: Are encoders worth the several thousand dollars they add to the cost of a mount?



#2 JMW

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Posted 23 February 2021 - 12:59 PM

I own a 900GTO mount that I have been using for a decade. I have upgraded to the CP4 controller, RAPAS and done maintenance to keep it running perfectly. I have a 1100GTO mount with absolute encoders on order. It adds about 50% more to the cost of the mount. I will be keeping my 900GTO so I can use it in my light polluted home observatory for EAA and take the 1100 to dark sky star parties where I can leave it setup for 4-8 nights at a time.

 

Here is my understanding of the advantages.

 

No periodic error, zero backlash on axis movements.

67+ million unique positions marked on each axis to allow for sub arc second accurate positioning of each axis at all times.

Closed loop system correcting axis position hundreds of times per second. Data can be use to respond to wind gusts other transient forces affecting axis position.

Using APCC Pro and APPM pointing model with atmospheric pressure, humidity and temperature data to account for atmospheric refraction, custom tracking rates can allow longer unguided exposures.

Mount position is always known as long as the clutches are locked down on the 1100 or 1600 mounts. Mach 2 clutches can be released and not affect absolute encoders.

 

The cost of the 1100 mount is not insignificant and another 50% increase for encoders makes the choice even harder for many. The value of the system is an individual choice and there are factors such as how much value it provides to the owner and the economic impacts of the purchase on the owner.


Edited by JMW, 23 February 2021 - 01:31 PM.

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#3 mmalik

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Posted 23 February 2021 - 01:14 PM

Bottom line: Are encoders worth the several thousand dollars they add to the cost of a mount?

Yes; but...

 

 

...my take is too many junk mounts are being created in the "middle" in the name of encoding, single axis or otherwise. Analogs in my opinion need to phase out. Mount industry needs a convergence in the middle around $5K with high (machining) quality & dual absolutes. Currently, 10K+ is the where (machining) quality and (dual encoding) precision lies. Regards

 

post-205769-0-55509400-1613922353.jpg


Edited by mmalik, 23 February 2021 - 05:57 PM.

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#4 yzhzhang

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Posted 23 February 2021 - 01:42 PM

Mount industry needs a convergence in the middle around $5K with high (machining) quality & dual absolutes. Currently, 10K+ is the where (machining) quality and (dual encoded) precision lies. Regards

 

I think currently a ture, useful pair of encoders cost about $5000... That's why all REAL encoder-based mounts cost that much and it's probably hard to reach $5k~


Edited by yzhzhang, 23 February 2021 - 01:42 PM.

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#5 Dynan

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Posted 23 February 2021 - 02:06 PM

After battling SDE (SubDivision Error) in a CEM60-EC, I went to the CEM60 Non-EC. My tracking troubles disappeared. I saw that it was hit and miss with encoders...some get lucky, some don't. It depends on factory manufacture. Sent the CEM60-EC in for adjustment and it came back as bad, or worse. Luck of the draw, apparently.

 

In detail: https://www.cloudyni...m60ec-problems/


Edited by Dynan, 23 February 2021 - 02:07 PM.

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#6 durak

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Posted 24 February 2021 - 10:48 PM

After battling SDE (SubDivision Error) in a CEM60-EC, I went to the CEM60 Non-EC. My tracking troubles disappeared. I saw that it was hit and miss with encoders...some get lucky, some don't. It depends on factory manufacture. Sent the CEM60-EC in for adjustment and it came back as bad, or worse. Luck of the draw, apparently.

 

In detail: https://www.cloudyni...em60ec-problems

 

 

It's important to remember that there are different tiers of encoders. Just like moist things in this hobby. For example, premium mounts like 10 Micron and AP are using high-quality absolute encoders that cost them approximately $5000 to buy.

 

The encoders that "mid-tier" mounts use are of lower quality/precision. 


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#7 andysea

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Posted 24 February 2021 - 11:12 PM

I will probably add them to the AP1100 at some point

I don't know if they add much, maybe in certain conditions they do. However they surely don't hurt :) Joking aside, I am wondering the same thing.

I already get superb tracking and guiding with my non encoder AP mounts but maybe the encoders will squeeze out just a bit more precision.



#8 Hobby Astronomer

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Posted 25 February 2021 - 12:12 AM

Best advise is see how far you can get with your polar alignment, oag, and phd2 before feeling like you need encoders.

 

If you can get your polar alignment down to less than one arc minute, your mount should really start performing.

 

Imaging without an OAG is worse than a bad case of hemorrhoids.

 

Phd2 has a new multistar guiding version. This is really great.

 

Many of the new CMOS cameras can do fairly nice narrowband images with 5-10 minute subs. If you go to a super dark site your subs need to be only 2 to 3 minutes long.

 

So max out what you have before you just blow $10K.

 

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

 

Now if you have the money for the 10 Micron here is what you can get. (Yes is posted this up earlier today on another thread)

 

This is a 3,600 second sub unprocessed uncalibrated.

 

OIII 3 NM Filter/ OAG, 23" Polar Alignment, GM 1000 HPS, PHD2 Guiding.

 

Thors Helmet NGC 2359.

NGC2359_3600sec_2x2_OIII_0005_BEST_clone.jpg


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#9 andysea

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Posted 25 February 2021 - 02:14 AM

I can't believe you didn't get any satellites in 1 hour!

To be fair tho one could get the same result with a very good mount that does not have encoders as long as they use an OAG.

 

Regarding polar alignment, I found sharp cap to be extremely accurate,


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#10 Hobby Astronomer

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Posted 27 February 2021 - 07:55 PM

I can't believe you didn't get any satellites in 1 hour!

To be fair tho one could get the same result with a very good mount that does not have encoders as long as they use an OAG.

 

Regarding polar alignment, I found sharp cap to be extremely accurate,

I agree with the importance of the OAG. Try the OAG every time before dropping the money on a high end mount.

 

However it seems would be hard to get the alignment to do the subs at 1,400mm focal length without a mount with encoders. The polar alignment ideally needs to be about 25" or less. If you are doing 60" your images will not me as round as you might like. With the encoders I have got the drift down to about 1 pixel every thirty minutes....Everything being round as a star and anything that looks like a grain of rice being a galaxy.

 

HA


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#11 Hobby Astronomer

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Posted 27 February 2021 - 08:00 PM

gallery_225131_300_54563.jpg

 

Here is a 30 minute sub with one pixel of drift.


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#12 andysea

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Posted 27 February 2021 - 09:51 PM

I used to image at 0.5"/PX with my AP900 at 2000mm FL and of course with an OAG. I'm pretty sure that I could have done subs of indeterminate length (with guiding) 

That being said, if we ever move to a location that has much better seeing than we get here, I will add encoders to the AP1100 for sure and start doing high resolution imaging again, hopefully with better results.

I believe that the encoders will provide the last bit of precision to take full advantage of good seeing.


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#13 orlyandico

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Posted 28 February 2021 - 01:29 AM

Actually I think the ultimate image quality would be obtained with a tip-tilt mirror (the simplest - and AFAIK only type available to amateurs - form of adaptive optics), since encoders can't counter the seeing (you would need guiding for that).

 

Assuming you can use AO, pretty much any mount with low to moderate periodic error would yield excellent results, since I recall that the old SBIG AO could handle up to 50" of displacement. Heck assuming a perfect polar alignment, you wouldn't even need to guide the mount (just guide the AO unit).

 

It's pertinent that the large professional observatories have both encoders/modeling and AO.


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#14 psandelle

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Posted 28 February 2021 - 01:02 PM

Actually I think the ultimate image quality would be obtained with a tip-tilt mirror (the simplest - and AFAIK only type available to amateurs - form of adaptive optics), since encoders can't counter the seeing (you would need guiding for that).

 

Assuming you can use AO, pretty much any mount with low to moderate periodic error would yield excellent results, since I recall that the old SBIG AO could handle up to 50" of displacement. Heck assuming a perfect polar alignment, you wouldn't even need to guide the mount (just guide the AO unit).

 

It's pertinent that the large professional observatories have both encoders/modeling and AO.

Crap! Now you're making me want to try out an AO!

 

Paul


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#15 EFT

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Posted 01 March 2021 - 01:46 AM

Actually I think the ultimate image quality would be obtained with a tip-tilt mirror (the simplest - and AFAIK only type available to amateurs - form of adaptive optics), since encoders can't counter the seeing (you would need guiding for that).

 

Assuming you can use AO, pretty much any mount with low to moderate periodic error would yield excellent results, since I recall that the old SBIG AO could handle up to 50" of displacement. Heck assuming a perfect polar alignment, you wouldn't even need to guide the mount (just guide the AO unit).

 

It's pertinent that the large professional observatories have both encoders/modeling and AO.

While this seems to be the promise of AO, it must not be the reality since AO has never really caught on at the amateur level.  I assume that it has something to do with the technological limitations of the equipment at prices people can afford for personal use.  If there were a device that could simply erase sky quality issues, it would seem that a lot of people would be using it.  It was the same with the Telescope Drive Master and the claim that it could essentially make a mass-produced mount work like a premium mount.  The encoder idea was correct, as can be seen from the increasing use of encoders on premium mounts, but the application was wrong so the device never took off.


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#16 orlyandico

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Posted 01 March 2021 - 10:11 AM

Tip-tilt only corrects for first-order aberrations in seeing, this is why the professional observatories use deformable optics to correct for higher-order aberrations.

 

I think the main problem with AO for amateurs is that in order for it to be effective, you need correction rates >20Hz or more, amateur AO can only do ~5-10Hz (0.1-0.2 second exposures) due to the limited aperture, so you're limited to fairly bright guide stars. On the large scopes there's a lot more aperture so very short guide exposures are more practical; the big guys also use artificial laser guide stars so they always have a bright guide star in the field.

 

That said, no mount even with the best encoders around can correct for seeing, so AO would still have benefits even if only the simple first-order aberrations can be corrected - assuming the limitations of finding an appropriate guide star are overcome.


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