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Need help understanding absolute encoders . . .

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#1 dawziecat

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Posted 26 May 2013 - 09:43 AM

I need some help understanding the real function of absolute encoders.

I already understand they are far too expensive to consider equipping the AP1100 I hope to have soon with them. That's too bad. :(

But I am unsure what their real function is anyhow.

I understand the mount always knows “where it is” when encoder-equipped. You can push it manually and it keeps track of where it is. Nice.

But I don't understand how it interacts with guiding during AP. The encoders keep track of the axes rotation far more accurately than the driving gears are able to deliver on their own. Is that correct? When it detects that the driving rate has deviated from true sidereal, due to PE for instance, it sends corrections to the motors to get back on track. Correct? Similar to a guider in that respect.

The encoders deliver far more accurate tracking, thereby making work far “easier” for the guider. However, with non-perfect alignment being a certainty and the presence of flex to some degree being almost as certain, it seems guiding will still be necessary. So, if you have to guide anyway, why buy such uber-costly “drive rate correctors” as absolute encoders?

Assistance solicited, please as I don't seem to fully “get it.”

#2 freestar8n

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Posted 26 May 2013 - 10:40 AM

You ask a good question. I ask myself the same things.

If someone is doing unguided imaging with a high end mount then the tracking might be improved with encoders - particularly if the whole thing is powerful enough to correct for wind gusts and help cancel them.

But if you are doing high res work then you will need to guide with off-axis guiding or equivalent - and you will be making corrections due to flexure between the optics and the encoder readings. So you are still autoguiding, but the encoders might keep things smoother between corrections.

Do they work? I don't know. It's like periodic error - I see people improve their PE plots, but I don't see them reducing the size of stars in long, guided images.

If I had a high end mount in an observatory I would like encoders on it - especially for unguided work with a well calibrated system. But to add on encoders to mid-range equipment - I don't get it. You need to guide anyway - so just guide well with OAG and low latency.

Frank

#3 dawziecat

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Posted 26 May 2013 - 10:53 AM

Whoops! I meant to post this to the mounts forum but since it is here, I guess I will leave it . . . unless a mod thinks it should be moved.

If you don't understand it, Frank, what chance do I have?

I want to do 30 minute NB subs at 2000mm f/8.0. I'm going to have to OAG the AP1100 to do that. The encoders nearly DOUBLE the price of an already costly (for me) mount!

I remain puzzled as to how the $6900 encoders would make my life more refulgent. I quite understand how they would lighten my burdensome wallet! :) :)

#4 orlyandico

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Posted 26 May 2013 - 11:13 AM

well the 10Micron mounts can do 20-30 minutes unguided at the focal lengths you talk about.

the encoders help, definitely, but mount modeling is also required due to the flexure, etc. that you mention.

AP mounts even without encoders can do this trick with APCC (supposedly) although i have not seen any 20-30 minute unguided subs at 2000mm with an encoder-less AP mount.

#5 David Ault

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Posted 26 May 2013 - 12:43 PM

I saw this video the other day (the encoder discussion starts around 11:28), and one of the points the Astro-Physics Rep. talks about is the encoder's ability to zero out backlash on the DEC axis (timecode 12:10). I'm Not sure it's worth the $6900 bill though.

Regards,
David

#6 vpcirc

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Posted 26 May 2013 - 02:25 PM

You're not likely to get 20-30 min unguided with any system encoders or not. JWalk can give his input he has an ap 1600 with encoders and APCC. I know his pointing error is less than 1 arc min and he's shooting 60 min subs with perfectly round stars at 3600 FL. That's hard to do without encoders. Where they really help is eliminating the amount of guide corrections. The mount is recieving the exact position several 1000's of times per second. Guiding can be thrown off atmospheric conditions. At longer focal lengths. Don't buy the flexure argument, that's normally an excuse for a poor mount setup and low end equipment. If you've got mirror flop issues like an SCT sure it's an issue, but most guys spending 15-20 k on a mount aren't using an SCT.






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