Self correcting mount

[Alaness]

Something that has been bugging me for the past few days - we have clocks that can correct themselves to be extreamly accurate. To get the same kind of accuracy from a telescope mount you need autoguiding.

Why don't we have telescope mounts that monitor and correct their own rotation? It'll need a link to an atomic clock over ratio or the Internet, but that isn't hard to do these days.

I'm guessing by the fact I can't find such a mount that I'm missing something, but I can't figure out what. Ideas?

[Digz]

And how would the mounts correct their own rotation - with a guide camera relaying information received back to the mount.......

They will need some sort of reference to the night sky so they know where they are and what corrections to make. Thats my take on it in any event. I know extremely expensive mounts are far superior at tracking but I dont know if they have a kind of built in guide facility.

[Alaness]

And how would the mounts correct their own rotation - with a guide camera relaying information received back to the mount.......

[Alaness]

I think you missed my point.

[Digz]

I think you missed my point.

Then please expand!

[ollypenrice]

Something that has been bugging me for the past few days - we have clocks that can correct themselves to be extreamly accurate. To get the same kind of accuracy from a telescope mount you need autoguiding.

Why don't we have telescope mounts that monitor and correct their own rotation? It'll need a link to an atomic clock over ratio or the Internet, but that isn't hard to do these days.

I'm guessing by the fact I can't find such a mount that I'm missing something, but I can't figure out what. Ideas?

You can buy them. Well, if you have the means! ASA do their direct drive. They need high precision Renishaw encoders and the RA shaft is the armature of a very slowly rotating motor which is controlled using feedback from these encoders. The system is widely used by professional astronomers as well, on the large telescopes.

There are complications, though. Polar alignment has to be of an alarmingly high order and, since the refractive effect of the atmosphere varies with altitude, the control software has to adjust the drive speed of the mount as it rises and falls in altitude during a run. (This means the mount does need to know where it's pointing but it gets this information from a normal planetarium just like that of the cheaper GoTo mounts.) After using their customers as beta testers for several years it now seems that ASA have got most of the bugs ironed out and lots of people are getting good results, even at phenomenal focal lengths. What we now need is for this kind of technology to filter down the the £4000 mount class and then to the NEQ6! I doubt I'll live to see that...

Olly

Edit; Obviously you can't have backlash in the gears so a no-gears system like the ASA armature shaft, or possibly a Mesu style roller drive, is what's needed. I'm not sure what problem it solves though. A Mesu roller drive, autoguided, is fantastically reliable.

[Alaness]

My mistake, I only noticed your first paragraph.

[harry page]

Hi

You can get this to add to your own scope http://www.harrysastroshed.com/TDM.html and I love it very much

But yes cost is a issue , but I am sure it will come down in cost

Regards Harry

[perfrej]

I second Olly and Harry. You need encoders and you need software that compensates for all the problems (temp, refraction, altitude...). In addition you need the mount clock set very accurately but that is the least of the problems (just connect the GPS and have the mount sync to that).

As for motor systems... The ASA way is one way to go and the Mesu/Gemini friction way another. Then there is belt drive implemented in one or more places. My 10Micron has belt drive and a worm, high resolution encoders and gives me half-hour subs at FL 1470mm, and that's without guiding and external computer help! On a side note; you do not have to set the polar alignment that well as the mount tracks in both axis and thus compensates for the errors (polar alignment, refraction and cone error - all calculated internally in the mount).

In fact, I have never guided that mount and I don't plan to unless I really want to go for long subs

Happy 2013!

/per

[andrew s]

Even then you need to correct for flexure in the mount, telescope and image train. You can only do this at the moment with ProTrack on Software Bisque Paramounts.

Regards Andrew

[Capricorn]

I am bothered by the term:

we have clocks that can correct themselves to be extreamly accurate. To get the same kind of accuracy from a telescope mount you need autoguiding.

I would argue that we do not. We have clocks that receive a signal then reset themselves to that signal, but that is not correcting themselves. Take a watch that gains 1 minute every hour, if you read a more accurate clock hourly all you need do it pull the winder out and put the watch back 1 minute, That is not correcting itself but accomplishes what you describe.

[ollypenrice]

I second Olly and Harry. You need encoders and you need software that compensates for all the problems (temp, refraction, altitude...). In addition you need the mount clock set very accurately but that is the least of the problems (just connect the GPS and have the mount sync to that).

As for motor systems... The ASA way is one way to go and the Mesu/Gemini friction way another. Then there is belt drive implemented in one or more places. My 10Micron has belt drive and a worm, high resolution encoders and gives me half-hour subs at FL 1470mm, and that's without guiding and external computer help! On a side note; you do not have to set the polar alignment that well as the mount tracks in both axis and thus compensates for the errors (polar alignment, refraction and cone error - all calculated internally in the mount).

In fact, I have never guided that mount and I don't plan to unless I really want to go for long subs

Happy 2013!

/per

How does temp play into this, Per? I haven't thought about it. I don't doubt that you're right, just curious. (I've no plans to buy an ASA since that would mean selling anything I might like to put on it!!!)

Olly

[perfrej]

Olly,

Temperature is a factor in the refraction model along with air pressure and some trigonometry/physics

/per

[andrew s]

You can also get temperature induced flexure e.g. Carbon tube on Alunimiun saddle plate however, the effect is quite small. - Andrew

[ollypenrice]

All these extraneous factors make me suspicious of unguided setups for long focal lengths but the fact remains that they do work once sorted - despite my suspicions! I think the day will come when they are the norm for permanent setups but I doubt it will be all that soon.

Olly

[andrew s]

Olly - People are already running Paramounts with 10 minute subs without guiding. Regards Andrew

[Ajohn]

There is a cheaper partial answer to this called PEC, periodic error control. Having an accurate time beat is only part of the problem - that could probably be extracted from one of the cheap radio clocks that are about. Even Liddl sell them at times.

On a typical set up there are two worse errors than the rate the drive is driven at. Steppers generally have a 5% or more error max in the actual step angle. The other error is the gears that are used in the drive. Nothing is perfect to the arc second level. Typical gears and worms+wheels can be lapped to increase their accuracy but it's not a straight forward procedure especially with gears.

When PEC is available the drive is trained by correcting it while it's tracking a star. The corrections are stored by software and later applied automatically when the scope is being driven. Stars could be tracked at various elevations as well to iron out atmospheric diffraction problems. I used to be interested in a drive called Littlefoot that is still about on the web but am rather surprised how expensive it is now that it's all ironed out. Not sure what it can do now but it did have PEC. I did manage to interest the person who developed it in very fast slew rates obtained by dropping out of micro stepping but not sure what has happened on that score.

I would guess that for various reasons that PEC would still benefit from auto guiding on a typical home set up but I suspect it would still allow much better exposures and more accurate aiming.

John