TDM Telescope Drive Master - Thoughts

[Astroscot2]

Hello all,

I was thinking of upgrading my non goto G11 mount with the new Gemini system next year with the idea that moving from stepper to servo motors will give me better tracking and the goto would be useful too. Ive noticed that when imaging at my Newtonian FL 1300mm approx the stars are not perfectly round in some exposures and valuable data is being lost during the few clear nights I get. Ive played aboud with balancing etc but no real improvement.

I started hearing a number of months ago about the TDM and immediately thought it was too good to be true. 1 – 2 arc second tracking on the lower end EQ mounts like my own and potentially unguided images.

I know its an expensive bit of kit but was wandering why so few people are talking about it, OK most imagers are imaging under 1000mm fl where guiding is less of an issue but the TDM sounds like the holy grail..

Would be interested in hearing your thoughts.

http://www.mda-telescoop.com/index.php?option=com_content&task=view&id=18&Itemid=40

Mark

[andrew s]

Mark - Form what I have read TDM does what it says it does - i.e. improve tracking in RA. However, it does not guide and can't prevent image distortion due to shifts due to seeing or polar misalignment (although it can be used with such systems). I would propose you look at guiding options as well both conventional and the newer adaptive systems.

I have always found goto a god send and using Tpoint and The Sky 6 makes placing objects on centre of the chip a real delight.

Happy Xmas Andrew

[steppenwolf]

I have used the TDM on an EQ6 mount and was very impressed with it, it did exactly what you would expect in that it controlled errors in the RA axis. It was beautifully manufactured too. If you have perfect polar alignment on a fixed pier, it has much to commend it but for what it is worth, here is a brief summary of my findings:-

I was very impressed with the simple operation and huge improvement in periodic error from a peak to peak of 13.86 arcseconds to just 1.41 arcseconds. However, it should be borne in mind that corrections are only made in RA and no corrections are made for errors produced by external influences like atmospherics, mirror flop, flexure and polar alignment. Declination (DEC) corrections will still have to be made using an autoguider so this is not a complete solution for perfect tracking.

The real benefit of using this system though is that when used in conjunction with an autoguider, much longer integration times can be used allowing you to guide on dimmer stars which can be a great advantage when using an off axis guider.

[harry page]

Hi

I have it on my Scope ( the beast )

Have a look here http://www.harrysastroshed.com/TDM.html

Very happy with mine , does what it says on the tin

What I like is that you just switch it on and it works , no hrs of training or retraining

As you say though not cheap , but worth it to me

Harry

[Astroscot2]

Thanks all, I would still guide but the the correctiosns should be more manageable and im less likely to loose subs.

Thansk Harry, I knew I had seen someone in the UK using one, 10 min unguided subs, who needs a parramount eh ! Looks the biz, glad is sorted your issues, think im sold.

[PortableAstronomer]

I found this in a nearby shop for 249 euros.

http://www.bresser-online.nl/Webwinkel-Product-5130025/TDM%E2%84%A2-Adapterset-EQ6-(new-version)-Montering.html

ok that page was in Dutch (for the picture), but the description of it in English here...

http://www.astroshop.eu/miscellaneous/explore-scientific-tdm-adapter-for-eq6--new-version-/p,15836

My question is, how easy is it to fit to the EQ6 mount?

Do you need to buy anything else, or do you just need some screwdrivers & other basic toolbox stuff?

I must say I am very tempted.

[E621Keith]

I found this in a nearby shop for 249 euros.

http://www.bresser-online.nl/Webwinkel-Product-5130025/TDM%E2%84%A2-Adapterset-EQ6-(new-version)-Montering.html

ok that page was in Dutch (for the picture), but the description of it in English here...

http://www.astroshop.eu/miscellaneous/explore-scientific-tdm-adapter-for-eq6--new-version-/p,15836

My question is, how easy is it to fit to the EQ6 mount?

Do you need to buy anything else, or do you just need some screwdrivers & other basic toolbox stuff?

I must say I am very tempted.

The links you posted were for the adapter only. It doesn't include the encoder or the encoder electronics box.

you will also need this, in addition to the EQ6 adapter

http://www.bresser-online.nl/Webwinkel-Product-5125383/TDM%E2%84%A2-Telescope-Drive-Master-Tracking-Unit.html

[PortableAstronomer]

Thanks Keith. That seems a lot more believably-priced.

So for about 1600euro or £1200-1300 you can get the complete system. Hmmm. Might have to wait another year for me, but still an interesting concept.

Knowing the world of specialist astro products though, they'll probably be discontinued soon, like the spc900 webcam was, and be seen on future Fleabay listings for 2x new price.

[pete_l]

I recall reading somewhere that the TDM system was bought up by Meade. It seems to be still available, but certainly isn't being promoted with any enthusiasm, and doesn't seem to have achieved much penetration in america.

There was an article on CN called "Closing a Can of Worms: Eliminating Periodic Error" which described something similar. Now, given the amount of effort and verbiage that goes into reducing P.E. and the importance given to low figures in astro-photo capable mounts, I'd'a kinda thought that something like this would be revolutionary - provided it works as advertised. Strangely, it hasn't taken the world by storm and made all conventional mounts obsolete overnight. So, unless it doesn't work, I'm at a loss to explain it's lack of uptake or promotion. .

Now, I'm not an early adopter - I firmly believe in letting other people make my mistakes for me. However I am an eternal optimist and as soon as I hear a few people with (say) an EQ6 or iEQ45 say "This is my set-up, I used to have a P.E. as shown here <wiggly graph> and after fitting a TDM I now consistently get this <virtually flat line>. And I used to get images like this <Jackson Pollock painting> and now I get images like this <HST photo>" then the proverbial cheque will be in the equally proverbial post. So far, I'm still waiting ...

[ncjunk]

I recall reading somewhere that the TDM system was bought up by Meade. It seems to be still available, but certainly isn't being promoted with any enthusiasm, and doesn't seem to have achieved much penetration in america.

There was an article on CN called "Closing a Can of Worms: Eliminating Periodic Error" which described something similar. Now, given the amount of effort and verbiage that goes into reducing P.E. and the importance given to low figures in astro-photo capable mounts, I'd'a kinda thought that soemthing like this would be revolutionary - provided it works as advertised. Strangely, it hasn't taken the world by storm and made all conventional mounts obsolete overnight. So, unless it doesn't work, I'm at a loss to explain it's lack of uptake or promotion. .

I wouldn't class it as revolutionary its just a system reading back actual speed and correcting for errors at a fast enough rate so that they dont become noticable at the imaging system. Its doing the same job as an autoguider in a sense but by comparing the actual speed against required speed.

The concepts easy enough and i keep meaning to find an encoder and try fitting it and programing a control system.

The problem for me is my eq6 can take my c11 and only a little bit more weight. I can guide any errors out sufficiently to take photos. so this system makes it easier to guide out errors but am i willing to pay 1600€ for something thats more acurate but still produces the same end result but just makes it easier?

I would be tempted but i want a 50kg capacity with 4 peak to peak so thats going to be a 5k mount then and not this.

I like this from an automation point of view and would love one just to play with the programming.

now wheres that encoder....

[pete_l]

Yes, that does seem to be the point of failure: that for most people autoguiding provides a "good enough" solution. They're both examples of closed-loop control systems, though personally, I like the elegance of a corrected guider that's not dependent on me picking out a guide star - or any other manual intervention.

So at present I'm sitting here just waiting for a critical-mass of users to say "yup, it does what it says on the tin" and I'll be in. But my skepticism is that this device has been around for some time, yet Thomas Bisque still hasn't thrown in the towel and is still making Paramounts, when by all accounts a £1500 strap-on can outperform a £12000 mount. Considering what a huge improvement it purportedly makes to PE, why don't more people consider it to be a "must have"?

[E621Keith]

I may be wrong in this but I think TDM was bought by Meade Europe which is a separate company from Meade USA for some odd reason (Apparently they split). TDM is sold by Explore Scientific in the USA which is owned by JOC. JOC also part own Meade Europe.

Anyway, I believe TDM uses a high resolution absolute position encoder, and reads position rather than speed. In order to achieve one count per arc second, it will need to have a resolution of 1.3 million counts per turn (A typical Maxon servo encoder does 512 CPT, while Renishaw encoder on Planewave Acension does 14 million CPT). High res absolute position encoder don't come cheap.

IOptron IEQ45 TDM upgrade cost £1368 from Altair. The Renishaw encoder equiped IEQ45(resolution unknown) in the USA cost $3.1k (£2k) more than the standard IEQ45. However that upgrade will probably cost £3k if Altair ever consider selling that in UK. When you see prices like these, TDM's £1.4k price tag and compatibility with a wide range of mounts are revolutionary.

Rather than a alternative to autoguider, it's more an alternative to PEC.

[ncjunk]

I may be wrong in this but I think TDM was bought by Meade Europe which is a separate company from Meade USA for some odd reason (Apparently they split). TDM is sold by Explore Scientific in the USA which is owned by JOC. JOC also part own Meade Europe.

Anyway, I believe TDM uses a high resolution absolute position encoder, and reads position rather than speed. In order to achieve one count per arc second, it will need to have a resolution of 1.3 million counts per turn (A typical Maxon servo encoder does 512 CPT, while Renishaw

Ok fair point about position but that would make a good friday night argument down the pub. I am moving it to X in Y seconds and checking its position and correcting (if pulses). or moving it to x position in y time, if i get absolute position feedback, but you'd still have to compensate if it got there too quickly or slowly . Either way what are you controlling if using Distance and Time?

I really do fancy building something like this and I'm tempted but I dont think i could buy one. I would rather save the money for the next level mount due to load capacity limitations.

[vdb]

I've been investigating this myself, to achieve good results it will cost you, if you build it yourself it's almost impossible to get good results, only if you buy, or the expensive encoder, or buy a cheaper one and make a gearbox based on friction, standard gears will ruin resolution ... And then you need a controller that can work together with this ... one that can is http://siderealtechnology.com/

[chrisshillito]

Considering what a huge improvement it purportedly makes to PE, why don't more people consider it to be a "must have"?

Because PE isn't the only error folks are typically faced with. The TDM can only be soley effective in a situation where there is zero polar alignment error, no optical alignment (cone error) or flexture issues. If you have any of these then you will end up having to autoguide as well and at that point the question has to be asked if the TDM will actually deliver any practical improvment to a user's astronomy compared with running autoguiding alone or indeed pec + autoguiding.

Autoguiding alone is ultimately a more forgiving approach than TDM alone as you don't need a perfect setup.

Given the cost of the TDM system there may well be other areas of a typical imaging rig where the average user woukd be better off spending their cash.

Chris

[ncjunk]

I've been investigating this myself, to achieve good results it will cost you, if you build it yourself it's almost impossible to get good results, only if you buy, or the expensive encoder, or buy a cheaper one and make a gearbox based on friction, standard gears will ruin resolution ... And then you need a controller that can work together with this ... one that can is http://siderealtechnology.com/

I think I'll start a thread on this.

You have already summed up my initial thoughts there. If you get a lower resolution encoder with the idea of adding gears you are adding errors to the system the moment the put the gears in the loop.

The trick would be to find what combination of angluar measurement device, gears, connectors etc would give a high enough accuracy but be versatile enough to compensate for any introduced errors. Or if there is some form of measurement that averages out or reduces these errors.

i'll start another thread later.

[pete_l]

Because PE isn't the only error folks are typically faced with. The TDM can only be soley effective in a situation where there is zero polar alignment error, no optical alignment (cone error) or flexture issues.

Yes, we're in complete agreement there, Chris.

The thing is, PE is an attribute of the mount alone and a TDM is meant to improve that one aspect of the mount's performance. It's arguable that cone error and PA can be "tuned out" and mirror flop - well you still have to deal with that if you use a separate guide scope.

You only really get a proper closed-loop control system with an OAG but that isn't the most popular technique for guiding. I presume because of the difficulty in finding guide stars.

From my perspective, anything I can do which improves a mount's performance is worth investigating. Though the lack of take-up of TDM-like technologies seems to be a red flag. People seem to be more willing to buy "old technology" solutions such as Meade's Star Lock for $7000 (mount only) than an add-on TDM for half of that.

[E621Keith]

Ok fair point about position but that would make a good friday night argument down the pub. I am moving it to X in Y seconds and checking its position and correcting (if pulses). or moving it to x position in y time, if i get absolute position feedback, but you'd still have to compensate if it got there too quickly or slowly . Either way what are you controlling if using Distance and Time?

I really do fancy building something like this and I'm tempted but I dont think i could buy one. I would rather save the money for the next level mount due to load capacity limitations.

I think the difference between a high res absolute position encoder and a equally high res incremental encoder (speed) is noise in the system can potentially result in a miss read (skipped a few steps). An incremental encoder cannot detect this error while a absolute position encoder can.

Lets not mention gear reduction, backlash error is a complete pain. Furthermore, the servo motors in mounts has a encoder already, the error coming out is a result of the drive train error (worm wheel and reduction gears) between the motor and the RA shaft.

[chrisshillito]

The thing is, PE is an attribute of the mount alone and a TDM is meant to improve that one aspect of the mount's performance. It's arguable that cone error and PA can be "tuned out" and mirror flop - well you still have to deal with that if you use a separate guide scope.

I'm sure the TDM does a great job of correcting RA axis tracking errors and no doubt would be particularly effective on the lower end mounts where the factory standard mechanics can be such that the errors produced are not at all periodic over any reasonable time period (such as synta mounts) Unfortunately for the owners of these lower end mounts TDM represents a significant investment when compared with the price of the mount itself.

Certainly polar alignment, cone error etc will be an issue no matter which method of control you choose. The difference is that with TDM alone (or indeed pec alone) these errors will ultimtely compromise your ability to track an object whereas with guiding alone they only act to produce field rotation.

Chris.