PE and Mounts question

[originaltrilogy]

I saw a link in a thread here to a closed loop system for removing PE from mount.
It was using reinshaw encoder on G11 with Sidereal Technology servo driver hardware. I think is like TDM

Question is, I have mount with PE of 4 arcseconds, what would closed loop system do for me that guidecamera does not?
Sorry if have asked wrong way.

[bojan]

Nothing - but you would pay much more for that, and you would be able tell the world "I have Closed Loop PE control on my mount".
(BTW, guiding camera system is also closed loop system)

[originaltrilogy]

Okay thank you.

So really no benefit at all?
I see ASA mount with reinshaw encoder and as a technical standpoint I like the idea to know where mount is all the time and adjust to be where it should.
But guidecamera does this also.
Is there any advantage to do with encoder instead of guidecam. Is update faster or cause less mount vibration or something?
Seems must be reason people spend so many dollars, or reason TDM was invented. Maybe is just marketing reason huh :P Wouldn't be first time.

[frolinmod]

Okay, I might be totally wrong here and if so, you guys need to call me on it, but here are my thoughts:

Does your mount track smoothly? So long as your mount's tracking is smooth, then the so called closed loop system (which is still open loop with respect to the stars unless it's using a guider, right?), won't help your guider do a better job. Only if the system can turn a non-smooth mount into a smooth mount will it help the guider.

Guiders work well so long as there are no sudden movements. When it comes to tracking, the only way to totally close the system is to use a guider of some sort, yes? Other things like PEC and ProTrack make the guider's job easier, but you still need the guider to close the loop. When the guider's job is easier, it needs to correct less often hence can use longer guide exposures hence can use dimmer guide stars. And with good PEC and ProTrack (which corrects for atmospheric refraction, tube flexure, polar misalignment, etc), for reasonably short sub-exposures (YMMV), you don't even need the guider at all and can go unguided.

For instance, my Paramount ME has a raw uncorrected PE of about three arc-seconds peak-to-peak (and smooth). PEC brings it down to about 0.6 arc-second peak-to-peak corrected (and smooth). When I turn on ProTrack many other tracking imperfections are also corrected. This makes the tracking very smooth. At FL=3911mm, if I've set the mount up well, I can go five minutes without guiding, but no longer. People who claim to go longer unguided must have a shorter focal length scope than I do.

With that in mind, I think it might be a total waste of money for me to purchase such an add-on.

Now if the add-on improved raw pointing too, beyond the 10.0 RMS, 12.08 PSD arc-seconds corrected I get with Tpoint, say to 10 arc-seconds raw (particularly if that is a MAXIMUM value rather than RMS!), that would be another story. I'm very much interested in getting good raw pointing BEFORE applying any Tpoint model.

Wow, this took me so long to compose that two other people posted while I was composing it.

[bmitchell82]

It would remove the need for a guide camera completely. This means diddly squat for most people but in the world of imaging once you get past the happy snap phase telescope optimisation starts kicking in.

eg. For me im a newt lover! OAG is definately the way to go normally. So once you add on a good size 11k chipset or even 16803 chipset plus your guider, plus your 3" field flattener plus your Adaptive optics (dependant on FL) plus your off axis guider plus your guide camera. the back space you require starts adding up (my system currently sits at 270mm from the Tube to chipset more like 300mm to the end of the CCD) and thats not including AO.

So removing the need for a OAG and Guide Camera is an attractive option because it reduces your back focus, hence reduces the need for 3.5" focusers and bohemith secondarys to get full illumination at the chip. it also lightens the load required to be supported by the focuser. One less thing to worry about and less cables.

Further more with the addition of the encoder thats running i believe at 25Hz, it can adjust for wind gusts which .... is needed on such a big parachute of a newt! Guiders can help, but no where near as fast as you need it to be effective!

Are they expensive. Yes. Are they worth 20 minute unguided subs... well Yes. Will they reduce in cost over time. More than likely! Think about computer equipment, 1995 486DX66 was close to 2000 dollars (about 4000 in todays money) now my Samsung galaxy S blitez it for processing, graphics size and display. Give it time new technology will come to fruition.

To stick your head in the sand and say it works now is just a silly notion, if that where the case we wouldn't be talking to each other on the internet nor would we have telescopes. most likely we would all be sitting under a shady tree somewhere ugging at each other and head butting rocks for fun! It wasn't that long ago that a 10" Dob was massive. now they are small!

[originaltrilogy]

OK is clearer now.
Mr Mitchell, what is the 25Hz reference to? How does mount know about wind gusts etc.?
Would still need adaptive optics to cope with seeing problems yes?

[bmitchell82]

Don't take the 25Hz as absolute gospil, but from what i have read about in relation to how these encoders work so well is they take a reading 25 times per second (25Hz) and adjust aka speed up or slow down the siderial tracking to compensate. This is how they get virutually a PE free system. Its not that the machining is that much better.

As a side shoot of this, if wind buffets the OTA, it is working that fast that the mount can withstand the buffeting. Unlike a guider that is at best with an oag or AO you are at 10 Hz with a good star and down to 3-5Hz on normal. So you see just by default it moves quicker and hence is quicker to react to wind buffeting.

As for AO no mount can sort that out. it is 100% to do with the light that is just about to hit your imaging chip so if your running at 1500mm focal length or better AO is a really good thing to have!

Brendan

[Poita]

According to the ASA site, they read the mount encoders 100 times a second.
If the mount is responsive enough I guess it could iron out most issues due to wind or bumping etc. fast enough.

[bmitchell82]

well there you go your running at 100Hz. its all down to the link between encoder, microchips and the motors. Ide say that it is more than capable of doing the adjustments.

[frolinmod]

Software Bisque says the Paramount ME checks and adjusts the position of the servomotors "about 3,000 times a second." For the Paramount MX they say it's "3,333 times a second." I kid you not.

[bmitchell82]

Even better... :)

[Poita]

Software bisque says lots of things.

[Logieberra]

Yes they do Peter, as does AP.

[billdan]

Another advantage of using encoders, is if you loosened your RA/Dec clutches and e.g manually followed the space station. The Goto Model in the system would be constantly updated and remove the need to recalibrate your star alignment.

Bill

[frolinmod]

A homing switch can also be used for that purpose. After the mount finds the home switch pointing is restored and it's as if nothing ever happened. But it does require the mount to find the home switch which can take a minute for it to do.

Another advantage of the absolute encoders is that there is never any ambiguity concerning the physical position of each axis, thus no parking position restrictions. A homing switch won't do that for you.