GTD - Gemini vs MESU 200e vs Fast linear buying advice?

[RayD]

A quote below from @andrew s from a post about PEC :

"I don't have any experience with your mount but with mine, Paramount ME, I use PEC as it minimises the work the auto quider has to do. In controll enginering terms it is better to remove error before you use a a feedback loop that requires an error to have developed before you can correct it. Regards Andrew"

This is exactly what I believe is the reason for the encoder on the RA axis, it automatically reduces the PE without having to run a PEC routine such that the guider has to do very little work, so it makes achieving a lower RMS value easier, albiet achieving it without is probably not an impossibility.  If what you are saying is correct then advice like Andrew offered is incorrect and PEC has no place at all if guiding?

Not every question can be answered with theory and mathematics.  Sometimes you just need to try it and see the results.

Edited May 1, 2020 by RayD

[vlaiv]

3 minutes ago, RayD said:

A quote below from @andrew s from a post about PEC :

I don't have any experience with your mount but with mine, Paramount ME, I use PEC as it minimises the work the auto quider has to do. In controll enginering terms it is better to remove error before you use a a feedback loop that requires an error to have developed before you can correct it. Regards Andrew

This is exactly what I believe is the reason for the encoder on the RA axis, it automatically reduces the PE without having to run a PEC routine such that the guider has to do very little work, so it makes achieing a lower RMS value easier, albiet achieving it without is probably not an impossibility.  If what you are saying is correct then advice like Andrew offered is incorrect and PEC has no place at all if guiding?

Not every question can be answered with theory and mathematics.  Sometimes you just need to try it and see the results.

Although I see your point, I have to say that using encoders is not removing error before it develops. It is the same thing as guiding does. Maybe on a finer scale, but then, question arises - how rough is your mount when you need to correct error at time scales less than 1-2s?

You have both mounts - Mesu200 and CEM120EC.  I know that these two mounts don't cost the same, but CEM120EC2 is comparable in price to Mesu200.

Do I need to ask which one do you prefer?

 

[andrew s]

@RayD just to be precise. PEC is a form of feed forward control where a model is used to make a correction before an anticipated error occurs. The same is true of mount/atmospheric models.

Shaft encoders are used in feedback control where the correction rate is so much higher than that frequency of errors encountered due to the  mechanics of the mount and scope it behaves like a feed forward system.

Either can reduce periodic error to very low levels with good quality mounts.

Regards Andrew 

 

[RayD]

10 minutes ago, vlaiv said:

Although I see your point, I have to say that using encoders is not removing error before it develops. It is the same thing as guiding does. Maybe on a finer scale, but then, question arises - how rough is your mount when you need to correct error at time scales less than 1-2s?

You have both mounts - Mesu200 and CEM120EC.  I know that these two mounts don't cost the same, but CEM120EC2 is comparable in price to Mesu200.

Do I need to ask which one do you prefer?

 

No worries.

Yes without doubt, on performance alone, I would have another Mesu all day long.  On practical design with through the mount cabling, built in wifi and GPS etc I would have the CEM all day long.  They both perform well, but the Mesu is ultra consistent and reliable with its results (and it doesn't use PEC).

Ultimately I'd love an ASA, but that isn't going to happen any time soon 😂

Edited May 1, 2020 by RayD

[andrew s]

4 minutes ago, vlaiv said:

Although I see your point, I have to say that using encoders is not removing error before it develops. It is the same thing as guiding does. Maybe on a finer scale, but then, question arises - how rough is your mount when you need to correct error at time scales less than 1-2s?

You have both mounts - Mesu200 and CEM120EC.  I know that these two mounts don't cost the same, but CEM120EC2 is comparable in price to Mesu200.

Do I need to ask which one do you prefer?

 

My last post crossed with @vlaiv. I agree with your points but to a degree it's a matter of choice both work.

For some types of mount the encoders are an integral part of the drive system control (direct drive, some friction types) and without them the system is not stable.

Regards Andrew 

[vlaiv]

I'm finding this discussion valuable myself as I'm also contemplating obsy class mount for my future obsy (oh will this Corona related state of emergency ever end so I can continue with construction?).

Two contenders are, believe it or not - CEM120 and Mesu200.

CEM120 for precisely reasons that @RayD mentioned - a lot of gizmos that are useful - USB3.0 thru the mount cabling with all other cables, decent capacity for my current needs and of course - half the price of Mesu200.

It also features some mechanical improvements over regular worm gear mounts - like spring loaded worm gear and such.

I'm not considering EC version at all. I'm a bit worried now by some of comments. Is CEM120EC that much better guided than regular version? Because if it is - then regular version must be rather poor.

[vlaiv]

2 minutes ago, andrew s said:

My last post crossed with @vlaiv. I agree with your points but to a degree it's a matter of choice both work.

For some types of mount the encoders are an integral part of the drive system control (direct drive, some friction types) and without them the system is not stable.

Regards Andrew 

Crossed my mind to mention that as well.

Mesu200 uses encoders, right?. As far as I know - any mount using servo motors need to have shaft encoders in order for servos to maintain precision spinning rate.

These mounts don't advertise use of encoders - because these are not absolute encoders. Neither are those of CEM120, but from that mount advertising - one might get the idea that they are absolute encoders.

[andrew s]

@vlaiv when I chose my last mount the key factors were, in order were: mechanics, firmware, software integration and finally support..

That's why I went for a Paramount (no encoders)

Regards Andrew 

 

[andrew s]

7 minutes ago, vlaiv said:

Crossed my mind to mention that as well.

Mesu200 uses encoders, right?. As far as I know - any mount using servo motors need to have shaft encoders in order for servos to maintain precision spinning rate.

These mounts don't advertise use of encoders - because these are not absolute encoders. Neither are those of CEM120, but from that mount advertising - one might get the idea that they are absolute encoders.

Yes all mounts with DC or servo motors have encoders built in to the  motor control but normally when talking about encoders people mean shaft (RA, Dec) encoders

Regards Andrew s

Edited May 1, 2020 by andrew s

[RayD]

1 minute ago, vlaiv said:

I'm finding this discussion valuable myself as I'm also contemplating obsy class mount for my future obsy (oh will this Corona related state of emergency ever end so I can continue with construction?).

Two contenders are, believe it or not - CEM120 and Mesu200.

CEM120 for precisely reasons that @RayD mentioned - a lot of gizmos that are useful - USB3.0 thru the mount cabling with all other cables, decent capacity for my current needs and of course - half the price of Mesu200.

It also features some mechanical improvements over regular worm gear mounts - like spring loaded worm gear and such.

I'm not considering EC version at all. I'm a bit worried now by some of comments. Is CEM120EC that much better guided than regular version? Because if it is - then regular version must be rather poor.

They are both very very good mounts within the price range.  I find the more modern design of the CEM superb, and the dovetail USB and power distribution is abslutely awesome.  However, the nuts and bolts design of the Mesu means that it just works, and works, and works and has done so for a number of years.

I love both but, being honest, for different reasons. 

The CEM is much easier to balance, but the balancing isn't as critical on the Mesu.

I agree, if the non EC version will work for you then there is no reason at all to get one with encoders.  I haven't tried the non EC so wouldn't comment, and went for the EC purely based on the low PE this model offers.  I don't think the PE on the non EC model is massively different, but I had the budget for the EC and thought it couldn't make it worse.

I must confess to not being a fan of the Mesu 200e fixings now being studs.  I'm not saying it is worse or performs any less well than my model, but it just doesn't sit right to my eye.

It's certainly a nice decision to have to make as they are both great mounts, but are actually quite different.

[Gorr_77]

gents, so in the end of day - perfectly stable balanced tracking Mesu, won't improve enough, to make visible difference after upgrading to encoders....if yes - lets say Renshaw Encoder - would Sitech managed it or Mesu will come with diffrent controller? 

CEM120 - Ray is happy using it - so what is a point to pay for EC? - or  is there a point to pay for Renshaw precision - to make visible difference?

 

Anybody currently using one? 

[Davey-T]

1 minute ago, Gorr_77 said:

to make visible difference after upgrading to encoders....if yes - lets say Renshaw Encoder -

Well that would at least double the price of the mount  😂

Dave

[RayD]

1 minute ago, Davey-T said:

Well that would at least double the price of the mount  😂

Dave

I reckon you're about right there!

[Gorr_77]

Hi Dave-T,

Yes, but - would you see the difference?

[Gorr_77]

Not on the graph and 300% zoom....- with your eye

[Gorr_77]

Davey-T, how is your HPS? did you ever taken apart? - just curiosity... 

[andrew s]

Just now, Gorr_77 said:

Hi Dave-T,

Yes, but - would you see the difference?

I don't think you would. It's PEC is low and slow and it is stable without them.(Based on reports on here.)

Regards Andrew 

[RayD]

7 minutes ago, Gorr_77 said:

Hi Dave-T,

Yes, but - would you see the difference?

Personally I think that if they were going to make a huge difference, Lucas would have added them.  A RMS value of 0.3"/p, which most of us with the Mesu200 see regularly, is perfectly adequate for most imaging the average amatuer is going to be doing on this class of mount.

Edited May 1, 2020 by RayD

[andrew s]

From a pure control engineering perspective a mount with excellent mechanics and firmware with On Axis guiding would be ideal. Closing the loop on the target is always the final cherry on the cake!

Regards Andrew 

Edited May 1, 2020 by andrew s

[Gorr_77]

14 minutes ago, andrew s said:

I don't think you would. It's PEC is low and slow and it is stable without them.(Based on reports on here.)

Regards Andrew 

that is right - well, i couldn't see it - on cem60 vs cem60ec - but i don't class myself as a leading photographer...

Edited May 1, 2020 by Gorr_77

[Gorr_77]

Somebody mentioned that adding a encoder will improve an error up to 3arec min = 0.03333333 deg .. big fork  class mount - but i don't know anything about it...

[andrew s]

If you want to get a real understanding of mounts and their control then read  "Telescope Control" by Trueblood and Genet. Some aspects are obviously dated but the core issues remain the same and you will see that direct drive, shaft encoders and the like are not new just cheaper now.

It covers everything you could imagine from shaft flexing, via control strategies and drive types to pointing  models etc.

Regards Andrew 

I lent my copy out and never got it back but got a second it's  so good, £20 on Amazon second hand.

[JeremyS]

7 minutes ago, andrew s said:

If you want to get a real understanding of mounts and their control then read  "Telescope Control" by Trueblood and Genet. Some aspects are obviously dated but the core issues remain the same and you will see that direct drive, shaft encoders and the like are not new just cheaper now.

It covers everything you could imagine from shaft flexing, via control strategies and drive types to pointing  models etc.

Regards Andrew 

I lent my copy out and never got it back but got a second it's  so good, £20 on Amazon second hand.

This is a good book. I remember when it first came out how amazed I was. It seemed like the future had arrived!

Still got my copy somewhere (no. it's not one I nicked from Andrew S 🙂 )

[Gorr_77]

11 minutes ago, andrew s said:

I lent my copy out and never got it back

it must be good

thanks for that, i will order one now...

and probably new mount as well....!!!!!

[RayD]

2 minutes ago, Gorr_77 said:

it must be good

thanks for that, i will order one now...

and probably new mount as well....!!!!!

The Celestron AVX is excellent, and with encoders will out perform the Mesu.................😂

Just kidding.  Be good to see what mount you end up getting.