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Equipment Discussions >> Mounts

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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: orlyandico]
      #5449940 - 10/01/12 10:36 AM

When they were designing the CGEM, they were generally operating within the constrains of the Altas mount and thus the size. Since the DX involved no physical changes to the standard CGEM, no additional space for motors was available and the stock motors basically take up all the space there is both length and diameter wise. Because of the way the motors are situated, even moving the electronics board further out to allow for a longer motor doesn't work. In addition, even at the undoubtably reduced price that Celestron gets the Pitmann motors (for the CGE Pro) at, they would easily add $500 to the cost of the mount since those motors retail at about $400 a piece. So, economically, it just doesn't work.

I certainly wish that someone could find some replacement motors, even if they were fairly expensive since some people would want them. Unfortunately, no one, including me, has had any success on that front. Matching relative size and specifications is very difficult.


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: Servo vs Stepper new [Re: photodady]
      #5449975 - 10/01/12 11:05 AM

Quote:

both tests show that the orthogonality is off by nearly 1/2 degree thus putting the last nail in the coffin for the usability of my particular CGEM mount for any serious astrophotography.

This lack of perpendicularity in the manufacturing quality control will permanently prevent accurate polar alignment




I don't know why you think these things. It's like asserting, without explanation, that if the OTA had a matte rather than smooth finish, it could never be focused.

Perfect orthogonality is desirable - and expected in a high end mount - but I don't see how it is required for polar alignment or imaging. Polar alignment only depends on the mechanical orientation of the polar axis and has nothing to do with what the dec. axis or OTA is doing. For unguided imaging the dec. axis plays no role, and for guided imaging 1/2 degree tilt would be negligible in guide corrections.

I'm puzzled as to why you used a small subset of terms in the TPoint model - particularly turning off Cone. Even if all the stars were on one side of the meridian (were they? If so, why?) you would want the cone term included. And during your laser tests - was tracking OFF?

As for the gear frequencies others are talking about - they would affect PEC and unguided imaging - but do they really affect guided imaging noticeably?

Frank


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jaddbd
scholastic sledgehammer


Reged: 05/18/05

Loc: Woodbine, MD
Re: Servo vs Stepper new [Re: photodady]
      #5450002 - 10/01/12 11:20 AM

Someone more knowledgeable might correct...

But I was always under the impression that orthogonality (or non-orthogonality) should not have an effect on your ability to polar align if you are using a drift align method.

John D
Maryland

Edited by jaddbd (10/01/12 11:27 AM)


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jrcrillyAdministrator
Refractor wienie no more
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Reged: 04/30/03

Loc: NE Ohio
Re: Servo vs Stepper new [Re: jaddbd]
      #5450010 - 10/01/12 11:28 AM

Quote:

Someone more knowledgeable might correct...




No need; you are correct. If orthogonality were an issue for polar aligning or imaging we'd be spending a lot of time discussing why an SCT (or any other design in which the mechanical and optical axes rarely coincide) can't be used.


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: jrcrilly]
      #5450061 - 10/01/12 12:14 PM

Ed, Derick also mentioned that the hard-wired ratio is scattered all over the code, hence it is impractical to have something like the Autostar where you can change the gear ratios any time.

It occurred to me though that it might be possible to modify the hard-coded ratios simply by doing a binary edit on the firmware binary before it is uploaded. That would allow the use of any gear ratio and thus much better servos would become available.

Not something I have the time to poke around with right now though, I've got enough leisure time tied up in the AP stepper mount...


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: Servo vs Stepper new [Re: rmollise]
      #5450318 - 10/01/12 03:03 PM

I been in contact with someone that works on fixes and design. While there should be some updates soon , the 8/3 error is not one of them. The updates if they come through sound exciting, but there has been no promises made.

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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: orlyandico]
      #5450353 - 10/01/12 03:21 PM

Quote:

Ed, Derick also mentioned that the hard-wired ratio is scattered all over the code, hence it is impractical to have something like the Autostar where you can change the gear ratios any time.

It occurred to me though that it might be possible to modify the hard-coded ratios simply by doing a binary edit on the firmware binary before it is uploaded. That would allow the use of any gear ratio and thus much better servos would become available.

Not something I have the time to poke around with right now though, I've got enough leisure time tied up in the AP stepper mount...




If we could use a different ratio, that could make things very interesting. Unfortunately, that kind of coding is over my head. I would certainly be interested to hear if there is someone out there that could do it.


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rmollise
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Reged: 07/06/07

Re: Servo vs Stepper new [Re: orlyandico]
      #5450511 - 10/01/12 05:24 PM

The reason I ask is the word _I_ hear is that Celestron is currently testing a firmware fix for this problem.

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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: Servo vs Stepper new [Re: rmollise]
      #5451007 - 10/01/12 10:29 PM

I believe that is a firmware fix for the dec cogging effect. Several versions of the fix have been tested and introduced other problems but did fix the cogging. They are close to releasing another fix for testing but it does not include a fix for 8/3 error.

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: Stew57]
      #5451026 - 10/01/12 10:41 PM

Hi Ed,

I got a brain wave. Is the CGEM compatible "enough" with the Atlas that it would be possible to drop the Atlas steppers and stepper board into a CGEM?


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photodady
journeyman


Reged: 09/03/12

Re: Servo vs Stepper new [Re: jrcrilly]
      #5451029 - 10/01/12 10:42 PM

If the users in this group can't, or refuse to, acknowledge the effects RA-DEC orthogonality has on the ability to polar align and ultimately its effects on tracking then I'm definitely in the wrong place.
The calibration routines in either the hand controller or in any third party software only have effects on pointing NOT on tracking accuracy.



Edited by David Pavlich (10/02/12 01:25 PM)


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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: photodady]
      #5451078 - 10/01/12 11:09 PM

Funny. People seem to have the same problem with the results on the Yahoo group. Like I said before, if you can show Celestron that there is a definite impact to the performance of the mount, then you will likely be able to get it replaced. They do listen to hard data.

Contrary to what some may believe, we are not all here to either be Celestron fan boys or simply pounce on people whose opinions (or data) we disagree with, but you can't expect people to be sympathetic when you appear to be wrong, appear to have unrealistic expectations, and appear to want only to vent about a particular product rather than do anything about it.

If it makes you feel better, you have now warned the world. Now I would suggest trying to resolve the problem and move on to do something that you enjoy in the hobby (assuming critiquing equipment is not what you are in the hobby to do).

Good luck.


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EFT
Vendor - Deep Space Products
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Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: orlyandico]
      #5451081 - 10/01/12 11:12 PM

I will have to look since I'm not sure if the steppers will fit. I have a couple of burnt out Atlas boards and I have an Atlas in the shop right now so I will try to take a little time and see if they can be fit.

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: EFT]
      #5451083 - 10/01/12 11:12 PM

I think it all boils down to unrealistic expectations.

Will non-orthogonality affect tracking? of course it will! but that's why we guide in both axes! would it be better if when perfectly polar-aligned we only need to do PEC? of course! but we can't have that at the price point of these mounts.

It's all very good and well to recommend "go out and buy a Mach1." That's exactly the same as the guy who wants BMW-class steering in his car but can only buy a Kia.


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zerro1
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Reged: 08/02/09

Loc: Smokey Point , 48.12°N 122.25...
Re: Servo vs Stepper new [Re: EFT]
      #5451185 - 10/02/12 12:14 AM

Quote:

I have an Atlas in the shop right now so I will try to take a little time and see if they can be fit.




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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Servo vs Stepper new [Re: zerro1]
      #5451267 - 10/02/12 01:51 AM

If I understand things correctly, polar alignment is achieved when the RA axis is perfectly parallel to Earth's rotation axis. And once polar alignment is achieved, the mount will track (periodic and non-periodic errors in the tracking gear train notwithstanding).

Since orientation of the dec axis is not part of the polar alignment or tracking equation, orthogonality errors do not impede polar alignment or tracking.

The important thing about orthogonality errors is that they will have a noticeable effect on pointing accuracy. This can indirectly affect polar alignment a couple of ways. First, if you are trying to polar align using Polaris, it is possible that a large enough orthogonality error may make it impossible to point directly at Polaris. Second, the all star polar alignment routine depends on an accurate pointing model (with some tolerance because it does a sync on the target star).

Saying it's impossible to polar align the mount is just flat wrong. A drift alignment does not depend on the pointing model at all. Thus a drift alignment will work no matter how much orthogonality error exists or whether you've modeled it.

Even on high end mounts that achieve close to perfect orthogonality errors, it is still possible (likely?) for the OTA to introduce it's own orthogonality errors. As you'd expect from my statements above, this can and does introduce pointing errors for mounts whose pointing models don't correct for it. But it does not harm their tracking performance in any way.

Now the 8/3 error is something that can effect the mount's imaging performance, but as has been said, it's effect can be mitigated through guiding. It will be interesting to see if Celestron fixes the issue. The only way that I can think of for them to do that would be to increase the number of PEC data points so that it can store 3 turns of the worm.

Just my two cents on the comments here,
-Wade


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freestar8n
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Reged: 10/12/07

Re: Servo vs Stepper new [Re: orlyandico]
      #5451280 - 10/02/12 02:16 AM

Quote:

Will non-orthogonality affect tracking? of course it will! but that's why we guide in both axes!




Non-orthogonality will not affect tracking or polar alignment at all because the dec. axis isn't involved in either. I would hate for people to come away from this thread with the misconception that the claim had any merit. It would affect goto accuracy in a 2+4 alignment, but it would have no effect on tracking and minimal effect on autoguiding.

Frank


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Kolenka
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Reged: 06/01/08

Loc: Seattle Area, WA, USA
Re: Servo vs Stepper new [Re: EFT]
      #5451285 - 10/02/12 02:32 AM

Quote:

Quote:

Ed, Derick also mentioned that the hard-wired ratio is scattered all over the code, hence it is impractical to have something like the Autostar where you can change the gear ratios any time.

It occurred to me though that it might be possible to modify the hard-coded ratios simply by doing a binary edit on the firmware binary before it is uploaded. That would allow the use of any gear ratio and thus much better servos would become available.

Not something I have the time to poke around with right now though, I've got enough leisure time tied up in the AP stepper mount...




If we could use a different ratio, that could make things very interesting. Unfortunately, that kind of coding is over my head. I would certainly be interested to hear if there is someone out there that could do it.




My concern is that the ratio is scattered all over the code. That means you can't just find "the bit that controls the ratios" and tweak it. It is possible, but to do it right means a rather lengthy analysis of the firmware. That could take quite a bit of work since you practically need to reverse engineer the whole thing to find all the references and the various ways it winds up in the final machine code.

The upside is that a person could probably ignore the handbox firmware and focus entirely on the motor control firmware. This assumes that the ratios are part of the motor control, and the handbox doesn't know them.

It can be done, but I wouldn't consider it a good use of someone's spare time unless they really want to reverse engineer the firmware. May be able to get a head start from the NexSXD folks, if they did more work than just reverse engineering the protocol between the handbox and the motor control firmware.


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: freestar8n]
      #5451327 - 10/02/12 04:16 AM

but Frank - nobody is "perfectly" polar aligned.

There would always be some activity on the DEC axis, even if minimal. Since everybody needs to correct in DEC anyway DEC guiding would also compensate for non-orthogonality.


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freestar8n
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Reged: 10/12/07

Re: Servo vs Stepper new [Re: orlyandico]
      #5451347 - 10/02/12 05:09 AM

Quote:

but Frank - nobody is "perfectly" polar aligned.

There would always be some activity on the DEC axis, even if minimal. Since everybody needs to correct in DEC anyway DEC guiding would also compensate for non-orthogonality.





But that means "tracking" is unaffected - because it only involves the RA axis turning, and polar alignment is unaffected - because it only involves the direction of the polar axis.

So it reduces it to issues of autoguiding - and that would indeed be affected due to the non-orthogonality - but the effect would be miniscule - scaled by the sine and cosine of 0.5 degrees. If you made a 1" correction in dec., then it would be off by 0.008" in RA, and 3.8E-5" in dec. This would never be noticed while guiding.

And even this assumes the guy's mount really is 0.5 degrees off - which is not clear since both his TPoint and laser measurements raise some questions on methodology and interpretation.

Frank


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