Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Mounts

Pages: 1 | 2 | 3 | 4 | 5 | (show all)
Stew57
Carpal Tunnel
*****

Reged: 05/03/09

Loc: Silsbee Texas
Servo vs Stepper
      #5376133 - 08/19/12 03:27 PM

Interesting article on cheaper servos used in the CGEM
http://www.photodady.com/blog/2012/08/11/celestron-cgem-mount-motor-drives/


Post Extras: Print Post   Remind Me!   Notify Moderator  
rmollise
Postmaster
*****

Reged: 07/06/07

Re: Servo vs Stepper new [Re: Stew57]
      #5376191 - 08/19/12 04:09 PM

Interesting, yeah, I guess. He says "You get what you pay for," but it doesn't sound like he knew what he was buying. Anybody expecting outstanding UNGUIDED performance in this mount or any of the others in its class has misplaced expectations.

You guide and you take good pictures and you are aware you don't own an AP or Bisque.


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: rmollise]
      #5376200 - 08/19/12 04:14 PM

Meh. This is why I chose stepper drive for my GoTo conversion - because my CGEM experience has really soured me on servos.

Steppers suffer from a lack of dynamic range (if you want precision, you can't get fast slews). My GoTo conversion is limited to 160X sidereal, which is pretty slow but surprisingly can be lived with.

Steppers are generally easier to work with, the circuitry can be simpler, etc - but they are open-loop. However one can buy steppers which ALSO have an encoder. I believe Taks do this.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Stew57
Carpal Tunnel
*****

Reged: 05/03/09

Loc: Silsbee Texas
Re: Servo vs Stepper new [Re: rmollise]
      #5376210 - 08/19/12 04:21 PM

Agree I can't fully support his conclusion especially if you factor price/performance. He does reveal the weak link in the CGEM as the cheap servos correctly. They are the culprit behind the cogging effect, 8/3 harmonics, and generally what ails the CGEM!

For the price I am quite content with my current CGEM but just wanted to set the record strait on the knock against the Atlas.

You don't always get what you pay for in this day and age, but you never get more. That is where most of us make our mistake.


Post Extras: Print Post   Remind Me!   Notify Moderator  
photodady
journeyman


Reged: 09/03/12

Re: Servo vs Stepper new [Re: Stew57]
      #5402779 - 09/04/12 11:18 AM

Thanks for using the link to my blog, I'm responding here because of the number of hits I received from this forum. I agree with most of the posts here. First and foremost I am a photographer FIRST, not an astrophotographer. I am relatively new to this area but I must say that I am a perfectionist and my background is in engineering and an aircraft technician.

I do use a guide scope system but I come from a perspective that any system used to improve upon, using any control system compensations, should ONLY be applied AFTER mechanical system optimizations. That being said I have found the CGEM to be more than adequate for short exposure astrophotography and for visual use. Even when using the guide scope it is still necessary to use software post-processing for star alignments.

Would you find it acceptable for your car to immediately veer to the ditch when you let go of the wheel? Of course not, you would you expect to be able to let go of the wheel for a few seconds without peeing your pants?

In my opinion the CGEM falls into the category of it being necessary to wear diapers (guide system) in order to make it acceptable for astrophotography for anything other than multi-short exposure or lunar photography.

Simply take a look at Celestron's pro mount and you will see that the RA gear is twice the diameter of the CGEM mount, the same is true for ANY other RESPECTABLE astrophotography mount. (period) The Bisque Paramount actually uses an eleven inch gear thus providing at least THREE times as much precision possible from the CGEM mount. Please show me ANY published high end fine art photos that were taken using a CGEM, the vast majority of them have been taken wit an AP mount.

The tradeoff here is, of course, CASH. So for the money the CGEM (or Orion Atlas) is as good as it gets - for the money. The Losmandy G-11 with an upgraded worm gear (and twice the RA gear diameter) will cost upwards of four thousand US dollars, the Celestron pro more than that.

Again, thanks for using the link to my site and I wish upon all – clear and dark seeing.

If there is something here that I am missing please don't hesitate to correct and/or educate me.


Post Extras: Print Post   Remind Me!   Notify Moderator  
austin.grant
professor emeritus
*****

Reged: 10/18/10

Loc: Shreveport, LA
Re: Servo vs Stepper new [Re: Stew57]
      #5402851 - 09/04/12 11:58 AM

Quote:

Unfortunately I am finding out that there are basic design flaws that will most likely prevent me from taking this mount from consumer grade into the level of precision that might be called laboratory or research grade.




....


Post Extras: Print Post   Remind Me!   Notify Moderator  
EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: photodady]
      #5402864 - 09/04/12 12:05 PM

Just a few things.

First, guiding is not using diapers, it is a common method for AP that is used by many people with much more expensive mounts.

I think that you would find that there are some Takahashi mounts that are considered excellent for AP use even though they do not have particularly large worm wheels in them. Large worm wheels are great (I think the old rule was something like the diameter of the RA wheel should be the same as the diameter of the OTA being used), but precision engineering and machining play a very important (if not more important) part as well. It won't matter if a worm wheel is 3 times the size, if it's quality is poor. Hauling around a mount with 11 inch worm wheels tends to be a bit of a pain. Thus, they are most often permanently mounted.

The CGEM/Atlas/EQ6 ARE respectable AP mounts within the confines of appropriate expectations. I'm not sure what you mean by "published" but you can look here www.jwalk.smugmug.com to see some pretty impressive images taken on a CGEM and there are many others out there doing both planetary and deep sky imaging with the CGEM and other mid-level mounts that is quite impressive. Some of them even using C11 OTAs. Most people who own equipment in the CGEM range don't feel the need to pay for or justify their equipment expenditures by "publishing" "fine art" images. I know people who have thrown $50K+ at the astroimaging problem and have taken some astounding images and gotten them published in some of the magazines. I know others who have spent a fraction of that amount to take images that are just as impressive but they don't worry about getting published. There is a significant element of operator/imager talent in imaging and post-processing that comes into play with AP. If all it took was a great setup to snap incredible pics, then all of us would be doing it.

For the record, the G11 does not have an RA worm wheel that is twice the diameter of the CGEM ring gear. Maybe about 50% larger, but still quite small in reality and it uses a worm that is at most 50% of the diameter of the CGEM worm.

There are pluses and minuses to everything. If you have the money to buy high end equipment and are really serious about the hobby, then I certainly recommend pumping that money into the economy. But the average person does not have to spend that much (comparitively) to do some great work and the CGEM/Atlas mounts can do it. Expectations are very important. It would appear that you went into the CGEM with unrealistic expectations and as a result, your results are not surprising.


Post Extras: Print Post   Remind Me!   Notify Moderator  
brianb11213
Postmaster
*****

Reged: 02/25/09

Loc: 55.215N 6.554W
Re: Servo vs Stepper new [Re: photodady]
      #5403012 - 09/04/12 01:36 PM

Quote:

Simply take a look at Celestron's pro mount and you will see that the RA gear is twice the diameter of the CGEM mount



Look, there's no way that I'm claiming that the CGEM is the greatest mount in the world, but the worm gear diameter isn't the big issue. It's the tolerance of the parts. There's a lot of backlash in there, and that makes work for the autoguider.

The worm gear diameter may have an issue on how much torque the drive can supply without wearing the worm/gear at an excessive rate but this has little to do with precision.


Post Extras: Print Post   Remind Me!   Notify Moderator  
jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: Servo vs Stepper new [Re: photodady]
      #5403037 - 09/04/12 01:45 PM

Quote:

Simply take a look at Celestron's pro mount and you will see that the RA gear is twice the diameter of the CGEM mount, the same is true for ANY other RESPECTABLE astrophotography mount. (period)




Don't read too much onto the worm wheel diameters. There have been plenty of poorly-performing mounts with great big gears in them. Bear in mind that whenever Synta makes a knockoff of a high-quality mount they retain the gear diameters - but not the performance. If you want a high-quality, precision version of the Atlas/CGEM it's available, and was for years before the knockoffs happened. Just go back to the original - the EM-200. Same gear diameters and ratios, but +-3.5 arcsecond tracking out of the box. It would cost far more than the price difference to upgrade the Chinese mounts to compete with the originals in performance.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Stew57
Carpal Tunnel
*****

Reged: 05/03/09

Loc: Silsbee Texas
Re: Servo vs Stepper new [Re: brianb11213]
      #5403038 - 09/04/12 01:46 PM

The rate of change in the PE is the determing factor in guiding. The diameter og the worm and worm gear may affect this but the precision of the machining determines this. There are work arounds for backlash. Balance and guiding in one direction only for the DEC can help. I took string and affixed it to the RA with a weight on it. Keeps a constant amount of torque on the RA axis no matter the position of the RA axis. Keeping a slight east heavy balance does the same thing.

While backlash can be dealt with, the servos in the cgem are stilled handled as steppers at low voltage/speeds. It makes the servos not more accurate than the steppers used in the Atlac hence my reference to the article. many erroneously believe the Cgem is more accurate and tracks better (no stepper jumps) than the Atlas. This article should dispell that.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Lorence
scholastic sledgehammer


Reged: 09/15/08

Re: Servo vs Stepper new [Re: Stew57]
      #5403052 - 09/04/12 01:52 PM

Quote:

Interesting article on cheaper servos used in the CGEM





You might find this interesting.

http://www.siderealtechnology.com/

No idea if the controller would be practical in a CGEM but it looks like it will control an LX200. Whether the end result would be worth the effort is unclear but I'm thinking about trying one.


Post Extras: Print Post   Remind Me!   Notify Moderator  
EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: photodady]
      #5403084 - 09/04/12 02:15 PM

Here's an excellent example of the CGEM with a C11HD being used for planetary work: http://www.flickr.com/photos/42637403@N06/7929032654/in/photostream/.

Post Extras: Print Post   Remind Me!   Notify Moderator  
EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: Servo vs Stepper new [Re: Stew57]
      #5403090 - 09/04/12 02:18 PM

Quote:

Many erroneously believe the Cgem is more accurate and tracks better (no stepper jumps) than the Atlas.




I agree. Little or no difference in regards to accuracy and tracking. The positives and negatives of both designs even them out.


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: EFT]
      #5403118 - 09/04/12 02:28 PM

A stepper that is micro-stepped is probably just as smooth as a servo. My current Go-To conversion microsteps at 1/64 - the biggest lumps in the fundamental are due to the gearbox.

And we all know the famous CGEM gearbox with its 8" amplitude 8/3. For comparison, the (surplus, 33-buck-but-220 new) steppers in my Go-To conversion have about 4" p-p gear noise.

At least, I cannot get the native PE of 15" p-p below about 5" using PEC - so I'm assuming that's due to the gearbox.


Post Extras: Print Post   Remind Me!   Notify Moderator  
psandelle
professor emeritus


Reged: 06/18/08

Loc: Los Angeles
Re: Servo vs Stepper new [Re: EFT]
      #5403131 - 09/04/12 02:35 PM

People with far more experience and bigger brains than I are already answering for the CGEM's ability to do AP (I've seen wonderful work with them), but I will make comment on the analogy thrown out by photodady, which I feel is a misleading one:

"Would you find it acceptable for your car to immediately veer to the ditch when you let go of the wheel? Of course not, you would you expect to be able to let go of the wheel for a few seconds without peeing your pants?"

For what they do, CGEMs do NOT "veer off the road" for visual, or for short focal length AP, or even when worked by a "master" for longer focal length AP. I think a more accurate analogy in the automotive world would be comparing a Toyota Tercel (I used to have one) and a Ferrari (used to have them, too - let's say a 328GTS, which is what I had when I had my Tercel).

Neither veered off the road (in general) when the wheel was let go (at least not if I had them balanced correctly). They both went from point A to point B very well (let's analogize that to GOTO's). And they both were fun to drive in their own way (let's call that short focal length AP). BUT, take them to the track, and the Toyota could not compare to the Ferrari. One would have to add a lot of "extras" (just like one would have to do for a CGEM) to get the Toyota even close to keeping up with the Ferrari, and, in the end, you get what you pay for.

So, there, I think, is a more accurate analogy. Right now, I have a Volvo station wagon because neither the Toyota nor the Ferrari could fit all the astro gear I have when I head to a darksite.

Paul


Post Extras: Print Post   Remind Me!   Notify Moderator  
jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: Servo vs Stepper new [Re: Stew57]
      #5403143 - 09/04/12 02:41 PM

Quote:

many erroneously believe the Cgem is more accurate and tracks better (no stepper jumps) than the Atlas. This article should dispell that.




Stepper jumps were a concern decades ago. The original Vixen Sky Sensor (NOT the Sky Sensor 2000) used steps that were too coarse for the drive ratio and caused "jumps". Ever since, manufacturers have known enough to avoid the issue by selecting step sizes appropriate to the gearing. It's just not a concern today.

That said, PWM driven servos are now used instead on most mounts, from the bottom of the range to the top. The article implies that this is a flawed concept, then proceeds to recommend two superior mounts using the same concept. It's not the scheme that limits peformance, it's the implementation. Being in the same price class, and from the same manufacturer, I'd expect the Atlas and CGEM to offer similar precision.


Post Extras: Print Post   Remind Me!   Notify Moderator  
ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Servo vs Microstepping Stepper new [Re: jrcrilly]
      #5403920 - 09/04/12 09:29 PM

John C is right. Recent high-end GOTO stepper based is microstepping based. High-torque (thus requires high current to drive) hybrid perm-mag stepping motor is used. Since microstepping drive method with sinusoidal waveform is used, the motor is tracking just like a regular perm-mag DC motor but with a great precision because the movement is timed.

High torque also eliminates the need for a high gear-down mechanical gearbox thus less coffee grinder noise and slops caused by multiple gears.

However, the drive electronics is not so easy. It has to handle ramp-up and ramp-down on the speed to avoid missing a step. The firmware has to handle both the position and precision timing for tracking including necessary fine-tune adjustment.


P.S. changed the title to properly reflect the recent art, not the old stepper technology.


Clear Skies!

ccs_hello


Post Extras: Print Post   Remind Me!   Notify Moderator  
orlyandico
Post Laureate
*****

Reged: 08/10/09

Loc: Singapore
Re: Servo vs Microstepping Stepper [Re: ccs_hello]
      #5404052 - 09/04/12 10:38 PM

I don't really see why there is this story floating around that precision microstepping is "hard" - you can do this with an Arduino and an A4988 driver, which is a single $10 chip.

Post Extras: Print Post   Remind Me!   Notify Moderator  
gnowellsct
Postmaster
*****

Reged: 06/24/09

Re: Servo vs Stepper [Re: orlyandico]
      #5404093 - 09/04/12 11:01 PM

Quote:

Meh. This is why I chose stepper drive for my GoTo conversion - because my CGEM experience has really soured me on servos.

Steppers suffer from a lack of dynamic range (if you want precision, you can't get fast slews). My GoTo conversion is limited to 160X sidereal, which is pretty slow but surprisingly can be lived with.

Steppers are generally easier to work with, the circuitry can be simpler, etc - but they are open-loop. However one can buy steppers which ALSO have an encoder. I believe Taks do this.




Steppers are very tolerant, but they are not main stream any more. I am 100% stepper on my current mounts. But I do push-to not go-to. Greg N


Post Extras: Print Post   Remind Me!   Notify Moderator  
gnowellsct
Postmaster
*****

Reged: 06/24/09

Re: Servo vs Stepper [Re: psandelle]
      #5404104 - 09/04/12 11:08 PM

Quote:

in the automotive world would be comparing a Toyota Tercel (I used to have one) and a Ferrari (used to have them, too - let's say a 328GTS, which is what I had when I had my Tercel).


Paul




I understand. You would drive the Ferrari for 90 minutes and then it would have to be taken to the shop. While it was in the shop getting fixed for three or four months, you drove the Tercel. Then you drove the Ferrari again for 90 minutes till it broke.

I understand because my brother used to own a Lotus and a white Ford pick up. He would use the white pick up to tow the Lotus to the shop and then drive it while it was being fixed. From your choice of second vehicle, I infer you had a AAA membership in good standing.

Greg N


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | (show all)


Extra information
36 registered and 55 anonymous users are browsing this forum.

Moderator:  Dave M, richard7, bilgebay, iceblaze 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 5433

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics