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Equipment Discussions >> Mounts

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rmollise
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Reged: 07/06/07

Re: Servo vs Microstepping Stepper new [Re: Stew57]
      #5404960 - 09/05/12 02:05 PM

Quote:

Look his expectations may have been a bit unrealistic but he did get to the bottom of some of the behavior issues with the CGEM.




No he didn't. Not IMHO. And... most people will find they have to guide even an AP or Bisque mount for long exposures, too. His was a case of badly misplaced expectations due to a lack of information about mounts in general.


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

Loc: NE Ohio
Re: Servo vs Microstepping Stepper new [Re: rmollise]
      #5405014 - 09/05/12 02:35 PM

Quote:

Quote:

Look his expectations may have been a bit unrealistic but he did get to the bottom of some of the behavior issues with the CGEM.




No he didn't. Not IMHO. And... most people will find they have to guide even an AP or Bisque mount for long exposures, too. His was a case of badly misplaced expectations due to a lack of information about mounts in general.




I agree. His discovery that controlling a servo by varying the supplied voltage is imprecise doesn't add to general knowledge; it's widely known. That's why no mounts use such a scheme. His discovery that the CGEM uses PWM to control the motor doesn't bring anything new to the table, either - all servo-driven mounts use PWM. His conclusion that the use of PWM is a negative factor for the CGEM would necessarily apply to AP and Bisque mounts, as well - yet those mounts perform very well. Folks still guide with those mounts, though. Even a couple of arcseconds of PE causes problems at some imaging scales - and there are more errors to deal with than just PE.


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orlyandico
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Reged: 08/10/09

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Re: Servo vs Stepper new [Re: ccs_hello]
      #5405091 - 09/05/12 03:18 PM

Quote:


You must have used a very old stepper+high gear-down combo.
Modern microstepping GOTO controller design (see EQ6 GOTO, say Orion Atlas) can go as high as 800x and in tracking speed fine adjustment in +-1% step.




actually I'm using a hybrid stepper with an 18:1 gear down. Nothing like the 120:1 to 150:1 of the old stepper drives, but still higher than I would like. I figure based on the torque curve of the Vexta stepper, that I should be able to achieve 250X - but that's at 24V, and I'm running 12V.

I believe the Atlas uses a very low gear reduction (4:1) which explains why it can reach 800X sidereal. Maybe I ought to try the 3.6:1 Vexta (same size) in my conversion. Not sure if it has enough torque though.


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ccs_hello
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Reged: 07/03/04

Re: Servo vs Stepper new [Re: orlyandico]
      #5405819 - 09/06/12 12:00 AM

As a reference, Atlas/EQ6 has the total gear ratio of 705:1. A hybrid stepper is fairly hard to drive to exceeding 300 RPM. It is possible but with reduced torque.
Also a commonly used design on microstepping drive is to "electronic gear shift". That is, when motor speed is getting higher, changing the 1/64 microstepping to 1/32 a the zero-crossover point, then 1/16, on on, until it reaches to 1/2 or full step drive mode where the torque is the highest. During all of these, do not miss any single microstep-equivalent count. This also gives better dynamic range.
BTW, low voltage high current stepping motor is the proper type to get.
Also, the tracking math within a popular microstepping GOTO controller: http://www.cloudynights.com/ubbthreads/showflat.php?Cat=&Board=atm&Number=5231717



P.S. sorry for the different discussion thread on DC perm-mag motor based servo vs. microstepping. I think it was what the original blogger's real focus area (DC perm-mag motor observations). Unfortunately, other statements dilute his article value significantly and IMHO totally unnecessary (After all, mount design is a complicated topic and cannot be describe in just one dimension). I chose not to tackle other statements (not enough cycle to spare.)

Clear Skies!

ccs_hello


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: ccs_hello]
      #5405851 - 09/06/12 12:21 AM

ccs, my total reduction is (18:1 x 192:1) = 3456:1

I understand the Atlas is (47:12 x 180:1) = 705:1

So my gearing is almost 5X slower - thus getting 160X out of it is not a bad figure (if I had Atlas reduction I'd be getting about 780X sidereal). I also am using low-voltage (4V) steppers. And I believe the controller (which I did not make..) is doing gear shifting already.

I don't really see the need for so many in-between speeds, I would guess 1/64 when tracking and full step when slewing is sufficient.


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Servo vs Stepper new [Re: orlyandico]
      #5405941 - 09/06/12 01:43 AM

Orlyadnico

I think the 3.6:1 ration should work fine on your mount.
What is your worm gear ratio ?

I am using the same controller with the combination shown here:
http://translate.google.com/translate?sl=de&tl=en&js=n&prev=_t&hl=de&ie=UTF-8&layout=2&eotf=1&u=http%3A%2F%2Fforum.rajiva.de%2Fforum%2FphpBB2%2Fviewtopic.php%3Ft%3D5828

Once you have the right steppers with a smaller gear ratio you will have no problems with fast Goto's.

regards
Chris


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: chboss]
      #5405973 - 09/06/12 02:24 AM

Chris, I was looking through the maximum torque curve of the 3.6:1 Vexta stepper and I'm not confident it is enough. My worm gear has 192 teeth.

Note that only very specific Vexta stepper models work for me! unlike say a Vixen GP or Orion SVP, where you can have larger external motors, the AP600 uses fully-enclosed motors, and the Vexta PX243, PX245, PX543, PX545, PK243 with SG gearhead are the only ones that will fit.

Here's what it looks like:




If I could get PX545 or PK245 that would be ideal, but what I got were PX243 (the last digit tells you how much torque the motor has). These are all NEMA17 with an offset spur gearhead. Thing is PX545 / PK245 are $220 each, new. Getting two of those would drive my GoTo conversion cost above $1000 USD. So I live with the 18:1 PX243 with small torque, and live with 160X GoTo slew speed.

Since the LFEP only uses 1-star align (and you have to manually Invert DEC when doing a meridian flip - not that the LFEP can do an automatic meridian flip anyway), if I have to do a long slew I am better off just de-clutching the mount, doing the meridian flip manually, re-syncing on the other side of the meridian. Then it's a short Go-To to whatever I want to look at. More accurate too.


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ccs_hello
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Reged: 07/03/04

Re: Servo vs Stepper new [Re: orlyandico]
      #5406147 - 09/06/12 08:18 AM

Ah ha. Now I don't need to guess if you are talking about a simple DIY (your own free code plus an inexpensive hardware) or a professional grade GOTO controller with a lot of love and care.
Rajiva's LittleFoot Elegance Photo (LFEP) -- microstepping based and Sidereal Technology's Servo II - servo based are designed for the retrofit/upgrade market and the features are getting better and better.
For a cost-driven mount, its GOTO component (usually came from an OEM design partner) tends to be more also optimized.
I'd say in ideal world, a better mount should use a better GOTO system proportional to the total built cost. But cost savers will try to standardize these electro-mechanical parts (not their core competencies anyway) to make mass production easier. I.e., lower cost for a better sales volume (and suggest using mount-tuning and autoguide to augment); or higher-grade but higher cost for a niche market.

Clear Skies!

ccs_hello


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Jason B
Proud father of 5!!
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Reged: 06/21/04

Loc: Mid-Michigan
Re: Servo vs Stepper new [Re: jrcrilly]
      #5406293 - 09/06/12 10:24 AM

Quote:

Quote:

I would consider the CGEM well worth praise if it does allow guided 10 minute exposures at relatively long focal lengths.




One of my observing buddies is doing some VERY nice work at 2000mm on a CGEM. Another uses an Atlas for narrowband work at quite long exposures. Both mounts can be capable imaging platforms. It just requires more work and greater effort (and more savvy) to use them than using a premium mount.

That said, my usual recommendation for a bargain imaging mount is a used CGE. Higher build quality, greater precision, more load capacity, and they can be found for $2000 or less. That's not all that much more than a new CGEM.




I routinely do 7-10 min guided exposures with my CGEM and my 10" F4 Newt or a 8" F8 RC. Not any issues at all. I very rarely throw a frame away. I use a larger tripod to handle the 10" Newt but other than that, all I had to do was adjust the backlash in the DEC axis the first week or so of use and have had no issues since. The mount has more than exceeded my expectations. I am careful to be balanced each time I change OTA's and use the hibernate function extensively.


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: Jason B]
      #5406357 - 09/06/12 11:00 AM

ccs_hello,

I started out with the Arduino and A4988 stepper, with a 4-line I2C display. I even got magnetic proximity switches (for homing) and a PS2 joystick as D-pad replacement.

Then I lost interest and decided that I wanted something reliable since dark-sky time is rare and the long drive is a hassle.

So I went with the LFEP. It's worth pointing out though that an LFEP alone is already 60% of the cost of an Atlas / EQ6. And that's not counting the Vexta motors, which are $220 each, new (although I got them for $33 each). (the LFEP is still the cheapest Go-To retrofit, compared to the SiTech, Gemini II, Boxdoerfer, or FS-2)

Although a Synscan / EQ6 retrofit kit at about $400 can also be used, although the ratios cannot be changed so you have to do some magic with the physical gears - not an option for me as the space inside an AP600 mount is very limited, there is no room for external gearing.

Still, a Pololu or EasyStepper board is $20, and an Arduino is under $50. And there already is the code out there that lets an Arduino emulate an EQ6 mount (so all you need is the Synscan controller and some steppers). I already got as far as encoder support and the Taki Go-To affine transform (on the Arduino).


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ccs_hello
Postmaster
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Reged: 07/03/04

Re: Servo vs Stepper new [Re: orlyandico]
      #5407414 - 09/06/12 10:33 PM

Quote:

... Although a Synscan / EQ6 retrofit kit at about $400 can also be used, although the ratios cannot be changed so you have to do some magic with the physical gears - not an option for me as the space inside an AP600 mount is very limited, there is no room for external gearing ...




Recent development on Synscan intelligent motor controller + EQMOD is that gear-ratio is no longer locked down to a fixed value (705:1). If you use EQMOD in advanced mode, such restriction now can be lifted.
(Just a minor mistake on them is that the "B" value has to be an integrer <-- see my earlier link explaining how the tracking timing loop is accomplished.)

Clear Skies!

ccs_hello


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: ccs_hello]
      #5407452 - 09/06/12 10:56 PM

Ok, good to know that the Synscan ratios can be changed. However if it requires EQMOD it is still not a generic solution for folks who want a stand-alone setup.

Or does everyone with an EQ6/Atlas use EQMOD exclusively? starting to sound like the Paramounts with their TheSkyX!


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Servo vs Stepper new [Re: ccs_hello]
      #5407456 - 09/06/12 10:58 PM

Hi Orlyandico

I see your point, the space restriction is severely limiting the motor choices...
The problem in your configuration is really the torque not the step size. The only way to confirm is a practical test. But the investment in such motors is probably too much for an uncertain outcome.

On my old mount I used a Goto speed of 160x with no problems which was enough for my photographic use. But if you want to use it for large automatic Goto's it is just a bit too slow.

regards
Chris


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: chboss]
      #5407472 - 09/06/12 11:10 PM

actually Chris there are some 3.6:1 Vexta's on ebay now for $90. So $180 for two.... hmm. Maybe I'll get them anyway for my next GoTo conversion. Or not.

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photodady
journeyman


Reged: 09/03/12

Re: Servo vs Stepper new [Re: Falcon-]
      #5449245 - 09/30/12 09:40 PM

Just to update; As promised I completed two separate RA-DEC orthogonality tests, one using TheSkyX TPiont software model and another using a simple laser pointer and 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, thus wreaking havoc with the mount tracking, without any remedy for repair or compensation.
Links to my findings:
http://www.photodady.com/blog/2012/09/27/cgem-tpoint-test-results/

http://www.photodady.com/blog/2012/09/29/cgem-ra-dec-non-orthgonality/


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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]
      #5449434 - 09/30/12 11:50 PM

Unless you purchased this mount used or waited over a year to do all this, I don't know why you would not contact Celestron, present them with this documentation, and get a replacement. This is assuming that the mount is simply unguidable at this point. If you are trying to do unguided work, then you are barking up the wrong tree in the first place. I can pretty much guarantee that none of the mounts at this level or below are orthogonal, but there are certainly some that are worse than others. I have only encountered one that I felt was particularly bad, but even that mount was useable.

Telling people that they should look to "Losmandy, Astro-Physics Inc, Software Bisque" is about a useless a recommendation as you could make. If everyone could afford those mounts, then there would't be a CGEM.


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rmollise
Postmaster
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Reged: 07/06/07

Re: Servo vs Stepper new [Re: EFT]
      #5449735 - 10/01/12 07:42 AM



Well said, Ed. Most folks wanting a mount in this class do not want or need an AP/Tak. And most of these mounts will guide very well.

Edited by rmollise (10/01/12 07:44 AM)


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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]
      #5449808 - 10/01/12 09:00 AM

That is true and understood by those that have been involved in the hobby. This typy info is not found out by reading the astro mags or almost every posted review. If you really dig hard enough you will find iy here but is it fair to expect a beginner, reading the ads, to really understand the limitations? Look at the expectations of the LX80.

The CGEM is what it is. A good into AP mount that carries a fair amount of weight. The mount has a fair amount of warts that are particular to it however, like most mounts in it's class. They should not be swept under the rug nor shouted down. The CGEM finds it's best attribute based on value not on it's superior abilities or absence of problems.

Celestron will not fix the 8/3 error and decided it will not try. Is a beginner expected to know what that is and how to measure it? They say the are working on a fix for the dec cogging effect. Again should beginner have to understand this and know how to check for it. The members here represent a small potion of total consumers.


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rmollise
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Reged: 07/06/07

Re: Servo vs Stepper new [Re: Stew57]
      #5449892 - 10/01/12 10:05 AM

How do you know they won't fix it? Did they tell you? Who told you? Even if they don't what it means is that, at worst, some subs would have to be thrown out.

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Servo vs Stepper new [Re: rmollise]
      #5449912 - 10/01/12 10:17 AM

Rod, based on the feedback of one of the Celestron engineers on TeamCelestron, they do not consider the 8/3 cogging to be significant to their customer base.

To wit, they have said that 90% of CGEM purchasers don't even know what periodic error is, much less how to measure it. And of the remaining 10%, 9 out of 10 of them will guide - hence they will not use PEC and will not hit the 8/3.

That leaves 1% of CGEM buyers who want a fix for the 8/3. It is not cost-effective for them (it would add at least $500 to the cost of the mount to fix the 8/3).

So.. it is what it is. I wondered aloud why they didn't put the CGE motors in the CGEM DX, so that the DX is more than just a CGEM with a beefier tripod and H-bridge chips. They said the better motors won't fit. So they are aware of these issues, but it doesn't make business sense for them to fix the issues.


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