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Stew57
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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/


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rmollise
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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.


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orlyandico
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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.


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Stew57
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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.


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photodady
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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.


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austin.grant
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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.




....


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EFT
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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.


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brianb11213
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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.


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jrcrillyAdministrator
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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.


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Stew57
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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.


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Lorence
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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.


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EFT
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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/.

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EFT
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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.


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orlyandico
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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.


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psandelle
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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


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jrcrillyAdministrator
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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.


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ccs_hello
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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


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orlyandico
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Re: Servo vs Microstepping Stepper new [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.

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gnowellsct
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Re: Servo vs Stepper new [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


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gnowellsct
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Re: Servo vs Stepper new [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


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gnowellsct
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Re: Servo vs Stepper new [Re: photodady]
      #5404121 - 09/04/12 11:21 PM

Quote:


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.




Well, $3800 unless you're counting shipping. ($3300 plus $500 for an Ovision).

A lot of top high end work is done on planets with lower end mounts.

There is something to your argument; in fact, I think it's right. Nonetheless, the selection bias in saying this is what the people who are in the magazines use is apparent. There is a certain class of people who have the wherewithal to go high end on everything which makes their choices easy. The best CCD systems, Peltier coolers, best optics, best software, one can go north of $100k without much effort, but in any case, $20 to $50k.

The people working on the less expensive mounts have also made other decisions which may make them less competitive in high end astrophotography.

Many, however, are doing work which equals or exceeds the pics Palomar used to send to the textbooks. But if you compare everything pixel by pixel the high end equipment is sure sweet when it is in the hands of people who know how to use it.

Greg N


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psandelle
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Re: Servo vs Stepper new [Re: gnowellsct]
      #5404153 - 09/04/12 11:48 PM

Greg N - actually, I had a great Ferrari mechanic and bought the 328GTS new, so except for tune-ups/oil changes, I never had a minute's trouble for almost 8 years (never should have gotten rid of it). But I couldn't put groceries or anything in it, hence the Toyota. Now a Lotus!!! That thar's trouble!

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photodady
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Reged: 09/03/12

Re: Servo vs Stepper new [Re: EFT]
      #5404175 - 09/05/12 12:04 AM

The CGEM mount is what it is, no more, no less.

Interesting that the link you provided shows that most of the photos, that had the info provided, were captured using a AP 1200 mount. I do understand that the machining tolerances are important but being machined with equal tolerances the larger diameter gear WILL ALWAYS be more precise.

A single ten minute exposure will exceed, by far, ten one-minute stacked exposures. With AP mounts this IS easily doable without guiding.

I can easily produce a Horse-head nebula photo that rivals one shot from the Hubble derived solely from a dark frame exposure given enough time and effort post-processing and an example to work from. That is the difference between multi-short exposure composites and true long exposure photos that still retain intact data that might be scientifically useful. Most amateur astrophotography, that you see on the web, is merely an artistic rendering subjectively highly manipulated, aligned, and warped on several levels in order to achieve the final result.

The SOLE reason I posted my reviews in my blog is that there is a lack of TRUTHFUL REALISTIC reviews on the web that would possibly have prevented me from wasting my money on a CGEM. By the way if you are interested mine is for sale while I put my name on the AP waiting list.

In the mean time I will be activating my SkyX TPoint trial and will be posting extensive data on numerous tolerances (or lack thereof) in many aspects of the CGEM mount, watch my blog for updates probably sometime later this month or possibly early next month.

http://www.photodady.com/blog

The latest development from AP with precision encoders on the AP 3600:
"The measured tracking error was well below 1/2 arc second for a 20 minute time period" (only in the wildest dreams of a a CGEM user)
http://www.astro-physics.com/index.htm?products/mounts/3600gto/3600gto

Good luck and clear seeing.


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Falcon-
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Re: Servo vs Stepper new [Re: photodady]
      #5404206 - 09/05/12 12:42 AM

Quote:

Most amateur astrophotography, that you see on the web, is merely an artistic rendering subjectively highly manipulated, aligned, and warped on several levels in order to achieve the final result.




Yes, true.

And so are most of the pretty pictures released for PR use from NASA, ESO, etc, and by those using mounts such as the AP3200 that may grace a magazine cover.

Astrophotography *IS* about the end result image, not scientific data gathering after all...

Quote:

The SOLE reason I posted my reviews in my blog is that there is a lack of TRUTHFUL REALISTIC reviews on the web that would possibly have prevented me from wasting my money on a CGEM.




I get the strong impression that what is considered realistic in this case is very much a mater of perspective. Nearly the entirety of my imaging has been done on a CG-5 mount! For me a CGEM's failings would be a wonderful blessing! From my perspective I *expect* auto-guiding to be required to achieve 10-minute exposures. I would consider the CGEM well worth praise if it does allow guided 10 minute exposures at relatively long focal lengths.

I can understand being disappointed if you come from the perspective that no guiding should be necessary, but to my mind that is not a realistic expectation at this price point.


(BTW - While I do not consider myself the equal of the top-end astrophotographers, I still do have a lot to learn with processing especially, but I think I am also justified in using myself as an example that good AP can be done with "poor" mounts.)

Anyway, just to be clear I am not trying to be confrontational, just trying to point out the disparity of perspectives.


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jrcrillyAdministrator
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Re: Servo vs Stepper new [Re: photodady]
      #5404218 - 09/05/12 12:57 AM

Quote:

I do understand that the machining tolerances are important but being machined with equal tolerances the larger diameter gear WILL ALWAYS be more precise.




A larger worm wheel diameter will provide greater torque but doesn't affect precision. For a given worm accuracy, it's the tooth count on the worm wheel that determines drive precision. The more teeth per 360 degree rotation, the less erroneous axis motion results from a given worm error.

Quote:

A single ten minute exposure will exceed, by far, ten one-minute stacked exposures.




It'll certainly exceed the stack in terms of random noise - a real image killer. Otherwise the result is pretty similar so the noise is by far the most significant difference. Stacking is an essential part of CCD imaging because of its ability to cancel random noise.


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austin.grant
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Re: Servo vs Stepper new [Re: photodady]
      #5404230 - 09/05/12 01:11 AM

Quote:



The SOLE reason I posted my reviews in my blog is that there is a lack of TRUTHFUL REALISTIC reviews on the web that would possibly have prevented me from wasting my money on a CGEM. By the way if you are interested mine is for sale while I put my name on the AP waiting list.






Speaking of REALISTIC, I don't think anyone here would agree that you had REALISTIC expectations. You didn't get burned by Celestron or the CGEM, you got burned by your own desire to be cheap. You wanted lab-grade results from a consumer-grade product.... That's silly.


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jrcrillyAdministrator
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Re: Servo vs Stepper new [Re: Falcon-]
      #5404234 - 09/05/12 01:17 AM

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.

Edited by jrcrilly (09/05/12 01:17 AM)


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EFT
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Re: Servo vs Stepper new [Re: photodady]
      #5404253 - 09/05/12 01:48 AM

Quote:

Interesting that the link you provided shows that most of the photos, that had the info provided, were captured using a AP 1200 mount. I do understand that the machining tolerances are important but being machined with equal tolerances the larger diameter gear WILL ALWAYS be more precise.




Actually, what is interesting is that I'm sure you had to look at each image to determine which ones were taken with a CGEM and which were taken with the vastly more expensive AP1200. The point is that, in the hands of someone who is good at AP imaging and post processing, both mounts can produce stunning images.

As noted by others, the "larger diameter gear WILL ALWAYS be more precise" statement is clearly incorrect.

It all comes down to expectations. If you had actually read a lot of the threads discussing the CGEM I doubt you would have bought it since there many threads where people have compained about the CGEM incessantly. Most of use who appreciate the CGEM mounts have problem of "truthful realistic" reviews of the CGEM to be just the opposite of you. So unless the only reviews you looked at were the ones in the big magazines, I'm not sure what you are talking about. I'm actually happy to see the number of people defending these and other similar mounts in this case.

Good luck with your further testing of the CGEM, but I doubt it will every live up to your expectations. But if you want any chance at that, then you had best consider tuning it up some to make it perform at its best.


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Stew57
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Re: Servo vs Stepper new [Re: jrcrilly]
      #5404256 - 09/05/12 01:52 AM

The blog post does reveal the CGEM in ways noone else has. I think the data he collected about the CGEM is pretty representative. While the expectations may be unrealistic the shortcomings are documented. usually one only gets subjective "feelings". The CGEM can do good work but many times the extra time and effort working with less mount is not revealed, just the "pretty" picture. I get the feeling he wants a mount that doesn't have to be fussed with and babied to work well. If that is the case the CGEM is out.

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orlyandico
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Re: Servo vs Stepper new [Re: Stew57]
      #5404317 - 09/05/12 04:15 AM

I think what truly premium buys you is less hassle.

My old AP600 guides very well but very jerkily when west heavy.

I'm putting an Aero quest worm on my CGEM and expect much lower PE. But it will still be balance sensitive and have declination guiding problems. Can these be worked around and good images taken? I suspect yes.

But the premium mounts would not be so persnickety...


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Jon Isaacs
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Re: Servo vs Stepper new [Re: photodady]
      #5404345 - 09/05/12 05:53 AM

Quote:

The latest development from AP with precision encoders on the AP 3600: "The measured tracking error was well below 1/2 arc second for a 20 minute time period" (only in the wildest dreams of a a CGEM user) http://www.astro-physics.com/index.htm?products/mounts/3600gto/3600gto.




I think no one here is surprised that the $27,000 3600GTOPE outperforms the $1500 CGEM in most every way by a wide margin. I myself am surprised anyone would even mention the AP mount when discussing a mount like the CGEM.

In terms of astrophotography it is well known, that the mount is the most important factor. In fact that is the reason A-P went into the mount business, no sense in building the best telescopes if there is nothing to mount them on. Roland Christen is a perfectionist.

As others have pointed out, to get the professional level photographs requires professional equipment. One can take satisfying photos with a mount like the CGEM if one has reasonable expectations. In the photo world, a good mount/tripod is imperative but the job of the mount is simple so one does not expect to pay more for the mount than for the camera and lenses...

Jon Isaacs


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brianb11213
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Re: Servo vs Stepper new [Re: Jon Isaacs]
      #5404370 - 09/05/12 06:28 AM

Quote:

In the photo world, a good mount/tripod is imperative but the job of the mount is simple so one does not expect to pay more for the mount than for the camera and lenses...



Well, supporting a camera and short tele lens weighing a couple of pounds for exposures under a second is a great deal less of a challenge from the support point of view than supporting a longish focus scope and camera weighing scores of pounds and expecting it to follow an apparently moving target accurately for tens of minutes.

Actually, with current highly sensitive digital cameras and stabilised lenses, for many types of "normal" photo work a tripod is more of an encumbrance than a help. My Manfrotto 028B spends far more time supporting a pair of binoculars for night time astronomy than it does holding up a camera.


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ccs_hello
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Re: Servo vs Stepper new [Re: orlyandico]
      #5404383 - 09/05/12 06:49 AM

Quote:

...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.
...



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.


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ccs_hello
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Re: Servo vs Microstepping Stepper new [Re: orlyandico]
      #5404394 - 09/05/12 07:00 AM

Quote:

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.



This is what microcontroller or what development platform is more suitable type of hardware debate. I'd also recognize that "microstepping" as a software module has existed for a long, long time. But I'd say this: a properly designed microstepping GOTO controller for astro application (or high grade, professional quality) is not so easy. I had a list of requirements previously in CN's ATM forum (and in another forum outside the CN). For some hobby-DIY self-developed attempts, I'd say they only meet some of them (and leave the rest of tasks for future work ) So you decide.

Clear Skies!

ccs_hello


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orion69
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Re: Servo vs Microstepping Stepper new [Re: ccs_hello]
      #5404540 - 09/05/12 09:34 AM

@photodady

You can't judge CGEM's AP performance using 9.25” EdgeHD on DSO targets and better yet, unguided!? It' hilarious really.
Blogs like yours are reason why I don't read blogs.

Knez


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Stew57
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Re: Servo vs Microstepping Stepper new [Re: orion69]
      #5404608 - 09/05/12 10:22 AM

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. A lot more in depth tahn a lot I have seen even here. Like he said the CGEM is what it is and now we know more of what it is.

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Stew57
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Re: Servo vs Stepper new [Re: photodady]
      #5404616 - 09/05/12 10:25 AM

I would like to see give the same treatment to the Atlas and IEQ45. I think that would be interesting read.

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Starhawk
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Re: Servo vs Stepper new [Re: psandelle]
      #5404638 - 09/05/12 10:39 AM

Ok,

If this is where you're coming from, visit this web site:

www.astro-physics.com

Call them and tell them you want a Mach 1 GTO mount. Trust me- this is what you are looking for. Spend some time on the site and choose your accessories carefully.

-Rich

Quote:

Greg N - actually, I had a great Ferrari mechanic and bought the 328GTS new, so except for tune-ups/oil changes, I never had a minute's trouble for almost 8 years (never should have gotten rid of it). But I couldn't put groceries or anything in it, hence the Toyota. Now a Lotus!!! That thar's trouble!




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Ray Gralak
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Re: Servo vs Stepper new [Re: Jon Isaacs]
      #5404719 - 09/05/12 11:38 AM

Quote:

Quote:

The latest development from AP with precision encoders on the AP 3600: "The measured tracking error was well below 1/2 arc second for a 20 minute time period" (only in the wildest dreams of a a CGEM user) http://www.astro-physics.com/index.htm?products/mounts/3600gto/3600gto.




I think no one here is surprised that the $27,000 3600GTOPE outperforms the $1500 CGEM in most every way by a wide margin. I myself am surprised anyone would even mention the AP mount when discussing a mount like the CGEM.
Jon Isaacs




I think the reason he mentioned AP mounts is that someone gave him a link to pictures that were supposedly done with a CGEM, but most were actually done with an AP1200. That said those pictures, although colorful, had a little too much noise reduction for my taste. The excessive noise reduction caused some stars to be misshaped (visible in some of the full sized images) and areas to be featureless hazes. In my humble opinion, although they are not bad, they are definitely not up to the best deep sky images of these areas that I've seen, mostly because of the excessive noise reduction.

-Ray


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Stew57
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Re: Servo vs Stepper new [Re: psandelle]
      #5404804 - 09/05/12 12:30 PM

I like the Lotus analogy as my brother in law had one (Elise). When it was on it was great but it wasn't always on shall we say. When you make something that light something has to go so it was rather uncomfortable and noisy. When you develop that much power out of a little toyota 1.8, the engine is stressed and tuned to the max. Same moter detuned is quite reliable.

When engineers want to carry a heavier payload with a inexpensive mount that is not exceptionaly heavy, well something has to give. The compromises made for affordabilty will lead to less performance and lower longevity.

The blog posts reveals some of the compromise. I for one would like to see the compromises made in the case of the Atlas and IEQ45 also.


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rmollise
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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
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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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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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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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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
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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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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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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
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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
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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
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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
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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

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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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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
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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
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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
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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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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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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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EFT
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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
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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
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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
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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
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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
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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
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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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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
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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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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
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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
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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
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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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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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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
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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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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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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
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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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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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orlyandico
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Re: Servo vs Stepper new [Re: freestar8n]
      #5451355 - 10/02/12 05:19 AM

Ah ok wrong terminology on my part. When I said "tracking" I really meant.. guided tracking, since I had assumed this is what all normal folks with normal mounts do

And you're right, the contribution of non-orthogonality would be tiny.


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Stew57
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Re: Servo vs Stepper new [Re: orlyandico]
      #5452277 - 10/02/12 04:56 PM

Drift alignment takes care of the tracking issue. Does the 4 calibration stars (well actually one as they are averaged) compnstae enough to take care of pointing accuracy enough for ASPA to be accurate? Perhaps that (orthogonality) is a variable in the wide discrepancy in user reported success when using ASPA.

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jaddbd
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Re: Servo vs Stepper new [Re: photodady]
      #5452564 - 10/02/12 08:02 PM

If you use a DRIFT alignment method (computer assisted or otherwise), you should be able to achieve accurate enough polar alignment regardless of orthogonality. The goal of DRIFT is to get the RA plane of the MOUNT perpendicular to the pole. Since you adjust the MOUNT to bring the DEC drift back to parallel when shooting East and at then at the meridian South, it seems to me that any angle of error on the RA plane would be irrelevant (since it is parallel to the plane you are trying to achieve) and any angle created by shooting a little high or low off the DEC plane would have to be very large to be of any measurable relevance.

That being said, I have 3 different scopes which I use (one at a time) on my permanent (drift aligned) set up, which all point slightly different when mounted, but all “track” and “guide” the same.

Also (responding to some banter earlier in the thread), my experience in thinking that you can just point and shoot a high end mount “unguided” is a bit of a misnomer. Even if you have excellent polar alignment, and very little periodic error, you still have to account for the fact that objects in different parts of the sky move at different rates due to refraction, and that rate may change position of the said object changes. There are software tools provided to adjust for this, but it is way easier and more accurate just to guide IMHO.

John D
Maryland


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rmollise
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Re: Servo vs Stepper new [Re: photodady]
      #5453566 - 10/03/12 01:46 PM

Quote:

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.




It doesn't and won't. But stick around and you'll learn the basics before taking the bit in your teeth again...


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EFT
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Re: Servo vs Stepper new [Re: EFT]
      #5454375 - 10/03/12 11:48 PM

I took a look at the possibility of putting the Atlas system into the CGEM. While maybe not impossible, it would not be easy. The first issue would be the motor mounts. I would be necessary to reposition the mounting holes on the Atlas steppers to fit the CGEM locations. That could probably be done and there is probably enough space for the motors.

The biggest challenge would be the motor board. The Atlas motor board is longer than the CGEM board but not as tall. Getting the board in the hole and attached to a face plate might be challenging. Worse yet, the Atlas motor cables attach to the board on end creating a further space issue. If it can all be fit, then a custom face plate would need to be made to attach the motor board to.

Perhaps not impossible, but definitely difficult. Considering the cost of a Synscan conversion kit (if you can find one anymore), it's hard to say whether or not it would be worth the effort.


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orlyandico
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Re: Servo vs Stepper new [Re: EFT]
      #5454590 - 10/04/12 05:42 AM

Thanks Ed. I suspected as much regarding the motor board.

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andysea
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Re: Servo vs Stepper new [Re: jaddbd]
      #5507487 - 11/06/12 08:19 PM

Quote:

Also (responding to some banter earlier in the thread), my experience in thinking that you can just point and shoot a high end mount “unguided” is a bit of a misnomer. Even if you have excellent polar alignment, and very little periodic error, you still have to account for the fact that objects in different parts of the sky move at different rates due to refraction, and that rate may change position of the said object changes. There are software tools provided to adjust for this, but it is way easier and more accurate just to guide IMHO.

John D
Maryland




I fully agree with you John!
Guiding seems to be the best way to achieve good tracking even with excellent mounts; and guiding with an OAG in my opinion is the way to go.
As long as the mount as a smooth and repeatable periodic error it will be easily guided out along with any drift generated by not perfect polar alignment.

Andy


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