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Equipment Discussions >> Mounts

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gmartin02
professor emeritus
*****

Reged: 04/11/05

Loc: Santa Clarita, CA
Re: CGEM not suitable for serious astrophotography? new [Re: Stew57]
      #5649146 - 01/28/13 08:10 PM

Quote:

The two biggest problems that plague the CGEM are motor related. The dec cogging effect and the 8/3 PE that makes pec ineffective. Both are currently being activly woked on by celestron. They have been working on it for awhile and have no firm timetable for completion. If you are the impatient type......




+1 on the dec cogging problem. I have a CGEM with the dec guiding issue (some CGEM mounts do this, others do not). I am unable to shoot astrophotos with my AT8RC because of this problem, but am able to use my Tak FS-60 & Orion 110 ED because of their shorter focal lengths (although I still have to "nudge" the dec axis sometimes to unstick it when shooting with the shorter f/l scopes).

I am patiently waiting for the firmware or firmware+motors/motorboard solution from Celestron for the CGEM (this fix is already in the new VX mount).

Greg


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: CGEM not suitable for serious astrophotography? new [Re: EFT]
      #5649153 - 01/28/13 08:13 PM

Quote:

Quote:

New firmware should be coming out soon for the CGEM DEC guiding bug.. but....There might..MIGHT... be new motors coming out for the CGEM.. similar to the VX line..




It is pretty likely that the new motors in the VX could simply be dropped into the CGEM if they changed the firmware to account for the new gear ratios. That's the only reason that we can't dump something else in there already. The big problem will be if they will be willing to supply enough motors for sale to satisfy everyone that might want to make the switch. Actually, if they would just allow us to change the gear ratios in the firmware that would allow anyone to drop in any motor that would fit.




The issue is that the new motors are is very short supply right now..

Also...

It isn't the motors that makes the Dec Guiding issue go away. It is the firmware, and the firmware WILL work on existing mounts as well. The VX mount has the new firmware. The issue is during all the hoopla to fix the cgem bug, which was fixed in a beta release, the vx firmware took precedence.. Then somehow the fixed cgem/cge-pro firmware got fubared..

The new motors have different gear ratios, and would also require different feedback loop parameters. That could be overcome with a software update and an update to the PRN value. The latter requires you send in your motor board to Celestron. There is also a voltage issue. The voltage issue could be worked around by reworking a few resistors to your motor board (after you sent it in) but it is better for most consumers to get a new motor board if (and only if) you want the new motors.

The new motors have a bit more torque and integer gearing.


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dickbill
scholastic sledgehammer


Reged: 09/30/08

Re: CGEM not suitable for serious astrophotography? new [Re: EFT]
      #5649171 - 01/28/13 08:21 PM

Quote:

Quote:

However, is it confirmed that PEC in the cgem is inefficient as ddady mentioned?
I used the PEC once with no great result but my drift in declination indicated that i was not very well polar aligned. So i'd like to know from those who are well aligned.




My understanding of the problem with the Celestron PEC is that it does not cover enough cycles to account for the 8/3 error from the motor gearbox. That is an issue for some people but not all. But PEC has supporters and detractors in general as well as disagreement on whether it should be used with guiding or not, so just like guiding, it is not a universal fix and will not magically turn a $1500 mount into a $15,000 mount.




Knowing that all the cgem errors (whatever their sources are) during a worm cycle are not perfectly periodic, my expectation is still that some of them must be periodic and the PEC routine must correct for them. So at least some smoothness must be obtained during the play back of the reccord even if non periodic glitches remain. In short the PEC can not be totally inefficient.


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: CGEM not suitable for serious astrophotography? new [Re: dickbill]
      #5649196 - 01/28/13 08:31 PM

it's not. you can get rid of most of the fundamental, i.e. if you have 25" of fundamental and 8" of 8/3, you can get rid of maybe 15" to 20" of the fundamental. this will drop the PE of course, but the 8/3 is untouched.

i.e. you cannot get less than 10" corrected PE.


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WarmWeatherGuy
Carpal Tunnel
*****

Reged: 08/27/11

Loc: Orlando, FL 28° N, 81° W
Re: CGEM not suitable for serious astrophotography? new [Re: orlyandico]
      #5649217 - 01/28/13 08:43 PM

I love my CGEM. I take 10 minute exposures at 2000mm with my C8 (f/10) all the time. I can get polar alignment down to 15" (1/4') using the All Star Polar Align, after a few lengthy iterations. I use a C5 (1280mm) for my guide scope though. If you use some tiny 50mm finderscope lens for guiding you may have trouble.

Here is a single 10 minute exposure I took of the Sculptor Galaxy using the CGEM in strong gusty wind:
http://www.astrobin.com/5015/

It really is a matter of what is meant by "serious astrophotography." Many people are thrilled with a blurry picture of M42 that they got with their 6" DOB. NASA would be unhappy with what you can do with a $50,000 mount.

The proof is in the pudding. Browse Astrobin to see what you can do with different mounts.


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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: CGEM not suitable for serious astrophotography? new [Re: wolfman_4_ever]
      #5649231 - 01/28/13 08:50 PM

Quote:

Quote:

Quote:

New firmware should be coming out soon for the CGEM DEC guiding bug.. but....There might..MIGHT... be new motors coming out for the CGEM.. similar to the VX line..




It is pretty likely that the new motors in the VX could simply be dropped into the CGEM if they changed the firmware to account for the new gear ratios. That's the only reason that we can't dump something else in there already. The big problem will be if they will be willing to supply enough motors for sale to satisfy everyone that might want to make the switch. Actually, if they would just allow us to change the gear ratios in the firmware that would allow anyone to drop in any motor that would fit.




The issue is that the new motors are is very short supply right now..

Also...

It isn't the motors that makes the Dec Guiding issue go away. It is the firmware, and the firmware WILL work on existing mounts as well. The VX mount has the new firmware. The issue is during all the hoopla to fix the cgem bug, which was fixed in a beta release, the vx firmware took precedence.. Then somehow the fixed cgem/cge-pro firmware got fubared..

The new motors have different gear ratios, and would also require different feedback loop parameters. That could be overcome with a software update and an update to the PRN value. The latter requires you send in your motor board to Celestron. There is also a voltage issue. The voltage issue could be worked around by reworking a few resistors to your motor board (after you sent it in) but it is better for most consumers to get a new motor board if (and only if) you want the new motors.

The new motors have a bit more torque and integer gearing.




Yes, the DEC cogging issue is different than the 8/3 issue that I was thinking of.

What is the source of information on the new motors? From the looks of them, I would be very surprised if they are not 12VDC motors (it would be silly to use anything else). From the photos I've seen, I would expect the new motors to be the same Igarashi motors with different gearboxes. A fix that would require not only new motors but a new motor board at a total cost of somewhere around $500 would be pretty useless. Even having to buy motors at the current $128 is outrageous. These motors can be obtained in the many thousands easily (actually that's really the only way to buy them) and any shortage would be because of a lack of desire to purchase them, not because they are unavailable.


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM not suitable for serious astrophotography? new [Re: dickbill]
      #5649319 - 01/28/13 09:43 PM

The efficiency of the PEC depends on the amount of 8/3 error. For some the 8/3 error is the largest error so pec is pretty much useless. My 8/3 is 75% of the first harmonic so PEC will only get that far at best. I have no cogging problemthough so it is not all bad.

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photodady
journeyman


Reged: 09/03/12

Re: CGEM not suitable for serious astrophotography? new [Re: RogerRZ]
      #5650219 - 01/29/13 12:35 PM

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.

This is 1/60th of the performance of the Astro-Physics mount, so then I should be satisfied if my Accord has a top speed of only 4 MHP?

Five minutes of accurate unguided performance is not that unreasonable.


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650276 - 01/29/13 01:02 PM

cars aren't mounts.

while it is true that 5 minutes unguided is not that unreasonable - at what focal length? an AP can't do 5 minutes at 3000mm.

if we take your statement as 5 minutes at 600mm - a reasonable focal length - then yes a CGEM still can't do that. i don't know if there's any mount less than the G11 that can. it simply is. you don't get that performance for $1500.


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Mike X.
scholastic sledgehammer


Reged: 06/28/10

Loc: Greece-Athens and Rome-Italy
Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650278 - 01/29/13 01:03 PM

Quote:

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.

This is 1/60th of the performance of the Astro-Physics mount, so then I should be satisfied if my Accord has a top speed of only 4 MHP?

Five minutes of accurate unguided performance is not that unreasonable.



No it's not but it depends on the focal length someone wishes to photograph with and the results that is satisfied with.

Here are my two cents of thought for what they count...I find it a little exaggerated to put a mass produced,budget mount with.an industrial sample quality control against a high end, expensive and highly refined mount with piece per piece quality control and customization.

The only thing the mass produced mounts have in common with some other mounts like the mach1 or the ddm60 etc...in my humble opinion is that they are...mounts...period.

Said that...I'm happy doing dso ap with a cg5....and I bet is way worst than the Cgem.

Edited by Mike X. (01/29/13 02:06 PM)


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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650295 - 01/29/13 01:13 PM

Quote:

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.

This is 1/60th of the performance of the Astro-Physics mount, so then I should be satisfied if my Accord has a top speed of only 4 MHP?

Five minutes of accurate unguided performance is not that unreasonable.




Unguided AP for anything but the brightest objects, fastest focal lengths and shortest exposures, particularly at this price point, is unreasonable. Depending on the scope and imaging setup, it can be unreasonable for even a much more expensive mount. Any amount of research on the CGEM would show that. I agree that about the only thing the CGEM and an AP mount have in common is that they are both telescope mounts.


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650311 - 01/29/13 01:20 PM

Quote:

The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.




Non-orthogonality of the axes has no connection with dec. drift at all. The axes could be off by 20 degrees and it would certainly affect GoTo accuracy - but once you had the scope somehow aimed at an object, and if the polar axis is aligned, and if you are smoothly rotating around that polar axis, then the tracking will be no different from that of a perfectly orthogonal mount.

Polar tracking just involves rotation around a single axis. As long as the rest of the geometry is fixed relative to that axis - there is no way to induce additional rotation in a different direction.

Frank


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jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650366 - 01/29/13 01:48 PM

Quote:

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.




That would amaze me; I've never heard of orthogonality issues causing drift of that magnitude. Sounds more like a severe polar alignment error.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650369 - 01/29/13 01:49 PM

Quote:

OK, you have a point. The CGEM drifts about one arc-second every twenty seconds, just in the declination axis, due to non orthogonality.

This is 1/60th of the performance of the Astro-Physics mount, so then I should be satisfied if my Accord has a top speed of only 4 MHP?

Five minutes of accurate unguided performance is not that unreasonable.




Your problems, whatever they are, are not caused by "non-orthogonality." Look to your polar alignment. How did you perform it?


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wolfman_4_ever
Pooh-Bah


Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: CGEM not suitable for serious astrophotography? new [Re: rmollise]
      #5650476 - 01/29/13 02:46 PM

That's exactly what I was going to say Rod.

This guys blog post screams user error..

The mount is only going to be as good, or as smart, as the user is.. I don't care what mount you are using.. As they say.. "stupid is as stupid does"


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gdd
scholastic sledgehammer


Reged: 11/23/05

Loc: N Seattle suburb, WA
Re: CGEM not suitable for serious astrophotography? new [Re: rmollise]
      #5650550 - 01/29/13 03:15 PM

Quote:

Your problems, whatever they are, are not caused by "non-orthogonality." Look to your polar alignment. How did you perform it?






I don't understand what non-orthoganality has to do with tracking accuracy though I can see it messing up goto accuracy. You don't even need to have a dec axis to track (think of AstroTrac).

Gale

Edited by gdd (01/29/13 03:16 PM)


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jrcrillyAdministrator
Refractor wienie no more
*****

Reged: 04/30/03

Loc: NE Ohio
Re: CGEM not suitable for serious astrophotography? new [Re: gdd]
      #5650567 - 01/29/13 03:21 PM

Quote:

I don't understand what non-orthoganality has to do with tracking accuracy though




If it were off by tens of degrees you'd see drift. If it were critical in small amounts, folks would have to worry about alignment of the optical axis of the telescope to the mount. Some do worry about that, but most do not.


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photodady
journeyman


Reged: 09/03/12

Re: CGEM not suitable for serious astrophotography? new [Re: freestar8n]
      #5650625 - 01/29/13 03:47 PM

"Non-orthogonality of the axes has no connection with dec. drift at all."

This is completely untrue, you are conflating optical axis-RA non-orthogonality with non-orthogonality of the RA-Dec axis.

For the former misalignment your statement is true and is called Cone Error, but for RA-DEC nonorthogonality the error is not constant, rather the scope will travel both below and above the intended path. The two extrema will be 90 degrees from the zero crossings. When the path traveled is viewed from the side it will form a lissajous pattern, or a figure eight.

The periodic error for the corrections using a guiding system in the declination axis will have a period of 24 hours. For my particular scope the peek-to-peek error is about 1/2 degree, or about the angular width of the Sun or Moon.

A similar effect occurs when viewing the Sun at the same time every day and plotting its position, I think this is called the Sun's analemma. This is also due to the nonorthogonality of the rotation of the earth's axis in relation to its orbital plane.

Sorry I just assumed that those outspoken in an astronomy forum would have at least had a basic course in astronomy.


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dickbill
scholastic sledgehammer


Reged: 09/30/08

Re: CGEM not suitable for serious astrophotography? new [Re: jrcrilly]
      #5650653 - 01/29/13 04:09 PM

Misalignement also translates into field rotation, that's easy to see.
The simplest think to explain a dec drift, as Rod said, is inaccurate polar alignment. Do not assume it is good just because 'before' it was good. My experience is that if you are going to image, check it, even if two days before it was indeed very good and you havn't move the mount.

First using the polar routine after top notch 2stars align + 2, the mount put the Esquimo dead center of the C925 at f10, 120X. So then i did the routine for polar alignement and had little drift in RA and dec thereafter.
Well, 2 days later i thought i should not realign since it was good before and i didn't move the mount. And this is when my declination drift showed up. In between I had lots of winds. I think the altitude can not change so easily under transversal winds so it's perhaps just the azimuth that has shifted, or perhaps something about the central spreader and screws that get loose, I'll know next time i realign, but anyways, as long your mount is not on a solid permanent pier shielded from the winds, polar alignement should be checked regularly IMO.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: CGEM not suitable for serious astrophotography? new [Re: photodady]
      #5650683 - 01/29/13 04:24 PM

Quote:

"Non-orthogonality of the axes has no connection with dec. drift at all."

This is completely untrue, you are conflating optical axis-RA non-orthogonality with non-orthogonality of the RA-Dec axis.

For the former misalignment your statement is true and is called Cone Error, but for RA-DEC nonorthogonality the error is not constant, rather the scope will travel both below and above the intended path. The two extrema will be 90 degrees from the zero crossings. When the path traveled is viewed from the side it will form a lissajous pattern, or a figure eight.

The periodic error for the corrections using a guiding system in the declination axis will have a period of 24 hours. For my particular scope the peek-to-peek error is about 1/2 degree, or about the angular width of the Sun or Moon.

A similar effect occurs when viewing the Sun at the same time every day and plotting its position, I think this is called the Sun's analemma. This is also due to the nonorthogonality of the rotation of the earth's axis in relation to its orbital plane.

Sorry I just assumed that those outspoken in an astronomy forum would have at least had a basic course in astronomy.




The telescope is only being driven in RA. If it is properly polar aligned, you won't get drift. Forget this as the source of your problem.


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