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CGEM not suitable for serious astrophotography?

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#26 tboconnor

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Posted 28 January 2013 - 04:56 PM

I should also mention - I currently have two scopes - a Meade 12 inch ACF that Im using for visual and some lunar/planetary imaging, and a WO Megrez 90, which is what I want to use for deepsky dabbling.

I dont currently have any auto-guiding gear, but was intending to add something inexpensive like the Orion miniguider - kind of just to dip my toe in the water.

Just for fun, I tried some unguided shots with the Meade on the CGEM - I used the polar alignment routine, and managed to get one or two shots that showed it can guide for around 20 seconds - although it was very hit and miss - some shots showed trails with 15 second exposures, while some where fine with 30 seconds. I really need to drift align, but I wanted to see how good the all-star alignment system was :)

I really dont expect to be able to get much joy out of shots from the Meade - its way over the payload of the CGEM, although for visual it works just fine.

#27 orion69

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Posted 28 January 2013 - 05:28 PM

No, of course not.
CGEM is not suitable for unguided astrophotography.
If you want unguided AP then buy some high end Direct Drive Mount.
Also CGEM is not suitable for long FL AP.
Also CGEM is not suitable for heavy scope AP.

But under 900mm and up to lets say 12-14 kg equipment?
Well that is another story. Of course there are CGEMs that are not working correctly and that has to be said. But if you get a good one you actually don't have to hypertune...

99% of my images are made from 30min guided subs with refractor @ 765-900mm.
Do you know how many of those 30min subs I had to toss after I got OAG?
None.
And my CGEM has never been hypertuned.

Is this serious enough for you?

#28 kbastro

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Posted 28 January 2013 - 05:51 PM

I have ta agree with orion69 on this one,, cgem's including one that I used, was ok,, but no where near exceptional and didn't have chance in hell of producing good astrophotos unguided at anything over 800mm f.l.

#29 WadeH237

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Posted 28 January 2013 - 06:15 PM

It looks like it hasn't been mentioned, but this has been discussed before.

#30 Jay Wise

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Posted 28 January 2013 - 06:44 PM

One look at orion69's website puts an end to this discussion about the cgem's ability in the right hands. It is said that some of Ancell Adams photos were taken with a modified Brownie! It is frequently the master not the instrument that counts.

JayW :waytogo:

#31 dickbill

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Posted 28 January 2013 - 06:51 PM

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.

#32 EFT

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Posted 28 January 2013 - 07:03 PM

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.

#33 orlyandico

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Posted 28 January 2013 - 07:14 PM

this horse has been beat to death before.

and "daddy's" claims are imho specious. you don't buy something for $1500 and expect it to perform like $6000. you get what you pay for as he himself said.

with that out of the way... a good PEC training on a mount that has small non-periodic terms (i.e. Celestron AVX!) will reduce the native PE by at least a factor of 4. If you accept that a normal CGEM has around 25" to 30" of PE, a good PEC (if there were no 8/3) would reduce that to perhaps 8".

your typical user with a DSLR or 8300 class CCD and imaging with a refractor would have perhaps 2.5" per pixel image scale. with 8" of corrected PE remaining, you could go unguided up to perhaps 2-3 minutes? not much more than that. but it is what it is.

unfortunately most CGEMs have at least 7" to 8" of 8/3.. in some cases much worse. so PEC training won't get you much below 15". long story short - you have to guide. all the time.

most folks are using CCDs anyway so guiding is not a huge extra chore.

funny story: I was puttering around last night with this:
Posted Image
and guess what. I was still guiding.

It's just too much effort to do the perfect polar align to go unguided for any length of time. Not to mention you need to factor in atmospheric refraction and all that. But with an eyeball polar align (mount in Park 3, I put a scope on and ensure it's pointed at this church stained glass rosette a couple miles away) i can do subs as long as i want, with guiding.

#34 EFT

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Posted 28 January 2013 - 07:16 PM

One look at orion69's website puts an end to this discussion about the cgem's ability in the right hands. It is said that some of Ancell Adams photos were taken with a modified Brownie! It is frequently the master not the instrument that counts.

JayW :waytogo:


Absolutely correct. If you need images better than those, then just download the Hubble shots. A tremendous amount of AP is time and talent (of which I am lacking in at least one ;) ).

Here are some more amazing shots with a CGEM and a CGEM DX: http://jwalk.smugmug.com/.

The raw AP talent of Jimmy Walker and Orion69 simply leaves me in awe. Their photos are every bit as good as the ones that are done by one of my club's members and he has spent probably 5 to 10 times as much on his system.

#35 orlyandico

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Posted 28 January 2013 - 07:25 PM

Ed, you forgot Jeff over on the CGEM yahoo group. I also greatly admire his long FL work. but he has a leg up with the SX AO.

One thing you do get with the spendy mounts though is less hair loss. Less tweaking etc. If you have to drive 200km to get to dark skies like I do, and the weather is uncooperative most of the time, actual clear imaging time is precious and at some point you'll tear your hair out trying to deal with the mount...

#36 EFT

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Posted 28 January 2013 - 07:32 PM

Yes. There are definitely other's out there. Too many for me to keep track of. Jimmy's photos are the ones I blew up to 8 feet wide by 8 feet tall.

What the people with the bigger bucks are doing is no longer driving anywhere. They just buy a plot over in southern New Mexico or Arizona at one of the new astro communities, build a remote observatory, and do all their imaging from many miles away. The CGEM is not particularly suited to that, but I know people who do it with everything from Meade classic LX200s to top end AP and Bisque gear.

#37 ghataa

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Posted 28 January 2013 - 07:43 PM

Maybe I should then just take my CG5 out back and shoot it.

George

#38 wolfman_4_ever

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Posted 28 January 2013 - 07:49 PM

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..

#39 EFT

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Posted 28 January 2013 - 07:59 PM

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.

#40 orlyandico

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Posted 28 January 2013 - 08:02 PM

.. and for that reason (the promise of sweet, sweet AVX motors :D ) I am holding on to my CGEM. Even though every time I turn it on, I find something annoying (like my recent problem with the ASCOM drivers).

but it is just so darn easy to use! ASPA is great, the pointing model is great... AP doesn't have a pointing model (simple 1-star align). Unless you bolt up a PC with APCC.

#41 gmartin02

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Posted 28 January 2013 - 08:10 PM

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

#42 wolfman_4_ever

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Posted 28 January 2013 - 08:13 PM

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.

#43 dickbill

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Posted 28 January 2013 - 08:21 PM

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.

#44 orlyandico

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Posted 28 January 2013 - 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.

#45 WarmWeatherGuy

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Posted 28 January 2013 - 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.

#46 EFT

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Posted 28 January 2013 - 08:50 PM

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.

#47 Stew57

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Posted 28 January 2013 - 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.

#48 photodady

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Posted 29 January 2013 - 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.

#49 orlyandico

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Posted 29 January 2013 - 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.

#50 Mike X.

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Posted 29 January 2013 - 01:03 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.

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. ;)






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