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CGEM not suitable for serious astrophotography?

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#1 tboconnor

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Posted 28 January 2013 - 09:44 AM

Hi all :)

Came across this blog post:

http://www.photodady...final-thoughts/

I was about to start to give astrophotography a more serious go, but it sounds like I might be in for some serious frustrations.

How do other CGEM owners feel about this?

#2 jrcrilly

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Posted 28 January 2013 - 10:27 AM

The orthogonality issues described make the mount less than ideal for unguided imaging. With guiding, they wouldn't be significant.

#3 neptun2

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Posted 28 January 2013 - 10:41 AM

I have seen many good pictures made by CGEM owners. It is relatively cheap mount so you can't expect level of precision and tracking like a mount that costs 4 times more money but if you do not overload it and have autoguiding i don't see any reason not to try astrophotography.

#4 RogerRZ

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Posted 28 January 2013 - 10:55 AM

I don't know what "serious" astrophotography is, but I would wager a guess that most "serious" (and a lot of not so serious) astrophotography rigs incorporate autoguiding as a part of their equipment.

#5 Footbag

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Posted 28 January 2013 - 11:10 AM

After reading through a lot of his posts, I realized a few things. First, he's well over the weight capacity for visual observing. In addition, he's using a long FL SCT.

In one of his early posts, he says that he would expect the CGEM to get 5m unguided exposures. If he means with the SCT, then he's expecting too much from the mount. My Mach 1 cannot do that.

I belive that drift alignment doesn't require orthogonality or initial leveling. He seems to think otherwise. Unless I'm misunderstanding.



Every mount is going to have every type of error. The closer you look, the more you will see. That doesn't mean all of your problems are stemming from the error you are observing.

#6 bunyon

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Posted 28 January 2013 - 11:12 AM

5 minute unguided exposures? Does any mount do that?

#7 EFT

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Posted 28 January 2013 - 11:14 AM

"Serious" is all in the user. Loaded to the max at f/10 or higher will be problematic as it can be for many mounts (and more important, many users). Unguided imaging is a waste of time and effort even on some much more expensive mounts.

Can the CGEM be used for some great astrophotography? You bet! I took images from a CGEM user this last year and blew them up to eight feet wide for backgrounds for my PATS/ASAE boths and they were stunning. They were taken with an FSQ106 at f/5. It takes a lot more to be able to do that with good results, even at f/5, than to just look at an image on a small computer screen. In my book that does make the CGEM suitable for "serious" AP.

#8 dickbill

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Posted 28 January 2013 - 11:17 AM

Wow, this guy is tough, but he might be right. I was surprised recently, after two stars align and polar align routines, to still get a fast drift in declination. I was at f20 and autoguiding was powerless. I am still new to the mount so i still keep my judgement for latter, but, since ddady did an extensive work on the mount, i expect him to be right.
Bottom line: spending lots of time in polar alignment to fix dec drift might not be a solution. But, can we hope to be saved by the imaging software? stacking AND derotation of subs when two stars are selected for stacking should fix a slight polar misalignment, correct?

PS: sorry f/20 for me is c9.25 x2, that is 4700 mm. At bthis fl, i could see a star drifting in declination immediately (and RA too)

#9 jrcrilly

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Posted 28 January 2013 - 11:21 AM

5 minute unguided exposures? Does any mount do that?

You'd need a perfect polar alignment (the blog entry linked to in the first post is complaining about the difficulty of achieving that with this mount). Then you'd need to limit the focal length to that for which the corrected PE is less than one pixel. Then you'd need to be using something more internally stable than SCT. My old EM-200 (with its excellent polar alignment scope) worked well unguided at 500mm.

#10 AstroRick

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Posted 28 January 2013 - 11:36 AM

Hi,

When I got into astronomy and astrophotography in 2010 I bought a CGEM mount and have gone through trials and tribulations to learn how to take astrophotographs. It is an ongoing process. I am indeed "serious" about learning how to do prime focus AP, but I am far from the level of someone at the professional level. So I would say that answering the question of whether the CGEM is suitable for serious astrophotography does depend on your definition of "serious".

In my ongoing learning process, I have tried to eliminate possible problems with the hardware so I could figure out what areas in my process and technique were in need of improvement.

* I purchased and installed a Hypertune kit from Deep Space Products to make it easier to balance the mount and to improve tracking.

* I purchased reticle eyepieces to better center alignment and calibration stars.

* I purchased a polar alignment scope to get the initial setup closer.

* I purchased a digital level as an independent measure of level. The bubble level on the mount is not accurate, when compared with the digital level. I also use the digital level to adjust the altitude by placing the level on the saddle and adjusting it to the observing latitude.

* Finally, I purchased some ultrahigh molecular weight polyethylene and cut a washer-shaped ring to sit between the tripod and the base of the mount. The ring decreases the friction which allows adjustment of the azimuth during polar alignment even when the mount is securely fastened to the tripod.

In a perfect world my ~$1500 would have bought me a mount that did not these enhancements to get it to work acceptably. In fairness to Celestron, some of it may not have been necessary, but I did it anyway in order to eliminate any hardware issues that may have existed in order to uncover flaws in my techniques.

As a result of all of the above, and I am now able to diagnose and correct issues that affect image quality and to collect images with round stars from edge to edge.

In hindsight, some of the struggle may not have been necessary if I had purchased a more expensive, higher quality mount. However it is equally likely that the cost of that mount would have prevented me from ever getting into astronomy to begin with.

I hope this explanation is of value to you.

#11 dickbill

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Posted 28 January 2013 - 11:48 AM

yeah well, i guess a mount of a quinzillion dollar can always do great unguided imaging at 500 mm, but so can do a cg5.
But i don't think anybody can expect a 5 min unguided at 2300 mm for any mount. I was well warned in these forums, when the cgem came a couple of years ago, that the PE of the cgem would not be any better than the older Atlas, but worse, maybe even no better than the cg5!
Several times i asked "2 min unguided at 2300 mm right?"
and the answer always was "nope, 20 seconds like the good ol' cg5'. So it's not reallistic expectation to ask for these kind of unguided performances. The deal was always that the cgem can autoguide a heavy load better than the cg5 at ~2000 mm. Now if that was not true, it would be very disapointing since that means going adaptive optics and more dollars spent.

#12 EFT

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Posted 28 January 2013 - 11:48 AM

I would also have to question the "work" that he did on the mount. While he states that "you get what you pay for" his expectations were far above what he paid for and would probably be above his next choice of a G11. Expectations (along with experience) are very important here. The CGEM is far from a perfect mount in both design and quality, but in the hands of the right person, it is an amazing piece of equipment. For those with the money, I always recommend going higher end, but a high end mount in the hands of someone less experienced can also provide poor results. There is an inherent amount of talent in astrophotography and some people just have it while others don't. Some people aquire the talent with hard work while others never seem to.

#13 Stew57

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Posted 28 January 2013 - 12:04 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......

#14 corpusse

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Posted 28 January 2013 - 12:05 PM

I don't see why unguided images is even important at all. As long as it can take unguided images for the 2 second exposures I take as I align that's good enough for me. The CGEM was the first mount I ever had and got it up to 10 minute exposures with a 8" and even 2 and a half minutes with an 11" scope.

While I consider myself a "serious" imager my picture quality may not be "serious" but I'd rank my location, then my skillset above the mount as being the reasons.

#15 dickbill

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Posted 28 January 2013 - 12:13 PM

Yes experience and patience is mandatory in this hobby and sometimes talent too. But at the amateur level i hope two things (beside adaptive optics).
Using software and smarter ccd: the soft would correct for evrything: drift, derotation, and perhaps ignore or substract some photons that fall into the ccd well during a single accumulation period.
Or Electron Multiplication CCD for the masses, so that subs of a few seconds can replace minutes-long exposure.

#16 rmollise

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Posted 28 January 2013 - 12:49 PM

OK...to put it as kindly as I can, the person who started that thread was...uh..."barking up the wrong tree." The CGEM is quite suited for serious astrohphotography. ;)

#17 TxStars

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Posted 28 January 2013 - 01:07 PM

Please define the following:
1) "Serious"
2) "Is"

"Seriously though", would'nt any type of astrophotography be "Serious" compared to casual visual?

#18 rmollise

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Posted 28 January 2013 - 02:34 PM

Not sure I follow you...but...

_I_ define "serious" as one's primary pursuit in amateur astronomy.

#19 photodady

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Posted 28 January 2013 - 03:22 PM

ABSOLUTELY!
The Astro-Physics 3600GTOPE cans easily do 20 minute unguided sessions tracking accurately to within ONE arc-second of a degree over the entire period.

http://www.astro-phy...coder-comple...

I'm not claiming that this is in any way an affordable mount, just that it is possible and commercially available.

#20 RogerRZ

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Posted 28 January 2013 - 03:32 PM

It is possible and commercially available to go 240mph with a car, it just isn't fair to expect a $30000 Honda Accord to do so.

A CGEM is a lot like an Accord. Pretty capable, pretty affordable, widely available...

#21 korborh

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Posted 28 January 2013 - 03:39 PM

Within 1" may not be enough for un-guided where the image scale may be 0.5"/px or less. So 1" on RA and DEC will show up as resolution loss compared to a well auto-guided one
.
Usually when un-guided performance is claimed, it is without quantifying the actual star FWHM compared to if it were guided. Un-guided long exposure for doing high-res does not make sense - auto-guiding is much easier, predictable, cheaper and will give better results with much less effort.

#22 cn register 5

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Posted 28 January 2013 - 03:40 PM

This guy turned up on the CGEM Yahoo group and gave the impression that he had an axe to grind. There were a lot of flaws in his claims.

He had a TPoint plot that alleged to show a mount orthogonality error but he had disabled cone error and I think that the cone error had been transferred to the mount orthogonality term - with a huge error term.

He flounced off when we pointed this out.

Chris

#23 dickbill

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Posted 28 January 2013 - 04:10 PM

Photodaddy, if you had been reading cn forums around 2005 when the mount began, you'd have seen plenty of threads on the expected performances and endless comparisons to the Atlas.
Me among others were plenty hopefull of better gear, ie better PE and longer unguided subs than the cg5, but even before the mount was available, the replies in the post were clear that it was not going to be the case.

#24 tboconnor

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Posted 28 January 2013 - 04:16 PM

Wow, thanks for the amazing information everyone! Ill get stuck into imaging with my CGEM now...Or at least when the clouds clear up :)

#25 Patrick

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Posted 28 January 2013 - 04:31 PM

I was looking through "Dady's" photo galleries and noticed he doesn't have a single gallery devoted to astrophotography. Hmmm....sort of puts a damper on his credibility in my mind. It looks to me like he's basically a photographer, not an astrophotographer.

Patrick






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