CGEM not suitable for serious astrophotography?
Posted 28 January 2013 - 09:44 AM
Came across this blog post:
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?
Posted 28 January 2013 - 10:27 AM
Posted 28 January 2013 - 10:41 AM
Posted 28 January 2013 - 10:55 AM
Posted 28 January 2013 - 11:10 AM
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.
Posted 28 January 2013 - 11:14 AM
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.
Posted 28 January 2013 - 11:17 AM
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)
Posted 28 January 2013 - 11:21 AM
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.
5 minute unguided exposures? Does any mount do that?
Posted 28 January 2013 - 11:36 AM
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.
Posted 28 January 2013 - 11:48 AM
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.
Posted 28 January 2013 - 11:48 AM
Posted 28 January 2013 - 12:04 PM
Posted 28 January 2013 - 12:05 PM
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.
Posted 28 January 2013 - 12:13 PM
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.
Posted 28 January 2013 - 12:49 PM
Posted 28 January 2013 - 01:07 PM
"Seriously though", would'nt any type of astrophotography be "Serious" compared to casual visual?
Posted 28 January 2013 - 02:34 PM
_I_ define "serious" as one's primary pursuit in amateur astronomy.
Posted 28 January 2013 - 03:22 PM
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.
I'm not claiming that this is in any way an affordable mount, just that it is possible and commercially available.
Posted 28 January 2013 - 03:32 PM
A CGEM is a lot like an Accord. Pretty capable, pretty affordable, widely available...
Posted 28 January 2013 - 03:39 PM
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.
Posted 28 January 2013 - 03:40 PM
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.
Posted 28 January 2013 - 04:10 PM
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.
Posted 28 January 2013 - 04:16 PM
Posted 28 January 2013 - 04:31 PM