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Question for Losmandy Imagers

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#76 Tom Picciani

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Posted 06 March 2009 - 12:39 PM

Just to clarify my last post. It is certainly possible to guide out error, whether periodic or not. But doing it after the fact often leaves evidence of the error. If you can see it in guiding, you can see it in the image.

Theoretically and in practice, using PEC can guide out the periodic elements of the error as the error occurs. It's advantageous to do it this way rather than just having PEC turned off and correcting the problem after the fact.

#77 rsbfoto

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Posted 06 March 2009 - 12:55 PM

Hi Dan,

If you are able to get a G11 tripod instead of the flimsy GM-8 tripod your GM-8 will instantly improve.

If you keep the GM-8 well adjusted 20-25 pounds are no problem for it.

#78 Charlie Hein

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Posted 06 March 2009 - 01:27 PM

In my experience, PE is much more important than you make it out to be. Remember, autoguiding corrects after the error occurs. If the error is not periodic and predictable, as in lesser quality mounts, then the error must be guided after the fact. It will show on the image prior to it being moved back to the correct position. This can reduce the sharpness of the image. This is the reason that a 20 arc second average mount is not nearly as desirable as a 10 arc second mount.


I'll certainly agree that my statement is a pretty simplistic viewpoint, and deliberately so. It's true that guiding corrects an error only after the error has occurred. However this is true of any mount that is being autoguided. If you have corrections to make and you use autoguiding to correct them there will always be this "lag" regardless of the quality of the mount. It is a function of the process of autoguiding, not the mount itself.

If the mount is guiding correctly (as in gathering enough samples to react as quickly to changes as possible without overcorrecting and thereby "chasing the seeing") I stand by the statement that almost any amount of PE can be well corrected - unless the error is ramping up and down stronger and faster than the system (guider and mount) can accomodate for. If the mount has a "spike" that ramps up hard and fast no guiding solution (including adaptive optics and PEC) can react to the event quickly enough to be effective.

If the system is working correctly and the error ramps up and down in a controllable fashion then the only real difference between correcting a 5 arcsecond error and correcting a 30 arcsecond error would be the number of individual corrections required to keep the star centered. As you say this will definitely result in an image that is somewhat less sharp than it could be if you use adaptive optics guiding or PEC, but it's also safe to say that this is a phenomenon that is common to every autoguided mount regardless of build quality.

When it comes right down to it at a certain point the "desirability factor" of a 10 arcsecond peak to peak mount versus a 20 arcsecond peak to peak mount ends up being almost a philosophical question:

If you are going to be guiding with either mount (this is a given), and you can guide effectively with either mount (this will take some amount of work to achieve in either case but is certainly possible in both cases), then does it really make a difference to you if the error is successfully corrected in 10 steps or 20 when the outcome will be indistinguishable?

The real question (in my mind) is if the extra quality in build makes the work required to get good guiding easy enough to warrant a difference in price, or in Dan's case, a difference in carrying capacity.

Perhaps the best thing to do is go to an EQ6/Atlas group and ask what sort of guiding times these people can manage. I think you'll find the Atlas, while a good, solid scope, isn't up to the standards of Losmandy, either the GM8 or the G11. But that's my opinion. For the most part, the error of the Losmandy 8 can be predictable and mapped iwth PemPro. And the Gemini can be purchased separately, although that will cost a few hundred more to go that route.


Bear in mind that it is these quick "spikes" in PE that can't be guided out well that are the real issue here. Although statistically speaking you are going to see more errors of this type on a mount like the Atlas, these quick PE "spikes" can certainly be a problem in the GM-8 and G-11 too. The now legendary "dreaded 76 second error" on the G-11 is a perfect example of this. A worm bearing block that is out of square with the worm causes a predictable "spike" in the PE at 76 second intervals. The amplitude of this "spike" can easily be large enough to wreck your guiding - even when using PEC to correct the errors. In fact, the Ovision worm was developed specifically to correct this problem with the Losmandy mount.

I guess what I'm trying to say is that even with mounts of the class of the Losmandy you may still need to do some serious tweaking to get things just right. My G-11 definitely has the 76 second error. However, I've been able to adjust the worm block so that the error is brought down to an amplitude that as far as I can tell doesn't even trigger a response from the autoguider and doesn't seem to affect my images even at just over 1 arcsecond per pixel resolution. These problems can be remedied, even in the Atlas mount. A quick read of the Losmandy_users and EQ6 Yahoo! groups will show that many people in both groups are doing so and getting better performance out of their mounts for the effort. Again, you have to ask yourself if the amount of work that may be required would be worth it, although no matter what there is going to be some level of effort required in rder to achieve good results - there is no such thing as a totally "plug-n-play" guiding system.

And finally, don't underestimate the quality of the new Ovision worm that can be purchased from France at a later date.


The Ovision worm is a really good option if you're having problems getting rid of the 76 second error on your mount. In the majority of cases it also improves the peak to peak PE figure of the mount over the stock worm and block(s).

That raises another point. If someone were to purchase an ovision GM8 worm assembly, is it the same as the G11? I could see someone upgrading a GM8 with the expensive worm then putting the original back when they upgrade to a G11. If that's possible.


I believe that the worm is interchangable between the GM-8 and G-11 but I can't say so with 100% certainty.

Charlie

#79 Charlie Hein

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Posted 06 March 2009 - 01:30 PM

Just to clarify my last post. It is certainly possible to guide out error, whether periodic or not. But doing it after the fact often leaves evidence of the error. If you can see it in guiding, you can see it in the image.

Theoretically and in practice, using PEC can guide out the periodic elements of the error as the error occurs. It's advantageous to do it this way rather than just having PEC turned off and correcting the problem after the fact.


Yes, I completely agree - and PEC is an option for either the GM-8 or the Atlas.

#80 Patrick

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Posted 06 March 2009 - 04:40 PM

In my experience, PE is much more important than you make it out to be. Remember, autoguiding corrects after the error occurs. If the error is not periodic and predictable, as in lesser quality mounts, then the error must be guided after the fact. It will show on the image prior to it being moved back to the correct position. This can reduce the sharpness of the image.



The amount of PE that can be 'handled' is more a factor of the imaging scope's focal length and the pixel size of the imaging device than the lag time of the autoguiding process. At the short focal lengths Alanon is currently showing in his sig line and with his current DSLR he should not have any problems autoguiding his way out of the Atlas's periodic error...without recording those errors. That's simply because the light sources will not cross over from one pixel to the next before the autoguider corrects the error. The autoguider uses sub-pixel correction algorithms.

I know this is a 'Losmandy' thread, but with his scopes and cameras, the lowly CG5-GT would be capable of doing a good job. Alanon has said on more than one occassion that budget is a big consideration. A GM-8 would also be more than enough to do the job very well, but is it worth the extra cost? I don't think so, UNLESS the plan is to image unguided. Then the mount's native PE would be a deciding factor.

If Alanon wants to upgrade his scopes in the future to longer f/l and heavier instruments and doesn't want to upgrade his mount as well, then the Atlas is the clear choice because it already has the capacity to handle those loads, whereas the GM8 does not.

That's my story and I'm sticking to it! :grin:

Patrick

#81 waassaabee

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Posted 06 March 2009 - 06:12 PM

So what I seem to be picking up here is a new, out of the box GM8 will image with 20 lbs. if well balanced. Some tinkering and modifications could push it to 25 lbs. Stouter tripod recommended.?.?. User maintenance friendly?

Right now I plan on continuing with the optics and guide setup I have, but I'm sure that will change over time. The WO and everything else is a touch over 13 lbs so as it stands the GM8 would fit just fine. Where's that stimulus...

Lowly CG5-GT? I resemble that statement... :roflmao:

Great info all around, and very informative.
Keep it rollin'!

#82 alanon

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Posted 11 March 2009 - 04:49 PM

Thanks to everyone! I had been at the hospital for some abdominal surgery since my last post, and I just now got home to read your responses. Like waassaabee said,"Great info all around, and very informative." :cool:

#83 alanon

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Posted 11 March 2009 - 04:54 PM

Lowly CG5-GT? I resemble that statement...


Why do we all associate with such lowbrow types? :rofl2:

#84 hersey0308

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Posted 11 March 2009 - 09:46 PM

hey gary - in the pic i have one 7lb plus 21 lbs counterweight high on the shaft to balance about 26 pounds on top. like other parts, the counterweights are compatible for gm8, g11 and the titan.
david

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