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The Venerable G11

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#1 Jeff B

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Posted 06 July 2014 - 05:35 PM

Having been addicted to my AP Mach 1 for the last 6-9 months, I used my trusty G11 the other night to carry an older AP 6" F9 Starfire and then an APM 6" F8 APOs.

Using the G11 after such an extended vacation from it and after my love affair with the Mach 1 really made me appreciate just how NICE of a package the G11 is, especially with Gemini.

It's easy for me to load in to then out of the back of the car. It's super easy to set up. Super easy to get a reasonnable polar alignmnet and the Gemini just plain works for me for visual, 6 stars, 3 on each "side" and gotos are quite accurate. AND it doe not sound like a can opener when slewing.

When put on the grass, damping times with either tube assembly were ~1/2 second...just plain excellent.

It just did its thing and taken as a package with the HD tripod and either tube assembly, it was a complete joy to use.

And it's a wholeeeeee loooooot less than the Mach 1.

So having had an ongoing love affair recently with the Mach 1, has really made me appreciate the "venerable" G11 for what it is, a well thought out and executed product at a great price.

Jeff

#2 blueman

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Posted 06 July 2014 - 07:49 PM

For visual the G-11 is a great deal at its price as is the GM-8.
Blueman

#3 Ed Wiley

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Posted 06 July 2014 - 11:44 PM

I love my G11/Gemini2/Ovision and my G8/dsc. Great scopes for the value. Would I like at AP1100? You bet, and I occasionally buy lottery tickets. :roflmao:

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#4 Hilmi

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Posted 07 July 2014 - 12:43 AM

While I started off with a shaky start, my G11 now does AP with approximately 15 Kg of weight on it with a rejection rate of 2 out of 45 exposures. And those rejected exposures were due to improper balance. While I had initially kept on cursing this mount, now I am in total love with it. Scott tuned this mount so well that it is unbelievable how well it performs now.

The Gemini 2 system is really good too, still has a few rough edges but there is a constant stream of updates coming with new versions getting released at least once every 3 months. Each release has further refined the system.

The ability to do field repair is also great.

#5 Whichwayisnorth

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Posted 07 July 2014 - 01:01 AM

I'm very tempted to pick one up used and do all the upgrades.

#6 orlyandico

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Posted 07 July 2014 - 04:31 AM

The new ones with precision worm have phenomenally low PE, Mach1-class. Of course, a mount is about more than the PE, but for the price you pay, the G11 is a smart choice.

#7 vahe

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Posted 07 July 2014 - 10:32 AM

I have one of the earlier models, 22 years old, it was introduced as “Celestron G11”.
The G11 has been work in progress for Scott Losmandy, unlike AP mounts it was refined over the years, little by little to what it is today.
Mine being one of the very first units has a bumpy ride, but it is manageable for visual, heavier loads tend to smooth out the ride considerably

Vahe

#8 Jeff B

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Posted 07 July 2014 - 02:27 PM

....heavier loads tend to smooth out the ride considerably

Vahe


Yes, I've noticed that too. There seems to be a "weight window" where the high frequency ringing is damped out and there is no low frequency rocking. For my G11, that "window" is in the 30-45 pound range, depending on the moment arm involved. Go below that weight and the system is almost too stiff, while above that weight it's too sloppy.

Jeff

#9 Scott99

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Posted 07 July 2014 - 02:54 PM

I actually took some friendly teasing from the general public at a recent star party because of my Mach1 - I only use it for push-to observing (which I prefer over goto). I had people asking why I wasn't using the "computer" on the mount. I told them I was "too lazy to do the alignment" during setup. I usually only do a simple polar alignment through the open peep-hole, it's enough for accurate tracking.

I wish AP still made a basic non-goto mount. The main reason I use a Mach1 over a G11 is because it breaks into two pieces for transport.

I also prefer the Losmandy and older AP design of having only one clutch knob per axis. 3 knobs is more difficult for the type of observing I do, with no added benefit for visual use. I would have stuck with my old AP 600 but the AP 160mm was a bit too much weight & torque for it.

the other nice thing about AP mounts is the somewhat smaller adapter system that's used to mate the mount head with the tripod.

#10 PeterR280

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Posted 07 July 2014 - 02:56 PM

I have the older AP 600E and love it. It's non goto but so easy to set up and run. it's also very stable.

#11 Scott99

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Posted 07 July 2014 - 03:08 PM

I have the older AP 600E and love it. It's non goto but so easy to set up and run. it's also very stable.


Peter - great mount! I do have a very early 400 mount, pre-QMD, that is extremely simple and easy to use. It'd be nice if they still made these, but I guess they need to re-coup all the investment they've made in the higher end technology, goto, etc.

shouldn't complain too much about the Mach1, for all its complexity it's fairly light and easy to use. I only have to adjust the 3 clutch knobs a few times during a typical session, and goto is only a couple of quick 2-star alignments away if I need it.

AP does have some nifty stuff too, the slots in the new tripod adapter plate are hugely useful. You don't have to move the tripod around anymore, you can just move the whole mount around in the slots to get on Polaris quickly.


#12 Peter D.

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Posted 07 July 2014 - 09:17 PM

I've been using my new (used) G11 G1 for three weeks now and I love it. It's a really solid mount for the money; a huge step up from my CG5, which I still like for its light weight and good goto visual performance. I chose the G11 over the CEM60 and I'm glad I did, considering the problems I've been reading about that mount on another thread.

One thing that bugs me about the G11 is I keep forgetting that "photo" mode must be switched on for guiding (but is way too slow for framing, requiring constant switching back and forth).

I'll get accustomed to that eventually, but another thing is that I have to re-sync every time I do a meridian flop (otherwise the goto's are off by several degrees); does anyone have any insight as to why that is needed?

#13 Dan Finnerty

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Posted 07 July 2014 - 11:11 PM

I'll get accustomed to that eventually, but another thing is that I have to re-sync every time I do a meridian flop (otherwise the goto's are off by several degrees); does anyone have any insight as to why that is needed?


For the G2, you have to separately build east and west models. I don't know it the G1 works the same way. Have you done that? It might explain your problem.

One of the more annoying "features" of the G2 is that it will not seed the other hemisphere's model with the index error from the first model, which would cut down considerably on the slew error to the first star of the new model. I should probably make that suggestion on the Gemini Yahoo forum, but somehow never get around to it... Rene Gorlich can actually be pretty responsive to useful suggestions (if that is a useful one).

#14 urbanMark

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Posted 08 July 2014 - 03:12 AM

I have one of the earlier models, 22 years old, it was introduced as “Celestron G11”.
The G11 has been work in progress for Scott Losmandy, unlike AP mounts it was refined over the years, little by little to what it is today.
Mine being one of the very first units has a bumpy ride, but it is manageable for visual, heavier loads tend to smooth out the ride considerably

Vahe



Ah, but that is the beauty of the G11. You can take the oldest G11 and completely update everything to the latest specs. I too have a G11 that says "Celestron" with "Losmandy" in smaller print, and it has every update except the high torque motors (still don't need them, and the old motors make less noise). With Scott's updated worm block I get uncorrected PE of +-2.5 arcsec! Certainly in premium mount territory. That said, the Mach1 is a nicer mount no doubt, but they don't call the G11 the "low end of the high end" for nuttin'.

#15 Peter D.

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Posted 08 July 2014 - 07:45 AM

For the G2, you have to separately build east and west models.


I've heard the term "build a model" in regards to goto software, but I'm not entirely familiar with it. It was my impression that the sky model is built into the firmware; for my Celestron CG5 I get great results with a 2-star alignment on one side of the meridian and then a 3rd star on the opposite meridian. I'm doing the same for Gemini 1; does "building a model" mean that I have to do a complete 3-star alignment on each side of the meridian (this will be difficult, as I often have limited visibility on one side of the meridian)?


One of the more annoying "features" of the G2 is that it will not seed the other hemisphere's model with the index error from the first model, which would cut down considerably on the slew error to the first star of the new model.


So that explains why there's a big error everytime I do a meridian flip, even after I flip back? Will this be corrected by doing a complete 3-star alignment on both sides of the meridian? Also, do the 3 stars have to be in a triangle, or just separated by 1 hour in RA as specified in the manual?

I've noticed that Gemini 1's goto's are inconsistent: sometimes they're spot-on, and other times they're out of the FOV. I also notice from these forums that some users align to many more than 3 stars: that's not very practical when I move the mount every night. I've read the manual, but it's not helping (there's only a brief paragraph on "Modeling Parameters"); how many alignment stars are needed from a practical perspective?

Thanks for your help; I'm really a newb when struggling with the Gemini firmware. It will all be worth it once I'm up to speed: I love the solidness, tracking and lack of backlash with the G11 as compared to my other mounts.

#16 tazer

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Posted 09 July 2014 - 07:43 PM

With Scott's updated worm block I get uncorrected PE of +-2.5 arcsec!


Out of curiosity is that worm block default on new mounts or is that a separate upgrade?

#17 Raginar

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Posted 09 July 2014 - 08:25 PM

Pete,

You build a 'model' of the sky. It knows where everything should be if your setup was perfect. By going to multiple stars on each side of the meridian, it can connect the dots and ensure pointing is accurate based on your 'imperfect' setup.

#18 Dan Finnerty

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Posted 10 July 2014 - 12:57 AM

So that explains why there's a big error everytime I do a meridian flip, even after I flip back? Will this be corrected by doing a complete 3-star alignment on both sides of the meridian? Also, do the 3 stars have to be in a triangle, or just separated by 1 hour in RA as specified in the manual?

I've noticed that Gemini 1's goto's are inconsistent: sometimes they're spot-on, and other times they're out of the FOV. I also notice from these forums that some users align to many more than 3 stars: that's not very practical when I move the mount every night. I've read the manual, but it's not helping (there's only a brief paragraph on "Modeling Parameters"); how many alignment stars are needed from a practical perspective?

Thanks for your help; I'm really a newb when struggling with the Gemini firmware. It will all be worth it once I'm up to speed: I love the solidness, tracking and lack of backlash with the G11 as compared to my other mounts.


Yes, that explains why you have a large error after a meridian flip, but not why you would have a large error when you flip back. I'm at a loss to describe that one.

Your mount has no clue whatsoever where it is on the surface of the earth, nor what its orientation is in space. When you enter lattitude, longitude and time, you pin down where it is on earth. And "when". Since the earth is constantly rotating, "when" (i.e. time) is essential.

Your polar alignment pins down the mount attitude. The software assumes that you have the RA axis parallel to the earth's rotation axis. There are databases with the coordinates of thousands of celestial objects, but the mount software knows little else. It has the mathematical equations to calculate relative positions for pointing, but only by being given position, time and assuming polar alignment.

In addition, there are mechanical error sources in the mount. Backlash, mechanical flexure, telescope optical axis not aligned perpendicular to the Dec axis (called "cone error"), etc. Identifying and correcting for errors in all these things (including position and time) are what modeling is all about.

The first, and often largest error that crops up is called "index error". When you set your scope/mount to the "Counter Weight Down" (CWD) position, the mount assumes ideal alignment. However, there is often an error in encoder value at this position that will cause a fixed offset in any slew. The first alignment star helps identify and correct for this index error. Adding additional stars helps the software more accurately identify this error as well as identifying other error sources. Mount alignment is one of the most important. As I recall it takes 3 stars to estimate azimuth/elevation errors in polar alignment. More stars give better estimates of the errors. Also, more subtle errors can be identified and corrected for with more stars, such as cone error. I don't know the detailed math behind the error statistics, some of it is addressed in the Gemini software descriptions. When you are doing modeling with the G2 system, it shows you the errors being modeled and as you add stars, more error values get populated.

For very high accuracy goto's you need a mesh of stars that cover the az/el range over an entire (eastern or western) hemisphere. It is an ideosyncracy of the G2 software that it builds and maintains two completely separate models, one for each hemisphere. They do not interact.

That is the source of my complaint about the index error from one model not being used as the start point for the other model. I hate having to use the guide scope (which I always forget to align when I set up my scope for the night) to get a star into the eyepiece where I can then precisely center for alignment. After a star or two, the pointing nearly always becomes accurate enough the star lands in the eyepiece and I no longer have to get down on my hands and knees to look through a finder scope. Since this initial index error does not get transferred to the other hemisphere's model, I end up back on my knees in the dirt once again.

In theory this two-hemisphere model is capabile of higher precision pointing, but in practice, for those of us who have to set up our scopes every night, it is a pain in the posterior. The Celestron is simply more user friendly in this regard. And still manages to be as accurate as the G2 software in my opinion.

As for whether to use more than 3 stars or not, let the mount performance be your guide. If 3 stars per hemisphere get the job done for you, then that is all you need. If all I plan on looking at on a given night are the moon and a planet or two, I might only do a one-star align to fix index error. If I'm lucky and they are all in the same hemispher, I don't even bother doing the model on the other side. With experience, you will learn what works well enough for you. Don't feel obligated to follow tedious "rules" if they prove unnecessary.

Another point. If you find the goto accuracy is better some nights than others, there are two things that can be causing that. First is a good polar alignment. Everything works better with good polar alignment. The models start out better and converge faster on an accurate solution with fewer stars.

Second, are you using a cross-hair eyepiece in your telescope when aligning? That makes a big difference. Just estimating center by eye is not enough for good modeling. You will inevitably make random mistakes that will perturb your models. Use the cross-hair for precision and accuracy.

A long winded reply to your questions. I don't have the energy this late in the evening to edit and correct spelling, so I hope this all makes sense! :grin:

#19 Peter D.

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Posted 10 July 2014 - 09:05 AM

Thanks for the detailed response, Dan; you're a great help, and it does make sense.

I knew about the impacts of Index and Polar Alignment errors, but not so much about the other error terms. I still don't entirely understand how the mount's algorithms model those other error terms, but your explanation helps. Actually, understanding the algorithms doesn't matter: I'll just add more stars to the alignment until I'm satisfied, as you suggest.

I'm using an illuminated cross-hair eyepiece for my alignments, but it's only a 10mm and my only barlow is 2X. I've read elsewhere that additional precision can be gained though the use of a 5X barlow, but I've never seen one for sale.

I now understand that the G11 uniquely requires a complete alignment on both sides of the meridian, but I'm mystified by my observations of significant error when I flip back: it's about as big as for the 2nd star in the first hemisphere, but not as big as the initial Index error correction on the first star. Perhaps I made a mistake somewhere.

It appears that the clouds will clear tonight so I can experiment some more.

#20 Hilmi

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Posted 10 July 2014 - 10:49 AM

I had the same problem, it was due to excessive backlash

#21 MikeCatfin

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Posted 10 July 2014 - 11:11 AM

With Scott's updated worm block I get uncorrected PE of +-2.5 arcsec!


Out of curiosity is that worm block default on new mounts or is that a separate upgrade?


It's a separate upgrade.

#22 Nebhunter

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Posted 11 July 2014 - 01:01 AM

I've been using my Celestron version G11 for years now. Upgraded the OPW and HP worm. Doing up the single locking bolt to hold DEC position causes the two plates to compress at the bolt. This will cause a deflection in the Dec after spending time to do a drift alignment. A better solution is a 3" C clamp and do up the clamp in the centre of the two brackets. You can see that in this photo. Look closely just above the RA lock knobs. Once the drift alignment was dialed in I put the scope with illuminated reticle on a star with tracking on. I then positioned the clamp and would tighten it in various spots along those side plates. I found a spot where I could do up the clamp good and tight and not cause any deflection from the pressure and the star stayed dead centre in the scope.
http://www.pbase.com...image/146029919

This rig is loaded - Tec 140 with field flattener and a big medium format Pentax 67 camera with vacuum back. Beside it is a large Pentax 400 edif lens with another Pentax 67 camera. Beside that an 80 mm Sentinel guide scope riding on the Skywatcher Guide Scope mechanism - I hate using guide rings. Everything is bolted to either a saddle plate or directly to a 20 wide DUP plate 3/4" thick. A 12 Casady saddle holds the load which all totalled up comes to right around 60 lbs.

I spent quite a bit of time doing drift alignment off and on during brighter moon phases. Doing a Pec training run on a star I put two 2x barlows in the Tec with 12mm Ilum reticle and let it run for 2 hours. After 60 minutes the reticle was just on the edge of the star for RA - that's at about 325x so my training errors had accumulated, but not bad for 60 minutes. After 2 hours the critical Dec stayed dead centre of the star. The new HP worm and one piece block are wonderful.

Gemini 1 version is bullet proof. Hot or freezing weather - no issues - just leave the hand controller on the power regulator to keep it warm. Using an ST-4 to autoguide - no issues. I consistently got strings of zero for RA and Dec with a 7 second iteration. Smooth is the word.

The only other upgrade was to install another large bearing into the RA and Dec assembly right at the top when each section is removed. Not really necessary, but it does help with the load at the top end.

A view from the underside showing the layout. All the cables were routed after trial and error for the dew straps, and ST-4 cables and tied off a specific spots so that the mount could slew to any position without a snag.
http://www.pbase.com...image/146029503

Final top view. Those guide rings visible on the 400 lens and guide scope are there to prevent flexure. Ring is bolted down to the dup and bolts used to just support the lens and scope during mount acrobatics. Flexure will kill you trying to find it. Shooting film with typical 60 minute exposures or longer in some cases requires a SOLID mount and setup.

http://www.pbase.com...image/146029507 If you click the mouse on the picture it will take you back to the gallery where other pictures can be seen. best viewed in Original size - select that by scrolling to below the picture.

So is the Losmandy G11 a great mount for the money? Hell yes.






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