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Is this performance typical of G11?

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#1 Hilmi

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Posted 17 January 2013 - 05:11 PM

Is guided performance of 1.6 arcseconds RMS typical of a G11 or am I just expecting too much out of my mount? I'm getting to the stage where a Mach1GTO is starting to sound cheap if it is going to resolve all my headaches. My stars at 1260 mm focal length are ovals and that is after guiding. I've put so much effort into this mount and I can honestly say I am not satisfied with the performance. Am I imaging at too long a focal length for this mount?

#2 Footbag

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Posted 17 January 2013 - 05:18 PM

I don't know exactly what problems you are having with the G11, but it is more then capable of imaging at 1260mm assuming you are within the weight limit.

What type of optics? Reflector? Maybe it's mirror flop or flexure? Is it slow steady drift? Or is it back and forth?

The Mach 1 is a great mount, but if you have a problem with your image train or mounting, the mount won't fix it.

#3 Hilmi

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Posted 17 January 2013 - 05:21 PM

I'm using approximately 2/3 the mounts rated instrument load. I'm using an SCT with an off-axis guider with the mirror locked down and an external focuser.

I'm willing to give this mount one more chance in the form of a replacement worm gear. If that doesn't fix it then that's about all I am willing to take from equipment that doesn't perform.

That's not counting all the little bugs like the occasional runaway declination guiding.

#4 orlyandico

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Posted 17 January 2013 - 06:01 PM

1.6" RMS sounds sub-standard.

My CGEM with 40" periodic error and 20" of 8/3 harmonic (which, I am assured by Celestron, is not typical - most CGEM's are around 20" and 7" of 8/3 harmonic) can do 1.8" RMS all day.

Is that 1.6" RMS with long-ish guide exposures? (at least 4 seconds?)

But........

Maybe the roundness of your stars doesn't have much to do with that 1.6" of RMS guiding error. I just started using my C9.25 (2350mm focal length) un-reduced with my Mach1. Due to a variety of issues (cheap $75 tripod, unbalanced load due to not having enough weights) with the C9.25 loaded my RMS guiding error has degraded to about 0.9" - but I still get round stars at native FL (image scale of 0.68"/pixel).

So if I can get round stars at 0.9" at double your FL (10-minute exposures) then I suspect at the much more friendly 1260mm, 1.6" RMS should be just fine.

A lot of folks use G11s for much longer focal lengths than 1.2m so I know the mounts are capable.

Can you post a PHD guiding graph?

I suspect you have some declination guiding issue..

#5 Peter in Reno

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Posted 17 January 2013 - 06:17 PM

It can depend on a lot of things. Bad seeing can make guiding RMS bigger. Or the quality of guide star in OAG can affect it. Or poor autoguider settings in autoguiding software can play a role. What are you using to measure guiding RMS? What autoguider software are you using?

It took me a while to tweak PHD settings to get highest guiding quality. It varies from mount to mount, scope to scope and location to location. Which looks worse: RA or Dec axis? Do you have graphs for both axis? If Dec looks worse, then maybe backlash? If the stars are ovals, what direction: Dec or RA?

If you are using OAG, what autoguider are you using? Poor quality guide star (too dim or crescent shape) can affect guiding and if your camera is insensitive, then autoguiding software can have issues. I have read that Maxim autoguider is pretty sensitive to strange shaped stars especially off axis from OAG. I use PHD and it does not mind the crescent shape stars.

You may need to do process of elimination outside of the mount before blaming the mount.

This is my typical PHD settings for C-8 EdgeHD at F/10 or 2000mm focal length using A-P Mach1 mount:

RA Aggressiveness: 60
RA Hysteresis: 10
Max Dec Duration: 75
Min Motion: 0.70
Calibration Steps: 125msec
Auto/Resist Switching
Extreme dithering and Settled at < 0.5
3 - 4 sec guiding exposure.

Peter

#6 orlyandico

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Posted 17 January 2013 - 06:18 PM

.. although that said.. I don't possess tons of cash, so buying the Mach1 only happened after many months of analysis paralysis.

But it has met and exceeded all my expectations. I have spent literally years tweaking and working on my CGEM and AP600. The Mach1 is the first mount I've experienced that simply works out of the box, and keeps on working even when abused (badly unbalanced load).

The first night I got it, I was so delighted with the guiding, I failed to notice that the CP3 controller had gotten so hot I scalded my hand touching it. Turns out the RA worm block was very tight against the worm wheel (must have gotten knocked that way during transportation). But it still kept on chugging and putting up its 3.5" p-p periodic error..

Sure it has its shortcomings. The worst is the 1-star alignment. No pointing model. But.. in my opinion it's better to spend $6.6K for one, than $3.6K for a G11 Gemini 2. Doesn't really help you, I know..

#7 Hilmi

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Posted 18 January 2013 - 03:51 AM

OK, it's not about tons of cash, it's about the frustration getting to the stage where you don't enjoy the hobby any more. On any typical night you spend 1 to 2 hours trying to get it to work.

Now with regards to some of the questions. It's not declination guiding, for Declination I get an RMS of around 0.5 +/- 0.1

I use MaximDL to guide, I can't use a separate program because my camera is integrated into the filter wheel and you can not address the camera from two different programs at the same time. This puts PHD out of the question.

I am getting slightly distorted stars on my cam (the OAG that's built into the STT-8300) But the reason I don't think that's the reason is that the problem is consistently with the RA axis.

If somebody can teach me how to pull the logs out of MaximDL I'll gladly post them here.

#8 orlyandico

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Posted 18 January 2013 - 07:47 AM

well Hilmi.. I know that feeling of tweaking it for an hour, then having to watch the guiding graph all the time while its exposing because you don't know if it's going to go crazy..

I am surprised that a G11 would behave that way, so many people have success with the G11. But I have never had one.

When I got my AP600, I thought my mount woes were over. But it is a 20-year old design, and it has its limitations. Needless to say, it wasn't "perfect."

The Mach1 is perfect. So far. Have only had a few hours' worth of use on it.

#9 Jared

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Posted 18 January 2013 - 10:23 AM

I'm going to just come out and say it. Get the Mach 1.

Is your experience with the g11 typical? No, but hardly unprecedented. Have others had problems getting the tracking accuracy out of the G11 that they really want? Absolutely. Needing to spend time tweaking, adjusting, replacing worms, issues with Oldham couplers and poor quality bearings... That is very common. Based on my own experiences with the Mach 1, all these problems just go away. I have used a Mach1 very successfully for years now coupled to an Eagle portable pier. It has handled a 48 pound imaging load at 2,100mm focal length with no problems. On one memorable night I even managed an image with 3.5" FWHM stars with 20 - 30 knot wind gusts! Why bother on that night? I had driven 400 miles for dark skies, and there was no way I was going to let the transparency go to waste!

#10 Hilmi

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Posted 18 January 2013 - 10:36 AM

Only if I find a buyer with only reasonable loss of money on my current mount. But I am really thinking about it. Even got the shipping quote

#11 Mert

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Posted 18 January 2013 - 10:59 AM

Can't help you much Hilmi with the G11, but 1 guy on our
astronomical asociation has 1.
Had a lot of problems with it where the RA-motor was bad.
Also the controller burned out.
I dismantled the whole thing, cleaned, relubed and mounted
everything again.Mecanically I have to say that is is a
VERY decently built mount.
The worst part IMHO are the worm-blocks, independant
square blocks where the worm rides in.
This is a very bad solution, where the Ovision worm
solution is much better, there are a lot of test reports
out there on the web.

What guiding aplication are you using?? PHD, Metaguide,
Maxim, whatever....????
I never had any sucess at all with PHD, then switched over
to Metaguide and bingo, all gooder, better and wonderfuller.

Just my 2 :penny: :shrug:

#12 Startraffic

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Posted 18 January 2013 - 01:41 PM

Hilmi,
I believe you have the new one piece worm with the brass high precision worm on your RA, Correct? If you do then I would suspect that the mesh isn't dialed in. It is easier with the OPW than the old worm block system, but it isn't a cake walk DAMHKIT. My G11 would do round stars using a Meade 10"f4 SNT with a 4x Powermate to a Meade DSI & a 90mmf4 Ioptron as a OAG & MaxinDL. It wasn't easy to get it to work, I had a LOT of flexture problems, balance problems, firmware problems, etc. I had been evolving my G11 from DCS's & was also in a observatory as you are. There were times I wanted to take mine to the dump as well. You're setup should work.
I was about to post this & remembered something else. Are you balancing slight heavy to the east? You don't need much, 500g is fine, but it needs to be there to keep the RA mesh snug. I think there is a mod in the Losmandy forum that helps address this, but I couldn't find it. It basically had a string wrapped around the RA & a weight on the end to keep the mount biased to the east. You would lift the weight, get the mount in perfect balance, then let the weight hang free, effectively biasing the mount to the east. I can't remember where I saw it or how it was done, but I do remember the hanging weight.

Jared,
I believe Hilmi's mount has G2, & the OPW, they don't use Oldham couplers, they have a different type, no more Oldham coupler stickage issues. The OPW also becomes a module when assembled, you adjust the entire module for the gear mesh & clearance.

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#13 Hilmi

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Posted 18 January 2013 - 01:58 PM

I have the OPWB and I have tried all the adjustment mojo I could find online. Even that guy who suggested I spill some chicken blood on the worm block while dressed in a loin cloth. Everybody says it works once you get the hang of it but no concise instructions.

My theory is that if this was a seeing induced problem it would hqve impacted both axis. Not just RA. Maybe as a last ditch effort I should go for the ovision worm. At least that comes with consice directions on how adjust and install

#14 Footbag

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Posted 18 January 2013 - 02:10 PM

Even that guy who suggested I spill some chicken blood on the worm block while dressed in a loin cloth. Everybody says it works once you get the hang of it but no concise instructions.


Are you sure your chicken was a virgin? :lol:

Sorry, I couldn't help myself.

#15 Hilmi

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Posted 18 January 2013 - 02:35 PM

So thats why it didnt work

#16 Startraffic

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Posted 18 January 2013 - 03:04 PM

Adam,
The chicken doesn't matter, Hilmi has to be the virgin, Uh Oh :foreheadslap: :shameonyou: :whistle:

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#17 Footbag

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Posted 18 January 2013 - 03:22 PM

Ok. Now we're just making the astrologers look sane. :lol:

Edit: O-vision worm for sale on A-mart. Located on Hilmi's side of the Earth.


#18 Hilmi

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Posted 18 January 2013 - 10:14 PM

Thanks, I took a look at the Ad this morning. Because he is including those McLennan gearboxes, he is selling that stuff very close to the price it comes from the manufacturer. Installation instructions for those gear boxes look terrifying. So I'm not so sure I want them.

Looking at buying direct from the manufacturer.

#19 contraf15

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Posted 21 January 2013 - 07:54 AM

Hilmi I have a G11 G2 that I bought new this month with the one piece worm. I am having exactly the same issue guiding with MaximDL, SBIG STi and OAG. Polar alignment is spot on, declination guiding is nearly a flat line, but I get spikes in RA every several seconds that stretch stars accross the RA axis of the imaging camera chip. The mount is perfectly balanced, and I have a 22 oz weight hanging on the east side of the declination shaft to keep the gears meshed east-heavy. The problem occurs with or without the weight. I have tweaked every possible guide setting in Maxim, guide speed on the mount, etc. and it does not improve. It must be something mechanical. I am going to try swapping the RA and DEC gearboxes and see if that makes a difference, but short of that I'm not sure what else to try. Please post if you figure it out on your mount...

#20 Hilmi

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Posted 21 January 2013 - 08:56 AM

I read somewhere that the gear boxes only contribute 0.16 arc seconds of error. I think its the worm or worm block.

#21 Startraffic

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Posted 21 January 2013 - 11:30 AM

Contraf15,
What is the time period of the RA jump? That should give you a clue as to where it's coming from.

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#22 contraf15

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Posted 21 January 2013 - 11:15 PM

That's one of the problems...it isn't periodic. I'm attaching a 15-minute log file from this evening. I was guiding with an ST-i, SBIG OAG through a Stellarvue SV80ST with a TeleVue 0.8x reducer. With the reducer the scope has a 384mm focal length, so the pixel scale for the STF-8300 is 2.9 arcsec/pixel. Even at that scale the RA deviations are obvious in the main camera images. The SBIG OAG provides a further reduction of 0.7x, so the guiding focal length is approximately 269mm. That gives a guiding pixel scale of ~5.7 arcsec/pixel. You can see in the log file sudden deviations of up to 4.2 pixels (I've seen as high as 9 on occasion). That's from 24 to 50 arcsec of sudden movement! Of course this makes exposures of any length worthless because it happens so frequently and is totally unpredictable. I must be missing something...any ideas??

Attached Files



#23 Hilmi

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Posted 22 January 2013 - 12:59 AM

I'm seeing similar issues, the guide plot looks so erratic! Bounces all over the place. Tried fiddling with guiding aggressiveness etc... no effect. Guide through pulse guide and through ST4 connection, no joy. Try using PEMPro, and PE is so low and the curve is very smooth. Can't recall the figures I am abroad now and I am far from my imaging laptop, but PEMPro is showing me a smooth curve and PE in line with the results reported for Ovision worms.

Can you try the same thing on your side Contraf15? Measure PE without guiding.

#24 orlyandico

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Posted 22 January 2013 - 01:13 AM

if there are sudden jumps that seems like slack in the RA gearing... or on the other hand maybe the worm is too tight and the jumping is due to stiction..

maybe try loosening the RA worm engagement. i have no G11 but on my CGEM, when the RA slack is very small (tight worm-to-gear mesh) there are a lot of problems. These go away when the worm is looser.

#25 contraf15

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Posted 22 January 2013 - 06:14 AM

I suspected the gear mesh might be the culprit, so last night I adjusted it across the full range allowed by the thumb screw on the one piece worm block. There was no change.

Hilmi I will try to get a PE measurement within the next couple of days.

Jeremy






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