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Is this performance typical of G11?

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#51 Hilmi

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Posted 23 January 2013 - 03:49 PM

And the exported log file.

Now either I am doing something very stupid, or these results don't correlate with what I am seeing when using the mount.

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#52 wolfman_4_ever

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Posted 23 January 2013 - 04:32 PM

I will absolutely contact Losmandy and OPT if I determine there is something wrong with the mount. At this point I haven't made that determination. With so many variables, it is far more likely that I am missing something. The list of possible culprits is getting smaller, but sending the mount back or having it replaced is not a trivial matter (especially if a replacement will behave the same way). The machining and pointing accuracy of this mount are far superior to anything I have previously owned, so I want it to work and will spend some time to get it working correctly. If the problems remain after I have exhausted the list of things I may be doing wrong, then I will conclude it is faulty and go from there.


:waytogo:

#53 orlyandico

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Posted 23 January 2013 - 07:25 PM

Can you show the raw data? Pempro final PE curve that you show is already smoothed and the large single excursions and non integer periodic stuff like the celestron 8/3 are no longer shown.

Your result does show that there isn't anything inherently wrong with your G11...

#54 NJScope

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Posted 23 January 2013 - 10:32 PM

Hilmi:

Let me expand my comments a little further so that I can better understand your problem with the G-11. Does your G11 jump in RA unpredictably (non-periodic) with all cameras cables disconnected from the Gemini front plate (not just turned off but physically disconnected)? If the answer is yes then there is a serious problem with the mount which can not be corrected with guiding. However, if all equipment is disconnected from the Gemini cable interface plate and you only see drift expected from imperfect polar alignment and worm gear periodic error then I think it is worth revisiting my suggestion about the guide camera interface cables. I came very close to returning my ST8XME to SBIG because of a similar experience while trying to use the internal guide camera. Even if it was turned off but the cable connected, RA would jump indiscriminately in the middle of imaging. Only after completely disconnecting all camera interface cables from the Gemini faceplate, did the Losmandy G11 behave normally, albeit with the uncorrected drift and worm gear PE. Once I installed the relay-adapter box from SBIG between the camera and autoguider input on the Gemini faceplate, all problems with random RA jumps disappeared even though the Gemini 1 Level 4 manual says that the SBIG relay box can be used but is unnecessary. BTW, the Losmandy optocoupler is not compatible with the newer Geminis so that my initial reference to the optocoupler rather than the SBIG relay box is incorrect (failing memory with age!).

#55 wolfman_4_ever

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Posted 24 January 2013 - 12:57 AM

cable sag strikes again! I've been bit by that monster before.

#56 pfile

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Posted 24 January 2013 - 01:52 AM

let me just say that i have the same setup as Hilmi (almost - i actually don't have the one-piece worm block) and i am almost as frustrated. i have been able to get round stars on some very long Ha exposures, but only near the pole. at the equator the RA errors are happening too fast for the OAG to compensate. without PEC the mount has 30 arcseconds pk-pk error. with PEC its on the order of 7-10 but the mount keeps forgetting that it has a PEC curve available. very frustrating.

#57 contraf15

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Posted 29 January 2013 - 12:10 AM

Well the weather has been terrible in South Central Texas so I haven't been able to test guiding for a while. But I have been able to do a few things.

Hilmi, I went back and read some of your posts from shortly after you bought your G11. I also bought mine from OPT with the one-piece worm block installed, and I initially had exactly the same issue you did. Namely, the Ruland coupler was not tightened down on the RA worm and the mount would slip when slewed in RA. I took apart the worm block and tightened it up the day after I put the mount together.

Over this last weekend I took it apart again to investigate further. It turns out that the coupler itself was not assembled correctly. Each disc in the coupler is secured by four tiny hex bolts, two of which are accessed through holes in the ends of the device and two of which can only be accessed after removing one of the end pieces. Two of these internal bolts on mine were so loose that they were close to falling out. This resulted in the coupling itself being flimsy in both torsion and lateral offset. If I understand the purpose of the thing correctly, it is to allow some motion laterally (in case the gearbox and worm are not perfectly aligned), but transmit all rotational motion (should be rotationally rigid). Mine wasn't even close until I disassembled it and tightened it up. You can see the hex bolts that were loose in this picture. This is just the beginning of my adventure, though...

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#58 contraf15

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Posted 29 January 2013 - 12:17 AM

While I had the worm block disassembled, I decided to inspect the worm itself. I found an area on it where it looks like the threads are actually damaged. You can see it on the attached pictures, though it doesn't show as well in the pictures as it does looking at the worm. I don't know if this is significant enough to affect tracking, and the sides of the threads (the part that contacts the teeth on the RA gear) look fine. But just to be sure, I decided to check out the DEC worm. It was perfect. No visible flaws. I swapped the worms and the gearboxes on the two axes. Still not the end of the story...

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#59 contraf15

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Posted 29 January 2013 - 12:35 AM

Now things really started to get interesting. When I went to reassemble the one-piece worm block, I must have over-torqued the hex bolt that clamps to the gearbox shaft. I think I stripped the threads because I could not get it to tighten down. As a last resort I decided to re-tap them using a slightly larger diameter. What I didn't do was drill a larger pilot hole first, and just my luck the blasted tap snapped in half so that I couldn't retrieve the part that was now embedded in the coupler. A quick search online showed that Losmandy doesn't even sell the Ruland couplers by themselves (just Oldham couplers). The manufacturer wants $107 for one and they don't ship for 10 days. I did find a distributor who sells them for $70, but again the wait is a couple of weeks.

So...I drilled and tapped a hole in the gearbox side of the Ruland coupler for a set screw and used a small 3/32 hex grub screw to connect the gearbox. It turns quite freely and there is no binding or pulsing. In fact, since the grub screw holds the flat side of the gearbox shaft, I think it is more secure than the old way. You can see it at the top of the coupler in the picture.

Now that everything is back together, RA slews sound much better than before. I think the loose coupler may have been the source of my random RA movements. Unfortunately I can't test it out until the weather is better. And of course when I do test it, I won't know if changes are from swapping the worms, the gearboxes, fixing the coupler, or breaking the tap and jerry-rigging the coupler. So much for controlling my variables!

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#60 Hilmi

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Posted 29 January 2013 - 12:47 AM

That's very interesting, I will go look at my coupler and see if there are any loose parts. It never occurred to me to check if it was assembled correctly. I had just contacted Ed about hyper-tuning and he was great, he was concerned about the several hundred dollars of shipping cost and offered to guide me through the process. I'm still trying to convince him to take a consulting fee. A very generous man.

I plan to take the mount apart, strip it and rebuild it this coming Thursday. I'll watch out for that potential problem you highlighted.

#61 orlyandico

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Posted 29 January 2013 - 02:06 AM

Hilmi, buy the kit and DVD from Ed in lieu of a consulting fee.

#62 Hilmi

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Posted 29 January 2013 - 02:25 AM

Unfortunately there is no Kit for the G11. He sells lots of other bits and pieces that will be of use to me :) Next time I need something, I'll first check if he sells it before looking anywhere else.

#63 orlyandico

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Posted 29 January 2013 - 02:37 AM

i ended up buying the DVD for the CGEM. i suppose the tips will prove handy for my AP600. had the same problem as you - shipping the AP600 to Ed would cost me $450 each way.

speaking of which.... there's some terrible DEC stiction on my AP600. i spent over a year fighting it and in the end gave up and got a Mach1.

now the AP600 sits there accusing me :tonofbricks: i'm hoping with Ed's DVD and some bearing scraping i can fix the declination. the RA is fantastic on the thing.. if i can fix the DEC i'm sure it will perform very well...

#64 EFT

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Posted 29 January 2013 - 10:30 AM

While I had the worm block disassembled, I decided to inspect the worm itself. I found an area on it where it looks like the threads are actually damaged. You can see it on the attached pictures, though it doesn't show as well in the pictures as it does looking at the worm. I don't know if this is significant enough to affect tracking, and the sides of the threads (the part that contacts the teeth on the RA gear) look fine. But just to be sure, I decided to check out the DEC worm. It was perfect. No visible flaws. I swapped the worms and the gearboxes on the two axes. Still not the end of the story...


It actually shows quite well in the picture. That worm is trashed. The damage is right in the area that the worm rides on the wheel and it is significant. It would be a periodic error, but it would be huge. In addition, if there is that damage, then there is likely additional damage like the worm not being straight. It takes a lot less damage than that to make a worm useless.

I would definitely replace that worm and if it still under warranty I would show them the picture and ask for a replacement.

#65 Startraffic

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Posted 29 January 2013 - 12:17 PM

Contraf15,
Oh yeah that worm is bad & needs to go back. I can possibly even tell you how it got that way. Look at the RA gear very carefully. I'll bet the axis "jumped across the worm at some point from not having the clutch tightened up during balancing, & the Worm not having enough "bite" to hold it in place. DAMHIKT IIRC the anodized aluminum is harder than the brass worm & chewed it like a file. Check the RA gear for damage as well, I "should" be ok but....

Clear Dark Skies
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#66 contraf15

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Posted 29 January 2013 - 12:50 PM

Thanks Ed and John for the replies. I'm going to send a picture of the worm to OPT and see what they say. I have been very careful with the mount (with the exception of the coupler buffoonery) and I know that the damage didn't happen here. I suspect it happened at the time of the sloppy one-piece worm installation.

#67 Hilmi

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Posted 29 January 2013 - 01:19 PM

Contraf15, I tried tightening the coupler I also have a bearing that felt a little rough so I moved it to the dec axis. Cant test anything, its cloudy.

#68 contraf15

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Posted 30 January 2013 - 08:34 AM

OK, finally had some clear skies last night to test things out.

The short version: I am ready to throw this thing in a lake. My CGEM tracked better than this.

After changing the RA worm, gearbox, and bearings, I continue to get trash results with RA tracking. Deviations were on the order of 10-15 arcsecs, and they were quick (unable to guide them out). I tried guide exposures anywhere from 0.5 secs to 4 seconds in an effort to get in front of whatever is causing the problem, but no luck.

The deviations appear to be periodic, on the order of every 120 seconds or so which corresponds to half the worm cycle. But this is with a completely different worm than before! I honestly don't know what could even cause such behavior. I am out of ideas, other than return the silly thing. But I don't know what part of it is defective.

Any suggestions?

Jeremy

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#69 contraf15

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Posted 30 January 2013 - 08:38 AM

Same results with PHD by the way, tried that too.

Jeremy

#70 freestar8n

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Posted 30 January 2013 - 09:23 AM

I don't know what the problem is - it could be inside the gearbox - I don't know, but something that fast would be best studied with video. If you have a web-cam or video camera you can connect to the telescope, you could use MetaGuide (free, my software) to study the behavior of the star in detail onscreen, and even record its position at video rate. It may give insight into the cause, but either way would be interesting to see.

I don't know if you are showing a passive log or a guiding log - but the star appears to stop and "catch" - then suddenly snap forward. I may be misinterpreting it though.

Anyway - something this fast would be helped by video, and there is free software for the purpose.

Frank

#71 Startraffic

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Posted 30 January 2013 - 10:05 AM

Jeremy,
PM sent.

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Startraffic
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#72 contraf15

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Posted 30 January 2013 - 12:57 PM

Hi Startraffic,

For some reason I didn't receive the PM. Can you resend?

Thanks,

Jeremy

#73 Startraffic

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Posted 30 January 2013 - 01:04 PM

Jeremy,
On its' way
Clear Dark Skies
Startraffic
39.138274 -77.168898

#74 EFT

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Posted 30 January 2013 - 07:45 PM

OK, finally had some clear skies last night to test things out.

The short version: I am ready to throw this thing in a lake. My CGEM tracked better than this.

After changing the RA worm, gearbox, and bearings, I continue to get trash results with RA tracking. Deviations were on the order of 10-15 arcsecs, and they were quick (unable to guide them out). I tried guide exposures anywhere from 0.5 secs to 4 seconds in an effort to get in front of whatever is causing the problem, but no luck.

The deviations appear to be periodic, on the order of every 120 seconds or so which corresponds to half the worm cycle. But this is with a completely different worm than before! I honestly don't know what could even cause such behavior. I am out of ideas, other than return the silly thing. But I don't know what part of it is defective.

Any suggestions?

Jeremy


I could be that you have the worm spacing too tight. That can cause the worm to catch and spring free and will accentuate imperfections in the worm. I would back off the spacing a little bit and see what happens.

However, if it is not the worm spacing, and assuming that you did not change the motor or the worm wheel, they are about all that is left. A motor problem doesn't seem likely but I suppose it is not impossible. A defectively cut worm wheel is a possibility. Overall, it sounds like it is time for a return. With the damage to the worm, it's hard to say what other damage the mount may have sustained, but when unexplainable things are happening with a mount that is usually better behaved, that tends to suggest that something bad happened to it.

#75 contraf15

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Posted 30 January 2013 - 07:55 PM

Thanks for the comments Ed. I did change the DEC and RA motors when I changed the worms. Is there a way to check the worm wheel? Tonight I am going to try guiding with a different camera through the ASCOM driver instead of the ST-4 port like I've been doing. I don't anticipate any change, but I have to try everything. I'll also isolate each cable just in case there is some crazy RF interference or something.

This is really frustrating after dropping almost 12 grand on new gear last month. Maybe I need to take up needlepoint instead.






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