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After months of tracking trouble, I found it

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#1 Hilmi

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Posted 19 November 2012 - 12:40 PM

I've been having months and months of tracking grief with my Losmandy G11, even auto-guiding wouldn't do. Last night I finally found the cause. The worm had some abrasions like it had been scraped against something lengthwise. So I decided to test the famed Losmandy interchangeablity parts theory and took the Dec worm gear and put it in the place of the RA gear.

My RMS is was an average of 1.6 arcseconds with error's jumping up and down sometimes as much as 4 pixels. Now I am down to an RMS of 0.6 to 0.8 without much care and thought to balancing the mount properly. This is at 2000mm using my OAG.

I am now contemplating what else I can do so that I can take images at 2000mm as my stars are still not perfectly round, but this is a giant leap ahead for me.

#2 Raginar

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Posted 19 November 2012 - 01:35 PM

So... You placed the dec worm gear (without the abrasions) into the RA worm gear? I'd replace the worm gear and see if that helps. Maybe an o-vision or just a new worm gear would be a good investment?

2000mm is still going to be a bear; have you tried a focal reducer?

#3 Steve Drapak

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Posted 19 November 2012 - 02:22 PM

Hi Hilmi,

Are you using a guidescope or off-axis guiding? I used a guidescope and could never get round stars until I went to off-axis guiding to get rid of flex(though this does come with it's own annoyances such as it being harder to find guide stars, unless you have the cash for the new on-axis guiders).

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#4 Hilmi

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Posted 19 November 2012 - 02:37 PM

Raginar,

I'm considering the Ovision, but after the damage from the new STT-8300 with built in OAG, my wallet is looking mighty slim now :)

Steve,

I'm getting the same amount of error with my OAG as I was with my guide scope. That's because my guide scope was mounted very rigidly.

#5 tboss70

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Posted 19 November 2012 - 05:25 PM

Now that your down to RMS of 0.6 to 0.8, how far is it jumping up and down? The same?

I have these troubles with my G11 as well but the last attempt at autoguiding gave me 10 minutes with almost complete round stars at 100% Of course that was at 900mm.

Im also considering the Ovision. I just haven't pulled the trigger yet. Are you using a stock G11 or do you have the OPWB?

#6 Dan Finnerty

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Posted 19 November 2012 - 11:57 PM

Congratulations on solving your mystery. A useful object lesson for the rest of us. 0.6 arcsec tracking accuracy sounds pretty darn good to me. It will be interesting to see if you can nail down any other error sources and resolve the cause of the remaining oblong stars.

#7 Hilmi

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Posted 20 November 2012 - 02:05 AM

Basically, I am still not able to image at 2000 mm. I am also finding that the RMS does occasionally go back up to 1, but it settles back down to 0.8 average during the long imaging session I had last night. I can't afford changing telescopes now, as I have already invested too much in my current platform.

My next step is to obtain the adapter I need to attach my Optec Lepus reducer to my camera which has STL threads. That should bring down the FL to something like 1200 mm. If I can image well at that focal length I will not bother to make any more adjustments.

#8 Startraffic

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Posted 20 November 2012 - 01:17 PM

Hilmi,
Try giving Scott or Russell an email. You've have a continual problem & they might be willing to send you a new worm. If not then a new high precision brass worm is $60 USD.

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#9 Hilmi

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Posted 21 November 2012 - 01:34 AM

Last night I did some backlash adjustment on the Dec axis, don't ask for how, I just fiddled with it and I took 4 minute subs in Ha with round stars. I think 4 minutes should cover a full worm period on this mount. Now I wonder if the mount came delivered with the high precision worm on the dec axis (the one I swapped out), because I heard that the steel worm could sometimes look about the same color as the brass one and could be confused for brass. The reason I wonder is because if it is the case, then it is worth it to get a new high precision worm gear to replace the damaged one, otherwise if the worm I have swapped out is also the high precision model, then there is no need to buy a replacement.

Now I just have to see how repeatable this performance is, I have had instances before of non repeatable high performance in guiding. If these results are consistent, then I am a happy man.

#10 pfile

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Posted 21 November 2012 - 12:12 PM

strange, i have the same setup as hilmi, camera, mount, and FL.

i still have not gotten my collimation fixed, so the fwhm of my images is a bit high, but for the most part i'm seeing round stars with the OAG built into the STT's filter wheel. not perfectly round mind you, but certainly good enough (aspect ratio in the low 90%s)

i would have never expected this since i've measured the PE of this mount at almost 30 arcseconds peak-to-peak. of course, when it comes to autoguiding, the absolute error is not as important as the rate of change of the error. i do have a couple of spots on the worm where the rate of change is kind of high.

i'm using Equinox Image on the mac. the guiding usually reports an RMS of 1.2-2px error, which seems too high, but i guess the results speak for themselves.

not sure what the image scale of the guide camera is.

#11 Startraffic

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Posted 21 November 2012 - 12:32 PM

Hilmi,
I dunno, My mount came with one of each, steel in the DEC Brass in the RA. When I changed the RA to the OPW I moved the brass to the DEC. I didn't ANY problem telling brass from steel. Any doubt, check it with a magnet.

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#12 tboss70

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Posted 21 November 2012 - 12:50 PM

Dont all new mounts come with the new high precision worm?

#13 pfile

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Posted 21 November 2012 - 12:52 PM

yes, new G11s come with the high precision worm, but not the one-piece worm block.

#14 Hilmi

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Posted 21 November 2012 - 02:25 PM

I have the OPWB, what I don't know is do both axis come equipped with the high precision worm. When I opened up the DEC axis, the worm was brass colored, so I assumed they are both of the same type.

My original worm definetly had physical damage, but I believe my biggest improvement came after reducing Dec axis backlash. I wanted to further test it tonight, but it has clouded over beyond any hope.

#15 Hilmi

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Posted 22 November 2012 - 03:23 PM

OK, after a bit of fiddling with the worm gear positioning in the dec axis and RA axis, I am now able to take extended subs with round stars, unfortunately, it seems to stall the dec axis when slewing. I got a bunch of feeler gauges so that I can try be a bit more scientific about it.






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