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Periodic error on G11


Jun 25, 2007

 


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#33344 Jun 25, 2007

Guys-On Astromart Dove Web has a G11 for sale with 1.77 arc sec of PE

(!)obtained by replaceing the Oldham coupler with a helical coupler,

enlarging the worm bearing holes with a Dremel and using angular

contact bearings held in place with Locktight. I presume he also has a

high precision worm. Also see Eric W. Benson's Astronomy and

Photography page, who probably initiated these changes. Maybe it's a

lot easier than we thought! I am awaiting part numbers from him as

Mcmaster Carr has no angular contact bearings smaller than 12 mm.

Charles



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#33370 Jun 26, 2007

Actually it is the other way round... I had a case of bad tracking out

of the box, with Paul K.'s and others help I was able to learn enough

to eventually diagnose the problem: severe 76s error + lousy worm

(even though it was a "high precision" worm). So after doing some

digging I found a doc file from webdove in the archives and followed

the trail. The original 76s effect it seems, was "discovered" the year

before by Wayne Malkin and others (can't remember who exactly),

although AFAIK they did not find a solution. Webdove did find an

effective solution although from the doc file as I recall it was not

clear why it worked (but he must of known?). At this point in time I

believe we have indisputable evidence that the problem is from

pinching the ball bearings.



The fix is quite easy, as Frank U. discovered, enlarging the bearing

pockets is not always necessary, in his case all he had to do was file

down the bottom of the block, any bumps or convex shape there will

when tighened down pinch the bearing since the wall is so thin on the

bottom. The last few worm aligns I have done I have paid no special

attention, just used a straight edge and feeler gauge, run the motor

with the scope on, no funny sounds or binding, great good to go.



As for couplers, in my opinion the jury is still out, I have more data

that I still haven't fully analyzed, but none of it was taken on a

really steady night. But put it this way, the helicals are still in my

mount and the oldham's are on the shelf. I had a night a few weeks

back where the seeing was 1.2" with 0.63" sampling, the autoguider RMS

error on both axes was often below 0.2", that's like wow, so why mess

with a good thing.



A nugget I have learned in the last 6 months, if you have jumpy

tracking or jumps in the DEC axis, put more grease on your

worms/gears. For over three months last fall I kept getting random

tracking spikes, and tried everything to fix it: (helical couplers,

worm realign, motor swap, major unbalance, perfect balance, ancient

telescope dance around tripod...) Then I spread on as much grease as

possible (cheap white Lithium stuff from Canadian Tire, and I didn't

even bother cleaning the old stock grease from the DEC axis off,

blasphemy!!!) while moving the mount back and forth until it oozed

from every gear tooth, haven't had the problem since :)



BTW Ken re your comment the other day about speaking to Scott

Losmandy, it absolutely baffles me that he still does not acknowledge

this problem (and the known solutions!). Does he not want to improve

sales and have a better product?!? Anyways...



Eric



--- In Losmandy_users@yahoogroups.com, "drgenovese92853"

drgenovese92853@...> wrote: >

> Guys-On Astromart Dove Web has a G11 for sale with 1.77 arc sec of PE

> (!)obtained by replaceing the Oldham coupler with a helical coupler,

> enlarging the worm bearing holes with a Dremel and using angular

> contact bearings held in place with Locktight. I presume he also has a

> high precision worm. Also see Eric W. Benson's Astronomy and

> Photography page, who probably initiated these changes. Maybe it's a

> lot easier than we thought! I am awaiting part numbers from him as

> Mcmaster Carr has no angular contact bearings smaller than 12 mm.

> Charles

>



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#40443 Nov 2, 2008

Hi Guys. Havn't posted anything in this group before. Havn't been

doing much deep sky in the last few years. acquired this late 90's

G11 a couple of years ago and have mostly been doing visual or

planetary stuff with it. Plan to do some serious deep sky imaging and

decided to start doing some fiddeling and testing. When I bought it

it had been modified with upgraded bearings-regular radial bearings I

think-and a stainless worm. I played with the worm/worm gear spacing

and there was small but significant rough spots on the worm and a

slight eccentricity in the worm gear(fount by just hand turning the

worm ). I adjusted the worm and blocks to have about 1mm of movement

at the end of the worm shaft at the loosest spacing, and then I spent

a couple of hours lapping the worm with #320 valve grinding compound,

then carefully cleaned it all off (without dissembly-did not want to

move anything). Last night I put my C8 on it with my old trusty Nikon

FE body and visually focused using a 2X finder magnifier, and then

did a typical 1 hour test shot with the polar axis misalligned a

degree or two. The seeing was excellent. At 2000mm the film plane

image scale is 1.72arcmin/mm.or 103 arcsec/mm, and the negative was

scanned. Remarkably the overall star trails are only .14mm wide(about

8.4 arc sec), but when the computer image is enlarged 800% and

processed, a much brighter central star trail is seen and the total

PE is about 30% of the maximum star trail width, or 2.5 arc sec, or +-

1.5 arcsec. Not bad! Charles







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