#56376 Oct 28, 2015
I know this has been discussed forever but I've been thinking -again- of picking up the Ovision worm - they are apparently now stocking them since they have a two-week delivery (last time I ordered it was months and I gave up and cancelled). ��Any pros and cons I should be aware of - I currently have the brass worm and I can live with the PE and usually guide it out but if a new worm will improve things, I'd like to go for it. ��
Ovision or OPW?��
Or new bearings and Belleville washer? ��Probably should try this first but with winter coming to northern Ohio, it may be spring before I get to actually test the results.
#56377 Oct 28, 2015
Good question... cost/benefit analysis is hard to do... here are some thoughts...
First step is: .measure your PE on your system presently.I (and others) were able to get to under 1 arcsec (RMS...actually got 0.6 arcsec) PE with a Losmandy precision brass worm ($100), and a Losmandy OPW ($500 with worm).. I put in 2 of ABEC-7 bearings at $~15 each.. David Partridge got that level with the original 2 blocks, and two of ABEC-5 bearings.. The Belleville washers keep the worm train in compression with at most 7 pounds of force... and this allows for thermal expansion and contraction of the different materials.
If the PE of the worm is "regular," that is, not jittery, then PEC (built into the Gemini 1 or 2...but you have to "train" it...pretty easy...) will have no trouble cleaning up the remaining variation.. I read that once you get down to about 1 arcsec error, you are normally in the range of the "seeing" fluctuations limit of the atmosphere. .
The problem of PE trouble is always attributed to the "worm" quality, but that is not the entirety of the issue.. In my mind the 2nd key item is the bearings... the quality we want is certainly precision (the higher the ABEC rating the better...top of scale is 9 but these are special order... 7 is now a stocking item) but also something not spec'd: bearing "rumble".. So that is the harder quality to assess, and you have to judge that from results.. Michael Sinescalchi, (helixgate.net/) who wrote extensively about his experiments to improve the G11, was not able to justify spending for higher than ABEC-5 bearings... but they are now very affordable. . .
I found the ABEC-7 bearings that I purchased to be very tight axially.. That is, when I put the worm into the bearing, and hold the outside of the bearing, I could not wiggle or wobble the worm axis.. But it was easy to wobble the worm axis with the original bearings.. So I think that putting in the ABEC7 bearings could, in a simple way, mimic the effect of the OPW in keeping the bearings perpendicular to the worm axis.. I did not try that... it is just a thought if you want to keep costs as low as possible on the path to lower PE. .(Maybe David Partridge's ABEC-5 bearings were also "tight" in the same way... perhaps he can comment. .)
The other issue is your hand assembly of the RA worm to its motor: .keeping the bearings, and the motor drive, very accurately on the same axis.. The OPW or Ovision keep the worm and bearings axial, solving the primary trouble of the 2 worm blocks.. But neither OPW or Ovision assembly has a way to keep the motor drive shaft (and its Oldham coupler) on axis perfectly with the worm.. You have to really look visually at that when you assemble the RA drive.. The G11 mounting of the motor seems to have some oversized holes allowing the motor mount to be shifted... I put some metal wrapping around the bolts to better center the mounting and to better hold it from moving around (pics are in my PDF writeup). .
I think of the coupler working: it has two "rods" that are brazed onto the drive disks and there is a plastic coupler disk that allows these drive rods to slip and still drive the axis.. But if the two axes (drive motor and worm) are not on the same axis, you will get the worm speeding up and slowing down ... in a harmonic behavior that is the same rate as the worm (or maybe 1/2 that period).. A Fourier analysis of the frequency elements could show that to be associated with PE of the worm...but it really isn't. . (The Orion Atlas/Synta EQ6 has a drive gear that is perpendicular to the worm... so it introduces yet other gear ratios to the puzzle). .
Anyway... like I said,... first see what your PE is on your system... then you can judge how far you should expect to get. .(Others state that the OPW and precision worm have gotten them around 2 arcsec RMS PE).. There are instructions in my PDF about how to make the (free) measurement... thanks to the great work of other amateurs.
The new bearings (2 at ~$15...for the old style separated blocks) and Belleville washers (cheap)... are an easy thing to try first... before you enter the $1k cost arena....
#56378 Oct 28, 2015
My understanding is that OPW worm is not available and won't be for a long long time so it is not really an option.
#56379 Oct 28, 2015
Ahhh.... good question...I just called Russell at Losmandy about that.. He has brass worms, but is out of OPW units.. You can Pre order them.. He says the plans are to make them in April 2016.. He says if any dealer were to have some in stock, it would probably be Oceanside Photo in LA..When I ordered mine, from an LA area dealer who had one in stock, I received the OPW with a separate brass worm.. I was unhappy on receiving the unit, since I read that the "OPW -with" that I'd ordered...the worm was supposed to be "Pre loaded. ". Mine was not...and anyway without a spring washer in the kit, there really is no way to preload the worm...and keep its load steady.But none of those pre loaded OPW were available at the time.I then tried out the system by installing the worm. ..I got about 4.8 arcsec rms.. That started me on the trail of PE reduction...Belleville
.My understanding is that OPW worm is not available and won't be for a long long time so it is not really an option.