Re: Help with DEC backlash/jumpy guiding
|
Hi Magnus, When you moved to another object, or did a meridian flip, the problem went away or got better.?? So I consider 2 possible causes.? One is the DEC of the mount, and the second is the guide scope/guide camera.?? If you are using an OAG, that will solve the 2nd problem and I only guide that way.? Any slight change in the guide camera or guide? scope position will glitch your autoguiding, and due to the weight of the autoguide camera cable, that force changes with RA/time, and with meridian flip.?? Or, as you mentioned, it could be the mount DEC axis too...as you said a possible? cause of the trouble in your last description.??That is: I think you must have a slight DEC axis physical hysteresis in the worm mounting.?? When you moved to another object, or did a meridian flip, the problem went away or got better.?? If you think about it, in a perfectly polar aligned system, the RA worm is constantly moving toward positive RA, which may keep a force on one worm bearing.? But the DEC worm should not be moving (ignoring refraction of the air, and seeing).? The DEC axis can wiggle.? And PHD will have a time lag or overshoot trying to compensate for that DEC hysteresis.? ? If you have a slight I mis-balance in DEC, its ring gear will be pulled against its worm gear, pressing the worm either left or right against one of the worm bearings...the other worm bearing is more loose.? Over the night, or pointing to other objects, that misbalance changes net force on the DEC worm.? The net force will be a function of the RA angle and DEC axis angles.? (The force on the RA worm is a function of the RA angle and the polar axis angle, but not the DEC angle.)?? Bottom line, in the Losmandy original design, the DEC worm is not kept in a condition of constant constraint during the night of imaging...even if you had a hanging weight wrapped on it.? Note that the highly rated Ovision worm system designed in a Belleville washer for this purpose...Losmandy did not.?? As the night temperatures change, the brass or stainless steel worm is going to expand or contract differently than the aluminum holding the worm blocks, and the aluminum worm blocks themselves. So if the system compressing the worm bearings was "tight" at one time/ temperature, it can become loose at another time/ temperature.?? Hysteresis might be curable in PHD2, and it might be tolerable with a wide angle, short FL scope image.? But if you are using a long FL scope like an SCT, it probably is worth trying to fix the hysteresis mechanically. You mention the possible benefit of a Belleville washer and that you have not found these.? I have a stock of them for the Losmandy worm bearings (and the Orion Synta Atlas too...same problem).?? Since they are flat, they easily mail from US to international destinations in a letter.? For?$5,?I can send you 3 of them, and you have 1 for RA, one for DEC, and one spare (though they never should break).?? You will find installation instructions in our Files section under my name.?? Some simple hand polishing is required with very fine sandpaper or emery cloth to polish down the perimeter of the far worm bearing, so it can slide in its worm block cylinder.? Pulling out the rear bearing can be a challenge but I show how to form a simple puller to do that.?? You lube that cylinder and assemble the washer behind the bearing, pressing the blocks together so the spring constantly pushes the bearing on the worm. That keeps the worm in place, and keeps the Oldham coupler pieces together (which you must do or that alone will cause glitches!), and also keeps both worm bearings "compressed" so they keep lowest rumble preformance. To get the lowest RA PE about 1 arcsec native PE (no PEC), I found a benefit from replacing my worm bearings too.? I used ABEC7 rated ones, under $20?in the US.? David Partridge in the UK found great results with ABEC5 rated ones he got in Europe.? ?We both used the brass Losmandy worm, but David used original 2 worm blocks and I used an OPW.?? Wow...a lot of writing at 5AM! Hope you solve your issues...we all await your images!? Michael? On Nov 25, 2017 12:09 AM, "jfev5mnsvqiyzpq756wvokt55jve7sbg2esr5mip@... [Losmandy_users]" <Losmandy_users@...> wrote:
|
Michael Herman