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measuring backlash


Daniel-K

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Why would you want to "set backlash" ? Normally gears in mesh will have taken up any backlash almost immediately unless they have to change direction of rotation. Do you mean "how do you set the righ amount of backlash between gears so they run smoothly without any binding?"

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That is there because, whilst RA drives only one-way - and guiding is accomplished by driving at siderial rate plus or minus a guiding pulse, Dec can be required to drive both ways. Therefore EQMOD has to know how much backlash there is in the dec drive so that it can compensate for it when changing direction.

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Unless you have very expensive anti backlash gears, all systems will have some backlash.

When gears mesh there will be a small amount of 'play' to prevent it seizing or binding. Take into account the mount has to operate in a wide range of temperatures and is made from varying materials which have different thermal coefficients.

Be careful with any adjustments. If you don't see a problem with tracking, I would ignore.

Typed by me, using fumms...

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so this doesn't need adjusting?

Who can say? If you believe you have a backlash problem that affects your guiding in dec then that setting is the one you would play with. I'm afraid that as with all things guiding its really down to playing around with the settings until you find a combination that works best for your particular setup.

Chris.

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I don't know about the EQMOD side, but I think the way to measure backlash (in the DEC axis, say) would be to use something like a dial gauge to test how much it is possible to turn the saddle whilst the worm is stationary.

Are there any units for the backlash figure in EQMOD?

James

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Hi,

During the day slew the scope in RA and Dec to a distant object, a TV mast or lamp podirections, with tracking turned off. Note the RA and Dec positions as reported by the handset or computer. Using the slowest slew rate, slew in the opposite direction one axis at a time until the scopes just moves from the fixed object. Compare the new RA and Dec with the ones from before, the difference is the backlash. Backlash is a measure of how much the motor needs to turn to take up the slack in the gears and start to move the scope.

My old Dob had a fair bit of backlash and I calibrated it out using the above method. My EQ6 seems to have very little and I have never bothered.

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The real solution is to get rid of worm drive and go to roller or belt with zero backlash. It is just incredibly nice using a zero backlash mount, trust me! They will not always be expensive.

On the EQs you can greatly reduce backlash following Astro Baby's guide to the EQ mount strip-down. Once you've done that the mount needs to know how much there is and many guide prgrammes measure it themselves. If you want to do it you just get the mount guiding and then press one of the direction keys (this will be Dec in reality) while looking at the guide star on the crosshair. You have to time how long it takes for the press on the button to take effect by moving the star. In reality you don't have enough hands so what I do is use auto and then try slightly altering the chosen setting to see what effect it has.

Olly

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