Telescope mount: Fixing the backlash
Category: Homemade Telescope Mount
Posted by: Tom How
The new telescope mount is in the observatory. The telescope mount is polar aligned. I have my first astrophotograph. What next? Tweaking!
With a project like the homemade telescope mount, nothing ever works first time. After each major test you have a dozen things that need improving and tweaking.
This time around, most of the problems seemed to involve backlash.
The DEC backlash was horrible. Really bad. Much worse than the test I did at Easter.
It is only when you’ve got an entire telescope hanging on your mount can you easily find the cause of backlash in the gears of a telescope mount. The large leverage makes things twist and bend that you never expected to twist and bend when holding them in the workshop.
Backlash on telescope mounts is caused by two things. On telescope GEM mounts driven by worm wheels and worm gears, there is always going to be a tiny bit of play between the meshing teeth. Having the gears meshed tightly just strains the motors, so you’ll always end up with a tiny bit of play. A thousandth of an inch play on a 7 inch worm wheel translates to an arc minute up the business end of the telescope.
The other cause of backlash is the mounting of the worm gear itself. This is an often overlooked and neglected source of problems on a telescope mount. In an ideal world, the worm gear should be mounted completely immobile with respect to the worm gear. However, this isn’t practical. Because of minor eccentricities in the worm wheel and gear, plus effects from thermal expansion etc, it is normal to “float” the worm gear, spring loading it against the worm wheel.
This mounting system must be very precise – whilst the worm gear is allowed to move in 1 dimension, it must not be allowed to twist and slide back and forth. It is only by putting a telescope on the mount and yanking it backwards and forwards and feeling the mechanism carefully can you detect the tiny bit of play in the mechanism that causes the backlash.
Once I’d identified the main areas of play, I detached the motors and took the worm gears back into the workshop for some adjustments.
During this process I found a rather glaring error. Each worm wheel is bolted to a bush which holds the set screw than clamps the worm wheel to the shaft. For some reason these three screws where loose. There are two explanations for this. It is possible the action of the mounts operation has caused the bolts to loosen and come undone. However my favoured explanation is that I’ve never tightened them! Normally these bolts are only tightened once both worm wheel and boss are on the mount, and perhaps I forgot! I will keep an eye on them. If they shake loose again it is out with the loctite!
For the next test I will inspect the trained backlash values in Autostar from the previous test. I will re-train the drive and see if there is an improvement in the values.
The level of DEC backlash I was experiencing will certainly be part of the DEC guiding problem. Once the DEC backlash is minimised I will be able to retest the guiding.
Any people wonder why commercial telescope mounts as so expensive!
With a project like the homemade telescope mount, nothing ever works first time. After each major test you have a dozen things that need improving and tweaking.
This time around, most of the problems seemed to involve backlash.
The DEC backlash was horrible. Really bad. Much worse than the test I did at Easter.
It is only when you’ve got an entire telescope hanging on your mount can you easily find the cause of backlash in the gears of a telescope mount. The large leverage makes things twist and bend that you never expected to twist and bend when holding them in the workshop.
Backlash on telescope mounts is caused by two things. On telescope GEM mounts driven by worm wheels and worm gears, there is always going to be a tiny bit of play between the meshing teeth. Having the gears meshed tightly just strains the motors, so you’ll always end up with a tiny bit of play. A thousandth of an inch play on a 7 inch worm wheel translates to an arc minute up the business end of the telescope.
The other cause of backlash is the mounting of the worm gear itself. This is an often overlooked and neglected source of problems on a telescope mount. In an ideal world, the worm gear should be mounted completely immobile with respect to the worm gear. However, this isn’t practical. Because of minor eccentricities in the worm wheel and gear, plus effects from thermal expansion etc, it is normal to “float” the worm gear, spring loading it against the worm wheel.
This mounting system must be very precise – whilst the worm gear is allowed to move in 1 dimension, it must not be allowed to twist and slide back and forth. It is only by putting a telescope on the mount and yanking it backwards and forwards and feeling the mechanism carefully can you detect the tiny bit of play in the mechanism that causes the backlash.
Once I’d identified the main areas of play, I detached the motors and took the worm gears back into the workshop for some adjustments.
During this process I found a rather glaring error. Each worm wheel is bolted to a bush which holds the set screw than clamps the worm wheel to the shaft. For some reason these three screws where loose. There are two explanations for this. It is possible the action of the mounts operation has caused the bolts to loosen and come undone. However my favoured explanation is that I’ve never tightened them! Normally these bolts are only tightened once both worm wheel and boss are on the mount, and perhaps I forgot! I will keep an eye on them. If they shake loose again it is out with the loctite!
For the next test I will inspect the trained backlash values in Autostar from the previous test. I will re-train the drive and see if there is an improvement in the values.
The level of DEC backlash I was experiencing will certainly be part of the DEC guiding problem. Once the DEC backlash is minimised I will be able to retest the guiding.
Any people wonder why commercial telescope mounts as so expensive!
