Heavy alt-az mount

[AstralTraveller (David)]

I need a bit of unusual advice.

The atmospheric chemistry group here are in to open air IR spectroscopy. The spectrometer consists of a modified 12" SCT and the spectrometer box containing the source, detector, beam splitters etc. The whole unit is about 1m long, about 40kg and is mounted on a solid aluminium bar. They take this out to the study site (eg a field covered in a particular fertilizer, a paddock of sheep, a pig pen etc) and aim the spectromter at a corner reflector. The IR beam travels through the air over the field, hits the corner reflector and then returns to the spectrometer and they determine the flux of the gasses of interest (eg methane, nitrogen oxides, CO2) coming off the field.

The spectrometer is mounted on a solid tripod and aimed with the use of a very sturdy-looking (and not cheap) atl-az head. The problem is that every time they get the beam hitting the reflector and then tighten the locking bolts, it moves off target. The backlash in the gears (worm and wheel) is huge and there is no obvious adjustment. It is taking them an hour or more to get it aligned. To make matters worse, if the wind direction changes they want to be able to swing the spectrometer around to a different reflector and then return to the first one if the direction changes again.

I have discussed with them some modifications which may help (eg deliberately unbalancing the instrument on the mount so the loading is always on one face of the gear, clamping the adjusting shaftes rather use the existing clamp.) However, even if that works, the scale on the mount would only allow pointing accuracy of about 0.5 degrees and that isn't good enough.

What they are hoping to find is an off-the-shelf solution. They need an alt-az head that can take 40kg and a fair amount of wind loading, be able to reproducably be positioned to within perhaps 10' of arc but without too many unnecessary bells and whistles. They don't need tracking, GPS, 25,000 object database, a coffee maker or even drive motors (OK maybe the coffee maker). Is there anything out there for them??

thanks,
David

[Baddad (Marty)]

Hi David,

You were kind of on the right track with your solution.

I, not having seen how the gears mesh, would not know if this is possible. Its also difficult to explain without diagrams.

To neutralize gear backlash in equipments I've dealt with, the following was employed.

Its done with spring tension on a two peice gear. The gear consists of two seperate gears on one shaft. One is locked to the shaft, the other can rotate on the shaft.
A spiral spring in between them and the shaft through the centreof the spring. The ends of the spring lock into each of the two gears. One in one side, the other in the opposing gear.

The two gears are rotated a quarter or half a tun in opositte directions to obtain tension in the spring.

This gear assembly is now mated to its drive or following gear. Taking care not to release the spring tension. In effect it becomes three gears for where there was two.

To clarify it. The two gears under spring tension rotate on one shaft as one single gear does. The gear cogs now place tension both ways onto the cogs of the mating drive or following gear. No more gear backlash.

If its clear as mud let me know. It is a difficult concept to explain. It is not all that easy when you see it at first but its not difficult to do.

I thought that the system would have had something to overcome the backlash. If some one tampers with the gears then backlash will be present. I wonder if this happened?

Cheers Marty