Optimizing your Meade LX90

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Piet Le Roux South Africa
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Optimizing your Meade LX90

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Post by Piet Le Roux »


The Az/Alt fork mount telescope is the ultimate viewing telescope due to its comfortable viewing angles and easy setup procedure but only if its goto and tracking are accurate. Unfortunately keeping your Meade LX90 with its Autostar or Audiostar controller in tiptop working order will require some regular maintenance and calibration.
Lets look at the main factors that contributes to inaccuracy:
1) Your OTA is not 100% level when it rotates 360 degrees.
2) Your drives have “play” (this is felt when the clutches are locked and a small amount of movement is felt when you gently rock the OTA sideways or up and down)
3) Your drives have not been trained properly.
4) Battery or axillary supply voltage are not kept on or above 12 Volt.
Getting your OTA level in all directions can be achieved with a spirt level mounted on your OTA, preferably on the pivot point. It is a bit tedious to adjust the tripod legs and OTA to get this right so don’t, after you have managed do this, loosen the Alt clutch to point at a pole star! Finding True North is not critical and any inaccuracy will be rectified after the first alignment star, you just have to be close enough to find the first star.
The drive consists of a motor and gearbox that drives a worm gear that drives a 154 tooth spur gear. Your drives are mounted with two heavy duty screws, the one is a pivot screw and the other a block screw, the drive must be securely mounted but must still be able to pivot freely.
Pivot.jpg
The worm gear has a Allen screw on the end of the worm block to adjust the endplay, this screw must be adjusted but before this is done remove all the old grease and grease the worm with white lithium or another suitable grease. The grease needs to be very tacky and stable so none of it gets into unwanted places like the optical decoders next to the motor. After the shaft is well lubricated force some grease down the Allen screw hole before inserting the screw. Tighten it till it reaches the shaft and then loosen it about a 1/8 turn. Degrease the spur gear teeth and grease with fresh grease. I rotate the dive after assembly for a full rotation of the OTA in both directions and then inspect it again to remove any excess grease.
Drive training eliminates the effect of “slop” introduced by the gearbox. This needs to be done to maintain accuracy because the encoder is coupled directly to the motor before the gearbox. When drive training is done the motor rotations needed, before the spur gear moves, are measured and given a training value. Every time that drive changes direction a certain amount of rotations are disregarded by the Az and Alt counters to maintain accuracy. So if training is not done accurately the mount will become less accurate every time direction is changed. Use a fixed terrestrial target and a astrometric eyepiece when doing drive training. I recommend that you use patched firmware obtainable from https://www.stargps.ca/downloads.htm. With patched firmware you can read the training values after you have done drive training, this is a very good indication of the condition of your drives : the default value is 100 so anything below that value is excellent but values above 150 are not so good. You can also edit the values from your handset if you have patched firmware. So you could check and adjust your training values on the fly by selecting a star and then move away from it with the Alt or Az drive and select it again by pressing enter : if it stops short your training value of that drive is too small, if it overshoots the value is too big. Motor calibration does not actually calibrate the motor but calibrate the LED’s and optocouplers used in the decoders that detects motor movement. The brightness of a LED can be effected by temperature and voltage and this adjusts it to optimum levels. It is good practice to do a motor calibration before drive training is done. The Az/RA and Alt/Dec percentage compensate for the “spin up” time needed ot overcome “slop” when changing direction by boosting the initial motor speed. I select it according to my training value : if the value is below 100 you need very little boost so 10% is enough, if the value is above 150 I would select 50% or more.
I have never used c-type batteries to power any of my LX90 scopes but have been using rechargeable 4.5 Ah 12 volt batteries. The axillary power input plug can be improved on and I am considering installing a heavy duty plug to the internal batteries wiring, because I don’t use them. In the meantime I have secured the plug with Velcro on the wire and scope to make it more stable after I had some problems with it that that resulted in voltage drops.

I included information and photos of one of the drives :
Declination axis adjustments.pdf
(385.22 KiB) Downloaded 219 times
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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