EQ-1M motor modification

[orly_andico]

I discovered that the EQ-1M motor from Orion fits "perfectly" on a Vixen Polaris, with no modding!

Problem is, it's designed for a 96-tooth worm gear and all the smaller Vixens have 144-tooth gears (e.g. 1.5X more). So the tracking is slow.

I cracked open the handbox and it's crystal-controlled. Good (stable) and bad (cannot tweak the speed). The crystal is a 3.58MHz, so I'd need to replace it with a 5.37MHz which doesn't exist. Citizen has a 5.33MHz - close enough - but 28-day lead time at RS! Digi-Key has a 5.5MHz - a bit farther out. These crystals are cheap (under $2).

Alternatively, if I can get two plastic gears with a 1.5:1 gear ratio, I can use that instead of directly coupling the EQ-1M to the RA shaft on the Polaris. Of course this will add backlash..

So..

1) where can I buy crystals? Sim Lim Tower has been mentioned before..

2) where can I buy plastic gears? Tamiya gears are too small.

[orly_andico]

The microcontroller in my EQ-1M is an Atmel AT89C2051-24PC.

According to my cursory research, the "correct" way of setting delays is with the built-in timers, which decrement once every machine cycle, which for the Atmel is CLOCK / 12.

So it does seem that the timer delay scales linearly with the crystal clock.

At 215X (W-O Zenithstar 70ED with 6mm Burgess/TMB + 3X Barlow) an object would cross the 66* FOV in about 1'20" with tracking off. With tracking on, unmodified EQ-1M, it took about 3'40" to cross the field (roughly 3X longer, since the EQ-1M only runs at 2/3rd the speed needed by the 144-tooth Vixen mount).

I swapped the 3.58MHz crystal for a 5.0MHz crystal (by linear scaling a 5.37MHz crystal will be needed). I calculated it would take roughly 15 minutes for an object to cross the field with the 5MHz crystal in place.

My results are good/weird: I now have to use the "South" switch setting but in 8 minutes the object has only moved about 1/6th across the field, an error that is 3X smaller than I expected!

So the 5MHz crystal is pretty much usable for my purposes (which is to use the Polaris as a glorified barndoor.. kinda like a poor man's Teegul/Sky Memo!) since it takes about 50 minutes for an object to cross the FOV of a 66* eyepiece at 215X.

p.s. I ordered the crystals (I got 9 of them.. for S$12) from RS-Online, Wednesday morning about 1:30 a.m. By 10:00 a.m. somebody had called me, and by 4:00 p.m. same day I had the crystals! for S$12 including delivery to my office :-)

Very handy.. no need to hoof it to Sim Lim Tower..

Also I don't think I need the 5.33MHz crystal anymore since the 5MHz one seems plenty accurate for my modest needs.

[orly_andico]

hi stuart

You are right, RA-only guiding would be trivial in this case although the hand box also has a 2X speed, so if the crystal is a bit slower, a few jabs at the 2X forward will do guiding (if the crystal is faster, the drive will have to be turned off every now and then... but the slack in the motor gearbox will partially negate guiding benefits).

By having the motor run a bit slow, speeding it up maintains contact on the gears and avoids backlash.

I can't use a divider because the Atmel CPU uses a directly-connected crystal and an internal clock generator. Of course the crystal can be removed and the Atmel driven with a clocking signal, but that would entail major modification (basically a clock generator board) which would not fit in the existing hand box.

As for your Mizar EQ-2 progenitor.. if the RA gear has 96 teeth then the stock EQ-1M will work fine with it OOTB. I also have some Meade motors and Autostar hand pad (and my cheap oldham couplers finally arrived from China!) so more hacking awaits..

but there's something to be said for the dead-simple EQ-1M. It's happy with its 4 "D" cells and there's no screwing around with alignment stars and what-not; I just use an indifferent "I think North is over there" polar alignment from the kitchen :-P