A Meridian flop on a Giant Refractor

by James Turley

As anyone who runs a GEM knows, when a scope crosses the meridian, you need swing the OTA around to the other side of the pillar/mount, and re-aquire your object (a pain for imagers, to be sure). This avoids a collision with the mount. For some perverse reason, I've always found this a fun (but sometimes annoying).

Last night, I was helping to run the Great 36" Lick Refractor. Our object was M13, high at zenith AND very close to the meridian. A difficult position for a 52 foot f/19 36" OTA, to be sure. The giant OTA soaring into air, was standing straight up. You slew this beast manually, so there's no GOTO beeping to tell you when the tube is ready to hit the pier. I was tasked with that function: to monitor the clearance of tube, and warn of the collision. Imagine that.

BEEP. Everything is manual. OTA sticking straight up, the 71 foot diameter inlaid hardwood observing platform floor needs to be at its lowest position. The object is, of course, to swing the 14.5 ton OTA around to the other side of the "pier", while avoiding hitting anything hanging off the pier or those massive DEC counterweights. Thus the floor needs to be raised (water driven hydraulic lifts, previously a wind-driven gravity system) while the tube swings around. As this happens, the dome slit paddle actuates the dome rotation, seeking a new slit position.

As the OTA swings around on a rising platform, the driver looks like some 1/6g moonwalker, being bounced on a moving platform. 90 degrees around, the driver needs to start lowering the platform, as the OTA climbs higher, else the massive OTA visual back would smash into the floor. Platform lowered again, he then seeks the DEC bearing of the object, M13. Waiting for the slit to arrive, the RA is adjusted.

BEEP. Oops, the tube is ready to hit the pier on the new side! Lock down. Watching the UT Clock on the console, we calculate the correct RA will arrive at our current locked Alt/Az position in 3 minutes. Sure enough, right on time, I watch M13 creep into the 55mm Plossl FOV, perfectly! Jeesh...this is better than my GP-DX flop! What a mount.

Not having been around truly big scopes very much (except Rich N's 7" :-), I was thrilled to be a part of this action. What fun! With the wind cold and howling through the slit, I watched a slice of the Milky Way reeling around the moving dome slit as that beast swung through the sky.

By the way, the Lick folk are now calling the 36" the "Largest Operating Refractor in the World". Since the Yerkes 40" in Chicago cracked. And the repair required the lens to be stopped down to 20". James would be happy.