Cascade Mountain - H44
Long awaited instrument almost complete
Last Updated - 12 Sep 2004 - 10 minute unguided image - Click to go there
The time has finally come where the new telescope at the Cascade Mountain facility will be complete and ready for operation. For a brief period starting about August of last year, the old scope was in operation however it was determined that there were problems with the optics and the German equatorial mount was simply not sufficient for the duties this instrument was intended for. Sometime in November of 2003 the scope was disassembled to make way for considerable improvements. An order was placed with Alan Bell of AB Engineering, Fort Wayne, Indiana to build a large German equatorial mount to replace the old one. This new mount will be delivered the first week of August 2004. Even with the considerable cost of this mount, we determined that Alan's design was by far the best in this class of mount. As a comparison, this mount is slightly more than 5 times the mass of the Astro-Physics AP1200 and will carry a very large payload. Total weight of the mount with pier is 1000 lbs and is strictly meant for permanent operation. The equatorial head alone is 600 lbs.
Summary of the German Equatorial Mount
Gearing consists of a custom made heat-treated steel alloy worm supported by preloaded ball bearings and high strength bronze alloy 15 inch worm gear for the polar axis, and a 12 inch worm gear made by Boller and Chivens for the declination axis. Both gears have a reduction ratio at the motor of 1:1440 which provides a tracking resolution of 0.05625 arc seconds per micro step of the installed stepper motors. This resolution far exceeds the optical resolution of the telescope and will be ideal.
The worm accuracy measurements are the lead and lag of the worm thread as a function of rotation. The error of the worm is approximately +/- 0.0001, and the hobbing error on the worm gear is approximately +/-0.0003.
All unpainted metal such as the counterweight shaft are cadmium plated to prevent rust. The counterweight shaft is "acme" threaded to allow precise adjustments of balance by simply rotating the counterweight(s).
For both the polar and declination shafts the bearings are preloaded ball bearings; that is the axial and radial play in each bearing is removed yielding smooth rotation of each shaft. The bearings are seated into the housings. The shafts and housings are both made of steel as to expand and contract at the same rate to not alter the preload of the shaft over temperature conditions.
The mount will be driven by Mel Bartels' computer drive system which also allows for remote site operation over a LAN or high speed internet connections. Pointing should be accurate to a few arc minutes using built in pointing model corrections.
Once the scope has been tuned for pointing accuracy (including GEM meridian flip) Maxim DL will be used for occasional semi-robotic operation using script files to instruct the instrument to slew to certain objects, image the object using CCD, then proceed to the next object in the list.
This mount can be built to the specifications (within the mount's limits) of the buyer. They can be purchased with or without computer control. Anyone seriously interested in a massive mount for professional or amateur use should consider AB Engineering for the job. Alan Bell has done considerable work for many institutions such as the Boller and Chivens scope at Stanford University (Stanford, California) and the 38 inch Fecker at Butler University (Indianapolis, Indiana).
The company also offers smaller mounts and custom optical systems.
To visit the AB Engineering website, click here.
Following are photos taken of the mount the day after completing assembly. Click thumbnails to see full size image. Note RA drive cover is removed in these photos:
The Following Graph is of the Periodic Error Analysis of the AB Engineering worm and main gear performed on 31 Aug 2004
After an average (or median) of the individual analysis files, a smoothed plot is stored in a final PE array as seen in this next graph. The Bartels Scope Drive system performs all the combining, drift subtraction and averaging to obtain the final PE array, or it can be refined manually using a spreadsheet, etc. It is evident from the graph that the worm and main gear are well machined by AB Engineering since the worst case point of periodic error is less than 4 arc-seconds in a four minute period of tracking, BEFORE any corrections are made. Any drift in the system is subtracted from the periodic error and corrections are made for that separately.
Needless to say, we are very happy with the ABE mount so far! Now that I have the PE and drift nailed, I can begin to do my work again!
The following is a 10 minute un-guided image of NGC 7814 in Pegasus that shows the superb tracking ability of the mount. Binned 2x2 single frame of 600 seconds. No manual or Auto guiding. Taken with a Starlight Xpress HX-516.
Click on image to see a combined 34 minute image of this galaxy taken with the 16" Cascade Mtn. telescope.
Summary of the Optical Tube Assembly
The telescope is a 16" (41cm) f/4.46 which provides a prime focal length of 1812mm, or 71.36 inches. The configuration is a Newtonian design. The primary mirror was figured by Bob Royce of R.F. Royce Precision Optics in Northford, CT who has built an incredible reputation of highly accurate optics of all sorts. He is considered by many to exceed the limits of even Zambuto with his extreme accuracy. After much research and time, Bob was chosen to work the mirror. We will be anxious for the time the closer planets are again in the night sky to image them and also see them visually through the scope, as Bob is well known for his "planet killer" optics.
The secondary is a 3.1 inch elliptical made by Protostar. It is held in place also using a Protostar holder and spider.
The new focuser is a variable speed, motorized Crayford made by Moonlite Telescope Accessories.
More specs, images, etc. will come as soon as the scope is assembled and in operation....
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