The Practical Observatory #4
by Dennis Allen
Back in the summer of 1985, the Muskegon Astronomical Society built a 12' club dome. In 2002, we had to replace the dome skin. This building served us well, but after thirty years the dome was showing it's age. The dome was difficult to open, became hard to move, and every panel had leaks. Rather than spending a crazy amount of time and effort laying down a new skin, we decided to replace the dome with a roll-off roof. Roll-off roofs are simple to build, easy to use, and require little maintenance. We already had 4-1/2' high cement walls, just had to put down the outside posts and fabricate the roof itself.
First, we tore down the old dome. Didn't have to worry about saving anything, so the tear-down went quickly. The next step was to make a new top wall sill. We used two bags of redi-mix to put down a new set of anchor bolts. FYI: Anchor bolts hold better when you drill and hammer them down. Once the sill was down, we dug holes for eight 12" sono tubes, two foot down. They serve as pedestals for the six outside channel posts and two side braces.
After the sono tubes were filled with cement, Menards delivered the 12' trusses, one regular and three 6/12 (internal 3/12) scissor trusses. The delivery also included ten sheets of 8' White McElroy corrugated sheet metal, six 14'x2"x4" for purlins and five sheets 7/16" paneling (looks like 8" rough cut with a primer coat). Harbor Steel then delivered two 20' and two 7' pieces of 3" hot-rolled steel channel.
To prep for the big work party weekend, I bolted eight 5" caster wheels to the two 12'x2"x6" truss sills. Kept the outside wheels 8" from the end trusses, the four inside wheels would sit under the internal trusses at 4' centers. Drilled and counter-sunk screw holes in the steel channel and rough cut the 4"x4" outside posts. As in our big roll-off roof rebuild, we're used heavy gauge post bases that mount with 3/8” anchor bolts. We screwed the channel to the posts, but left the anchor bolts off until we could get the channel squared.
It was a long three day weekend. On Friday we got the trusses up, with three 2"x4" purlins on each roof side. We went with a 9.5" overhang, the length of a rib-to-rib section on a sheet metal panel. We did a two truss test push of the roof, to make sure the wheels were sitting center in the channel.
To keep the roof from blowing off, we installed four chain binders (also called load binders). One for each corner of the building. Bolts hold a short section of chain to a roof truss. Each chain binder then secures the chain to a big eye-bolt in the top wall sill. I taped the chain binders to their chains, so people will always know how to lock the chain binders back down.
I spent Saturday putting down the outside post anchor bolts. I managed to get all the bolts down, but the cement was so full of gravel I ruined several drill bits doing it. Gary spent Saturday framing out the gables. He used the upper door leftover from rebuilding the big roll-off roof. Since the lower door wasn't actually centered, he had to mount the upper door a bit north of center. Also had to recede the lower door, so that both doors would match. On Sunday we hung the sheet metal roofing and the paneling for the gables. Oh, we found that 8" white PVC trim covers the 5" caster wheels perfect, hanging off the 2"x6" truss sills.
I spent a couple of days fixing mistakes and making adjustments. Fortunately, we used screws instead of nails so adjustments were relatively simple. Took time to caulk, paint, cut/fit trim, weather-stripping, and fill sill gaps with storm door bottoms. Used half of a 50' roll of 24" flashing to cover the soffit.
After the last heavy rain, we had water coming into the building from the south channel. Must be a slight dip on the south wall. I drilled several drain holes in both channel and so far the building is dry.
Note: After a couple weeks, the outside posts will tend to shrink. Make sure you check, and if necessary, tighten down your post bases.
So far, so good. The building is pretty maintenance free. Added a couple red and white lights. We decided to house the Celestron CPC-1100 in the new building. We had fun with the C11 all summer, even ran it from a laptop. Nice unit, every club should have a C11.
In June we put in a 3' steel pier for the C11, with 3/8" anchor bolts. The C11 is mounted on a heavy duty home-made wedge. Nice. The pier is a bit tall for visitors, so we got a small step ladder with a hand rail. We also bought a GSO 2" focuser, allowing us to rotate the eyepiece around for the visitors.
This spring we found water in the northeast corner of the building. Been a wet year. Thought it was coming from the east vent, so I boarded up that vent. The building stayed dry most of the summer, but started getting wet again this fall. I suspect the sod has built up too much around the C11 building, funneling runoff into the building. On the chance the water is coming from the channel, I found a 1/2" steel drill bit and bored out all the channel drain holes. I also caulked the seam between 7' and 20' sections of channel. Let's see if this keeps the C11 building dry.
The 3' pier has been a bit tall for visitors, so we decided to cut nine inches off the pier and get it powder-coated. Next spring we plan to run a new electrical line to the pier. Straight outdoor line to the outlet, so we won't have to worry about the floor getting wet.
We wanted to run a new electrical line to the C11 pier, but found the old line cemented into the wall. A new line would require a new outside line all the way around the building. The old line still works, however, so a new line can wait until we replace the walls. We were able to run an electrical line from the northwest corner of the C11 building to a new outside pier.
We had six inches of rain in one day, resulting in water along the east wall once again. So we dug up the sod around the C11 building and put down a yard of gravel. The gravel is now below the floor pad, so hopefully the floor will remain dry from now on.
Earlier this year we took the C11 to a local optical shop to be cleaned and collimated. This summer we tried prime focus photography on the C11. Been able to take short 10-20 second stacks, but the drift prevents anything longer. Thinking the GSO 2" focuser might be too heavy for the scope, we replaced it with a simple 2" adapter. We like the new adapter, but the mount still has drift. This fall we plan to send the mount back to the factory for a rebuild.
The C11 building has been dry and working good. Last fall we received a donated Celestron CGE-1 mount. Instead of rebuilding the C11 fork mount, we decided to fix the CGE-1 mount. A telescope repair shop in NH installed a new motor control board. That got the mount working, but still had alignment problems. Most of the time, the mount would overshoot the alignment star. The people on cloudynights.com recognized this alignment issue and a new set of cables fixed it. The CGE-1 mount now works nice. Several times this summer, I would turn on the C11 right after sunset and catch Venus, Mercury and even a 22 hour old crescent moon. I love that last alignment feature. In fact, haven't done a new alignment all summer.
Without the old wedge, the C11 now sits lower than before. So sometime before next spring, we'll get our CNC shop to fabricate a new adapter plate. A plate with longer bolts, allowing us to raise the C11 several inches.
I'd like to thank Ron Northman, Dennis Stroh, and Rocky Frees who helped in this project. Especially want to thank Gary Gangwer, for getting the roof itself up and covered. Would also like to thank David Grimard and Randall Mergener for help with the gravel.
Images Copyright © to the photographer listed. All Rights Reserved.
[Return to Top]
[Go to Home Page]
2015 Muskegon Astronomical Society, Inc.
This web page was last updated 07/29/21