Posts Made By: Mark Eschbacher

Charles,

I have been to their shop, and yes it IS worth the trip.

My impression is that they do a good job listing what they have in stock on the website. I'm sure there are things there that are not listed, however, I'll bet they are the more oddball items. (When I was there they had just taken delivery on a WWII Norton Bomb Site and were verifying that the self destruct charge had been removed! They said it was not for sale at that time.)

There is nothing like seeing all this stuff in person to get the ol' creative juices flowing!

I found the staff to be very helpful in a "laid-back" way, so be sure ask if you have questions and don't hesitate to tell them what you have in mind.

However, be aware that they are only open to the public on Saturdays (they spend the rest of the week working on internet orders).

Enjoy!

Mark

A closer view of the alt and az bearings

The focuser end showing staves visible inside

And lastly the bottom end

This weekend I am at precisely that point in my 16" K-B truss dob. I used the spreadsheet available on the web to get a rough mirror box height then added 2 inches. (Be sure the spreadsheet value used is measured from the BOTTOM of the cell frame to the top of the mirror box/balance-point) I feel much more comfortable with empirical data, so I intend to find the final balance point using an improvised balance beam.

First, I plan determine the CAGE'S balance point and place a pencil mark on that point. Next I will find the fully assembled mirror box's balance point and mark it also. I will then find the balance point of a 2x4 I will use as a balance beam. With the mirror box and cage hanging from the beam by ropes or straps that I can move back and forth on the beam, I will keep the BEAMS balance point on the fulcrum so that the moment arm of the two sides of the beam cancel each other. When the entire assemblage is in balance the distance from the fulcrum to the ropes will equal the distance from the cage's and mirror box's individual balance points (previously marked) to the scope's balance point.

Once the scope's balance point is known, I can trim the box down to the proper height. I figure the moment arm of the removed wood should be negligable.

Does anyone see a flaw in this plan? I guess those perfectionists out there could repeat the process after trimming and make the whole process iterative to the "sixth sigma"

Vince,

I have just recently completed this step of the process with my f/5 16". Like you, I didn't have a convenient 1 mile view since my property is surrounded by 70' trees. I have a low profile focuser, and that combined with my variety of eyepiece brands, barlows, and glasses/no-glasses viewing left only 1/4" on either side of my drawtube. Therefore, I opted to use the accurate but cumbersome process of building a sliding rack from scrap 2x4s that would hold my secondary cage.

The rack could slide on holes drilled through the rack that fit over 4 of my uncut 6' truss tubes. By sliding the rack holding the cage, I could bring the zenith stars to focus. The bottom of the truss tubes were firmly pocketed into a similar non-sliding rack that held the mirror box pointed at the zenith. With this set up I was able to use small plastic spring clamps (99 cent bin at the hardware store) clamped to the poles to hold the cage in place at focus, try out all eyepiece permutations, then measure the mirror box - cage separation. I took care when building the racks that the uncut tubes were equidistant from the central axis of the scope, then measured the separation at each pole to arrive at an average.

I took Kreig and Barry's advice and cut the poles 2" long. After installing the tube clamps on the mirror box, I assembled the scope with the overly long tubes and went back outside. Nothing came to focus; but I quickly determined I was OUTSIDE of focus (WHEW!). Three rounds of trimming 1", then 3/4" then 1/4" brought me right back to my original measurement and spot-on focus with the drawtube and collimation bolts set at their mid-points.

Sure it's cumbersome, but reassuringly accurate (plus you get a sneak-peak prior to first light)!
Using the actual components eliminates the "OOPS, I forgot to include the focuser board thickness" type of errors I always seem to run across when just measuring.

If I were to use the measure-only method I think I would cut the tubes 4" long and whittle them down 1" at a time until you can get something in focus, then measure exactly what to trim to put you where you want to be.

Mark Eschbacher

"The hurrieder I go, the further behind I get."

I have just completed a 16" (first light anticipated this Saturday, more info to follow) using the drywall screws to hold the split blocks to 1/2" Apple Ply Plywood as directed in the Kreig/Barry dob book. I too was concerned about holding power so I selected drywall screws that were 1/4 inch too long and cut the tips off with bolt cutters prior to installation. I thought this would allow more threaded surface area to bite into the wood and reduces the amount of taper that seemed to me more likely to tear out under load.

Another thing to consider is that there is a significant bolt that will go all the way through the split block and plywood that is used to tighten the truss tube in place. So the only time the drywall screws are used as support is when the knob/bolt combination is not tightened. I have found that with solid altitude bearings and no handles cut into the bearings or box, that the split blocks are the best handholds for carrying the mirror box. In my case the mirror box weighs 65 pounds, so it is advisable that if, like me, you are using the blocks as handles when lifting, that you tighten the knobs first; even if there are no truss tubes in the slots.

That said, mine have been quite solid even when lifting without tightening the bolt first. Probably because the stress on the screws is attempting to shear the screws, not pull them out. However, I would not recommend this if your 1/2" plywood has a softwood core as is typical of what is available at a standard hardware store. I have had these screws strip out on a previous scope using an oak veneered softwood plywood, but it still works because the bolt holds everything in place.

If you decide to use my modified drywall screw method I recommend smoothing the ragged edge of the clipped screw with a file and applying some paraffin to minimize tearing of wood fibers when installing the screws. And don't forget to pre-drill the screw holes, but no wider than the shaft of the threaded part of the screw.

Mark Eschbacher

And a closeup of upper cage showing tiger maple focuser boards. The eyepiece is a Surplus Shed military (note olive drab color)60mm plossl that has so much glass and so little metal rim that the image fills your field of view even though it has about TWO INCES of eye relief!

"Port hole"?... fughedaboudit! This is like hanging your head out the window 8O

Shure it has "sweet spot" issues and a bit of comma; but I love it.

I am a member of the Lehigh Valley Amateur Astronomical Society located in Allentown, PA. Our club has been working for several years on installing a 40" f/16 cassegrain observatory at our dark sky site. Figuring and polishing remain to be done on the optics but the mechanics are mostly complete. Photos can be seen at the following link.

http://www.lvaas.net/copper/thumbnails.php?album=27

A very ambitious project for which the dedicated members who donated time and have overcome seemingly endless problems should be rightly proud of!

I am curious as to where this scope would stand (when completed) among privately owned scopes. I seem to remember a web site by a group in california nearing completion on figuring a very large (40-60")mirror a few years ago.

I feel a little exposed being the contrarian here... but based on my recently completed 16" truss Dob, I feel best installing the mirror in the cell, adjusting the sling, and then CAREFULLY lowering the box down around it before attaching the two assemblies. This way the mirror is only moved a few inches from the storage crate to the cell and only a few inches off the ground before it is surrounded by the supporting cell. This way I see less chance of dropping the mirror, dripping nervous sweat on it, exposure to errant spittle as you curse the lack of finger room, chipping the back edge on the retaining clip top-nuts, or being startled at an inopportune time by your cell phone going off in vibrate mode. 8O

Naturally, all this is done after the construction is complete. I agree that you don't want to risk dropping anything on the mirror, and the mirror is HEAVY, so it is easier moving parts around without the mirror until all is assembled.

I currently have my box and mirror/cell separated so that I can work on blackening the inside of the box, add a mirror face fan, and add a beautiful dark wenge veneer to the top of the box. I just set the box on a rug, supported by the collimation knobs, and removed the bolts holding the cell to the box and lifted the box off. The mirror is safe in it's cell and covered with a piece of cardboard...just in case.