Mounting a Smaller Telescope on the CI-700 Mount

I gave up on building a telescope around that 17.5" mirror would work acceptably on the CI-700 mount -- it just wasn't feasible.  I looked into building a Dobsonian mount telescope, but found that I could have a professional do it for not too outrageous a price.  As a result, the mirror is at Swayze Optical for testing and, depending on what they find is the quality of the mirror, a refigure of it.

In the meantime, I decided to put the 8" f/7 reflector that I have on the CI-700 mount.  It weighs about 25 pounds, and wow!  Big Bertha was as much overweight for that mount as the 8" reflector is underweight.  A mount that is more capable than required for a telescope is really, really pleasant to use.

This 8" reflector started out as a project of my father and me when I was in junior high.  I have since replaced the tube and a few of the parts, and it really is optically quite respectable.  Mounted on the CI-700 mount it works beautifully!  Smooth, stable, no vibration (except the wind really gets going).

Last night, I tried the digital setting circles with it.  Now, I don't think that I was exactly aligned on Polaris, but it did a pretty credible job.  I set the digital setting circles using Vega and Arcturus -- only two stars, when the more you use, the better it gets.  Then I asked it to find the Hercules globular cluster, M13 -- and at 56x, it was on the edge of the field.  That's certainly good enough.  I asked it to find M51, the Whirlpool Galaxy, however, and the results were not so wonderful.  I may have to re-read the instructions on this.

Calling It Quits On The Telescope Rebuild

I was hoping to report victory on the telescope rebuild, but I'm just surrendering.

I cut 51 inches out of the middle of the tube:

And yes, it took 19 pounds of it.

But the cutting process wasn't pretty, and neither were the results:

Worst of all, when I was done I had the same problem with the lower cage that I had with the aluminum one -- too flexible.  I am beginning to think that this is a variant of the 50 pound bike theorem.  In case you aren't familiar with it, it was first explained by a friend (okay, a gal that I was convinced that I was madly infatuated with in 12th grade, before she went to MIT), based on the problems of bicycle retention in the Boston area:

1. A 20 pound bicycle is so valuable that it requires a 30 pound lock and chain to keep it from being stolen.

2. A 30 pound bicycle is in less demand, so it only requires a 20 pound lock and chain.

3. A 40 pound bicycle is barely worth anything, so a 10 pound lock and chain are sufficient.

4. A 50 pound bicycle doesn't require a lock and chain.  Who would steal it?

This reflects bicycle and lock technology of the 1970s; today the situation is probably somewhat different.  But I think the problem here is similar:

1. A big Newtonian reflector will either be heavy and stiff or light and floppy.

2. You can have a big reflector, but you either need a $6000 mount, or it is so hard to keep in collimation that it doesn't matter what mount you put on it.

Perhaps I should just make a Dobsonian mount, and give up on using this astrophotography.  Or perhaps I will just sell the parts to someone who has more energy to devote to building a Dobsonian.

UPDATE: CloudyNights has the ad here.

When Did These Quick Release Push Button Pins Get So Expensive?

For a number of years, ScopeRoller has sold a product that we call the Quick Release Toesaver; it replaces a stop that goes on the end of the declination axis for German-style equatorial mounts.  The advantage that our product provides is that it is very quick to remove and reinstall when taking the counterweights off the declination axis.  Here is what the version for the Losmandy GM8 mount looks like, installed:

 

(Corvette not included.)

Anyway, you will notice that it uses a push button quick release ball lock pin.  You have to press the button to pull the pin out -- and the force required to remove that pin otherwise is absurd -- like 8200 pounds.  It is not going to happen by accident.

These pins are patented, and they aren't cheap.  The last time I bought some was at least four years ago, and even in quantity, they cost almost $9 each.  I just shipped the last in stock Quick Release Toesaver today, and I found myself looking to buy more of the pins.  Zounds, have they become expensive!  Like $28 each!  To make a profit would drive the cost up to a level that there would be few sales.  I am thinking of switching to the much less expensive faspins, which look like this:

This would be substantially cheaper, and I could even perhaps knock down the product price a little bit.  I suspect that for the vast majority of customers, this would work well enough.  I suppose that I will have to get a couple of these locally to experiment with, and then make a decision.

Weight Reduction Program Again

Now that I am no longer focused on pain, I can resume focus on telescope weight reduction.  The reason that I went with the solid tube was that the truss tube approach failed for two reasons:

1. The lower cage was made of a .125" thick piece of aluminum -- and it was simply not stiff enough.  The truss was incredibly stiff, and exposed that the aluminum was flexing.

2. I stupidly decided that I should minimize weight of the tubes by putting the truss connectors at the very top of the bottom cage, and the very bottom of the top cage.  Because most of the weight of the telescope was the mirror at the bottom of the bottom cage, it enhanced the flexibility problem of the aluminum tube.

The alternative solution is to use the Sonotube with the truss connector at or below the center of gravity of the lower cage.  The Sonotube, after fiberglass reinforcement, weighs .37 lbs/.inch.  The six truss connectors weight 2.6 ozs. each; the six aluminum tubes weigh .23 ozs./inch.  By cutting out 53 inches of Sonotube, I lose 19.61 lbs.  The truss connectors and tubes add 5.85 lbs.  That's a net reduction of 13.76 lbs -- which would get the telescope below 50 lbs -- quite an improvement over what I had before I started the current project, and light enough for the Celestron CI-700 mount.

There remains one uncertainty: cutting 53 inches out of the middle of the tube may create problems with how to mount the Sonotube to the Losmandy dovetail plate.  It will take about 20 inches of mounting plate to attach the telescope to the dovetail plate.  I could stiffen a piece of aluminum C-channel (which would weigh only 2.83 lbs.) by bolting some 1/8" aluminum plate pieces into the C-channel.  Alternatively, I could replace it with a piece of 1/4" steel plate that is only 2" wide (although that would be 6.5 lbs, losing some of the gain from going to trusses).  I can't imagine a 1/4" piece of steel flexing enough to be a problem.

UPDATE: I can't really do anything yet.  I am not supposed to lift any weight until the stent is removed.

Why Do Never Have A Moon When You Need One?

Okay, I solved the problem of the good finderscope being too close to the tube for me to look through by using a 3" x 2" piece of aluminum tubing.  To reduce the weight a bit (and every ounce helps), I milled some holes in the top and bottom of it which also had the advantage of making it easier to tighten the screws that hold it to the main tube, and that hold the rings to the aluminum.

Lousy picture -- the HP Photosmart is just not enough aperture, even with a flash.

The downside is that more weight means I need more counterweights, and this has resulted in what can only be called an expedient counterweight solution:

The good news is that I need to rotate the entire tube assembly a bit to make it easier to use, and that will move some of the weight closer to the fulcrum, perhaps reducing the counterweight requirements a bit.

Now, as to the title: the problem is that getting the finderscope aligned with the main telescope is usually the most annoying part of setting up a telescope for me.  The ideal way to do this is aim the telescope at the Moon; center the eyepiece on the Moon; then adjust the finderscope so that the Moon is centered.  I could use any celestial object, but the field of view, even at low power on this beast, is tiny.  The chances of finding something in the sky with the main scope, then adjusting the finderscope, are tiny.  The Moon, because it produces so much sky glow, is surprisingly easy to find.  And yes, I have aligned them using a radio tower five miles away -- and that isn't far enough to solve the parallax problem for such a powerful scope.

Unfortunately, new Moon was last night, so it may be a couple of days before I can get this straightened out.

I was thinking of abandoning the telescope tube rings, but then I realized that if I had some that were 1/2" thick, they would lower the position of the tube relative to the fulcrum by 1.5" -- which would reduce the counterweight requirements, as well as making it easier to adjust the telescope tube's position on the mount.  I have contacted some gun rights acquaintances here in Horseshoe Bend that have a welding shop; I am going to look at cutting the hexagon out of aluminum, then having them weld the pieces together.

Still, it looks good.

The Dual Layer Ring Approach Did Not Work

At least the aluminum wasn't expensive to do an experiment.  The difficulty is that it is hard to bend two pieces of aluminum precisely enough on a brake to make one a close fit to the other.  Nor did I have any confidence that screwing them together would solve the problem, or produce an adequately stiff ring for such a heavy telescope.

I still think that a single 1/4" thick piece of aluminum might be sufficient, but there's no way to bend something that size with the equipment that I have, or that makes sense to buy, for a one time construction project.

In any case, until I can actually use the telescope, and verify that the optics are worth investing serious money in a carbon fiber composite tube, I am not going to spend more than $500 on tube rings.

Last night I discovered that trying to use a finderscope without crosshairs is nearly useless.  This is a University Optics 8x50mm finder that was part of the original Dobsonian telescope.  It looks like it was cobbled from parts; the diagonal looks like it might have been broken at one time, and been epoxied back together.  I have not seen any UO finders of this type with eyepieces that lack crosshairs, so it is possible that this is just a standard .965" eyepiece that someone found and put into this finder.  I have no idea what power this eyepiece gives with this finderscope, which may be why I am having trouble lining it up with the main scope -- perhaps it is a very narrow field of view.

New illuminated finders are pretty expensive, so I think rather than just abandon the Celestron 7x50mm straight-through illuminated finder, I will use a piece of aluminum rectangular tubing to raise the rings that hold it up several inches so that I can look through it without having to put my head through the tube.

If I can persuade myself that the optics are good enough, I'll spend the money on a carbon fiber composite tube.  My wife would rather that I spend the money to buy something off-the-shelf -- but the price of off-the-shelf telescopes this size is approaching $8000 -- and they are generally 70 pounds or more -- simply too heavy for the Celestron CI-700 mount that I have.  It makes more sense to buy a carbon fiber composite tube like this one, or from this maker in order to get the weight of the telescope between 60 pounds.

Math Errors, and Turning Lemons into Lemonade

I started work on making the hexagonal tube rings for Big Bertha last night, and having completed the first four half hexagons, my wife suggested that I verify that they were going to fit.  And they did not.  They were too small...by about 85%.  Let's see, sin 60^o is .866.  Where did I go wrong?

A regular hexagon contains six equilateral triangles.  For some stupid reason, I assumed that the height of each triangle was the same as length of each side of the triangle...which is clearly wrong.  (Well, maybe in a non-Euclidean geometry somewhere, but even there, I doubt it.)  The height of each triangle is sin 60^o times base of the triangle.

In addition, the bend operation ends up making each side of the hexagon about 1/8" shorter than where I put the bend marks, for reasons that I can intuitively see but have trouble articulating.  This amount seems to be same even when I make hexagons that are supposed to be 2" on each side; it isn't proportionate to the dimensions, but a fixed amount.

Rather than just recycle the aluminum, I have decided that it makes more sense to complete this set, creating a ring set for a 17.5" outside diameter telescope tube.  I suspect that if I offer it at my materials cost (about $20, including the screws holding the inner and outer layers together, and the thumbscrews that hold the hexagon halves together), there will be someone building a 14" to 16" reflector who will jump at the chance.  I will also get the experience of building this before starting on the one for Big Bertha.

UPDATE: Part of why I am confident that someone will buy my "lemonade" if it comes out okay is the price of factory rings this size: $429 per pair.

Bending 1/4" Thick Aluminum

One of my readers suggested an interesting way of solving the problem of needing to bend 1/4" aluminum into a hexagon: notch the aluminum where you need to bend it.  I could use my vertical mill and an 1/8" diameter end mill to put a 1/16" deep, 1/8" wide notch in 1/4" aluminum plate, then bend the aluminum on the metal brake.

I would only be bending 3/16" aluminum (which the brake should be able to do) at the notch, and because I could place the notch quite precisely, I would get much more exact results than simply marking the line in 1/8" inch aluminum.  I wouldn't be able to use the 1/8" aluminum sheet that I have, but I suspect that it would be stiffer than the layered approach I discussed recently, and perhaps no more labor, since I would not have to drill and tap the individual layers.

UPDATE: This did not work as I expected.  I had some 1/4" aluminum to experiment with, and the results were disappointing.  I used my new 1/8" end mill (which notched the aluminum beautifully) to cut a .125" wide, .125" deep slot, then tried to bend it to a 60^o angle.  Even though it was only a 1/8" thick layer that I was actually bending, it produced a slight break, rather than a smooth bend.  I thought that perhaps the problem was that I needed a wider slot relative to the depth, so I next cut a ..19" wide, .100" deep slot -- and that did not want to bend at all.  I think I will go for the two layers of 1/8" aluminum instead.  I can drill and tap the layers for 8-32, 1/4" long screws right next to each vertex, and have something impressively strong.  (The shear strength of a single 8-32 steel screw is > 2700 pounds.)

UPDATE 2: I am beginning to think that what make have happened is that aluminum work hardens, and cutting the notch with the end mill may have work hardened the aluminum underneath the notch enough to make it too brittle.

More Little Discoveries

I actually did roll Big Bertha out last night....but discovered that I could not actually look through the finderscope to get the telescope aimed at Saturn.  Why?  Because before, the finderscope was mounted low enough on the upper cage that I could look through it because my cheek was actually inside the tube.  Now, that's not possible.  One solution was to machine something that lifted the finderscope away from the tube.

The other alternative is to use a right-angle finderscope (and I have one of those lying around).  The downsides are:

1. I have to move one of the mounting rings on the current finderscope bracket down an inch, because the right-angle finderscope is a bit shorter.

2. The right-angle finderscope is a very nice University Optics 8x50mm, but the eyepiece lacks not only illuminated crosshairs -- it lacks crosshairs.  I think I will be able to move the illuminated crosshair eyepiece from the current finderscope to the new one -- perhaps with a little bit of making some sort of adapter on the lathe.

I confess that having switched to a straight through finderscope some years ago, I find the idea of switching back to the right-angle finderscope increasingly attractive.  While it is an unnatural feeling to use one of these on a telescope, having the finderscope sitting 5" away from the scope tube isn't wonderful, either.

On the plus side, hunting for the right-angle finderscope this morning in the telescope garage also brought to light some of the self-adhesive black flocking material and another Losmandy counterweight.  Right now, I have corrected the additional weight of the solid tube with a bucket that holds one of those citronella candles -- it's a bit silly looking.

Big Bertha In Daylight

Big Bertha in previous incarnations has never worked out terribly well in daylight because of the open tube structure; stray light is always going to be a problem.  I was therefore at first unsurprised when I used it to look at the television towers on top of Big Basin (which is a number of miles away) this afternoon.  But even at 222x, I was quite pleased with the optical quality.  Now, if only it gets dark to see something before I have to go to bed!

During church this morning, I suddenly realized a solution to a problem that has been bothering me about making hexagonal telescope rings to hold Big Bertha to the mount.  It would be impossible to use this metal brake that I have to bend 1/4" aluminum -- but 1/8" thick aluminum is probably too flexible.  But what I bent 1/8" aluminum into the inner part of the lower half of the hexagon, and another 1/8" thick piece to be effectively the outer layer of the hexagon?  Then bolt them together at the vertices of the hexagon, so that I get most of the stiffness of a 1/4" thick ring, with the ease of construction of the thinner pieces?  Even better, I have a large sheet of 1/8" thick aluminum that I could use for this purpose.

I have therefore been experimenting with making a small set of hexagonal telescope rings (like about 2" across) just to get some practice at making them on a small scale first.  And yes, 1/8" thick rings feel stiff enough, but because of the square/cube law, they almost certainly would not be stiff enough for the size that I need for Big Bertha.

I feel the need for a metal shear, but by the time I see the cost and size, I will probably just go to my friendly metal store and pay them to use their shear instead.  The bandsaw that I have just doesn't do that good a job on metal (it is really a woodworking tool), and cutting small slices with the chopsaw is impractical.

Lessons in Precision

I explained to a ScopeRoller customer a couple of weeks back that the cost of precision is a sliding scale, and an asymptotically increasing scale at that.  "Fifth of an inch precision is free; hundredth of an inch precision costs a bit, thousandth of an inch precision costs a lot."  I had that lesson myself over the weekend -- but that cheaping out on precision has its own costs.

I mentioned that I was not happy with the ability of Big Bertha to hold collimation, and that I suspected that the spider holding the diagonal mirror was the problem.  As the telescope went up in altitude, I could see the laser beam from the collimator moving as well.  I decided to replace the .0325" thick aluminum legs, which were held in position by tension, with .040" thick steel legs.  These should be at least four times as stiff as the aluminum legs because of material and extra thickness.

In addition, the cylinder that connected the legs to the diagonal mirror holder was made of acetal, and left about an inch of the 1/4"-20 screw that holds the diagonal mirror holder unsupported.  My thought was to replace the acetal cylinder with a full length aluminum cylinder instead.  Because aluminum is about 20 times as stiff as acetal, weight for weight, I could go to a somewhat smaller cylinder and have far better stiffness.  I had hoped to find a piece of aluminum tubing 1/4" ID with about an 1/8" wall, but the local metal supply store had nothing like that, so I took a piece of scrap .811" aluminum rod, turned the ends, then bored a 1/4" hole through the middle with the lathe.

Here is the spider, waiting for the flat black paint to dry:

The trick here is that the legs needs to be 120^o apart, so the attachment holes on the cylinder where the legs mount need to be 120 degrees apart.  I decided that yes, I could use some of my precision gadgets to make them 120^o +- 0.5^o -- but why bother?  I could just measure with a protractor, and that would be good enough.  Right?

Wrong.  It turned out that they were far enough off that the cylinder was not centered in the tube -- not even close.  So I ended up doing the extra work to do it right -- and it wasn't really much more work than doing it sloppy and wrong.  Here's a picture with another piece of aluminum in place, not the one that I drilled:

It sounds a bit like a Rube Goldberg contraption, but it isn't.  The tilting table on the right is at 90^o; mounted on the tilting table is a rotary table made by Sherline.  It has a coarse measure that reads to the degree (and you can certainly interpolate quite a bit more finely) and a vernier measure (the red handwheel) that gives marks down to 0.1^o.  Mounted on the rotary table is a 3" three jaw chuck, which held the cylinder I was trying to mark.  With this, I was able to determine that my old tapped holes were as much as 10^o off of where they should be.

Use precision tools for precision work; use brute force tools for brute force work.  I could have tried to drill and tap the holes with the vertical mill, but because the cylinder was 3" long, and the chuck really does not hold something that long terribly well under any real load, it was not likely to be a success.  So I settled for marking where the holes should be with an end drill for angle and length down the cylinder, and then drilled and tapped the holes on the drill press.  Once you have marked the hole's center with an end drill, the twist drill in the drill press does a pretty impressive job of following that hole, even if you aren't exactly on location with the twist drill.

Anyway, here it is installed:

Yes, there is a slight bend to the arms; better to have some compression rather than tension, and by bending the arms slightly, I was able to get the diagonal centered to better than 1/16" -- perhaps even as close as 1/32".  (The curve may also smooth out diffraction spikes -- the four or six arms that you often see in observatory astrophotographs.)  This is about as much as I can expect or even need for this part of the problem. 

There is no longer the gross and obvious miscollimation of the diagonal as the telescope rises in altitude.  The sky did not clear here last night until I was exhausted and ready for bed, but perhaps tonight I will roll it out and see how it goes.

Since The Weather Is Bad Anyway...

I mean, it snowed here on Wednesday!  And the weekend has been nothing but clouds and rain.  Once I successfully adjusted the spider legs to get the diagonal mirror centered in the telescope tube, I was able to collimate the telescope quite quickly.  But when I changed the angle of the telescope so that it was no longer horizontal, the collimation was off again.  I am pretty sure that the reason is that there are two parts of the diagonal holder that are too flexible:

1. The legs are made of .0325" aluminum -- and that is probably too flexible.  I can replace them with 20 gauge steel, which would be the same thickness (and optical path interference) but three times as stiff.  I may also go from 2" front to back to 3" front to back, which would improve stiffness even more.  The steel will weigh more, but only an ounce or two.

2. The central body of the diagonal holder is made of acetal, which is easy to machine, but has about 1/20th the Young's modulus (flexural deflection) of aluminum.  I think I will pick up 3" of .25" ID aluminum tubing, perhaps 1/8" wall, and use that as the new central body.  That should improve stiffness substantially with no  real difference in weight.  (The existing acetal part is fairly thick, so probably about the same net weight.)

Spiders

The structure that holds the diagonal mirror in a Newtonian telescope is called a spider, because it typically has several legs.

Last night, I was finally ready to do collimation of the optical path, and I discovered that no matter what, I could not get the diagonal mirror centered in the tube!  Why?  Was it this far off in the old tube?

No.  I had to cut down the mirror cell bottom plate because the inner diameter of the tube is 19.875" instead of 20.25"; I forgot that the spider required similar surgery.  It was easy to forget, because the legs of the spider are made of .030" aluminum (in the interests of minimizing weight and diffraction of light), and they are flexible enough that they just bent to fit.  But they did not bend symmetrically.

My first concern was that I was going to have to start from scratch on this, but it turns out that there was enough spare room where the legs attached to the center point that I was able to redrill the holes in the legs 3/16" over and solve the problem.  The picture below was taken with my little HP PhotoSmart camera, and it isn't terribly sharp, but you can see where I moved the holes:

I may buy a ring roller at Harbor Freight today to make tube rings.  One of the reviews indicated that one of the plastic parts broke almost immediately, which doesn't surprise me.  I will be rolling 1/8" or 3/16" thick aluminum, so I doubt that I will be stressing it much, and it sounds like other than that one part (which I can probably machine a replacement for out of aluminum), it is an adequate tool.

UPDATE: After spending a bit of time, I discovered that because the spider legs are held in tension (rather than compression, as it more typical of spiders), it is very dependent on the order of tightening to keep the body centered.  After a little experimentation, it seems to be well-centered, and collimation went very well.  But I am a bit concerned that it might not hold collimation as it moves across the sky.  Perhaps a more conventional (that is to say, commercial) spider would be a better choice for this.

Frustration: I bought the ring roller at Harbor Freight -- but it appears that some previous purchaser had tapped the taper pin that holds the handle to the roller before putting the handle in place.  There seems to be no way to remove the taper pin now without drilling it out.  So tomorrow I will go back and ask for one that has the taper pin not yet inserted.

Advantages of Fiberglassing the Sonotube

I had mentioned that I was going to put the polyester resin on the edges of all the holes, big and small, that I made in the Sonotube, to reinforce the edges and prevent fraying.  This worked like a charm.  I discovered that it also made the holes just a little small -- which meant redrilling the holes for the small ones.  I actually found the bolts that hold the mirror cell in place were threading into the holes!  I would not count on threads in this stuff holding any real load, but at least the bolts weren't flopping about in there!

The eyepiece focuser hole, however, required some filing -- and unlike Sonotube, which doesn't file well, the composite material I produced actually filed very nicely indeed -- better than the paint, which chipped a bit.  A very nice result.

Big Bertha, 3.0

I still need to do a bit of adjusting and collimation, and some touch-up paint, especially on the straps.  I was hoping to get to that tonight, because we have a clear sky, but I had orders to fill, so....

But everything is back in the tube -- and it appears that it is slightly heavier than it was before.  Oh well.  At least it should be rigid.

The following two pictures show what happens when you use a flash, and don't use a flash.  From the first picture, you would never know the inside of the tube is black.

Lack of Blogging...

My wife had a shoulder repair operation yesterday at St. Luke's, and pretty much the whole day was spent with getting her checked in, surgery, post-op.  I probably should have brought my laptop, but I wasn't expecting the entire day to get consumed.  I will say that I am always very impressed with the medical and support staff at St. Luke's (and every other hospital here in the Boise area).  Friendly; courteous; concerned.

She is doing okay, but considering that they went in and removed bone spurs inside her shoulder, she is in a bit of pain.  I stayed home yesterday and today to take care of her; I just can't imagine what single people do in these situations.

The evenings have been lost in the great telescope rebuild.  I discovered that all the work I put into trimming the C-channel pieces down was a mistake; there was not enough clearance for attaching the steel straps without a lot more precision in cutting than I could easily do with a bandsaw, so I started over, and it was much faster.  When drilling the attachment holes in the steel straps, the straps are thin enough that they briefly went red-hot as I drilled them!  (More oil next time.)

The new versions were faster to make, in spite of breaking the 1/8" end mill that I was using to cut the slot.  I finished with a 1/4" end mill, which looks less elegant for an .030" thick piece of steel strap, but it isn't like you can find a 1/8" end mill in Horseshoe Bend.

The upper picture shows the static end of the strap; the lower picture shows the screw that tightens it down.  Unfortunately, the thumb screws I bought at Grainger just weren't long enough.  A 1" long screw thread sounds good enough, but once the captive nuts were on it, that was not enough travel to be useful, so it now has a conventional hex head bolt.  I may replace those when I can order a thumb screw with a longer thread.  They work well -- although it takes a while to crank them down enough.  Of course, that also makes it less likely that I will overcompress the tube with the straps, so I guess that's a positive.

The mirror cell has turned out to be a "I was too clever for my good" moment (as have several in the project).  It turns out that having the mounting brackets separate from the bottom plate was a great idea -- except that the bolts holding the brackets to the bottom plate can't get past the mirror once the brackets are mounted inside the tube.  So I guess I will do what I was trying to avoid -- tap the mounting brackets so that I can screw the threads in from the outside of the tube.  This is a blind operation, but at least I can see the brackets from the rear.  Some other mirror cells have you trying to find the threaded holes in a circular casting where there are no real clues as to location.  The slots in the mounting brackets were for 1/4"-20 bolts, so it is easy enough to tap these for 5/16"-18 bolts -- which, fortunately, I have some in black oxide finish that I can use for this.

But that will be tomorrow night, or maybe the night after.  I actually have ScopeRoller orders rolling in, and I need to get some of these filled.

No Wonder Ken Dauzat's Rings Didn't Respond...

Ken's Rings is now closed as of April 15, 2012 and is no longer taking orders.

Unfortunate.  He made a nice product.

How To Attach The Tube (Cont.)

Here's a diagram of a simple to build tensioner for nylon webbing, stainless steel strap, or even belts.

The fixed side attachment doesn't move; I tap this side of the C-channel for a 1/4"-20 bolt.  Depending on the strapping material, I may need a large washer to distribute load across more surface area.  For nylon webbing, I would triple the layers for this attachment point.

The adjustment side attachment consists of a 1/4"-20 tapped hole and a bolt (probably a thumbscrew) with two nuts locked in position at the end of the bolt.  The way that I have found works most easily for this is to drill through a hex head nut into the bolt, tap the hole for 6-32 threads, and use a 6-32 set screw to lock bolt and nut together.  The strap goes through the slot in the side of the C-channel, and is held by two washers inside the captive nuts.  The strap can rotate between the washers, so it isn't a tight fit there.  You have to the get the length of the strapping pretty precisely correct for a snug fit, and then you turn the thumbscrew to clamp everything down.  This also gives some room for strap stretch over time.  If you reach the limits of the bolt, you can redrill the holes on the fixed side attachment, but I am considering this a short-term solution to the problem.

Limiting yourself to a thumbscrew not only simplifies tool-free adjustment, but also reduces the change of putting so much force on the tube that it damages it.

UPDATE: Good news: I asked my pastor this morning where I would steel strapping material on a weekend.  He just happened to have a 50 foot roll of .030" thick, 1 inch wide steel strapping material.  I started work on the device above to use some of this.  I discovered that the vertical mill definitely works better with the longer screw holding everything together; I also discovered that once again, the single most important factor in milling is getting the workpiece really well clamped in position.  Anything that lets it jostle in the vise is a problem.  I also discovered that the 1/4"-20 thumbscrews I had...were something metric, not 1/4"-20.  I guess I will buy two of those tomorrow.

Telescope Project Marches On

There was some pooling of paint at the bottom of the tube when I painted the first coat with the tube on end, so for the second coat, I went back to the suspension approach that I used when applying the fiberglass resin.

The method of holding the tube in the air at that stage wasn't very good, because sometimes the tube started rocking, and then the aluminum tube holding it would go flying off the chairs.  So I came up with a way to prevent the aluminum tube from getting too far afield:

Unlike the first coat, which did not stick spectacularly well to the fiberglassed Sonotube, the second coat stuck quite well.  It still wasn't very even, so I sanded it after the second coat as well, which again evened out the surfaces a bit (although taking off a bit of paint in the process).  There were still some pretty big depressions caused by air pockets in the area where I had used fiberglass cloth, so I mixed a bit more resin, and tried to fill in the holes.

I still won't call the result, after the third coat, "beautiful," but I think it will do, at least until I can talk myself into believing that the big money for a carbon fiber composite tube that weighs 13 pounds less makes sense:

I mentioned a few days back that I was going to epoxy some steel rod on either side of the center line of the tube where it attaches to the dovetail plate to prevent rocking, which would eventually damage the tube.  Then, yesterday, I mentioned that it would be better to have tube rings that lock onto the outside of the tube and screw directly to the dovetail plate.

I am still waiting on a price quote on those rings, and I started thinking.  I noticed that at the edges of the dovetail plate there are a bunch of 1/4" holes.  Hmmm.  Could I mount supports in those holes to prevent rocking?  My first thought was to machine some 1/4" (or perhaps slightly longer) plugs that would provide the support?  I could even just use 1/4" hex head bolts; the heads would provide just the right elevation to prevent rocking (although at the risk of marring the surface of the tube).

But the more I thought about it (and was glad that enamel dries so slowly), it occurred to me there was an even better solution that gives me the flexibility to rotate the tube, not drill permanent mounting holes, solve the rocking problem, and involves minimal use of materials.

1. I take the C-channel which was used to mount the old scope to the dovetail plate, and cut off two 2" sections (preferably the ones that have 1/4" through holes already).

2. Shorten the legs of the C-channel down to 1/16" inch (since I no longer need long legs for stiffness of the section).  Now I have a round tube to flat base adapter.

3. Drill and tap 1/4"-20 holes in each side of the two sections.

4. Buy four leather belts at the thrift store.  (I need about 60 inches total length, and preferably 1 3/4" wide belts.)

5. Use a 1/4"-20 bolt and a washer to hold one end of each belt to each side of the C-channel.

6. Use the belt buckles to secure the tube to the C-channel sections.  (This means that I will need to get roughly similar belt buckles when picking out belts.)

7. Perhaps drill some holes in these belts to get them in a position where I can tighten down the tube without crushing it marring the surface much.

Now I have the 60 pound or so telescope load held by two belts, which is, I think, sufficient to prevent anything from moving or working loose.

An alternative would be to look for some flexible 2" wide stainless steel straps, and apply felt to the inside to protect the tube.  But then I have to figure out a way to secure the steel straps so that they can be loosened without having falling completely off.  The Cave Optical mount that I had long ago used this approach, with a screw brazed into one end of the strap, so that you could loosen the straps at one end, rotate the tube, then retighten.  If I could find something like this (with roughly 60" long straps) that had some way to tighten and loosen tension, this would be preferable.  But the belts might be a quick way to get the telescope operational, and then worry about rings at a later time.

UPDATE: Or Velcro?  This claims that the closure shear strength is 11.0 pounds per square inch.  I can buy a 15 foot by 2" wide piece of Velcro at Home Depot for $28.97, what they call industrial strength Velcro.  If I had ten inches of overlap of hook and loop (and if I understand what they are claiming for the closure shear strength), that would be 220 pounds per strap.  That seems more than enough.  It would not be as elegant as aluminum rings, but it would be light, I could pick up the Velcro tomorrow, and put the telescope together tomorrow afternoon.

UPDATE 2: Or perhaps use nylon webbing with buckles.  The webbing has a tensile strength of 5500 pounds (probably more than the buckles that come with it or where I would attach it).

UPDATE 3: The more I think about it, nylon webbing and Velcro are likely to stretch under load, and that sounds a bit dangerous.  If I can find some steel straps at Home Depot, I could put a bolt through the adjustment end, and use a wing nut to tension it.

Roll Your Own (Rings, That Is)

The quick but inflexible solution for attaching a telescope to the dovetail plate is to drill holes in the tube and turn bolts through the tube into the threaded holes on the dovetail plate.  The more elegant solution (because it lets you rotate the tube into more useful viewing positions, with a little bit of effort) is to use rings like these.  I have bought Ken Dauzat's rings before for another telescope, and I was very pleased with them.  I have asked for a quote for the monster tube, and I am shuddering a little at the likely price.

But I saw this device for rolling your own rings from either tubing or flat.  It's $169, but that doesn't seem like  a particularly high price if it comes out the same price as buying one pair of rings.  Does anyone have experience using such a tool?