Posts Made By: Doug Scobel

I'm trying to find commercial providers of mirror cells suitable for a 16" truss Dob project I have going on. Webster sells a "Cadillac" for $800.00 but that's a little steep. Anyone know of others?

Yes I know I could make one, and it may come to that, but I'd like to know what's available commercially.

Thanks,
Doug

Hi, all, what are folks using as a substitute for the discontinued Wilsonart #50 finish laminate (typically in the Ebony Star color/pattern)? I've been scouring the yellow pages and Internet for cabinet makers and suppliers that may have some old stock but to no avail. Not even my local Wilsonart outlet has any. Has anyone had success finding any? I just need a sheet about 2' square. I don't care about the color - it will be on the bottom of the rocker box.

I could kick myself. Should have bought a couple sheets back when you could get it at Home Depot. Of course, hindsight is 20-20.

Thanks,
Doug

Has anyone used tape to attach laminate to the bottom of the rocker box? Good or bad results?

My motivation is that I want to use FRP, but it's not as durable as Ebony Star, and will eventually have to be replaced. I'm thinking something like double-sided carpet tape, something that would make removal later easier. The laminate just needs to stay put, so that it rotates on the pads instead of spinning on the bottom of the rocker box. Gravity should keep things nice and flat and in contact.

Conventional wisdom says to use contact cement, but that's pretty much permanent. I've heard that you can use a heat gun and you should be able to get it off intact, but I would think you'd still have a gooey mess to clean up once it's off.

Opinions?

Thanks,
Doug

Regarding a focuser, I recently obtained a nice dual rate Crayford style focuser from Moonlite Telescope Accessories (www.focuser.com). Nice, smooth action, although not quite as nice as say a Featherlite. But at only $265.00 it's a bargain. I'm really happy with mine, fine focusing is a snap.

The primary (pun intended) tool you need is patience and the ability not to get frustrated. Bad, or I should say unpredictable things WILL happen, and you need to be able to objectively analyze what's going on to figure out what to do when things don't go according to plan. This is where the help of someone else or a group with mirror making experience is invaluable. All the books and all the web sites simply can't cover it all - there are too many variables.

If you want to save money then don't do it. You'll end up spending nearly as much as what a finished mirror costs. But if you enjoy building things yourself, there is nothing like the personal satisfaction you get when you finally get a good mirror, accurate across the entire surface to within a couple millionths of an inch, that you made yourself literally with your bare hands.

That being said, if you are working alone, then you have two chances of getting that nearly perfect mirror on your first try - slim and none, and slim just left! Seriously, there is too much to be learned with just one mirror, unless you are willing to have the project go on for years. My personal suggestion is that you try to make an OK 6" mirror, then go for a better 8".

I made my first mirror when I was 14, a 6" f/9 (it was supposed to be an f/8, but I had trouble getting the curve deep enough), by myself using Thompson's "Making Your Own Telescope". It probably came out no better than 1/2 wave - I don't know - I really didn't know how to test well yet. After using it for a while, I re-ground it to f/4.5. This ended up a little better, maybe 1/3 wave. Next I made an 8" f/8, which ended up about 1/4 wave. Then I refigured my crappy 1 (yes one!) wave Coulter 13.1" f/4.5 to about 1/4 wave. Now I'm finally refiguring my 8" f/8 to be as good as I can get it - I won't settle for less than 1/10 wave.

My point is most people NEED the experience of easier projects before going on to more difficult ones. There's as much (if not more) art as science when it comes to figuring, which simply takes time to learn.

Regarding getting a blank and tool with a pre-generated curve, it may save a little time, but you kind of have the cart before the horse. Rough grinding the curve is probably the easiest part of the job. Fine grinding without leaving larger pits behind is a little harder. Polishing it out completely is a little harder yet. Figuring is by far more difficult, by maybe an order of magnitude. If it's your first mirror then I would say grind the curve yourself - you'll learn from it.

Regarding kits, Willmann-Bell sells nice kits and supplies, well packaged, and they have friendly service. Their mirror making supplies unfortunately are not on their web site, so you'll need to either call them or get one of their catalogs.

Thanks to all who responded. It turns out that indeed the lap was too hard. I've since moved upstairs to the kitchen (to my wife's chagrin) where it's warmer, plus I'm using softer pitch. Things are now working much better, I actually dug a hole in the middle of the mirror by using a too long stroke (which would have still left a hill in previous efforts)! I'm finally on my way. Yippee!

Dave –

An often overlooked but critical aspect of collimation is ensuring that the focuser is square to the optical path. If it is not square, then you will have great difficulty in getting the rest aligned. In fact, this is where you have to start, before you align anything else.

I struggled with collimation in my 13” f/4.5 dob literally for years until I discovered that my focuser was a good 5 degrees off laterally. Once I got it straightened out, the rest just fell into place.

This is where a laser collimator really shines (pun intended!). You can measure from the end of the tube to the projected dot to ensure that it is square longitudinally, and ensure that it hits the center line of a dowel or threaded rod inserted through the mounting hole of your exactly centered (side-to-side) secondary support to ensure squareness laterally.

After you’re sure that the focuser is square, insert the secondary and get it positioned correctly under the focuser, then adjust it’s rotation and tilt, and then finish up by adjusting the primary. You can do these with a Cheshire or a laser, or both.

Best Regards,
Doug

A Newtonian has an even number of reflections, so the image is not reversed, just rotated. Simply rotate your chart to match what you see in the eyepiece and you're there.

This goes without saying, but I'm going to say it anyway. Whatever you do, MAKE SURE THAT YOU HAVE A FOOLPROOF METHOD TO KEEP THE FILTER ON THE END OF THE TUBE. For obvious reasons, it must not be allowed to fall off, blow off, get knocked off, be pried off, etc. etc. etc. And remember that it is darn near impossible to make something foolproof, because fools are too ingenious!

Doug

Thanks for all the input so far guys, but what I'm driving at is what material works best as a telescope tube? e.g., fiber insulates very well, and so would be resistant to temperature changes, while aluminum is a good thermal conductor, so it would cool down to ambient or even below ambient more quickly. So what's better? The bottom line is, what material would give me the best images most consistently?

Thanks,
Doug