The Big Think

I’m still in the design phase of the telescope and I keep going back and forth over whether or not it’ll be motorized. There are definite plusses to having a motor on it (easy tracking, astrophotography, coolness, etc), but I still remain nervous that I would mess up the beautiful design by hanging an ugly and inaccurate motor off of the side. And what about power? Would I have to run an extension cord to the house? Batteries? I’m starting to think that going drive-less is the way. But then I got a look at this picture:

and thought “that’s it!” I’ll make a solar panel to drive the motor!

[beat]

on a telescope.

[beat]

(rolls eyes at self)

Had a rather long night last night with a bout of insomnia from 4-6:30am. No matter, though, because I was able to get some good mental design work done on the telescope. I have a good rough idea of the case that I’m going to build for it. It’s roughly modeled after a beautiful paduak and curly maple briefcase I saw. Still have to figure out a way to do the inlay work, and the exact design/materials for the inlay.

Speaking of inlay, I’ve come to a decision about a spectacular detail on the tube of the telescope, but I’m going to keep that one under my hat. Grin.

It may not look like it, but “work” is proceeding on the design. 99% of it is mental at this point (in more ways than one), but once I get the design figured out in my head and go through a virtual imaginary assembly, I should have most of the build issues worked out. That’ll save me grief and time once the construction begins. I still anticipate that it’ll be 4-5 years until the whole project is completed. It’s been a real joy to just be able to spend months thinking and incorporating different ideas that I see into the design. No rush to commit to anything or start building immediately- I’ve got plenty of other stuff to build in the meantime.

Speaking of which, time to go to the shop this afternoon. The sides of the ent. cent. are almost finished (except for the trim and doors). Next step is the center case, then I’ll go back and face frame the whole thing. Then build doors. Construction stalled this past week because of other commitments, but it should get back on track this week.

Here’s a nice wooden telescope.

So go ahead and return the Sketchup book to the bookstore or Amazon. I know all my friends and family members went out and got it after my post the other day. I came across it at Barnes and Noble tonight on 30% discount, which is only $1.50 more than I could get it through Amazon. So I went ahead and bought it. Looks like it’s a lot of good review followed by several chapters that go over concepts that still flummox me. I’m really looking forward to working my way through it, particularly with the online YouTube additions.

Found some telescope plans online today. Just posting here for future reference.

At some point it becomes a fascination for many people to get a better instrument. Unless you have a large pocket book, the only practical way to make a large instrument is on your own. Depending on your resourcefulness and ingenuity, you may save yourself considerable money. The down-side of this is, however, a good bit of time and work. But if the idea of making something accurate to a few millionths of an inch with nothing more than two bits of glass, some common tools, and some jury-rigged testing apparatus appeals to you, then a bit of work won’t stand in your way!

I’m a huge fan of Google Sketchup. I’ve been using it since before Google bought it and was ecstatic when Google made the previously $700 version free for anyone. There’s still a “pro” version but it pretty much only adds esoteric functions like output to 3D printers and CAM program drivers.

I used Sketchup to design my studio and got to know the program pretty well (LOTS of trial and error), but whenever I go back to it there is a lot of frustration as I relearn various little things about the program. I’m not a designer my trade and have very little in the way of artistic talent, so any program that can help me translate my ideas into good looking and accurate 3d models is a boon. Unfortunately, even the best 3d program has got to be powerful enough to do useful work, and that power comes at a price. Things get technical and hard to grasp quickly in a 3d program, and using one only occasionally like I do means that when I do go back I end up spending a lot of frustrating time relearning the same things.

Well, there’s now a great new book for people just like me: Google Sketchup for Dummies. I’m adding it to my wishlist (since Christmas is so close!) if you’re looking for ideas. The best part of the book is that the author has posted 62 how-to videos to go along with the book. The vids are free on YouTube, but there’s much more in the book itself. I think this is a great idea for future authors. Way to use technology to extend the usefulness of your product!

In related news, I’m homing in on the general design of the telescope and will be using Sketchup to make a mockup of it sometime soon. I’ll post screenshots (or the file if you have the free Sketchup program) soon.

A few years ago I came across the three volume work Burnham’s Celestial Handbook. I hadn’t heard or read anything about it, but it was really cheap (I think it was five bucks for all 2100 pages in three volumes), so I picked it up. It has sat unopened on my bookshelf for several years. I thought that it was mostly just a sky survey with coordinates for finding various objects like stars, nebulae, and Messier objects. Pretty dry stuff, but a good reference for only $5, and certainly useful for the upcoming telescope project.

In doing some reading this past week, I caught a few references to the Handbook and the slavish devotion that many admirers of the night sky place in it. I brought it off the astronomy bookshelf and gave it another look. It turns out that when I briefly perused it in the bookstore a few years ago I had only opened it up to the star charts and lists of coordinates. There are whole sections that are full of essays, commentary, history, and all kinds of other amazing writing about the night sky. It’s a treasure trove that took one man, Robert Burnham Jr, over a decade of patient solitary work to construct.

I did a little more digging and unearthed this article about Burnham himself and his sad life.

I’ve pulled all three volumes off the shelf. Volume three is currently open on my desk and I’m amazed by what is in there. Burnham chose to organize his book around the constellations. He takes a certain constellation, Orion, for instance, and describes the technical aspects of it (location, stars, dual stars, colors, etc), and then goes into the history of the constellation as seen from the eyes of many different cultures. He also throws other anecdotes around each constellation or star grouping. It’s really fascinating reading. I’m looking forward to digging through it.

I spent two hours last night reading and learning about optics and telescope mirrors. Took some good notes in my new leather journal Erin gave me a few months ago. It’s got a short leather strap that acts as a tie-up to close the thing. Very Henry(Indiana)-Jones looking.

I’ve always been confused by the relationship between focal length, mirror size, and f/ratio on telescope mirrors. Often designers and scope nuts go off on tangents on the benefits of an f/5 scope vs an f/8 or f/11 scope. I’ve never internalized what they mean by this so I quickly get lost when they start talking about it. I understand f/ratios when it comes to photography, but mirrors are a little different. So I decided last night to figure it out once and for all. And I did!

In a nutshell, the smaller the f/number (called the “f/ratio”) is, the shorter the telescope tube will be. What makes a smaller f/number? A more curved mirror. Think of it this way: if you have a flat mirror the light that hits it and bounces off will never come to a focus (f/infinity). Grind a little bit out of the center of the mirror glass, making it more curved, and the light rays will reflect off of it and meet somewhere far away from the mirror (lets say 100′). As you start to grind more and more material out of the mirror, making it more curved, the light rays will start to be focused to a common point that is closer to the mirror. Eventually, the mirror will be so curved that the reflected rays will focus, oh, about four feet away (just an example).

So let’s say you decided to stop grinding your 8″ mirror once the light rays focus four feet (48″) away. Congratulations, you have an f/6 mirror. Why? Because 48″ (the focal LENGH) divided by 8″ (the diameter of the mirror), is 6… so they would call your mirror an f/6.

It’s easy to grind a mirror without much curvature to it, but this will mean a really long telescope. For example, an 8″ mirror that focuses light 120″ away would be an f/15 mirror… and the scope would have to be 10 feet long! Remember, you have to actually look at the light at the point it all focusses together, which means some sort of secondary mirror or eyepiece at that focal point, which means a long telescope tube to support all that hardware. Much better to spend more time making the mirror more curved so you don’t have a behemoth of a telescope.

So why not keep grinding away until you have a bowl-shaped mirror that can focus, say, 12″ away? In this case, a 12″ focal length mirror that is itself 8″ in diameter would be an f/1.5 mirror. But you’ve taken off so much glass from the middle of the mirror that it is quite literally bowl-shaped. These extremely curved mirrors are not only hard to make accurately, but the can be weak in the center (flexing!) and they suffer from something called “spherical aberration”. Basically, the image won’t look very good because the light rays are bent so dramatically.

Most telescope makers/mirror grinders recommend a happy medium of around f/5 or f/6. So if you’ve decided on an 8″ mirror, you get a final telescope length of around 40-48 inches. I think I’m going to go with a 6″ mirror in mine because the raw materials are a little cheaper. An f/5 or f/6 mirror means the final tube length will be 30″ to 36″ long, or about three feet (5×6 or 6×6). I think this will strike a nice balance between size, ability, portability, and cost. A 6″ mirror ground to f/5 would cost around $150-200 if I purchase it outright. If I do it myself I’d pay about 1/3 that price. Of course, grinding a mirror can take 50 hours, so I’m not really “saving” that much. I’m still not sure if I’m going to grind it myself or buy. I’m not really to that decision point just yet.

But I have decided on a six inch reflecting telescope along the lines of this basic design:

Light goes in the front, reflects off the curved mirror, and converges at the secondary mirror where it is reflected to the eyepiece. It’s called a “Newtonian reflector” and is a good balance between capability and size. It’s also not too hard to build. Most Newtonians have round tubes that you can construct from- no kidding - cardboard cement tubes you buy at the hardware store. I may get a cardboard tube to test the optics and focal length, but the final design will have a wooden tube, probably octagonal or multi-segmented. Inlay? Decoration? Reinforcement to avoid wood movement? Time will tell.