The Way It's Made - In Theory - part 1

It's all fine and good to say the Newtonian is just a couple'a mirrors and an eyepiece, but everyone knows you gotta build this thing so it's gonna stay together properly, and with any luck look good too. This means figuring out the fiddly details of how these different elements are attached, and what they need to be able to do once they are attached. After all, you can't just glue a mirror on the bottom of a tube and hope to see much.

Let's start with the the Primary Mirror.

This is, in essence, the telescope. It is slightly concave in structure, which means it collects a wide field of view and concentrates it down to a focal point, in the shape of a cone. So, it's taking a relatively large image and compressing it down into a small point. The specific curve of the mirror determines the focal length, which means some mirrors will have short focal lengths (mirrors built on a small sphere) while others will have longer focal lengths (those built on larger spheres). The University of Florida has a good page on basic optics:

UFL

Of course, we don't need to know all that stuff. Don't get bogged down with all the hoopla surrounding this, that and the other thing. The only thing to really be concerned about is how long the telescope will be. I know mine is going to be 10" across (I know, I said earlier it was supposed to be an 8" telescope, I'll get into that later), but the length is still uncertain. That will depend on the focal distance of the mirror I end up getting. I'm aiming for about 30 - 40 inches or so. I'm not overly concerned with the exact length of the finished telescope. I will work with what I can get without breaking the bank, or having to search the world over.

As with everything else in this world, quality is determined by how much you spend (unless you end up getting a really sweet deal on something that otherwise would have cost a bundle - and that happens sometimes, so keep your eyes open). Cheap mirrors are subject to spherical and chromatic aberrations that will detract from the quality of your image. To what degree this is true I cannot yet say, but as this blog grows I will share all the info I can.

For the adventurous, the mirror can be made from scratch. Why not check out this page?

Can You Do It? (yes, you can)

But, as the site says, with the cost of pre-made mirrors coming down in recent years, then you're not really saving money if you make your own. Being the sort of do-it-yourselfer that I am, though, I would never try to talk a person out of making their own. The main reason I'm not going to make my own is that I don't really have access to the resources where I am. Besides, I want this project to go reasonably quickly, even though I don't have a great deal of time on my hands.

Now, what was up with that 8" - 10" discrepancy I brought up above?

The mirror itself is the Optical Aperture, which means that this will determine how much you see when you look into the eyepiece (well, actually the secondary mirror and the eyepiece also have roles in that, but that comes later). The tube of the telescope is generally larger around than the mirror - for one thing, the mirror needs "room to breathe" (a closed-in mirror is more subject to fogging apparently), and the larger tube means you don't end up with an image of the tube itself ringing the outside of your field of view. This is referred to as vignetting, but there are a number of possible reasons one could end up with vignetting; it could also be due to problems with the shape of your mirror, and other aspects. Another important reason to have space around the mirror is because it is subject to distortion if it is crammed into a small space. It needs to be mounted on a cell, which is a particular kind of construction that does several things. I will discuss that in the next post.

So, that "breathing room" around the mirror translates into an 8" telescope having an actual outside diameter of about 10" on a finished unit. If you walk up to a fellow who is out peering at the stars with a Newtonian telescope and ask him what size it is, he will tell you the diameter of the mirror, not that of the tube. If his telescope tube is 40 inches long, and the guy next to him has a telescope that is 24 inches long, they will both say they have 8" telescopes - if the mirror in both cases are in fact 8" across.

So, I guess that's all I have to say for now. Please come back for another gander at how this li'l project of mine is going.