Secondary Mirror Arrived in the Mail

Well, my first ever telescope part arrived in the mail (yesterday, actually). This little mirror is a Celestron 1.1" Secondary. I went into primary and secondary a little bit in earlier posts, but now I'll focus a bit more on that - pardon the pun.

Here is what my mirror looks like, just out of the package. I carefully lay it out on the tissue it was wrapped in, sitting atop a nice, comfy cushion. I don't want this thing to get scratched up before I even use it. I currently am keeping it in the packaging it arrived in for safety.



When I took that picture I forgot that when you focus on a mirror you will get one of two things; either the mirror itself in focus and the reflection blurred, or vice versa. So I took another one with part of the window frame being reflected in the mirror.



As sharp as the image appears, my camera is not that great, and does not do justice to the clarity and sharpness of reflection this mirror provides.

When I said above that the mirror is 1.1" that refers to the measurement on the narrow axis. Since the secondary mirror is placed at a 45 degree angle from the primary, the optical field is distorted - i.e. it appears shorter on one axis than on another. The elliptical shape addresses both the issue of foreshortening, and that of providing as much field of view with as little obstruction to the primary as possible.

I'm sure we all get it - you tilt the mirror and the shape appears to change. Still, I'm a very visual-minded person, so I like to use images to illustrate my point.

The mirror as it appears directly front-on:


The mirror as it appears when tilted:


That second view is roughly the same angle as that it will be placed at in the telescope, and, as you can see, the area of vision appears round rather than elliptical.

Notice also that the edge is angled. This is to maintain a good thickness of the mirror glass (important for keeping a sharp image - glass is subject to warping rather more easily than one might expect) while permitting for more visibility around the mirror. I'm holding it upside down to show the angled edge, but in reality it would be positioned the other way, so you see only a very small edge.

That's it for now. As I progress so will the blog. I don't have a "deadline" for this project, so I don't have a time line for it. However, as I learn more I will post more.

******

UPDATE

To answer the question posted by Albatross (and for anyone else who has wondered - like me, for instance), this is how you clean your mirrors:

Mirror Cleaning

The short story is, avoid cleaning them as much as possible. Dust will not inhibit viewing. If a mirror actually needs cleaning, the link above gives a great description of how to do so safely.

The Way It's Made - In Theory - part 2

I mentioned the Cell above, but didn't really say what it is, or what it does. In short, it does several things to ensure that the mirror is doing OK.

First, it holds the mirror in the proper position - which is to say, facing up the tube toward the secondary mirror.

Second, it provides a gentle cradle, if you will, to prevent pressure on the mirror which might distort the surface and give you a poor image. Yes, it's that sensitive. I couldn't believe it when I read about that. It's glass. Putting it in a clamp is gonna distort it enough to blur the image? Wowsers!

Third, it provides an open surrounding that allows free air flow, which helps keep the mirror from fogging.

Finally, it is constructed in such a way that allows the user to subtly move the angle of the mirror to do something called Collimation. In a Newtonian telescope, all this really means is that you line up your mirrors and eyepiece such that you end up with the optimal view.

I have some plans for the cell that I will post here soon.

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.

Collecting Info

I started out collecting info a bit at a time... hesitantly. To be honest, I didn't really know what I should be looking for first. After finding some less-than-helpful sites I began to come across links that were actually taking me somewhere. Wikipedia is a great place to start for many things. They won't have everything you're looking for on the subject, but you can get a good overview, as well as a few good links from the References and See Also at the bottom of the page.

Wikipedia entry.

Another good way to learn about stuff is to go online "window shopping". I spend a lot of time on ebay just searching for items to see what the description has to say about it. The thing about ebay is that the seller wants to give as much info as possible (usually) in order to increase the likelihood of sales. It also means you learn more about what is actually available for you to purchase toward your project, and how much you can expect to spend.

The single best way to learn anything is to immerse yourself in the culture. I am the sort who likes to join forums. Forums tend to have collections of seasoned enthusiasts, as well as lots of active discussions that you can learn from. The other great advantage is that you can actually get involved. You can not only partake in a discussion, you can start your own. Here is a site I found:

Telescope Junkies.

From there, I also learned about a really cool surplus supply site where one can occasionally find fantastic deals on telescope equipment.

Surplus Shed.

Not to get ahead of ourselves, but, again, just knowing what is out there helps to learn more about the project. After a bit more absorption of info it will be time to plan the actual purchasing of parts.

It will also soon be time to decide exactly what will be constructed from scratch and what will be purchased. If you plan to take on such a project you will need to decide that for yourself. I've pretty much decided that I will purchase the parts that require precision for good results - the mirrors and the eyepiece(s) - and build the rest.

Next time, I'll go into planning and designing elements of the actual scope.

Reflector Telescope Project

This blog will follow the process of researching, planning, and eventually constructing a Newtonian Reflector Telescope. Primarily, the blog is intended as a place for me to keep my own info, but I also hope it will help others to see that this is a project that can be done - even if you're starting where I did... which is to say right at the beginning. As with the progression of the stages, the blog will follow the same format. It will begin with an introduction to what a Newtonian telescope is, then it will break down the elements and discuss the construction in theory. From there it will progress into the collection and assembly of parts, until a finished telescope can be shown here.

I only barely understood how a reflector telescope worked, let alone how to build one. At the time of starting this blog, I only know a little more than that. However, what I have learned so far is that it is not only possible to make your own reflector telescope, it seems quite possible to do so for a reasonable amount of money.

The first thing I needed to know was the basics. What IS a Newtonian Telescope, and what makes it different from any other telescope? Well, I knew enough to understand that - being a "reflector" telescope - it had a mirror rather than a lens. The basic idea, in a nutshell:



Contrast this idea with the more commonly known Refraction (or "Refractor") Telescope:



The images above came from here.

As one might guess, it is called a Newtonian Telescope because of the fact that Sir Isaac Newton is credited with having been the first to turn one toward the stars. In fact, here is what his personal telescope looked like.



Pretty cool, huh?

Well, that's it for the very basics. Next entry I will go into learning more about specifics of how the thing works, rather than just the general idea. It's more than just putting a couple of mirrors inside a tube and looking through a hole after all. I'll be back with some links and discoveries I have made that - I hope - will elucidate things, and make this project come together.