Long time, no write.

Not that I think there are many people following this, but I wanted to make a quick post to let people know that I do intend to continue this blog. I have recently gotten some 10" diameter tube with which to make the body of the telescope. Problem is, I have yet to get the primary mirror. I'm thinking that may happen some time over the summer. Had some priority expenses come up over the last while (not the least of which was a dental bill for my daughter) but should be back on track soon.

Spherical Aberrations.

It seems mirrors build on a sphere are not as precise with regard to focus as parabolic mirrors are. The page linked to below shows this much more clearly than I could explain it.

Spherical Aberration


I will end up having to spend more on a parabolic mirror, but in the end if I had to get more equipment to correct for spherical aberration then I will be spending more anyway. So, in order to keep things simple and still relatively inexpensive I will be looking for an affordable parabolic. I have my eye on one or two... just gotta do a little more investigating before I hit the "BUY IT" button.

,,, or, not quite yet...

I did a little research before buying the mirror I had in mind. Turns out because it is spherical, not parabolic, I am advised against buying it. It seems there are issues with spherical mirrors that require special corrective measures in order to obtain good focus. After I find out more about that I'll post info here.

May Be Ready To Get Going Again

I'm looking at ordering a primary mirror soon. This will depend on what I find out about the quality of what I have in mind. This is just a quick note to say that this blog has not gone the way of the dodo.

Little Mirror Test

I decided to purchase a used (and somewhat scratched up) little primary mirror to play around with and get used to the whole telescope making concept before moving onto the bigger stuff. This thing was so inexpensive.

How inexpensive was it?

It was so inexpensive ....

Nope, there's no way I can make a joke out of that, so I won't bother. But let's just say I've spent more on a quick lunch downtown. Really. It came from the Surplus Shed (which I think I mentioned in an earlier post).

It's a 2.5" spherical mirror with a 24" focal length. This means I will be able to construct a cute little telescope as a practice piece. I also have a particular plan for it. I want to do something a little different. I'll let you all in on that after I get the thing in the mail and start the plans for building it.

Telescope Building Info Links

If you're interested in learning more about telescope building, here's a good link.

Here's another. And I am told this one is one of the best. I have yet to give it a good read. I'll be commenting on it as I get through it.

Oh, by the way, don't let the naysayers discourage you. When researching the construction of a telescope you will always encounter those who say don't bother trying to build one, just buy one. Well, it depends on what you want in the end. Do you just want a telescope, or do you want to have the experience of building a telescope? If you chose the latter of those two then you should not allow yourself to be discouraged. These comments usually come from people who do not appreciate the significance of what one gains from building something.

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.