More about Mirrors

Saladsamurai

Moderators: Feel free to move if this is not the appropriate spot

I have been looking at some mirror grinding kits for a telescope making kits; here are three listings for a 10" mirror that are the same in all aspects except for the "wave" aspect. What is this?

I am sure it has something to do with the fact that they are "semi" pre-ground (i.e. they have started the process for you). Do you think that the amount of work they save me is worth the extra 75-100 dollars for the mirrors?

Here is the ad: http://www.newportglass.com/educkit.htm

Many thanks,
Casey

chroot

If a mirror (or tool) is 1/4 wave, it means that its surface is accurate to within one quarter-wavelength of green visible light.

Ideally, telescope mirrors should be at least 1/4 wave accurate. Higher-end optics are 1/10 wave accurate.

- Warren

Saladsamurai

Fantastic Chroot, thank you. Does anyone have any experience with making telescopes that knows of any decent DIYs online or books?

I have a lot that I aquired through google, but if anyone has actual used one with reasonable success (or failure) I would love to hear about it.

Chronos

It's definitely worth $100 for a pre ground 10" mirror. The materials alone would cost nearly as much. Parabolizing the mirror is the labor intensive part, but pretty cheap if you have the time. Be aware that you need a test stand and patience to finish polishing it.

Saladsamurai

That is what I figured. Thanks Chronos, Does the testing require specil equipment? I am not sure what that entails. I recently joined the ATM of Boston, so I could probably find someone who would be willing to test it for me.

Does it require fancy electronics?

chroot

Traditionally, mirror grinding is done via a purely manual process involving pushing a tool (a chunk of glass) across your work (another piece of glass). No electronics are involved -- just lots of grunt work.

- Warren

Saladsamurai

I understand that the grinding is done manually; I was refering to the testing. I do not quite understand the testing procedure.

Chronos

Building a good test stand is more effort than expense or technology. The most basic setup is a point light source, razor blade, and a stable, movable track for the blade holder. Interpretting the shadows and figuring out the right polishing technique is the hard part. Abuse your fellow club members for assistance. An experienced finisher will save you many hours of frustration. Mirror figuring is still more art than science. A ten inch mirror is a very ambitious first effort, but I think you will enjoy the effort if you can get a little expert help. I like an undersized star lap for figuring, but, that is merely my preference. It is fast and I lean impatient. The risk is grand canyon sized scratches if you become inattentive.

Saladsamurai

I may go with a 6" to start out on. Better to foul up a cheaper mirror, not to mention see if I can tolerate the project.

Chronos

Wise choice, especially for a first instrument. The difficulty of correcting a mirror increases roughly with the square of the diameter. A six inch mirror is still large enough to provide very nice views - especially with a CCD camera. I am sorely tempted to try my luck with an 8" Schmidt-Cassegrain [albeit I expect to buy the secondary]. Size does matter with scopes. A huge, cumbersome instrument is simply not practical. Portability and ease of setup are the most important features in a home scope - unless you live in the country and don't mind building a backyard observatory. It is no accident that Meade and Celestron SC's are so popular. Aperature size is irrelevant if you hate using your scope.

Burnsys

Saladsamurai i am thinking about building a telescope too.
What i heard is that lower than F8 telescopes are much more complicated to build becouse the mirror has a much pronunced curve.
I was thinking to start with a 6' scope also.
People i talked to says it takes about 6 months of manual polishing just to finish the main mirror , i think i am just to anxious.!

Chronos

6 months is about average for a first mirror [6"]. The grinding part can be completed in less than a month. Takes about another month to polish the thing out. Then you find out it is horribly misfigured, but can't bear the thought of regrinding the thing. So you spend about 3 more months in a futile effort to forcibly polish it into shape. Finally you cave in, regrind with 220 grit, and get it figured in about 6 weeks [my experience]. F8 is a good choice for a first mirror. The correction from spherical to parabolic is not that difficult for a beginner. A 6" F6, on the other hand, is surprisingly more difficult - and frustrating - to correct. A few degrees temperature difference can spell the difference between a successful polishing session and a turned down edge disaster. Personally, I think it is probably about as easy to make an SC corrector plate [which is surprisingly less difficult than it first appears]. Still, there is something special about looking through your imperfect optics for the first time. And you totally understand why stars at the edge of the viewing field appear to be gravitationally lensed.