Hi,
I'm just getting into astronomy through my flatmates very cheap 80mm diffractor. Its got to the point where i'm getting annoyed with it so i'm looking at getting a better telescope.

My plan is to grind my own 6" mirror over the summer and build a newtonian reflector on a dobsonian mount... but i have some questions.

I've read lots of articles about the tolerances between spherical and parabolic mirrors. At the moment my thinking is that i'll get the mirror spherical with a ratio of about f/8 and then just attempt to parabolise it as much as i can. Does that sound sensible?

I'm also a little worried about the mount, as looking at saturn through my flatmates telescope at 66x magnification, the planet only stays in the field of view for about 40 seconds, and its not really big enough to be visible. How easy is it to manually track something with a dobsonian mount at powers ~100x?

I think thats it for now. Thanks!
Tom.

The reason Saturn stays in your freind's 80mm scope (REfractor, not diffractor) field is likely because the eyepiece is of questionable quality and has a very narrow field of view. However, 40 seconds doesn't seem terribly short. At 70X in my 10" dob, Saturn will stay in the field for at least that long, and tracking is pretty easy for me, though it does take a bit of practise. Last month I was in Arizona with my 10" dob and I was able to track Saturn at 250X.

How much astronomical knowledge do you have? What about woodworking/mechanical skills? Grinding a mirror can be problematic. Considering how many inexpensive 6" dobs there are, you might consider just buying one of those.

Buying a mirror and then buying the needed polishing tools, You need a second blank for the polishing tool with a convex curve to match that of the mirror or else the pitch will have different thickness across the tool resulting in a horrid polish when you try to parabolize it, is not cheap. Then you need the polishing pitch and cerium oxide or rouge. Then you have to build or buy a foucault (cheap to make at least) test rig and learn how to read the shadows seen in the mask over the mirror. Then you have to recoat the mirror. It's a steep learning curve. A very satisfying one but certainly not money saving over buying an f/10 6" mirror in the first place. That, while spherical, will perform very well optically with no need to alter it in any way. Even an already parabolized 6" f/8 mirror would likely be cheaper and perform better.

I ground my first 6" mirror in 1954. Though I ground it from flat blanks of plate glass. Took me from the fall of 1953 to spring of 1954. The learning curve slowed me way down. I later could do one in only a couple hard weeks of work but that first one was a steep learning curve with no one to guide me.

I think any serious amateur should build at least one scope in his life but in the 6" size you won't save any money. At least not here in the US.

Rick

Well i've found a company here in the uk that sells a 6" mirror making kit with everything in it for about £70, which seems pretty good. They'll also coat the mirror once its ground for about £10.

I'm a physics student, so i'm hoping i can cheekily use some of the stuff in one of the university labs to test the optics, but failing that i'm sure i'll be able to fumble something together. I think its the kind of thing i'll enjoy

Thanks for the responses, the info about the mount was helpful as i've never actually used on in practice.

Tom.

I think this thread is worth reopening after a few months, based on my personal experience.

I made a 6" f/8 mirror when I was in high school. The Edmund Scientific kit in the year 1964 cost $12 USA, compared to $60 for a finished mirror. Considering 45 years of inflation, the kit price was pretty competitive with your current UK source. I had the benefit of free coating, thanks to a family connection.

Mass production economics has brought the prices of commercially made mirrors way down, in some cases underselling the kits, but the optical quality may be iffy. If time and a steep learning curve are no issue, you have the potential to achieve virtual perfection.

I was too impatient to persist in achieving near-perfection. I had a stubborn bump in the center of the mirror which in hindsight I attribute to a defective pitch lap. Since the secondary covered most of it, I quit when the rest of the surface roughly approximated the desired paraboloid. The result was a mediocre mirror which nevertheless gave me much observing pleasure on a tight budget.

If I had it to do again I would use a Dob mount. It gives comparable stability at about half the cost and weight of an equatorial mount. With good charts and a good finder I can star-hop even under the severe light pollution here in greater Washington. At a better site about 50 miles away it is fairly easy with plenty of practice.

Back in the '50s, your renowned countryman Patrick Moore was routinely observing the planets with a 12.5 inch reflector on an alt-azimuth mount, at magnification as high as 400x, if I remember correctly.

I'm going to second what Hornblower said, although my Edmund kit arrived in 1968 ("Pitch Lap Mat: Time Saving Wonder").

I'm going to stick my neck out and say that for most of us seeing at the site tends to outweigh the quality of a handmade mirror. That includes home-grown silvering.

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If they can get you asking the wrong questions, they don't have to worry about the answers.

The 6 inch mirror is a good one to start with, from everything I have read. Some even go as small as 4 inch and make it F10 or longer. As you go to a higher F ratio, the need to parabolize the figure becomes less and less important .

Years ago, such places as Edmund Scientific sold mirror kits, but a quick look at their web site revealed no items.

Likewise, you could find books on the subject, with all kinds of facts, figures, how-to suggestions, etc.

I suspect that, just like in R/C flying , Amateur Radio, and other hobbies, "do-it-yourself" is fast becoming a thing of the past in amateur astronomy. I would love to be proven wrong, but IMHO, people just don't have the spare time to devote to their hobbies that they did in the past, so purchasing a telescope is a far more attractive alternative to trying to create your own!

Good luck ! Jim

Ok, sorry to prove you wrong, but this is simply not true! Here in Colorado Springs, the local amateury astronomy club holds Star Parties all the time, and more than half the telescopes available are home built, many quite recently.

Here are some resources for anyone looking to get started:

General Information / Guidance:

http://en.wikipedia.org/wiki/Amateur_telescope_making

http://www.telescopemaking.org/starting.html

Mirror Kits and other supplies:

http://mirrorworkshop.mtbparker.com/purchasingInfo.html

http://www.e-scopes.cc/

http://www.newportglass.com/ngwkitp.htm

http://www.willbell.com/ATMSupplies/ATM_Supplies.htm

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If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Well, I'm glad you can prove me wrong, at least for Astronomy ! Thanks for the links. I will have to look at them very carefully. I am always hungry for new information.

Here on the "Right Coast" almost all the telescopes we see are from the major producers, Celestron, Meade, and Vixen . There are some fairly well-to-do people around here, and I suspect that they probably have more money to spend on commercial equipment, then they do time to spend building stuff!

Thanks again for the links. It should make for interesting reading. I'm even thinking of getting a larger diagonal mirror for a scope I have. Maybe I will find a source !

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Keep Looking UP !

Tristate Astronomers http://www.tristateastronomers.org