Regrinding a 4.5" f/8 spherical mirror to parabolic

[clubjoker]

Hello!

I was given a Taso 4.5" reflector as a present, about three years ago. I know it's not the fanciest, but it has great sentimental value as the scope that kindled my interest in astronomy, and so while I have other more "refined" instruments, this one is my favourite. I'd love to upgrade it, and learn something about telescope making in the process.

One possibility is refiguring the mirror, which I believe to be a spherical mirror, perhaps regrinding it to a parabola. I realise that good quality mirrors like this are now readily available and cheap, so it wouldn't make sense in pure economic terms. But that's not the point.

This would be my first ATM project, and certainly the first attempt at mirror making. Would it make a good project for a beginner? Would there be much beneift visually in going parabolic? Would it even be feasible to change the mirrors figure like this? Is there a simple test I can do to check whether it actually is spherical - it may be parabolic already

So many questions already. Would appreciate your thoughts and words of advice.

Wide clears skies to you

John

[clubjoker]

Oops, forgot to say in the post, I think it's specified as f/8, will check the stated focal length which I think is written on the side of the scope and report back!

[Peter Drew]

At that aperture and focal ratio the difference between a sphere and a parabola is measured in fractions of the wavelength of light so only refiguring by polishing would be necessary. If it is already a good sphere there would not be a great deal of difference in the performance. A good economic way to start mirror making though.

[Naemeth]

One thing you may need to check is that the mirror is thick enough to take a regrind. Finished mirrors are thinner than their blank counterparts.

[bambuko]

In my opinion there would be very little improvement to be gained (my TAL-1 is not parabolic and it's great).

As Peter Drew says at f/8 and 4.5" there is little difference.

Also you would be attempting the most difficult part of mirror making (figuring) without benefit of experience you would gain along the way if you started from scratch.

Having said all this - it would be a great project and whether it makes sense (or not) it doesn't matter.

You would be doing it for a fun of it.

But I would go for making the mirror as opposed to trying to improve the one you have?

[pete_l]

I don't think this would be a good idea.

For a start, you'd have to strip off all the existing coatings from the mirror. Then do the most difficult part of mirror making, then get new coatings put back on the mirror.

The cost would be higher than the cost of a commercial parabolic mirror and the chances of success would be very small.

[clubjoker]

Thanks all for your words of wisdom.

Sorry for the slow reply. Being new to this site, I was completely oblivious to your replies until I check in now - I'd been expecting email updates if anyone replied. Must check my settings

So it sounds like I might be better off starting from scratch on a new build. I'd love to have an 8" f/6 dob. I'll get some books and start reading.

[Peter Drew]

Good plan! 

[Rustysplit]

I think you have made the right choice. An 8" f6 would be a good spec for a dob. I will keep an eye out for your build thread.

[clubjoker]

Yep, perhaps I'll just go for a paint respray on the Tasco, and see what I can do to improve the focuser and mouting rings, leaving the optics alone. I put a centre spot on the mirror at least so it's well collimated.