Potential mirror for DIY build - help needed

[Tinderboxsky (Steve)]

Two of these mirrors, and a similarly mounted flat mirror, were recently disposed of by the local CSIRO Science Education Unit. They were destined for the tip if there were no takers.
I put my hand up for one of the mirrors, sight unseen. With more time available, I have been thinking about making another telescope and thought it would be worth exploring whether the mirror would be suitable for a DIY project. I am a keen re-cycler and was thinking this might be a goer.

The unit has been made by Optical Works, London UK. The mount is extremely heavy and well engineered. As you can see there is fine adjustment for each axis. Unfortunately, a Google search turns up records to suggest this company closed some time ago.

The mirror dimensions are 314 mm diameter and 42.5 mm thick – a 12 X 1.75 inch in imperial measurements. Engraved on the back of the mirror is “17/9/84 8FL OWL”. The coating appears to be in very good condition.

Does anyone know anything about these mirrors and their original use? As mentioned above, it has come from the CSIRO.


As a possible DIY project, I now know the focal length, relatively long by today's trends. I assume my next step is to also understand the actual shape of the mirror. I would then be in a position to work out and decide whether it is a goer.

I would appreciate peoples’ comments to help me work out whether I am on the right track and any thoughts on the pros and cons as a DIY project.

Cheers

Steve.

[Rod]

Hi Steve

I am unclear from your description what the focal length is. Can you clarify?

Have you been able to do a ronchi test of the surface?

Rod

[Tinderboxsky (Steve)]

Hi Rod,

The focal length is 8 feet. I have been able to check this with a simple light source and reflected image on a white board. The mirror is engraved with 8FL which I take to be "8 feet focal length". So with the mirror being 1 foot diameter, I interpret this to be an F8 mirror. Is this right?
I have not done a ronchi test yet - I am still in the understanding and working out how to do it stage.

Thanks

Steve

[Rod]

Hi steve,

What distance was the light source from the mirror? If your light source was the Sun which you can think of as at an infinite distance then yes the focal length is 8 feet. If your light source is at the same distance as the reflected image, then then you are at the radius of curvature which is twice the focal length. In that scenario your focal length would be 4 feet.

The ronchi test is the simplest way to do a rough check of the mirror surface. You can tell quickly whether it is a sphere, has a turned edge or other defects. Here is a description:

http://stellafane.org/tm/atm/test/ronchigrams.html

Once you are sure of the focal length, the next step is to test the mirror surface. If you see slightly curved lines on the ronchi test then you may have a parabola and need to either learn to do a Foucault test or have someone test the mirror for you. There is plenty of information on the net about how to do a Foucault test. Personally I found I needed someone to go through the process with me before I got consistent results.

If you have a parabola then you can make a Newtonian. If it is a sphere then you would need to parabolise the surface which would mean stripping the coating polishing in the correct curve and then replacing the coating. Spherical mirrors can be used in conjunction with other corrective lenses or mirrors but I would not recommend that unless you are keen to learn ray tracing and invest a lot of time developing skill in optical fabrication.

Hope that helps

Rod

[Tinderboxsky (Steve)]

Hi Rod,

Yes the light source, a red LED, and the in focus reflected image were at the radius of curvature. This was exactly 16 feet from the mirror giving an 8 foot focal length.
Thanks for your thoughts re testing the mirror. I'll keep exploring.
Out of interest, I am still hopeful someone will recognize the mount and know the original use for these mirrors.

Thanks

Steve.

[Rod]

Hi Steve

A 12 inch F8 newt would be a nice telescope but very long. One more exotic option you could consider is Ed Jones' Chiefspiegler. It's like a catadioptric herschelian. Many of Ed's designs can be built with off the shelf lenses. The folded nature of the design would mean you could avoid using a ladder. His web site is here:

http://home.fuse.net/eljones/

Sorry I don't now what your mirror was used for. The mount makes me think of a coeliostat (solar telescope) but I don't know enough about them to be sure.

Rod

[Tinderboxsky (Steve)]

Hi Rod,

Thanks for that. I'll let you know how I go.

Cheers

Steve

[cometcatcher (Kevin)]

A 12 inch F8 would make a fantastic planet scope. I've often thought of making one for that purpose.

[mswhin63 (Malcolm)]

Originally Posted by cometcatcher

A 12 inch F8 would make a fantastic planet scope. I've often thought of making one for that purpose.

Totally agree, I would love to build such a scope as well.

[GrahamL]

My wife spotted this and believes that test stand would make a fab make up mirror

Are the mirrors spherical ?

[Tinderboxsky (Steve)]

I don't know the shape of the mirrors yet - am working through this now.

Might have to strengthen the bedroom dressing table if used as a makeup mirror - it feels like at least a 20 kilo bag of cement!

Steve.

[el_draco (Rom)]

Seriously funky! It looks like its part of a test bench setup. Would not mind one of them muyself!

[Wavytone]

Looks like it came from an optical test bench.

Another application for which this would have been ideal is Schlieren photography of the airflow round an object in a wind tunnel or shock tube - which CSIRO used to do in the 1950's-60's.