What would you do with this mirror?

[Whrrr]

I have just acquired a 6" f2 mirror and am thinking of building my own Dobsonian with it.

From what little I so far understand, this is a very "fast" mirror - what sort of applications would suit such as mirror or am I barking up the wrong tree?

With thanks,

Mark

[Doc]

The main thing that strikes me is coma, there's going to be loads of it. Also it will be rather small and compact which is not a bad thing. It will be an interesting project.

[johninderby]

Do you have any idea of what this mirror was used for originally? It sounds like it may have come from some specialised bit of gear.

John

[Rob]

Hang it in the shed!!.. thats going to evil on coma.. my god

[GazOC]

Other than as a "curiosity", I don't think its worth making a Dob/ Newt out of that mirror. Sorry!!

[Doc]

I remember seeing a F3.5 dob on cloudynights. It was something like a 20" but was only about 4 foot high.

[GazOC]

I think I've seen them down to 3.3 or so but the problems/ issues making and using the mirrors increase exponenetially rather than linearally (sp!) as the focal ratio decreases. f2 is just too fast.

[johninderby]

On the subject of ridiculously short focal length dobs.............

Mike Lockwood 14.5" F/2.55 Newtonian

http://loptics.com/fast14p5.html

John

[Trull]

Could you not fit a corrective secondary mirror to this on the back of a meniscus lens and trepan a central eyepiece aperture through the mirror to make a sensible schmidt cassegrain?

[barkis]

F2 is a bit fast, almost Schmidt camera like. the focal plane will be quite curved, so not too practical for a visual telescope.

I made an 8" f3.3 mirror a long time ago. It made a superb richest field scope if you ignored the comets towards the edge of the field.

I found myself using it regularly, though I had 12" f6 in an obs.

The beauy was, I could hold the OTA under my arm with a low power EP in, and pan around the sky, while sitting in a chair in the garden.

Ron.

[FraserClarke]

Is this the one off ebay?

If so, it's a sphere, not a parabola. It was either the primary for a small schmidt (unlikely), or a test sphere of testing convex optics (e.g. 'Hindle sphere test'). I thought about buying it as a test sphere, but decided not to.

Certainly not suitable for a dob type telescope in its current form. You could regrind to something like f/5 or longer...

[BinocularSky]

AFAIK there isn't an eyepiece that works at f2,

[Whrrr]

Yes - it is indeed the one on ebay and I was surprised to win it at that price. It seemed like a good idea at the time and perhaps one of those things that may come in handy one day! As for the Dob, I think I will look elsewhere... Can someone recommend a good place to buy mirrors in the UK?

Thanks,

Mark

[BinocularSky]

Can someone recommend a good place to buy mirrors in the UK?

Galvoptics. Or, for bigger stuff, Oldham Optical.

As with all suppliers, it is good practice to test your mirror(s) upon receipt -- even the best can sometimes let "lemons" through QC.

[spaceboy]

If all else fails you could always use it as a shaving mirror

[Whrrr]

...taking care to avoid the Great Red "Spot".