Using a photographic mirror lens as a compact scope?

[johnha]

Hi All,

I have a 35mm photo mirror lens (Tamron SP 500mm F/8 82mm front element) and a 10mm eyepiece convertor for it. From a quick look on the web it looks like it may be of Klevtsov-Cassegrain design. When new this was several hundred pounds worth of camera lens.

I have used it on and off as a 'scope, and when comparing it to my 4.5" newtonian at 45x the image quality has been very similar. Has anybody compared something like this to a typical compact 80mm scope or have an opinion on such a comparison?

Obviously it isn't as versatile as a dedicated scope (and lining it up without a finderscope is tricky). I'm using it on a standard photo tripod/ball head as I can't attach it to my EQ2 (I presume there will be an adapter to do this?).

Lens details: Tamron SP Adaptall-2 500mm F/8 Model 55B

Thanks,

John.

[michael8554]

I've some theretical pro's and con's:

Effective aperture is 500/8 = 62.5mm (does this work with camera lenses?)

7 elements compared to 1 (meniscus) on a Matsutov or 2 on a doublet will reduce contrast.

Light and compact.

The suggested comparison should be interesting.

[johnha]

Yes 500/8 works the same for camera lenses, although there's usually a variable diaphram to control the aperture (but not in the case of a mirror lens).

Correction, the front element is 82mm diameter, I presume the aperture is masked down to 62.5mm at some point.

John.

[michael8554]

Yes John, 82mm front element but there's a central obstruction (the secondary mirror).

So that's why it's f/8 and not f/6