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[Bernard Foot]

Now that I have a method for rear-mounting filters, I tried out a Tamron SP500 mirror lens (500mm, fixed aperture of f/8) using a Baader U. Unfortunately my bandpass filters (380BP20, 345BP25, 315BP20) cannot be rear-mounted and so I cannot comment on the ability of the lens at shorter wavelengths.

Here are some comparison photos of the SP500 against the Focotar 2. The Focotar images are heavily cropped, and so image quality is poor.

The exposure using the SP500 was about 4x the exposure using the Focotar-2. I say "about" because it's not clear what the effective aperture is because of the second mirror which blocks off some of the front opening of the lens.

Saturation has ben increased in the images.

First, images WBed for the Focotar-2:

SP500:


Focotar-2:


Secondly, both WBed against the same point on the road surface:

SP500:


Focotar-2:


The second pair of images are remarkably close, although it is noticeable that the writing on the front of the van does not show up on the Focotar-2 image.

I would asess the SP500 as being adequate for photography using general-purpose UV filters like the Baader U.

Edited by Bernard Foot, 07 April 2021 - 15:30.

[dabateman]

Yes that was about my experience with the Tamron 500mm adaptall with tripod mount. It may see just below 400nm.
I tried to spec it but can't make sense of the data. There was an odd 365nm bump that I don't think is real, but some odd reflection.