Mirror Lens for UV?

[Dmitry]

I have old soviet mirror lens MTO-500.

Scheme can be found at https://commons.wikimedia.org/wiki/File:Lens_telephoto_2.png

 

Rear element can be detached to alter working distance and focal length. And to eliminate some glass on path of UV light.

 

Anybody tried such lens for UV?

[nfoto]

Yes, as a matter of fact I spent today shooting with a selection of mirror lenses on my Nikon D3200 (internal Baader U filter). Some of my mirror lenses are loaned at this time, though, so only a few 500 & 1000mm were available.

 

The 1000mm f/10 MTO didn't do too well, but the 500mm f/8 Makowsky-Katoptaron TS 500E showed more promise. The same applied to the venerable Reflex-Nikkor 1000mm f/11 (first version with filter wheel; I removed some of the internal filters for experimental purposes and for today's testing didn't insert any filter at all).

 

Here is one of the last of the dandelions outside my new house. Almost at the near limit for the 1000mm Nikkor. The flower head was shaded and the exposure time accordingly pretty long (f/11, ISO 400, 4 sec).

 

 

A little earlier, I tried to shoot a clump of dandelions about 110 m away. Much shorter exposure time of course, but lots of heat waves to reduce image sharpness.

 

[Dmitry]

Oh, nice to know. Makowsky-Katoptaron is pure mirrors only design.

[nfoto]

Yes, but the reflective layers on the glass surfaces do influence UV transmission of this all-mirror design.

[JCDowdy]

Specialist UV/Vis/NIR mirror and catadioptric lenses exist:

 

Nye Optical f1.4 / 150mm UV-VIS-IR mirror lens

 

ZUFAR 4/350mm quartz fluorite catadioptric lens

[nfoto]

Existence is one thing, getting hold of an actual sample is something different.

 

Of course I wouldn't mind having some of these exotics of Klaus in my possession, but for now the Katoptaron has to make do. That lens was calibrated by the maker himself and still smells of cigars :D

[JCDowdy]

Doubtless so!

[OlDoinyo]

The Makowski (a 2-mirror Schiefspiegler design) has OK bandpass, but the image quality is somewhat lacking (at least in the example in my possession.) It also has a tendency to focus drift with changing temperature. Just why there is any bandpass limitation with this optic is a puzzle to me, as the designer lists transmission down to 300 nm. Perhaps mine has crud on its mirrors...

 

There are some kit reflectors on the market which are also mirror-only and might be adapted for photography.

 

The Questar telescopes are catadioptric, but are allegedly usable down to 300 nm. They are very expensive, though.

[JCDowdy]

This one is really cool looking too: Telespect 1:11 f=500mm

How close can the Makowsky-Katoptaron focus?

 

I have an all mirror off axis parabolic collimator, perhaps I should think about attaching .........

[OlDoinyo]

The Telespect is just a later version of the Katoptaron. These optics don't focus much closer than about 20 feet unless you use extension tubes or the equivalent.

[nfoto]

One can add extension to the Katoptaron, but the already awkward handling will be exacerbated.

[enricosavazzi]

Yesterday I happened to be testing a Spectra-Tech Reflachromat 15x microscope objective, which uses two mirrors in a Cassegrain configuration and no refractive elements. I have not tested it in UV because for now I am still trying to get a reasonably good image in VIS. So far it has eluded me, in part because contrast is extremely low (cleaning the mirrors and installing a conical front baffle will be my next steps) and in part because tube length and magnification are not consistent with each other (this is a finite version of the objective, not infinity corrected).

 

My web page on the Reflachromat 15x is of course unfinished and not linked to the rest of my site, but if you are curious about this objective you can see it here:

http://savazzi.net/photography/reflachromat.html

[OlDoinyo]

I read the page on the Cassegrain optic...but it makes repeated references to diffractive optics. Such things do in fact exist, but it was my impression that they are exotic and pricy. Perhaps you meant to say refractive?

 

It is amusing that they had to add in reverse chromatic aberration to make it compatible with the eyepieces!

 

A Cassegrain optic will of course have annular bokeh and will not accept a diaphragm nor any other aperture-varying device. A 15x microscope objective will have about a 13mm focal length. This would make direct use on most cameras virtually impossible; the focal plane might actually be inside the barrel.

[enricosavazzi]

I read the page on the Cassegrain optic...but it makes repeated references to diffractive optics. Such things do in fact exist, but it was my impression that they are exotic and pricy. Perhaps you meant to say refractive?

 

It is amusing that they had to add in reverse chromatic aberration to make it compatible with the eyepieces!

Yes, refractive it is, of course. Just my mistake.

A Cassegrain optic will of course have annular bokeh and will not accept a diaphragm nor any other aperture-varying device. A 15x microscope objective will have about a 13mm focal length. This would make direct use on most cameras virtually impossible; the focal plane might actually be inside the barrel.

Annular bokeh is unavoidable, since the primary mirror has a hole in its center (the Katoptaron and some reflective optics for semiconductor manufacturing sidestep the problem by using off-center mirrors). A variable aperture is however possible, e.g. a diaphragm mounted just in front of the primary mirror. It usually works also just in front of the objective/telescope itself (this does not work with the Reflachromat objective because the aperture is so close to the focal plane). The bright bokeh rings become thinner, so the ring shape is even more evident.

 

The problem of focal plane with subject at infinity being close to or inside a microscope objective is common to all microscope objectives. Even a Cassegrain telescope needs a convex (i.e. divergent) secondary mirror to avoid this problem (unless the secondary mirror is very large).

 

The main practical problems I have at the moment with this objective are the tube length (the 142 mm tube length indicated on the barrel does not give 15x) and the lack of contrast. Unless I will find solutions, I am afraid this objective is a beautiful dud. I found very little information on how to use these objectives.

[dabateman]

Enrico,

I just got a Spectro-Tech Reflachromat 15x 0.58 NA marked as 160 tube and 0 correction. This is added to my crazy UVC transmission microscope idea. I am not sure if yours will be similar but mine doesn't need such a long tube to work.

On my Em1 I got 2mm (2 ticks of a mm rule across the width of the sensor (approximately 9x assuming 18mm width) with dual m42 adapter + 12-19mm helicoid at 12mm with M42 to M52 step up ring a raf camera M52 adapter and M52to RMS adapter (19.25 +6+12+2+5+2= 46.25 mm). I was able to get roughly 15x (1.25mm in view) with just dual adapter and conical M42 to Rms tube from Raf camera. I am not sure its exact length.

 

Funny on the Em1mk1 and just a 11W led desk lamp, may have been 6W as I don't think its that bright, I was hand holding it at ISO 200 1/200 shutter speed. This was my first handheld microscope at 15x. I was able to hand hold higher magnification with a 17-31 helicoid as well, but shutter speed dropped to 1/20.

So maybe not be a dud, just not behaving as expected. I still need to test it in UV though. But its just two mirrors, so I doubt it will be an issue.

Sadly my mirrors are dirty and someone clearly tried to clean it, leaving odd marks on the first mirror. But not horrible considering how little I paid for it.