MTO-11CA aka "Russentonne" (transl. "Russian barrel")

The MTO is a very nice and cheap 1000mm f 1/10 lens suitable for terrestrial and astronomical
use as lens for a SLR (by way of M42-TAdaptor-Your SLR system) or (by M42-1.25" adaptor + eyepiece)
spotting scope. By design a folded mirror telescope (Makusov) it is very compact and transportable.

By attaching a 100mm extension you can also turn this lens into a 2000mm! For this to work the
 'stop' must be removed, see link below.

Unfortunately the optics of the normal CTO-11CA lens are nearly always in a very uptight state. 
As this uptightness deforms the mirrors it causes rather bad results. To get better results from 
this lens it you need to 'relax' it before use. Series "A" lenses are already relaxed by the 
vendor (and are unsually twice as expensive).


Relaxing the optics is easy:

(Translated from German, source http://www.strickling.net/russento.htm
as usual you are doing anything described herein at your own risk, I never experienced
a problem myself following this procedure but your milleage may vary)

Prerequisites: Screwdriver set, tripod, camera or eyepiece, clear night

There are 3 possible steps, step 2 is the most effective.

1. Setup of the Makusov plate - With a screwdriver you unfasten the front ring (that
holds the Mak plate) to the point that it just does not jiggle but still helds the plate.

2. Point the CTO-11CA to a bright star (or any other point light source), turning the 
focus from sharp to just a bit unsharp. A good lens setup should show you a diffraction pattern that 
looks like a 'ring'. If you get a 'trianglular' or 'eliptical' pattern instead you need 
to relax the main mirror plate:

Unscrew the grub screws at the back of the lens (the side that points to the camera). 
Remove the rear cap carefully, you should see the main mirror from behind. This mirror
is hold in place by a washer spring. Now untighten the threaded ring until it just
does not jiggle but still keeps the mirror in place. Put the rear cap back on and
screw in the grub screws.

3. Additionally you can also untighten the mounting of the correction lenses, however
this step is usually not needed.