Vixen VMC200L

Technical data:

Construction: Catadioptrical reflector
f=1950mm, f/9.75
Primary mirror: f=500mm, f/2.5
Secondary mirror: d=72mm, magnification 3.9x
Distance primary secondary:: ~330mm
Central obstruction 80mm diameter (40% of d)
Theoretical resolution (Dawes): 0.63"
Theoretical min mag: 13.3
Tube length ~525mm incl. focusser
Weight: 5.9kg
Finder scope: 7x50

The VMC features a catadioptric "Field Maksutov" type design with two spherical mirrors and a meniscus corrector element behind the secondary mirror passed twice by the light:

The open tube allows shorter cool down times compared to 8" SCTs but makes the scope more sensitive to tube current if not cooled down properly.

Adaption to Losmandy GM-8 with G11 head.

Collimation experiences

The VMC has a very robust seconday/corrector holder and spider vane and holds collimation very well. Nevertheless I managed to totally screw up collimation when playing with a holographic focussing aid (which didn't work for the VMC by the way...) and the VMC needed a complete recollimation.

The fast primary mirror makes the VMC quite 'nervous' regarding collimation, small movements of the mirros/collimation-screws have huge results.

I tried the "standard collimation procedure for Rumak kind maksutov cassegrains"

  1. Adjust the secondary at the extrafocal out of focus pattern
  2. Adjust the primary at the intrafocal out of focus pattern
  3. Start again with 1. until finished, do only small corrections in each

When I was finished I had quite nice airy discs but:

  • color
  • coma
  • astigmatism

so something went wrong.
I inserted a chesire in the focusser and the error was obvious: The "optical axis" I adjusted was not parallel to the ideal axis, so I tried the idea to align the secondary as close to the optical axis as possible with a collimation tool in the drawtube first and finish collimation at a star adjusting the primary only afterwards. I used a Chesire to adjust the secondary/corrector-element (no full aperture corrector in a Field-mak) on axis (assuming the draw tube's adjustment is correct) as good as possible. This might be even more acurate with a laser.
Then I collimated the primay at the intrafocal out of focus pattern of a star leaving the secondary alone. Success! Color and coma were gone. I still had some astigmatism. Fortunately it was caused by too tight push/pull screw pairs at the primary mirror holder deformating the cell slightly. I loosened the pairs, recollimated and finally the VMC was in tune

It's possble that there's no 100% accuray in aligning the secondary with a Chesire or laser but it should be a perfect way to start: Align the secondary first, then adjust the primary at a star and then do the very last touch-up of secondary and primary at the extra-/intrafocal star image.

The primary mirror holder of the VMC is quite sensitive regarding too tight fastened adjustment-screws resulting in severe astigmatism. The same is heard about the VMC's "brother" VC200L using the same mirror holder.