I am currently in the design phase of my next Newtonian astrograph (10" f/4.7 with sandwich carbon tube and Wynne corrector). While much has been discussed about mirror cell design, I feel that the secondary mirror holder has not received enough attention, in particular in the context of imaging.
Holding the secondary mirror properly is mechanically complex since there are many degrees of freedom that should be both finely adjustable and lockable (hereafter by secondary optical center I mean the point on the aluminized surface of the elliptic secondary mirror which is offset from the geometrical center by the proper amount):
- motion of the secondary optical center along the optical axis of the primary
- motion of the secondary optical center in the plane orthogonal to the optical axis of the primary (two degrees of freedom)
- rotation of the secondary around the optical axis of the primary (with the secondary optical center fixed)
- tilt of the secondary optical surface with the secondary optical center fixed (two degrees of freedom).
So overall there are six d.o.f. to deal with (three translations, three rotations). While 2. can be addressed with the centering of the secondary holder, using the tensioning nuts of the spider assembly, the remaining four d.o.f. should be taken care of by the secondary holder.
The common design found in most commercial telescopes (central pull screw, three collimation push screws) has two main issues:
- rotation of the secondary (point 3.) can be adjusted roughly, not finely, and cannot be locked. It is just held by friction of the collimation screws tips onto the secondary holder flat surface.
- tilt motion of the secondary (point 4.) cannot be done independently since adjusting the collimation screws does NOT preserve the position of the secondary optical center.
As a result, secondary collimation is not as stable as it should, and adjustment is not straightforward (the effect of the collimation screws is a bit unpredictable).
There are also mechanical issues like resistance to vanes torsion and to the torque created by the one-sided secondary mirror load.
I am aware of alternate designs (such as pushed forward by MitchAlsup on this forum, or the Strock design) but, as far as I understand, they don't allow to adjust some of the degrees of freedom (1. and 3.) hence to precisely dial field illumination and tilt (which is crucial for imaging, not really for visual).
Are there other designs that address these issues (commercially available or not)?
Edited by Dan_I, 25 May 2023 - 05:17 AM.