2" vs. 1.25" Dielectric Star Diagonals

by Roland Christen

Hi folks,

The question has come up why I recommend 2" over 1.25" dielectric mirrors. 
here is my response in a private Email:

Included here are two images showing the surface figure of a 2" diagonal.
MaxBright Center
MaxBright Surface
The dielectric coatings have one drawback, they are not accurate near the edge. Near the edge, the coatings lose accuracy very rapidly (severe turned edge). This is due to the large number of layers and the stress that results at the boundary. In a 2" diagonal, this is no problem because we make the blank quite oversize, so the actual clear area is very accurate. Also, you only use the center portion of the 2" mirror for a high power image anyway. There is room in a 2" diagonal to do that. In a 1.25" diagonal, there is little if any extra glass to allow for the turn down edge (the size of the turn down stays the same regardless of overall area). Also, you are using a relatively larger portion of the clear area to form your high power images, so the chance for optical defects is much greater. I also strongly advise anyone with a Newtonian NOT to get their diagonals coated with dielectric coatings because with a Newt, each star image uses almost the entire area of the mirror and the chance for optical defects like astigmatism is very great. Dielectric coatings are great and they have their place, but their limitations must be understood. The two images show a typical mirror from the batch we are assembling now. These mirrors have the latest in low stress dielectric coatings, but the interferograms do show the edge defect that arises with this type of coating. The oval area that was measured is the actual clear aperture of the mirror, approx 1.75" wide by 2.5 inches long. The actual glass is quite a bit larger to avoid having any of the edge defect in the clear aperture. Roland Christen, ASTRO-PHYSICS
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