2" vs. 1.25" Dielectric Star Diagonals
by Roland Christen
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.
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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