FAQ1:
What are the advantages of a larger secondary obstruction?
Answer1:
Cassegrain
telescopes in the f/12 - f/15 range normally have secondary obstruction sizes
ranging from 30 - 35% of the primary mirror diameter.
The secondary mirror diameter is not necessarily the baffled secondary obstruction. Usually, our Ritchey-Chretien telescopes are baffled with a secondary
obstruction ranging from 38 - 42% of the primary diameter. Our Classical Cassegrain telescopes range in obstruction size from 33 to
approximately 38%. Cassegrain telescopes need obstruction sizes this large to obtain
sufficient off-axis illumination for a reasonable field of view. Cassegrains with fast f-ratio primaries and high amplification
secondaries, such as are common in the popular Schmidt-Cassegrain telescopes,
can have smaller obstruction sizes. However, the trade-off is significantly increased field curvature.
There
are some rare exceptions where professional telescopes are designed to have very
small secondary sizes down in the 25% range, but they are generally designed for
very narrow-field studies with small detectors, and quite often have field
correctors to minimize the field curvature problem. Unless there were very specific requirements for a small-obstruction
telescope, Optical Guidance Systems would not consider making a Cassegrain
telescope with an obstruction size smaller than approximately 30%.
Telescopes with obstructions smaller than that would be for very narrow
fields.
Obstruction size is important in
preventing excess diffraction (which reduces contrast). However, our experience shows
that, in comparing a telescope with
‘seemingly’ good optics and a 25% obstruction with another telescope with a
33% obstruction, the latter performed better, having higher contrast and less
diffracted light around bright stars and planets. The reason was that the optical quality (overall surface smoothness, or
“RMS” value) of the telescope with the 33% obstruction was significantly
better. All
of our optics are guaranteed to be better than 1/4 wave front peak-to-valley,
but equally important in diffraction-limited optical systems is to have a very
good RMS value. The worst RMS figure that we have seen in Zygo interferometer testing of
our optics was 1/20 wave RMS.
The best was 1/40 wave RMS.
The average mirror usually comes in at around 1/30 wavefront.
FAQ2:
Is it true, are R-C
telescopes difficult to collimate?
Answer2:
Every OGS R-C optical set
is made-to-order and must meet the rigors of precise collimation, which requires
1) that the optics are installed and locked into our research-quality optical
cell mechanism, 2) that, precise indexing and orienting of the optics is
achieved, and 3) that the OTA is laser collimated to the ¼-wave performance
that we guarantee.
And please note, we do guarantee collimation! OGS has been
building (and collimating) R-C telescopes for over 18 years, and we can tell
you, yes, there are times that we have spent several hours (sometime days)
collimating an R-C; and that, using our professional RC-purposed lazar
collimator platform with 40” optical flat. But once we are finished
collimating an R-C, we guarantee that our customers will receive an excellently
collimated telescope that will stay properly collimated for years to
come.
What
if an OGS customer feels the need to slightly “tweak” the collimation of
their R-C telescope for some unforeseen reason? That is not a problem, as we
have available an affordable lazar collimator kit with instructions. Plus, by
our always-collimated guarantee, we are always willing to support our customers
with any walk-through help that they may need.
FAQ3:
What is the interferogram test that is
provided with every OGS telescope?
Answer3:
Please click here to learn
more>>
FAQ4:
What size domes does OGS recommend for observatory
telescopes?
Answer4:
Generally
speaking, the minimally appropriate dome sizes needed to accommodate two people
and one computer station are as follows:
OGS
