What are the advantages of a larger secondary obstruction?
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.
Is it true, are RC telescopes difficult to collimate?
Every OGS RC 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) RC telescopes for over 18 years, and we can tell you, yes, there are times that we have spent several hours (sometime days) collimating an RC; and that, using our professional RC-purposed lazar collimator platform with 40” optical flat. But once we are finished collimating an RC, 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 RC 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.
What is the interferogram test that is provided with every OGS telescope?
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What size domes does OGS recommend for observatory telescopes?
Generally speaking, the minimally appropriate dome sizes needed to accommodate two people and one computer station are as follows:
Telescope Size Dome Size
12.5″ 8′ or 10′
(For specific information concerning this matter, or simple to have a conversation with an OGS system developer about your planned project, we welcome you to give us a call at your convenience.)