Mike Jones article.qxd:Layout 1 - Astronomy Technology Today

More documents

Recommendations

Info

• In-House Testing• In-House Service• 100% Satisfaction• Over 100 Years Combined ExperienceWe Stock More Than3,500 Different Scopes,Parts, and Accessories!Zeiss, Meade, Celestron, Takahashi, Stellarvue, Vixen, Lumicon,Denkmeier, Vernonscope, Lunt, GTO, Thousand Oaks, JMI, Pentax,Sky Instruments, Proxima, Skywatcher, Coronado and Many More!26437 Ridge Road • Damascus, Maryland 20872www.handsonoptics.com1-866-726-7371
Baffle Optimizationfor CassegrainTelescopesBy Mike JonesCassegrain telescopes require lightshielding, or baffles, to prevent unfocusedstarlight from passing around the secondarymirror, through the primary mirrorhole, and directly reaching focus. This unfocusedlight is in the form of plane waves,rather than spherical waves converging tothe desired focal point. The effect is to addbrightness to the image without structure,which uniformly reduces image contrastfor both visual and imaging usage.Stray light in any telescope can be easilyseen using the stacked eyepiece techniqueI described in the October 2007issue of Astronomy Technology Today 1 .Thetelescope is aimed into the daylight sky ata safe angle from the sun, and a mediumfocal length eyepiece with good eye reliefis inserted into the focuser. A second eyepiecewith a real focal plane, such as an orthoscopic,Kellner or Plossl design, is heldover the first eyepiece, and focused to givea sharp image of the exit pupil from theFigure 1. Stray light in an unbaffled Cassegrain (red light from star, green light random).first eyepiece. In a Cassegrain, if you cansee skylight around the secondary mirrorat the exit pupil, the baffling is not designedproperly.An example of an unbaffledCassegrain is shown in Figure 1. The randomlygenerated rays in red are from aninfinitely distant object and are thus allparallel, and come to focus as seen. Tenthousand randomly directed rays werethen spawned from a plane in front of thetelescope to simulate stray light and plottedin green. It is easy to see the paths unfocusedstray light (plotted in green) cantake to arrive at the focal plane withouthaving been focused by the telescope mirrors.Optimal design of Cassegrain bafflingis defined as the design that minimizescentral obscuration of the telescope’s primarymirror while fully shielding thespecified focal plane. This turns out to bea more complex problem than it first appears.The lengths of primary and secondarybaffle tubes drive their diameters,and their diameters drive the resultingcentral obscuration and light blockage ofthe telescope entrance pupil. The secondarymirror baffle opens up toward the primary,and will in general always be largerthan the secondary mirror itself. If the primarybaffle is too short, the secondarybaffle must be larger to block light, whichincreases obscuration. As the primary bafflegrows longer, the secondary baffle diametercan shrink, but then the primarybaffle begins to intrude into light from theinner portion of the primary mirror, againAstronomy TECHNOLOGY TODAY 51
Page 1: Don’t Miss A Single Issue!www.ast
Page 5 and 6: BAFFLE OPTIMIZATION FOR CASSEGRAIN
Page 11: BAFFLE OPTIMIZATION FOR CASSEGRAIN

Mike Jones article.qxd:Layout 1 - Astronomy Technology Today

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?