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cold space
member
Reged: 09/09/07
Posts: 69
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Does using a star diagonal on the back of an F10 Meade scope make the focal length more?
And if it does what focal length would my F10 scope be now?
Regards Matt.
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deSitter
scholastic sledgehammer
Reged: 12/09/04
Posts: 778
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No, but like a filter, and unlike a mirror diagonal, a prism diagonal lengthens the optical path from objective to final image. This is just in the nature of light transmission on passing from one medium to another of differing refractive index - same thing that makes a stick in water appear bent. So you need more out-focus with a diagonal or filter in place.
-drl
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mclewis1
Thread Killer
Reged: 02/25/06
Posts: 2454
Loc: Colorado
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Actually the answer is yes ... but just slightly. Moving mirror designs that also have a magnifying secondary can come to focus in a variety of locations, there isn't just one focus point. It's one of the benefits of the design. Refractors, Newtonians and other fixed objective/mirror type scopes come to focus at only one point so deSitter's comments would indeed be correct if we were considering that type of scope.
You can prove this by putting an eyepiece directly into a visual back of an SCT and bringing it to focus and then adding a diagonal or extension tube and bringing that combination to focus ... the actual focus points will be quite different.
As for a change in F ratio, well yes but just a little bit (F10.1 to F10.3 or something like that). Not enough to worry about. Meade and Celestron spec their SCTs focal length with a standard configuration of visual back and a 1.25" diagonal ... but it's not an exact measurement, the F10 number is often from a rounded up or down value. I believe though that most larger F10 SCTs can come to focus at a point that would measure F11 but you'd have to turn that focuser knob an awful lot to achieve it.
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Mark
C11, C6, APM/TMB115, and AT80ED
Tandem mount CGE and CG-5A, WO EZ-Touch and AT Voyager
25x100s, 8x56s, T-Mount Light, and Mark 1 eyeballs
Modded 350D, SPC900, Mallincam
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deSitter
scholastic sledgehammer
Reged: 12/09/04
Posts: 778
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The final f/ratio of SCT systems depends on the spacing of the component mirrors. That of a refractor using a prism diagonal - or equivalently interspersing a glass block with parallel faces - does not. The only effect is to lengthen the total path. When you use a prism diagonal with an SCT vs. a mirror diagonal, you again must lengthen the path to get focus, but this means changing the distance between primary and secondary.
-drl
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cold space
member
Reged: 09/09/07
Posts: 69
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Thanks for the feed back,
Regards Matt.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12588
Loc: Cumberland, R I , USA42N71.4W
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Actully adding a diagonal on the back end of an SCT changes the focal lenght considerably. My C5 changes from F=1250 with no diagonal to F=1360-1375 with a diagonal. Actually an SCT scope has a different focal length for every eyepiece, unless of course you are using a set of parfocal eyepieces.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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waterskies
member
Reged: 08/18/07
Posts: 85
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How do you calculate the focal length shift with a diagonal based on eyepiece?
Quote:
Actully adding a diagonal on the back end of an SCT changes the focal lenght considerably. My C5 changes from F=1250 with no diagonal to F=1360-1375 with a diagonal. Actually an SCT scope has a different focal length for every eyepiece, unless of course you are using a set of parfocal eyepieces.
edz
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12588
Loc: Cumberland, R I , USA42N71.4W
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use several eyepieces that have field stops small enough so there will be no vignette. Drift time those eyepieces both with and without the diagonal. Compare the values. By proportion of the drift timed field of view, you will find the difference in magnification. Since the eyepiece F does not change, that leads you to the information needed to recalc the F of the scope.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Jeff Young
Post Laureate
Reged: 08/04/05
Posts: 3172
Loc: Ireland
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You can also calculate the increase if you know the prescription of the primary and secondary mirrors. The math can be found here (along with a perhaps more useful chart showing the effects on various common SCTs and a few MCTs):
http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EFLMAKCASS.pdf
Cheers,
-- Jeff
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Nikon 18x70s / UA Millennium Colorado:
Solarscope SF70 / TV Pronto / AP400QMD Coronado SolarMax40 DS / Bogen 055+3130
APM MC1610 / Tak FC-100 / AP1200GTO Tak Mewlon 250 / AP600EGTO
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