David: It won't work for cassegrain telescopes. These are systems of two active elements and the final focal length depends strongly on the distance between the two elements. two lense equasion. 1/F = 1/f1 + 1/f2 -(d/f1f2) That's how the sct can change it's focal plane position, by actually changing it's focal length and focal ratio. The specs are nominal for a normal usage with a diagonal and standard eyepiece, adding filter wheels, flip mirrors, and camera adaptors changes the position of the focal plane. To come to focus will change the focal length and the focal ratio. Not a problem with the Newtonian because it only has one active element the diagonal is not active. Refractors have all the active elements tied together at a fixed distances. Also moving the placement of focal reducers and doublers in the optical train can change the final focal length because they are active elements. DT --- On Sat, 5/8/10, David Rankin <David@rankinstudio.com> wrote:
From: David Rankin <David@rankinstudio.com> Subject: Re: [Utah-astronomy] Calculating exact focal length (Was: Imaging Calculator) To: "Utah Astronomy" <utah-astronomy@mailman.xmission.com> Date: Saturday, May 8, 2010, 9:26 AM That's a good point. I should add an input for focal reducers and barlows.
On 5/7/2010 10:08 PM, Chuck Hards wrote:
Tele-extenders and focal reducers have a multiplying factor printed on them (most do) For example, .85x. Assuming no eyepiece in the system, simply multiply the objective's focal length by that factor. Example, 8" f/10 = 80" = 2032mm focal length. 2032 x .85 = 1727mm = ~ f/7. You've changed your original system from f/10 to f/7.
Filter wheels and other accessories like extension tubes with essentially no magnifying power add or subtract nothing to focal length. They just take up "IN" travel.
On 5/7/10, Patrick Wiggins<paw@wirelessbeehive.com> wrote:
Looks nice. But I've always wondered how one calculates the exact focal length of a given setup. I suppose one could just use the manufacture's stated focal length of the OTA. However, once one starts adding accessories to the back like focal reducers, tele-extenders, extension tubes and filter wheels the advertised focal length becomes useless.
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