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Old 02-01-2013, 03:25 PM
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DavidLJ (David)
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Affect of star diagonal, etc. on imaging field of view.

Experimentation shows that with a SCT the introduction of a star diagonal and/or a filter wheel into the optical train measurably alters the imaging camera's field of view. For example, the attached image shows a composite of three images of a small part of M 41 imaged with the same SCT and camera but with and without the use of a star diagonal and filter wheel. No tele-extender (Barlow) or focal reducer was used. The black background image shows the field obtained at prime focus (i.e. without using either the star diagonal or the filter wheel. The red image shows the field obtained when just a star diagonal is introduced into the optical train. And the blue image shows the field obtained with the use of both a star diagonal and a filter wheel. The general rule seems to be that the farther back the camera is placed from the rear end of the OTA the greater is the magnification and the smaller is the imaging field of view.

Recognising that a SCT is focussed by shifting the main mirror in the OTA, I wonder if this general rule also applies to a refractor or a Newtonian where the main objective is fixed in place and focus is achieved by moving the camera to and fro? I'd like to resolve this question for the purpose of a project that I'm currently undertaking but I don't have access to a refractor or a Newtonian to experiment further. Can some kind soul please advise if the introduction of a star diagonal and/or a filter whel similarly affects the camera's imaging field of view in the case of a refractor or a Newtonian?
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Old 02-01-2013, 04:46 PM
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Merlin66 (Ken)
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In the Newtonian and Refractor etc the focal length is fixed. The plate scale therefore doesn't vary....you fit all the dangle dolly stuff just inside the fixed position of the prime focus.
With a SCT and some Mak's the effective focal length varies with the main mirror position up to 10-12% ie an f10 system can end up f11 with all the "bits" attached. (This also impacts on the effect of a reducer on an SCT, the attached Spreadsheet shows the result....)
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Old 02-01-2013, 05:31 PM
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DavidLJ (David)
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Thanks Ken.

As I suspected but very good to get your confirmation. Again thanks.

David
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