Thread: Focal Reducer
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Old 01-08-2012, 04:09 PM
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bojan
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Well, you don't really need it.. but it will help a lot.
It is because lower the f number of the objective, higher will be the surface brightness density of the extended objects (like nebulae) on the sensor, so you can get away with shorted exposure time for the same effect on the sensor, signal/noise wise.
However, the image of such object will also be smaller (or, FOV will be wider), which is a good thing for DS's, since they are usually quite large. Also, coma will be reduced (because focal reducers are also coma correctors).

For planetary photography, since the planets have relatively much higher surface brightness density, you need better resolution (because planets are small and you want details), but there is no need for so much light to keep exposure time reasonably short - so focal reducer is not recommended for them, you will actually want quite the opposite - barlow lens.
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