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F ratio is to all intents and purposes irrelevant in this conversation – it is simply the mathematical ratio of focal length / aperture. It’s a useful measure in some equations, but not here.
There are only 2 factors that are useful: Focal length, which determines image scale and Aperture which determines the number of photons available to play with.
A telescope with a focal length of 1000mm with an aperture of 250mm will give a brighter image that one of focal length 1000mm and an aperture of 150mm. End of. It only gets complicated when you start to compare different apertures and focal lengths. Luckily for us, if you double the aperture you quadruple the photon count which more than offsets the usual corresponding increase in focal length. That’s why aperture rules!
It is also true that if you reduce the focal length, the resultant image will be brighter too. This is true for stars also. Stars are not quite point sources on Earth due to that astronomically annoying thing called an atmosphere. If we were out in space, the average star would be a mere 1 milli-arc second across, but down here they are fuzzied out to a thousand times that.
So, for Jacquie, the focal reducer will make a positive difference not just in terms of brightness of star but also in the likelihood of increasing the number of comparison stars on the same frame which will come in useful later. For the Pluto event, bigger is better, but you can certainly get meaningful results with an 8”.
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