Quote:
Originally Posted by thegableguy
The ED80 had a 0.85 flattener on it so it was essentially f/6.4. The reflector is f/5. This SHOULD mean that the newt would be significantly brighter, like two thirds of a stop... but it clearly isn't.
Anyone have any suggestions ...
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This is a common (astro)photography myth. The f/ ratio has NOTHING to do with absolute brightness.
Using the same ISO value and exposure time, a larger ABSOLUTE aperture in millimeters / inches will ALWAYS show fainter stars and therefore, the same stars appear brighter. Period.
But a smaller f/ratio ('brighter') with the same absolute aperture results in a larger FOV, so the entire image appears brighter because a larger portion of the sky (i.e. more stars) are on the image.
In your case, the newton should show much fainter stars if you use the same exposure time and ISO value, which is (20/8)2 = 6.25 times brighter which is about 2.6 stops or two stellar magnitudes.