Quote:
Originally Posted by mura_gadi
Hello,
The total brightness of an object is determined from aperture only, and the f ration only affecting the magnification when used with eyepieces/lenses.
So, with a given eyepiece used the shorter F ratio will magnify higher and cause the object to be bigger but dimmer. (Same light spread out over a larger area). If you could set the 130 to have the same magnification at f7 and f6.5 you should see no difference in brightness.
Steve
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Yes the camera reference applies because the aperture does vary when you open up the aperture of a camera lens.
But practically speaking with regards to telescopes with a fixed aperture, if you add a reducer you will reduce exposure times and get a wider view. Hence the popularity of 8-12 inch scope with fast F ratio like F4 or less.
More of the light from the scene is allowed onto the sensor and when you have a long focal length its the same as cropping the field of view.
So the extra light is coming from the wider field of view.
The faster the F ratio the more difficult it is to have the image aberration free and the more sensitive to collimation etc.
Greg.