Hi Andy,

It seems we are pretty much agreeing but talking slightly apart.

What I'm saying is for the same size image of a particular object (on a CCD/film/eye) it makes no difference to the brightness whatever the native F ratio of the scope is.

If you were to require a barlow lens on a shorter F ratio scope because the object required magnification, then the longer F ratio scope will take less time to image (all else being equal) because now there is one more lens in the light path. Assuming the object size on the CCD in both cases is the same.

If the object to be imaged requires a focal reducer on a longer F ratio scope then the shorter F ratio scope will provide the shorter exposure time (all else being equal) because the longer F ratio scope now has an additional lens in the light path.

IOW, for the same size image to be seen on a CCD, it will require pretty much the same time for both scopes to form the same brightness image. Since the addition of a single lens won't impact the time by a huge percentage.

The shorter F ratio shouldn't be called brighter, it should be called wider.
Only the diameter of the main lens/mirror determines the exposure time given that the formed image will be the same size.

E.g. I noticed on your site where you described your experience at imaging at F10. The image wasn't as good as you'd wanted it, but, it should also be noted that it would also be larger than if you used F6, almost twice the size in fact.

The main reason I posted this is that there seems to be the misconception that a scope of lower F ratio will give brighter images. Sure it will, but it gives smaller ones too and thats only for the same set of eyepieces.

Most people I know choose a scope along these lines. As big a diameter as they can afford or is practical then buy eyepieces to get either a specific FOV or a specific magnification. Given that situation:

If one chooses the eyepieces to be the same magnification for both scopes then the images will be the same brightness for both scopes.

If one chooses eyepieces for the same FOV for both scopes, then both scopes will have the same brightness.

The only difference is that the longer FL scope will have longer length eyepieces. This also has huge advantages for people with glasses. It means their eyepieces will have more eye relief for the same price. The images will however appear identical in brightness.

So why say the lower F ratio gives a brighter image?

It's confusing to beginners and adversely affects people who wear glasses because the eyepieces are more expensive to produce with longer eye relief at shorter focal lengths.

All because there is a misconception that it will produce a brighter image.

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MrObvious