umadog

I think this is oversimplifying things a bit.

All eyepieces perform better in just about every respect at longer focal ratios, and also it's easier to make more accurate lens surfaces - for both eyepieces and telescope objectives - at longer focal lengths.

Yes, I agree. You'll notice more astigmatism from poorly corrected eyepieces in a faster scope, more coma, etc. The benefit of longer focal ratios is that you get a nice well-corrected view to the field edges without needing a coma-corrector or expensive eyepieces. A slow scope can get away without a fine-focus knob on the focuser. These are the benefits of a slow focal ratio. They are real benefits and are most apparent when looking at star fields.

For planetary observing, however, the focal ratio is one of the least relevant things. Don't forget that at high power with a small object in the centre of the field there are no problems with eyepiece astigmatism or coma. However, what I was really getting at is that when the issue of "planetary scopes" comes up one often sees comments about long focal ratio scopes being better for planets because the central obstruction is smaller. This is such a over-simplification that it's usually wrong.

"f-ratio is important. A 6" f/11 might very well best an 8" f/5 on most typical nights observing planets"

Focal ratio is not relevant it's the size of the obstruction that matters. So long as the secondary obstruction is under 20% of the primary diameter by area, the scope behaves like an unobstructed instrument. More than 20% and you start to see its effects. The effect you see isn't due to light loss it's due to increased diffraction caused by the large circumference of the secondary. This decreases contrast. However, there's no reason a 25% or even 30% obstructed scope can't perform very well. Why? If the scope is already a large aperture instrument with good optics then even with a hefty central obstruction it can still show superior contrast and detail. Optical quality and aperture matter more than focal ratio. I see this every time I observe Jupiter at f/4.

Focal ratio isn't the only thing which influences secondary obstruction size. The design of the instrument matters also. For example, my current f/4 has an obstruction of 19.5%. I used to have an Orion XX12, which is f/5, and that had an obstruction of 23%.

Here's Jupiter at f/4.5: www.cloudynights.com/ubbthreads/attachments/4889931-j2011-10-29_12-31_rgb_tba.jpg I guess it's been Barlowed. Hence the f/23.7 reference.