I'm researching planetary eyepieces and I keep running into the statement that an eyepiece shouldn't be used with a telescope that has the same (or very similar) focal ratio as the eyepiece. Like you shouldn't use a 4mm eyepiece with an f/4 telescope. I don't really understand why.I'm thinking of buying a Meade 5.5 uwa eyepiece to go with an f/5.9 Orion 8 inch dob.Would there be any problem with that? Would the combination not work well because the focal ratio of the eyepiece is so similar to the focal ratio of the telescope?Thanks, Scott.

...I keep running into the statement that an eyepiece shouldn't be used with a telescope that has the same (or very similar) focal ratio as the eyepiece...

I'm researching planetary eyepieces and I keep running into the statement that an eyepiece shouldn't be used with a telescope that has the same (or very similar) focal ratio as the eyepiece.......

Scott, this is a very good thread, and perfect for people jumping into the hobby. The math can be confusing at first, with terms like focal length, focal ratio, afocal projection...and astro folks...! You get it. Read jallbery's post (#4) carefully. I think he may be hitting mark with your, honestly rather understandable, confusion.For what it's worth, a quick math hack, I like to take the focal ratio of the scope and multiply the number by 2. Then find an eyepiece that has a focal length that is roughly twice that number. I find the 2mm exit pupil produced by such a combination to be perfect on all nights and in most observing situations.Good luck!

QuoteScott, this is a very good thread, and perfect for people jumping into the hobby. The math can be confusing at first, with terms like focal length, focal ratio, afocal projection...and astro folks...! You get it. Read jallbery's post (#4) carefully. I think he may be hitting mark with your, honestly rather understandable, confusion.For what it's worth, a quick math hack, I like to take the focal ratio of the scope and multiply the number by 2. Then find an eyepiece that has a focal length that is roughly twice that number. I find the 2mm exit pupil produced by such a combination to be perfect on all nights and in most observing situations.Good luck!I find that a 7mm exit pupil is optimal for observing some objects, a 0.3mm exit pupil is optimal for observing some objects and a 2mm exit pupil is optimal for observing some objects..A 2mm exit pupil provides 50x in a 100mm telescope. This is a poor choice for viewing the planets, closer double stars, small planetary nebulae, globular clusters, large nebulae like the Veil and the California...I say this: On most nights, for most objects, something other than a 2mm exit pupil will provide the best view. Jon

Actually, I've read that having an EP equal to your focal ratio gives you the theoretical maximum resolving power of the scope, so I've always considered a 1.0mm exit pupil to be a good thing and an important benchmark.