Why So Many Planetary Eyepieces?

[pbyrne]

I'm asking this merely out of curiosity as I'm not much of a planetary observer.

I see that some manufacturers sell their planetary eyepices in focal lengths from 2mm to 9mm, with 3mm, 4mm, 5mm, etc in between.

My question is this: Will the views through the scope be so much better in a 4mm EP than a 5mm EP? Or a 6mm than a 7mm?

One other question comes to mind: Does anyone use a 2mm eyepiece? I would imagine the magnification would be too great in any scope.

Thanks for listening.

Paul

[Doc]

Say your scope had a focal length of 1000mm so your 4mm would give you x250 and your 5mm would be x200. That extra x50 might be the difference between splitting a double star or not splitting it. Or might be the difference in seeing the cassini gap or missing it altogether. So yes thats why there are so many different focal lengths eyepieces.

As for the 2mm eyepiece, these are pretty rare and as you say very hard to use due to it's very short eye relief but nnot impossible. you woulld need excellent seeing though.

[DemonicOmelette]

I would guess there's not many people who buy EVERY size (unless they have more money than sense). They offer all the different sizes in order to offer choice. Maybe you find the eye relief slightly too short with a 4mm, but get on fine with a 5mm..... Just get the ones that fit you the best!

There's not a lot of difference between a size 8 shoe and a size 9, but one fits, and the other is too small. I'm not going to buy both!

[brianb]

I'm asking this merely out of curiosity as I'm not much of a planetary observer.

I see that some manufacturers sell their planetary eyepices in focal lengths from 2mm to 9mm, with 3mm, 4mm, 5mm, etc in between.

My question is this: Will the views through the scope be so much better in a 4mm EP than a 5mm EP? Or a 6mm than a 7mm?

Bigger but not necessarily better.

For most people, about 30x per inch of aperture - i.e. an exit pupil of about 0.8mm - is as much as is helpful. You get the exit pupil by dividing the focal length of the eyepiece by the focal ratio of the scope.

So, 0.8mm in a f/10 scope takes a 8mm eyepiece; in a f/15 scope takes a 12mm eyepiece; in a f/5 scope takes a 4mm eyepiece.

This illustrates why there is such a range.

Also, with poor seeing, it is quite often the case that you can't get a sharp image with 30x per inch & have to drop down a bit. So powers a bit lower are often useful.

One other question comes to mind: Does anyone use a 2mm eyepiece? I would imagine the magnification would be too great in any scope.

With perfect seeing, impeccable optics and a small scope, 50x per inch may be useful, but more likely for double stars than for planets - planets have low contrast detail which is easily lost with too much magnification. Now a 2mm EP in an f/4 scope gives 50x per inch. But for most people a 2mm EP will be far more powerful than is ever any use.

Also a longer focus EP used with a Barlow will probably be more comfortable anyway.

[johninderby]

For high power planetary observing, eyepieces that go up in 1mm increments are very useful. You may find for example that say a 4mm eyepiece gives good sharp views and a 3mm is too much, but an hour later the 3mm is giving sharp views and then a bit later you have to go up to a 5mm to get clear sharp views.

I use a 3-6 Nagler zoom for high power observing as it's so convenient and you can get just the right magnification without changing eyepieces.

John

[Macavity]

Within a single manufacturer type a series would be useful. I sense tho' one man's 6mm might be another's 7mm (or at least 6.95 mm?). I do know that my (lunar!) expectations re. an eyepiece's TRUE field of view differed quit a bit from the calculation based on the spec. I was later fairly convinced by someone's claim that this was due to (laudable) attempts to correct for various distortions in the image...