Author Topic: Is it ok to use the same focal ratio in scope and eyepiece?  (Read 268 times)

laucongsnagal

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Is it ok to use the same focal ratio in scope and eyepiece?
« on: December 31, 2017, 01:04:07 PM »
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.



Lamar Davies

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #1 on: January 08, 2018, 02:57:49 AM »
Don't understand this. Focal ratio of eyepiece ?

aninerti

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #2 on: January 08, 2018, 05:02:37 AM »
Quote
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.


Scott:

Do you have any links? Something's not right.

An eyepiece whose focal length is equal to the focal ratio of the telescope provides an 1.0mm exit pupil. A 1.0mm exit pupil is appropriate for a number of situations, too small for some, too large for others.

Bottom line: The Meade 5.5 UWA will provide 218x in your 8 inch F/6. That's a nice magnification for viewing the planets in decent but not perfect seeing. Go for it..

rackramasca

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #3 on: January 10, 2018, 02:46:52 AM »
We don't typically speak of eyepiece focal ratios, only their focal lengths.

And there is no problem using the Meade 5.5mm UWA on an F/6-ish Dob-- in fact, it is a good fit for a high power eyepiece.

I suspect you've misunderstood a common rule of thumb: that under average seeing conditions, the shortest focal length (i.e., the highest power) eyepiece that is practical is (in mm) about equal to the focal ratio of your telescope. Under exceptional conditions, you may be able to go as high as half your focal ratio.

This is roughly equivalent to another rule of thumb-- the maximum useful magnification under exceptional seeing conditions is roughly 50X your aperture in inches. Under less favorable conditions you may be limited to 25X your aperture.

So a 200mm F/6 scope has a maximum useful magnification of about 400X under ideal conditions (50 times aperture in inches). THis is a 3mm eyepiece (1200mm/3mm=400). Under average or less seeing, you may be limited to 200X (25 times aperture in inches), which would be a 6mm eyepiece.

Or we can just say-- it's an F/6 scope, so 6mm for average or less conditions, half that (3mm) for superb conditions. It will work out for any F/6 scope.

And these are just rules of thumb. They are grounded in real physics having to do with the resolution of a scope of a given aperture and what the smallest details that can be resolved are. At some point, more magnification just means a fuzzier and dimmer image that reveals no additional detail, Different people have different ideas about the point at which extra magnification doesn't help.

Paul Rivera

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #4 on: January 13, 2018, 10:53:52 PM »
Usually I would say that the highest power you can expect to work fairly well is one that has the focal ratio and the focal length the same. It also gives (conveniently) a magnification equal to the scope objective diameter (in mm) and the exit pupil is 1mm.

There is the proviso that when you get to about 5mm and f/5 that after that things get difficult. F/4 scopes suffer from coma and eyepiece that are shorter tend to need to be good - so cost more. It is a sort of step after which items are pushed to their operating limits.

Half the reason people seem to talk of f/5 and faster comes from the imaging field, but even in that I suspect that f/6 would be as easy and a lot less "difficult". I get the feeilng that "faster" has become a bit of a mantra and quite a few are unsure why.

But the numbers being similar mean little (nothing) what matters is how an f/4 scope performs and how a 4mm eyepiece performs. But a 6mm eyepiece in an f/6 scope should be a really nice combination.

What you may have heard is that after you get an eyepiece "shorter" then things can start to be a problem. As in a 4mm eyepiece in and f/5 scope means you are getting into the area where magnification is up but you can find clarity is down. Equally an 8mm eyepiece in an f/10 scope should be good also. It is all being able to judge things and the numbers are a sort of indicator.

carewemi

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #5 on: January 16, 2018, 02:42:15 AM »
Quote
...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...

Fanny statement!

Eyepiece anytime works with the same focal ratio as telescope where it was installed...
Focal ratio for telescope FL/aperture-of-entrance
Focal ratio for eyepiece FL/aperture-of-exit (exit pupil diameter)
Focal ratio for telescope = focal ratio for eyepiece

Tye Paez

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #6 on: January 16, 2018, 11:03:08 PM »
Quote
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.......

Would you quote a few such statements verbatim and identify the source? Thanks.

James Clayton

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #7 on: January 20, 2018, 05:46:22 PM »
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 <em class="bbc">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!

David Johnson

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #8 on: January 21, 2018, 07:38:27 AM »
Quote
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!


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

highdanmyne

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #9 on: January 22, 2018, 11:07:50 PM »
Since the focal ratio of the eyepiece is intimately connected
to the apparent field width, the rule of thumb seems peculiar,
and ripe with possibilities for hasty generalization.

What about a 100-degree EP versus an Ortho....and
thus, what about your target? The rule seems to buckle.

Various exit-pupil generalizations seem to have more sensible
footing in reality, though even then things get more complicated
when you consider the target.

bandretaco

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #10 on: January 31, 2018, 12:47:44 PM »
Quote
Quote

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!


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
Well, there is certainly plenty of truth with your statements, Jon. I suppose my comment could be misconstrued as a blanket statement. However, what I really mean is, at a 2mm exit pupil nothing looks bad, whether planets or deep sky. And honestly, as a planetary sketcher, I prefer the color and brightness of Jupiter and Saturn at 2mm, as opposed to 1mm. I realize this can easily open a can of worms regarding exit pupil preferences among observers, so I'll simply close with the typical "IMO" and YMMV".

byhodete

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #11 on: February 02, 2018, 07:21:44 PM »
Quote

Just how wonderful are the Pleiades in a 20 inch scope with a 2mm exit pupil? How about the California Nebula or Barnards loop in any telescope? Just how wonderful is that 0.6" double in the 10 inch with a 2mm exit pupil.

If you are viewing Jupiter in a 60mm, do you prefer the 2mm exit pupil to the 1mm exit pupil.

I make these comments because magnification and the exit pupil are the one thing we can change when looking through a telescope, the one tool we have. If you can even see Barnard's loop with a 2mm exit pupil, I'd be plenty surprised. I have a heck of a time seeing it with 7 mm exit pupil in a 80mm scope.
Jon

Tim Massey

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #12 on: February 03, 2018, 01:12:14 AM »
If you want quick and easy experience with different eyepiece focal lengths and the resulting exit pupils in your telescope, get yourself a decent zoom eyepiece and a 2x Barlow and have to it.

For your f/5.9 telescope, a zoom eyepiece that has 8, 12, 16, 20 and 24mm settings (for instance, the Baader Zoom) would provide 1.4, 2, 2.7, 3.4 and 4.1mm exit pupils. Put that zoom eyepiece in a 2x Barlow, and the exit pupils would be 0.7, 1, 1.4, 1.7, and 2mm.

Mike

Duane Berhane

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #13 on: February 03, 2018, 03:10:44 AM »
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.

tailipoma

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Re: Is it ok to use the same focal ratio in scope and eyepiece?
« Reply #14 on: February 04, 2018, 11:42:27 AM »
Quote
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.


The image exists at the focal plane, it is fully resolved at the focal plane and the magnification really has nothing to do with the theoretical resolving power of the telescope.

The role of the eyepiece is to magnify the image at the focal plane so that it matched to the observers eye. Any attempt to justify a 1mm exit pupil, has to involve assumptions about the resolution of the human eye. But the human eye is a very complicated thing and it's resolution is not fixed, it depends on how much light there is. With a dim image, the rods are ganged up and the resolution is very poor. With bright images, the cones in the visual center are activated and the resolution is much better. And of course we are all individuals, someone with sharper eyes can fully resolve the focal plane at lower magnifications than someone with poorer eyes.

This is a very complicated subject with many variables to consider. Fortunately, there is a very practical solution.. Forget it all and just look carefully into the eyepiece and see what you see. Swap eyepiece, zoom in and zoom out. You eye will tell you the optimal magnification. For very close double stars, this can require exit pupils of 0.3mm or even less.

Jon