Eye pieces

[kmce]

Hello all.

Could someone help me with eye pieces i am a tad confused Some seem to be measured in mm and some with the sign " i am assuming the smaller the mm the better and show objects bigger, is that right?

[Doc]

Yes but the image will be dimmer as the hole is smaller and not a lot of light can enter.

You will find as well the smaller the hole the more atmospherics play a part to destroy the image.

Bigger eyepieces let in more light, so give a more contrasty and bright image.

[haitch]

There are usually 2 important measurements on an eyepiece (EP). One being the focal length (FL) usually given in mm and the field of view (FOV) in degrees. The FL governs magnification - divide the FL of your scope by the FL of the EP to determine magnification eg 1000mm FL scope with a 10mm FL EP is 1000/10 = 100 so magnification of 100x.

Higher magnification is not necessarily better most planets will need a fairly high magnification to see any detail but even with them you will reach a point where you are spreading out the light so much that the image begins to break down or the disturbances in the atmosphere mess up your viewing.

For deep sky objects it is often a balancing act between fitting the object in the field of view (for larger objects), not spreading out the light too much so it remains visible and spreading it out enough to darken the sky background (most pertinent in light polluted locations).

The FOV helps with the fitting it in the objective bit - the higher the FOV the more of the sky will be visible in an EP of a given FL. It also helps if your mount is undriven as an object will stay in view for longer without moving your scope (as is the case at lower magnifications too).

[Ben Ritchie]

Some seem to be measured in mm and some with the sign "

For historic reasons, eyepiece focal lengths are measured in millimetres, while the eypiece barrel size is measured in inches - 1.25" and 2" barrels are the most common, but there's also a Japanese 0.965" format too.

The focal length determines the magnification, the barrel size what format focuser they fit.

[haitch]

For historic reasons, eyepiece focal lengths are measured in millimetres, while the eypiece barrel size is measured in inches - 1.25" and 2" barrels are the most common, but there's also a Japanese 0.965" format too.

The focal length determines the magnification, the barrel size what format focuser they fit.

Doh - completely forgot about barrel size!

[ronin]

What scope have you got?

What do you use it for?

A simple rule of thumb is that an eyepiece focal length in mm equal to the f number of the scope will work best if it is magnification that you want and are trying to get. I would disagree if the scope is something like a f/5. At f/5 you need a good 5mm eyepiece to get the best.

If you start down the track of more and more magnification then the image simply gets too poor to be of any use. So you have to compromise between magnification and quality of image.

If you are looking for new eyepiece suggestions then budget and use come into it.

[Daryl]

There are usually 2 important measurements on an eyepiece (EP). One being the focal length (FL) usually given in mm and the field of view (FOV) in degrees. The FL governs magnification - divide the FL of your scope by the FL of the EP to determine magnification eg 1000mm FL scope with a 10mm FL EP is 1000/10 = 100 so magnification of 100x.

Higher magnification is not necessarily better most planets will need a fairly high magnification to see any detail but even with them you will reach a point where you are spreading out the light so much that the image begins to break down or the disturbances in the atmosphere mess up your viewing.

For deep sky objects it is often a balancing act between fitting the object in the field of view (for larger objects), not spreading out the light too much so it remains visible and spreading it out enough to darken the sky background (most pertinent in light polluted locations).

The FOV helps with the fitting it in the objective bit - the higher the FOV the more of the sky will be visible in an EP of a given FL. It also helps if your mount is undriven as an object will stay in view for longer without moving your scope (as is the case at lower magnifications too).

Thats a great explanation and you've made things clearer for the likes of me

What visual gain to the user is a 2" EP over a standard 1.25.. for example the 28mm 2" UWAN is almost double the price of a 16mm 1.25 UWAN, having not looked though one I cant visualize what the extra money is buying, is it just a simple case of doubling the viewing area?

[John]

Thats a great explanation and you've made things clearer for the likes of me

What visual gain to the user is a 2" EP over a standard 1.25.. for example the 28mm 2" UWAN is almost double the price of a 16mm 1.25 UWAN, having not looked though one I cant visualize what the extra money is buying, is it just a simple case of doubling the viewing area?

The 2" format allows a wider field of view. A 28mm in the 1.25" size would be restricted to a field of view of around 55 degrees but made in the 2" size the UWAN can deliver an 82 degree field of view showing a wider expanse of sky and some go even wider than that.

You are paying for a lot more glass and a lot more figuring and polishing, coating etc. The 28mm UWAN weighs 1 kg !.

[haitch]

Thats a great explanation and you've made things clearer for the likes of me

What visual gain to the user is a 2" EP over a standard 1.25.. for example the 28mm 2" UWAN is almost double the price of a 16mm 1.25 UWAN, having not looked though one I cant visualize what the extra money is buying, is it just a simple case of doubling the viewing area?

Thanks

As John says - you reach a point with 1.25" EPs where the physical size means you can't fit any more FOV in. For example a 1.25" 40mm Plossl will only be able to show the same piece of sky that a 32mm can. It will just be like looking down a bit of a tube to a smaller image of it. Once you start going to wider angle EPs this point occurs at shorter focal lengths. If you, for instance, look at the Meade series 5000 SWA (68 degree) and UWA (82 degree) ranges you will see that the point that they need to step up to 2" to still achieve that FOV is at a shorter FL as the FOV angle widens.

[dmahon]

What about "pupil size" - where does this come into it?

[haitch]

What about "pupil size" - where does this come into it?

Now that's not something I fully understand yet. I know it is the diameter of the light beam leaving the EP and entering your eye and is dependent on the F ratio of your scope (focal length divided by aperture) and the focal length of your EP.

Basically the faster the scope (smaller f ratio number) the bigger the exit pupil and the longer the focal length of the EP the bigger the exit pupil so fast scopes with low power EPs are more likely to cause exit pupil problems.

You run into your maximum exit pupil when it reaches the same size as your own eye's dark adapted pupil. Your pupil is dependent largely on your age - as you grow older your pupils' ability to increase in size in the dark diminishes. It is therefore dependant on you as much as your scope/EP.

That said I have a fast 14" f4.5 Dob and 32mm & 38mm 2" SWA EPs which I love. Being 41 I should have a max pupil size of about 6.5mm and my 38mm SWA gives an exit pupil of around 8.5mm in the dob. As far as I can tell, at night, beyond your limit you are just wasting some light that doesn't get to enter your eye. It doesn't stop the EPs giving me nice wide, low magnification, views of the sky.

Some people report problems with the secondary mirror starting to become visible when the exit pupil is too large but I've only ever noticed anything like that in bright daylight where the 38mm seems to have a very slight central dimming if my eye isn't positioned perfectly.

Anyway the above is just my personal thoughts - as I said I'm no expert.

I've just been and found the scope calculator in the beginners advice section so if you want to do your own calculations here it is...

http://stargazerslounge.com/beginners-help-advice/111213-useful-line-calculator-scope-magnification-exit-pupil-size.html