Best eyepiece depends on scope's focal ratio?

[tims]

I recently read (in "The Backyard Astronomers guide" I think it was called) that the best eyepiece for any particular scope can be roughly worked out by multiplying the focal ratio of the scope by 2 - therefore a 20mm eyepiece is best for an f10 scope.
I can't remember exactly why this was so - something about how the eye operates? Does anyone have any idea why this may be so?

Thanks

[astro744]

It has to do with exit pupil with 2mm being ideal for good brightness and contrast.

Exit pupil = focal length of eyepiece divided by focal ratio of telescope

eg. 20mm eyepiece / f10 telescope = 2mm

Exit pupil also = aperture of telescope / magnification

[Manav (Yugant)]

Astro - You're like a knowledge pool I should have you on my speed dial

[astro744]

I'm just happy to help out.

[tims]

Thanks Astro

So, using the example above, if I have a 40mm eyepiece and put a 2X barlow in front does this still give me the desired 2mm for the exit pupil?

[astro744]

A 40mm at f10 would give 4mm exit pupil and when combined with a 2x Barlow would give twice the magnification and yes, half the exit pupil, ie. 2mm.

You could build your eyepiece collection around exit pupil alone and if your eyepieces all had ther same apparent field of view (AFOV) the magnification and true field of view (TFOV) would be in step.

For an f10 instrument with 1.25" focuser your maximum exit pupil will be 4mm, (40mm Plossl). An f10 instrument with 2" focuser can accept say the Tele Vue 55mm Plossl and this will give you an exit pupil of 5.5mm.

A nice range of exit pupils would be say 5-7, 3.5-4.5, 1.5-2.5, 1-1.5, 0.5-1. The actual number isn't that critical and you may want to cover more in the 0.5-1 range for planetary observing to cover a range of seeing. The 5-7 range is for wide field roaming under dark skies or simply as a low power finder eyepiece under any sky conditions. 4mm is usually a good wide field roaming power if the sky background isn't dark (suburban/city lights) so as to give you more contrast. 2mm is great for bringing out faint galaxies (ie good contrast) and gives you a nice dark background and there is still plenty of light in the object.

0.5mm is about the smallest you want since the image really gets quite dim beyond this point but not impossible. In fact under exceptional seeing you could add a Barlow and work in the 0.25-0.5 range and an object like the Moon could handle it.

[tims]

Thanks Astro, great reply and much appreciated . Another question, slightly off topic to my original query but I'm trying to figure out what premium eye pieces to buy without breaking the bank -

The Televue Ethos'es and the like are expensive - am I too far off the mark in assuming that a 17mm Ethos with a good 2X barlow (giving 8.5mm) isn't too different (optically) than using say an 8mm Ethos by itself?

I remember reading somewhere that some contrast is sacrificed when using a barlow but is this and other optical differences neglibible comparing the barlowed 17mm and the 8mm by itself?

[astro744]

Your best choice of eyepieces will depend on what telescope but ley me say a premium eyepiece will work well in all telescopes.

You mention the Ethos but it is at the top of the Tele Vue range in terms of price and that is because it offers a 100 deg apparent field. The focal lengths available are currently 6,8,10,13,17 & soon 21mm. The 21mm is likely to be the largest available due to field stop restrictions in a 2" barrel. There are numerous options in the Tele Vue range and for the cost of one Ethos you could easily buy all the Plossls or a couple of Radians or Panoptics. However if 100 degrees is what you want then the Ethos is there and I think the new 21mm one will be a joy in any telescope.

The entire Tele Vue range is designed to work down to f4 (see http://www.televue.com/engine/page.asp?ID=196) Of course at f4 there is significant coma but that is a function of the primary mirror in say a Newtonian and NOT the Tele Vue eyepiece. An inferior eyepiece will show you lots of other abberations, most notably astigmatism and severe curvature of field. However some inferior eyepieces work quite well at f8 to f10 or greater but not at f4 to f6.

What makes an eyepiece good? Well price is a good indicator but there are many mid range eyepieces from many suppliers that may work well in your telescope.

A good quality Barlow is invisible but it does extend the eyepiece eye relief a little. It is a % of focal length and the longer f.l. eyepieces get affected more so much so that the long eye relief is difficult to use (a real problem with a Barlowed 40mm). A Tele Vue Powermate has four lenses and is similar to a Barlow except that it maintains eye relief and lets the eyepiece perform as designed. There is also no vignetting with a Powermate as there can be on very short Barlows. An inferior poorly made Barlow will degrade the image significantly.

A 17mm Ethos will perform best with a 2x & 4x Powermate (in lieu of Barlow) and in effect become an 8.5 & 4.25 Ethos. However, there will be a lot of weight and length added to the focuser and an 8mm and (possible future) 4mm Ethos would be much more manageable. There are also an extra four elements of glass added to the optical path which reduce transmission ever so slightly but perhaps only an issue if your observing at the limits of detection.

My advice to you is try and get to look through as many different telescopes and eyepieces as possible at events such as star parties and if that isn't possible then ask yourself how important 'premium' is to you not just for astronomy but for other things you may be interested in. If you cannot afford more than one premium eyepiece right now then buy just one and get more later. Without knowing your telescope I cannot comment on specific focal lengths that may best suit you but here are some suggestions:

If you want to introduce Barlows or Powermates then may I suggest the following:

32mm Plossl
2.5x Powermate (12.8mm)
5x Powermate if you need shorther focal length to get greater magnification. (6.4mm)

This combination will give you a quality low power eyepiece with mid to high power options and comfortable eye relief all round.

If you want a wider AFOV then may I suggest the following:

24mm Panoptic (exactly the same TFOV as 32mm plossl but at greater power.
2.5x Powermate (9.6mm)
5x Powermate if you need shorther focal length to get greater magnification. (4.8mm)

The 24mm Panoptic is one of the most versatile eyepieces around. It has the maximum field stop in a 1.25" barrel (same as 32 & 40mm Plossl) but at greater power, smaller exit pupil (for greater contrast) and wide AFOV (68deg). It is simply a joy to use in any telescope and works well in all.

If you get the 2.5x Powermate ask for the newer one with filter thread as Tele Vue changed the barrel design since the original model did not accept filters.

If you have your heart set on an Ethos then get one now and add more later much later if need be. The Ethos can be split into two series; 21, 13, 8 and 17, 10, 6 if you have say a shorter focal length instrument. Note if you add a 2x Powermate to 21, 13 & 8 you get 10.5, 6.5 & 4 which is very close to the second series (once a 4mm is released).

The 21mm Ethos will be a jaw dropper in any telescope and will cost as much as a telescope but then again it is half the view. A 13mm would be a nice complement to it and is also a nice focal length in any telescope. An 8mm would be a galaxy buster and nice planetary eyepiece on many telescopes.

There is plenty to choose from and whatever you choose, enjoy!