what eyepiece(s) would you get?

[troypiggo (Troy)]

I'm thinking of getting eyepieces for an ED80 and ED100 for astro viewing rather than astro photography. Question time.

2" or 1.25" - which would you choose and why? I was thinking the 1.25" are much cheaper and would be fine for just general viewing?

What length eyepieces? I understand the length of eyepiece to scope focal length gives a magnification factor. What one or 2 magnifications would you think would be good for general purpose? Would you get one for widefield/nebulae/constellations etc, and one higher mag one for planets/moon? I have no feel for what mags will yield good results.

Also thinking of a prism(? right angle thingy) for viewing. Any tips or pointers on that? 2" vs 1.25"?

[Wavytone]

Star diagonal - get a 2" one - sooner or later you will one day want to put a 2" eyepiece behind it. Also the length of most refractor tubes are designed for use with a star diagonal, without one you may find the tube is too short and you can't focus the eyepiece.

Eyepieces - I have an orion 102ED, which is fairly short at f/7. I have basically three groups of eyepieces:

(a) a set of Vixen LVW's - big, heavy and you have to be careful to hang on to the tube when you take one out - otherwise the scope will flip down, the clutch on my mount is not stiff enough to hold it. Stunning to look through, though.

(b) A set of Edmund RKE's - small, cheap and lightweight 1.25" ones, compare favorably with orthos on my scope.

(c) few Vixen LV's - small and light 1.25" with excellent eye relief, mainly for having a quick look.

For practicality the small lightweight 1.25" ones are great as I don't have to worry about rebalancing the scope when swapping, and it will safely stay put when an eyepiece is pulled out. I use the heavy LVW's for "special occasions" when I am in the mood.

[troypiggo (Troy)]

Thanks for that. What mm length would you recommend? 10mm, 12mm, 25mm etc.

[Bassnut (Fred)]

For the very few times I bother actually viewing thru an ED80, I found a cheap 2" mid field eyepiece more fun that a fancy 1.25".

[Wavytone]

That depends on the focal ratio of your scope and how good the objective is.

The maximum magnification to expect from a refractor is 2X per mm of aperture. So for mine, 200X. With a 700 mm focal length, I'd need a 3.5 mm eyepiece to get that magnification. In practice though the shortest I have is 8mm and the views through this are excellent.

At the other extreme, the lowest useful power is defined by the diameter of the exit pupil your eye can accommodate. I opted for 5mm (I'm not young) and this means the lowest useful magnification is the aperture of the lens divided by the exit pupil - in my case 20X. As before, with a 700mm focal length that means a 35mm focal length eyepiece. The nearest I have is a 30mm Vixen NLVW.

Between max and minimum you will need at least one, maybe two eyepieces (total 3 or 4), so that the ratio of focal lengths from one to the next is about 1:1.5 or 1:2. In my case I chose 8, 13, 22, 30 for this reason.

With my scope, the LVW 42mm eyepiece gives an exit pupil that is too big, so I saw no point in buying it. Conversely, I skipped the 17mm in the LVW series (too close to 13 or 22 mm), and I frankly don't need to push the refractor all the way to see Airy disks - the 8mm provides fine views of the planets at 142X.

Depending on your scope, you could go for say 6, 12, 24 or perhaps 6, 10, 18, 30 depending on what is available in the type you choose.

And yes using a small refractor for a "Rich Field" view with a widefield eypiece (in my case the 22 or 30) is always good for hours, even after 30 years of stargazing.

Lastly when buying really short focal length eyepieces below 15mm, stay away from the ones with really short eyerelief - such as orthos and plossls. Aside from being a pain to use, they are also a ***** to keep clean.

Even if you only buy one really good eyepiece, the long eye relief types (Vixen LV, LVW, Orion Stratus, Baader Hyperion) are well worth the extra $ in the short focal lengths.

[troypiggo (Troy)]

Thanks guys. Wavy, that's the sort of info I was after. Excellent. Guess I should have posted a little more info in my OP.

The ED80 I have is 600mm, the ED100 is 900mm. What I might do is look at the detailed numbers for each scope, then try to find a mix that will suit both on average (for now).

So from what I gather in your post, I'd be looking at max mags of 160x for the ED80, meaning a 3.75mm eyepiece (I'll round off later, just doing the maths now), and 200x for the ED100, meaning a 4.5mm eyepiece.

You lost me a bit with "at the other extreme" and "exit pupil your eye can accommodate", but if I work on 5mm like you, ED80 gives me 16x and 37.5mm, while the ED100 gives 20x and hence 45mm.

So now I might do a bit of a search around for eyepieces in the, say, 5mm to 35mm range. I'll probably be looking at the ED pieces as a min.

Does that sound like I'm "getting it"?

[Wavytone]

Yes, you've got it. 5 to 35 mm will be fine. At f/7 or f/9 pretty much any eyepiece will be fine, you could buy a cheap set from http://www.andrewscom.com.au/site-section-10.htm for example.

The other thing is to try to buy a parfocal set, so you don't have to refocus every time you swap one.

The eye pupil is the aperture of the iris of your eye when dark adapted. While optical textbooks suggest 7 mm for youngsters, as you get older this gets smaller. In addition, if there is some light around your eye may not be fully dark adapted, in which case its better to assuming a slightly smaller value than 7 mm. You can also measure your own eye pupil too.

The exit pupil of the telescope is the quantity ExP shown in the figure on this page
http://www.telescope-optics.net/eyepiece1.htm

Essentially, you have to get your eye positioned so that ExP coincides with the iris in your eye in order to see the full field of view provided by the eyepiece. Eye relief is the distance ER between ExP and the eye lens of the eyepiece.

If ExP is larger than the eye pupil some of the light will not enter your eye, in which case you're effectively seeing with less than the full aperture of your scope.