eyepiece's for smaller appeture ?

[rory]

found a snippet in "illustrated guide to astronomical wonders" in the choosing eyepiece section. it states that smaller appeture scopes and slower focal ratios compress the range of useful eyepiece focal lengths.

goes on to say for example with a 90mm f11 refractor they use a 30mm widefield finder e/p. with the next reasonable next step up is the 10mm (x100)

thus eliminating middle range focal lengths ?

so am i wasting my money looking at 15/18/22mm eyepieces ?

be intersested to hear from other small appeture users on there most used magnifications.

[umadog]

I would think a 15 mm could still have its uses. A jump from 33x to 100x is pretty big for a small scope. The 10 mm is about the highest useful power you will get so you want one eyepiece in between. Here's why:

I don't know exactly what they mean about "compressing" the useful range, but it does cut it short. What you need to understand is eyepiece exit pupil. Smaller exit pupils will provide a dimmer image. The exit pupil is the diameter of the light beam leaving the eyepiece. A 7mm exit pupil matches the size of your maximally dilated pupil (your eye's pupil) so the image will look bright. Going larger than 7 mm won't make the image brighter. Going smaller will make it look dimmer but will increase magnification. The formulae you need are:

exit pupil = eyepiece focal length / telescope focal ratio

magnification = telescope focal length/ eyepiece focal length

The problem with f/11 is that it's really slow so you will get small exit pupils and the image will look dim. Even a 30 mm eyepiece will have an exit pupil of 2.7 mm, which is verging on medium power. The 10 mm gives an exit pupil of less than 1 mm, which is *tiny*. Here is a guide for choosing eyepieces based on exit pupil: Useful Magnification Ranges for Visual Observimg - How To

If the telescope optics aren't good then small exit pupils won't work well. In practice I would say your highest useful power would be best served by an eyepiece of roughly 13 mm. You already have a 12 mm so don't go higher power than that: it's ideal. Just buy one in the midpoint between those you already have.

[rory]

umadog, thankyou very much for your input.

[rory]

im reading that optimum exit pupil for dso's is around 2mm.

so quick calculations reveal around 22mm giving x45 mag would be a 2mm exit pupil.

am i thinking along the right track with this ?

[umadog]

A 2mm exit pupil isn't a bad place to aim for but remember that it also depends on the DSO and it depends on the scope. It's magnification that allows you to see details, so with a larger telescope you can get a good degree of magnification and retain a >3 mm exit pupil. With a smaller telescope you have a tighter balancing act between magnification and brightness. To run some numbers as an example:

a. 10" f/4.7 gives about 120x with a 2.1mm exit pupil.

b. 90mm f/11 gives about 42x with a 2.1 mm exit pupil.

You also need to pay attention to the field of view you are getting and to the magnification itself. For example, under bad seeing conditions 150x will be too much for any object regardless of the exit pupil.

DSOs with high surface brightness take magnification well but those with low surface brightness do not. Some DSOs are large and you may want to look at them using both low power (to see the whole thing at once) and high power (to concentrate on their details). Globulars respond well to high power. Some galaxies respond poorly to high power.

[rory]

thanks umadog, very helpful indeed.cheers...

[Qualia]

Thank you, very useful links and info, umadog.