Re: Eyepieces

[sashark (Sasha)]

Just wondering if anyone could give a newbie some advice on what eyepiece would be suitable for my telescope...

I would like to get something that gives the highest possible magnification for my scope as I've heard some eyepieces only work well with longer focal lengths?

My telescope is a Saxon 909AZ3 with a focal length of 900mm...

Currently have eyepieces in 25mm and 10mm

Thanks!

[barx1963 (Malcolm)]

Eyepieces are a source of endless talk in this and othe forums. The issue to be aware of is that more "magnification" is not always a good thing. If you have used your scope for any astro work yet and tried the 2 eyepices there is a noticeable difference between the 2.
The main reasons for this are twofold. Firstly, and this applies to all eyepiece designs, the image created by the scope and eyepiece is affected by atmospheric factors which are also magnified by the higher power. Also for dim objects you are spreading the available light over a larger area effectively dimming it.
Secondly, there are factors relating to the eyepiece design. The ones supplied with your scope are plossls and one of the design characteristics of these is that higher power ones have a much smaller exit pupil and eyerelief. Without getting technical this makes them harder to use as you need to position your eye much closer to the eyepiece and more precisely, and that can be uncomfortable. More advanced designs can improve this situation, but that can cost $$$!
My advice would be, use the scope a bit and play with the 2 pieces that came with it and see how you go before spending too many dollars. And above all, don't be tempted by those "kits" you see around the place as they are often overpriced and of dubious value.

Cheers

Malcolm

[mental4astro (Alexander)]

Magnification is the quotient of the scope's focal length and the focal length of the eyepiece.

So, if your scope has a focal length of 900mm, and your eyepiece is 25mm, then the magnification is :

900 / 25 = 36X

But there are practical limits to the max & min. The max has two factors affecting it, the first is the size of your objective lens or primary mirror. A rule of thumb is 50X per inch of aperture. So if your scope's diameter is 114mm (4.5"), then the max magnification is 225X (assuming the optics are decent quality). BUT, the ultimate limiting factor, regardless of the size of your scope, is the atmosphere. Typical conditions allow for 100X to 150X, and it doesn't matter if your scope is 4" or 20". Good conditions will allow upto 250X. Exceptional conditions are 400X and over, but this happens maybe once a month.

There is another factor, but this affects viewing comfort, and that is the EP design. Plossl EPs were the bees knees of EPs, 30 years ago. They were expensive to produce then, but today they are chip as chips to make. They still are a good design, giving good image quality when made well. Their main draw back is the eye lens (the lens of the EP you look into, as opposed to the field lens which faces into the scope) gets progressively smaller as the focal length shortens, and you need to place your eye closer and closer to the eye lens until you need to look through a pin hole and park your cornea on the lens too to look into the thing.

Today there are much newer designs that also employ newer glass types. These new designs don't see the eye lens get smaller - in fact some new high power EP field lenses are bigger than that of a 25mm plossl. The eye relief (how close you need to put your eye to see the whole field of view in the EP) is longer too. Again, new designs today can be as easy to look into as a 25mm plossl. New designs also allow for a wider aparent field of view, where plossls give a 52deg AFOV, new designs give from 58deg all the way up to 120deg.

Price is a tricky thing though, but some careful research will find you can get damn good high power EPs for as little as $70 new (the TMB Planetary Type II models). Their AFOV is only 58deg (still better than a plossl) but their ease of use is akin to your 25mm plossl. They pop up every now and then here in the IIS classifieds, but are snapped up real quick. The 9mm is a real gem in my kit!

One little known gem is the Celestron Ultima LX series. Around $140 with post from the USA (some retailers here in Oz have them for more than $500). Only thing is they are REALLY BIG! I have the 8mm, and is one EP that is very easy on the eye - eye strain with EPs is one thing that's never mentioned!).

Then, the sky's the limit with $$$$.

Before you decide which EP to get, get to a star party or Astro club meet and try a few out. You can best judge what best works for you.

Mental.

Edit: Pippedat the post by Mal!

[Wavytone]

Hi Sasha,

Borrowing an earlier reply, with all lengths in mm here are the relations:

Focal length of the scope = F = 900mm
Eyepiece focal length = f
Aperture of the scope = A = 90mm
Maximum pupil of your eye when dark adapted = a, which we will assume is 6mm (in average conditions).

Focal ratio = R = F/A = 10, or alternatively F = RA
Magnification = m = F/f = RA/f
Exit pupil of the beam from the eyepiece = e = A/m = f/R

Lowest useful magnification occurs when the exit pupil = the eye pupil, i.e. e=a. Or, when choosing your lowest power eyepiece f = Ra, which for your scope is 60mm. Your scope is quite small however, so there not much point trying to squeeze an eyepiece longer than the 25mm one you already have.

For a person with normal vision, the maximum useful magnification occurs between 1.1X to 2X per millimetre of aperture. For refractors and f/15 Maksutovs it's 1.5, possibly 2X if you have truly superlative optics. For SCT's and Newtonians, 1.1X to 1.5X. This is the highest magnification at which you should just be able to discern the diffraction pattern around bright stars. At higher magnifications, objects get dimmer and fuzzier and you won't see any more detail.

So for your refractor the maximum useful magnification is m = 1.5A or about 135X. The eyepiece focal length required is f = R/1.5 or perhaps even R/2, which suggests an eyepiece in the range 6-7mm is the shortest useful one.

Given that you have 10 and 25 mm, I'd look for one around the 6-7mm mark and perhaps another about 15mm. Since the scope is f/10, pretty much any eyepiece will work well enough so there really isn't much point splashing out a lot of money for posh ones.