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HOW TO SELECT AN EYEPIECE

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HOW TO SELECT AN EYEPIECE

The choice of eyepieces is somewhat subjective, depending on the telescope, intended targets, budget, etc. The eyepiece is "half the telescope", in a sense, so it pays to select eyepieces with a practical eye¡ªto avoid duplication of magnifications, to select the correct eyepiece for the scope type, and to make observing more fun.

Here are some methods by which to choose additional eyepieces for your telescope:

The magnification method:

You say, "I love to see things magnified, but I can't afford to have so many eyepieces that the change in magnification from one to the next is hardly noticeable. How would I choose which magnifications provide meaningful magnification differences?"

Well, start out with a very low power for the widest field in your telescope. That¡¯s usually from 30X to 50X. In a 10" f/5 telescope, a 32mm eyepiece would be a good lower power eyepiece, providing about 40X magnification.

Use 1.4X as a multiplier to produce the next higher power. You can convert to focal lengths by merely dividing the low power eyepiece by that same 1.4.

So, the set would proceed: 32mm, 23mm, 16mm, 12mm, 8mm, 6mm, producing magnifications about 40% apart. The raw difference between the 32 and 23mm is small, so, in this case, I¡¯d skip the 23 and go straight to the 16.

The collection didn¡¯t have to start with a 32. If the maximum field size is not an issue, this telescope could have gone: 25mm, 18mm, 13mm, 9mm, and 6mm to 7mm.

The exit pupil method:

Your eyes¡¯ pupils don¡¯t open past a certain size. If you use eyepieces that produce larger exit pupils than your eyes can receive, you are essentially stopping down your telescope to a smaller diameter as the edge-of-field rays don¡¯t even make it into your eye. Also, in a reflector, the shadow of the secondary gets bigger at lower powers, and may become obtrusive. The upper limit to exit pupil is about 7mm, best visual acuity is often 2 to 3mm, and people¡¯s eyes are often not able to field an exit pupil smaller than 0.5mm without having minor pieces of flotsam (called floaters) in the eye interfere with the view of details.

A decent set, by exit pupil, would be 7mm (lowest power), 5mm, 3mm, 2mm, 1mm, 0.5mm.

Multiply your telescope¡¯s f/ratio by the above exit pupils to obtain the focal lengths of eyepieces to form the set. In the case of the hypothetical 10" f/5, the eyepieces would be: 35mm, 25mm, 15mm, 10mm, 5mm, and 2.5mm

The Compromise method:

If you look closely at the 2 above methods, it's obvious the number of eyepieces could be reduced by using a good Barlow lens to double the power (of the eyepieces used with it). The first set could have been a 25mm and 18mm and a 6mm, with a 2x Barlow lens.

That would be much less expensive than buying all separate eyepieces.

As a matter of fact, the turbulence in the night atmosphere usually limits our viewing to powers under 200X to 250X, so having eyepieces that produce 50X, 100X, 150X, 200X will guarantee you have an eyepiece for 90% of your viewing. Add an extra high-power eyepiece, or use a Barlow lens on a lower power one, and you¡¯re set. In the example just given, 3 eyepieces (50X, 100X, and 150X) and a Barlow would cover all the powers AND provide all the lesser-used high powers for exceptional conditions.

Whichever process you use to figure out what eyepieces you need, just be aware that you don¡¯t have to have a whole kit full of eyepieces to view nearly every object visible. Three and a Barlow will do quite nicely.

But there are other methods, equally valid:

The Eye-Relief Method:

This is the way any person who must wear glasses to look through the eyepiece has to select eyepieces. Fortunately, few of us have such severe astigmatism that we have to view with glasses on. You can refocus the telescope to correct for near-sightedness (myopia) or far-sightedness (hyperopia), but that won¡¯t work with astigmatism. Though there are additional astigmatism-correcting lenses that can be added to a few different eyepieces, by and large people with severe astigmatism must view with glasses on.

Since wearing glasses forces the eye to move back away from the eyepiece, this means that most eyepieces will lose field of view. Those eyepieces whose position of best viewing (called Eye Relief) behind the eyepiece (where your eye is close enough to see the entire field of view, but not too close) is farther back from the lens are known as Long Eye Relief eyepieces. There aren't as many models of these as there are of the shorter eye relief types, but there are some. Glasses wearers usually stick to long eye-relief models. Some good news for the glasses wearer¡ªalmost all eyepieces with focal lengths longer than 26mm would be considered Long Eye Relief. At long focal lengths, glasses wearers have the same choice as non-glasses wearers.

The Use-Matching method:

This overlaps with the method of choosing by Apparent Field-Size Preference, so I¡¯ll discuss them both:

Many people find that, after observing a while, they are using primarily low powers to view large objects, like star clusters, and higher powers mostly for lunar and planetary viewing. That makes a large apparent field of view desirable at the lower powers, but makes smaller apparent fields of view acceptable at higher powers. A judicious selection of eyepieces would probably have the largest apparent field of view (and perhaps the most expensive eyepiece) for your low power(s), and a smaller apparent field of view (which often makes the eyepiece less costly) at higher powers. A 65 to 82 degree apparent field of view is very desirable when looking at large nebulas or star clusters at low power, but a 40 to 52 degree apparent field of view is quite acceptable at high powers, where the objects viewed are smaller. And some of the high-power eyepieces with smaller apparent fields of view have exceptional optics (like the Parks Gold Series Pl?ssl for example).

A user may prefer to have the ultra-wide apparent fields of view at all powers, or may prefer the narrower apparent fields of view at all powers. If so, then Apparent Field-Size Preference will become an important component of choosing eyepieces.

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