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- Category: Eyepieces
Telescopes are not the tube: they're the glass!
Think of a telescope and you think of a long tube with stuff on the eyeball end.
That long tube is actually the least important part of the telescope: it's just there to keep the other equipment in place, hopefully without vibrating too much.
The most important part of the telescope is the lenses and/or the mirrors. The tube is just there connect them together. Solid tube connectors are cheaper and they block out stray light, but some scopes just have struts between the optical ends of the scope.
The two ends are known as the objective (the object/target end) and the ocular (the eye end).
You could use a paper towel roll if the size were right. In fact, some vintage inexpensive dobsonian telescopes used essentially larger paper towel rolls as their structure.
Two ends: Objective (object) and Ocular (eyeball)
So, the important part is the glass, and both ends have a relationship that controls all the optical characteristics of the scope.
Generally, you want to have some flexibilty with the images you see, so you can change some of the glass to get a different view.
The Objective and the Tube are generally a unit
The objective lens or primary mirror actually drives the diameter and length of the tube, so the tube (or other support system) is generally a permanent component of the objective lens cell or the primary mirror cell.
The Ocular end is the Variety end
The other end, the ocular side - the user end - is where all the customization (and a most of the fun) occurs. It ends with interchangeble eyepieces.
Focuser
Because you are likely putting a lot of different equipment on the ocular end, there is almost always a focuser. The objective/primary has a fixed focal plane that the eyepiece needs to be positioned at, the focuser allows you to move the eyepiece precisely to that plane. That's how focusing works moving the eyepiece to the focal plane. Some scopes (like Schmitt-Cassegrains (and cameras) have fixed "receiver"; focusing moves the glass to move the focal plane to the sensor/eyepiece).
Eyepieces: A Labyrnth of Fun (and Money?)
Magnification is the focal length at the objective side of the eyepiece divided by the focal length of the eyepiece. A 1000mm focal length hitting a 25mm focal length eyepiece gives you 40x. Want higher power, put in a shorter focal length eyepiece. 10mm gives you 100x.
That number on the side of the eyepiece is the focal length.
If you buy a new entry level telescope package, it generally will contain two or three eyepieces.
One of the most common questions asked is "Are my included eyepieces any good?" The short answer is (and I seldom give short answers): NO!
Are they sufficent for getting started? Yes.
Are you going to get the best experience with these cheap eyepieces? No.
If you're enjoying the hobby, will you want to buy better eyepieces? Absolutely.
Here's a little secret: you can spend a lot more money on eyepieces than on telescopes. But you don't have to and you shouldn't.
I've rotated through dozens of telescopes, but I have a set of good eyepieces and I hang on to them.
If money were no object, here's the recommendation: buy a Televue Nagler 31, and a bunch of Televue Ethos eyepieces. They $700 or more, each. I own the Nagler, but have cheaper alternatives for everything else.
The eyepieces that came with your telescope kit, may be worth $30 collectively (beginner scopes have a lot of competition for price, the eyepieces and the mount/tripod get a lot of "cost reduction").
The good news is that there is a lot high-value, high-performance eyepieces between your "cheap" eyepieces and the bank breakers.
Note: there are two standard diameters for "real" telescope eyepieces: 2" and 1.25". Some cheap telescopes only accept 1.25" eyepieces. Check yours.
- The old standard was 0.96", these are mainly used by collectors who enjoy the vintage experience.
So what's the difference between a $40 Celestron 32mm eyepiece and the $700 Televue Nagler 31mm?
- Diameter - the 2" barrel of the Nagler can show a wider swath of sky than the 1.25" barrel of the Celestron. The 1.25" barrel essentially clips the image delivered to it by the telescope. For "point objects" (Jupiter, Saturn, Mars, planetary nebula - anything small), this isn't as big a deal. For big objects and wide fields - it is a big deal. Check your focuser!
- Quality - better glass, better coatings, better correction for fast focal ratios
- AFOV - Apparent Field of View. This is the big one and where most of the pricepoint jumps. It's how wide the view looks to your eye. It deserves explanation (Apparent Field of View)
Barlows
Barlows are magnification multipliers that allow you to use an eyepiece for multiple magnifications. A 2x Barlow with a 24mm eyepiece is provides the magnification of a 12mm eyepiece and maintains the properties of the eyepiece. Even a premium barlow is cheaper than most premium eyepieces and they can be used with many eyepieces. They're not only a magnification multiplier, they're an "eyepiece" multiplier.
- Details
- Category: Eyepieces
Compared to telescopes, eyepieces are simpler. They have only a few of characteristics that really matter:
- Barrel Size: 1.25" or 2". All scopes accept 1.25" diameter eyepieces; most good scopes have focusers that accept 2" eyepieces.
- Focal Length: often the only number you see on an eyepiece is its focal length. It matters.
- Apparent Field of View: this is how wide the view in the eyepiece looks. Wide is better. Narrow is inexpensive; wide is more expensive.
Eyepiece Focal Length
The magnification through a scope is the quotient (mathemtical result) of the focal length of the telescope divided by the focal length of the eyepiece. An inexpensive 4-inch refractor might have a focal length of 1000mm.
A 1000mm focal length telescope using a 25mm eyepiece yields 40x magnification.
A 1000mm focal length telescope using a 10mm eyepiece yields 100x magnification.
There is a practical limit on the focal lengths of widely-available eyepieces which limits the magnification range and the absolute (true) field of view - the width of sky you can see through the eyepiece. Call the range 40mm to 3mm. For a 1000mm scope that would give you a magnification range of 25x to 333x.
Note that your maximum magnification is limited by a number of factors. A rule of thumb is that the maximum magnification for a telescope is the aperture (diameter) in millimeters times 2.
A 1000mm telescope with an aperture of 100mm will have a maximum useful magnification of 100 x 2 = 200x.
Given that the maximum useful magnification of a 1000mm scope is 200x, it makes no sense to have any eyepiece with a focal lenght of less than 5mm. If the scope is an inexpensive achromat refractor, its highest magnification, due to the limits of the glass used, may only be 100x. An eyepiece short than 10mm would be of no use.
So, for your telescope you'll want to have a low power, wide field eyepiece, an eyepiece for near the highest useful magnification, and then maybe some in-betweeners.
That's your focal length range.
The next decision is how much do you want to spend. Eyepieces vary quite a bit in price even at the same focal length. Assuming decent quality, the primary factor in cost is the Apparent Field of View. Wide is expensive. Ultra Wide can be Ultra Expensive. What is Apparent Field of View?
Apparent Field of View - AFOV
AFOV is the angle the eye sees looking from one edge of the image to the opposite edge.
There are two primary benefits to wide AFOV:
- at a given magnification, wider AVOF shows more sky
- wider AFOV is a more intimate, intense view of the sky
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- many compare the narrow AFOV eyepieces as "looking through a straw" and the ultra-wide AFOVs as "like looking through a porthole".
The disadvantage to wide AFOV eyepieces are they are far more expensive than the narrow AFOV eyepieces.
For example a basic narrow 32mm (a wide field) eyepiece might retail for $50, the premium wide field wide AFOV, the Televue Nagler 31mm retails for about $700.
A combination of AFOV and the focal length of the eyepiece determines how much sky you can see for a given scope. Using the 31mm range (the wide end of the eyepiece spectrum), an eyepiece with double the AFOV will show four times as much space. Having the capability for wide views is a big deal.
Here's an expansive deep sky object, the Veil Nebula. It's a supernova shock wave that traces most of a circle. The two circles in the image represent the Fields of View from a 4-inch refractor with a narrow AFOV (44-degrees) and an extra-wide AFOV (82-degrees):
The wide eyepiece can capture the full limits of the circumference of expanding shockwave. The narrower eyepiece - it's the same magnification - captures only part of the nebula. In a manual mount, you can push the telescope around to examine the parts - that's actually kind of fun. With a GoTo scope you can push some buttons to move the scope around - that's not quite as fun. With the wide angle, you can see it all at once.
Insert a higher-power eyepiece to scrutinize pieces.
The second aspect of a wider AFOV lens is that you can see the same target at higher power - it spreads the image over a much wider area. You feel closer - it's a better more "in-person" experience. The middle circle below is a 20mm 100-degree eyepiece (the others are the 31/32mm) encompasses the full nebula. Through the eyepiece the targets look 50% bigger than the other eyepieces. You feel closer.
Telescopes have a maximum magnification capped by their diameter in millimeters times two. A four inch scope has a maximum useful resolution of around 200x (4 inches x 25.4 mm/in x 2 = 200). That's a maximum limit; most are more limited than that, inexpensive refractors in particular.
The point of this is that maximum useful magnification varies by scope