![]() And note that you’re seeing more at the exact same magnification, which in this example is 1x. You’ll see their face, shoulders, and chest. Then look at the person through a toilet paper roll tube. The easiest way to understand this is to do a simple thought experiment: think of looking at someone through a straw. AFOV is a feature, a specification of the individual eyepiece how that eyepiece interacts with your particular telescope will define the TFOV that you will see.Īt any given but equal magnification, the larger the AFOV, the larger the TFOV you will see. With a dob, if you use an eyepiece with a wider AFOV, especially at higher magnifications, the object you’re looking at will stay in the field of view longer before you have to nudge the scope to recenter the object as the earth turns and moves the object out of your field of view.Īpparent field of view is different from, but has a great deal of impact on, true field of view – TFOV. One area where the increased AFOV really matters is if you have a non-motorized scope, like a dob. This is what you want, this is the most important thing you’re paying for when you spend extra money on an eyepiece. Apparent Field of ViewĪpparent field of view (AFOV) is the big deal, the “killer app” of eyepieces. The first two of these features is displayed with the specifications for each eyepiece. Well, there are a few other important attributes that every eyepiece has that you should understand before buying: 1) apparent field of view 2) eye relief and 3) better correction. ![]() But which one should you buy at that focal length? Now you have your maximum magnification locked down in terms of knowing what focal length eyepiece to buy. To be prepared for this, you should have a good Barlow on hand, like the X-Cel LX 2x Barlow, to increase the maximum magnification your eyepieces are capable of. On rare nights, the atmosphere will quiet down, and that maximum magnification will increase. If your local seeing will allow you to get to 200x, then 1200mm divided by 200x = 6mm. Continuing with our example of the 8” dob, 1200mm divided by 150x = 8mm. To figure out the focal length of the eyepiece that will give you that maximum magnification is, you can take the focal length of the scope and divide it by that maximum magnification. The best way to find out what magnification your local seeing will generally support is to talk to members of your local astronomy club. The image will deteriorate and look less sharp than it did at a lower magnification. However, the seeing conditions will cause the view will break down. Whatever you’re looking at, say, a planet, will certainly look larger with greater magnification. If you go above this level that the atmosphere limits you to, you will get what is called “ empty magnification ”. On the other hand, if you take your scope down to New Mexico, you should be able to get right back up to, or even past, that 200x level. However, here in the Denver metro area, the wind whips through the saw-toothed Rockies to the west, causing all sorts of turbulence in the atmosphere which limits the seeing to about 150x. I was able to get to 200x easily, and sometimes even get up to 250x, the maximum possible with my 5” scope. As an example, I used to observe in Manhattan, and there was a steady flow of air (called a laminar flow) coming in from the west. However, the atmospheric turbulence that is always present, called “ seeing”, generally limits you to about 200x maximum, and perhaps less depending on the seeing conditions at your particular location. ![]() Continuing to use the 8” dob as an example, 50 x 8 inches = 400x, and 203mm x 2 comes out to just about the same amount, 400x. The rule of thumb in calculating the maximum magnification that a telescope can deliver is 50 times the aperture in inches or twice the aperture in millimeters. ![]() Depending on your scope’s aperture, the scope itself may be capable of giving you 300x, 400x, or higher. In figuring out how high of a magnification to get, you need to acknowledge the limits that the atmosphere places on you. So, the first analysis is to fill in magnification “gaps” you have – like a high-powered eyepiece, or more of a mid-powered eyepiece in between the two. For example, if you have, say, 25mm and 10mm Plossls that came with your 8-inch dob, they’re giving you 48x and 120x respectively. To figure out the magnification of your eyepieces, you take the focal length of your telescope (1200mm for an f/6 8-inch dob) and divide it by the focal length of the eyepiece. When you’re selecting a new eyepiece, the first and most important consideration should be how it fits in with the eyepieces you already have – particularly, your magnification scheme.
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