# Why do refracting telescopes have a wider field of view than reflecting telescopes?

Many websites say refracting telescopes have a wider field of view than reflecting telescopes, claiming it is because their angular magnification is less. Astronomical objects are therefore easier to locate using a refractor instead of a reflector of the same length.

I don't understand how angular magnification would relate to field of view. Shouldn't it just relate to the magnifying power of a mirror (in reflecting telescopes) or lens (in refracting telescopes)?

• What makes you believe the premise of your question is true? You haven't presented any details in your post. Edit it. – Bill N Apr 20 at 12:37
• @BillN I tried editing the post accordingly, I'm not sure if it still sounds incomplete? I assumed most of these terms are already known – XXb8 Apr 20 at 12:44
• It's normally the other way around. – Rob Jeffries Apr 20 at 17:37

Shouldn't it just relate to the magnifying power of a mirror (in reflecting telescopes) or lens (in refracting telescopes)?

There isn't a single "magnifying power of a mirror ... or lens." A simple telescope has at least 3 parts: an objective optical element (closest to the observed object), an eyepiece optical element, a "structure" for maintaining the geometry of the objective and eyepiece.

The objective, usually with a fairly long focal length, produces a potentially-real image in the region near the eyepiece. The objective can be a mirror or a lens, hence the distinction between reflecting and * refracting* telescopes, respectively.

If the eyepiece is a positive focal length lens, the image should be produced between the objective and the eyepiece. If the eyepiece is a negative focal length lens, the image should be slightly behind the eyepiece. This means that the optical path distance between the objective and the eyepiece must be close to the focal length of the objective.

The size of the image produced by the objective is much smaller than the original object viewed! This is counter-intuitive. On the other hand, the image is much closer to your eye, so it occupies a larger angular space in front of your eye, hence, the angular magnification. For an example, hold a penny in your fingers at an arms length from your eye. Then bring it closer to your eye. The penny image appears larger, but it really isn't. The eyepiece acts both as a magnifying glass to expand the image and, more importantly, a focusing device so that your eye lens can produce a sharp image on your retina. The angular magnification is $$M_{\alpha}=-\frac{f_o}{f_e}$$ where $$f_o$$ is the focal length of the objective, $$f_e$$ of the eyepiece, and a negative value of $$M_{\alpha}$$ means the image is upside-down.

With a reflecting telescope, the optical path easily can be folded, so a longer focal length can be used in the same physical length. That means that it is possible to get a larger angular magnification system in the same physical length compared to the refractor.

With larger angular magnification, a small real angle will occupy the same visual field as a medium real angle at small angular magnification. For example, if the width of the visual field of the user through the eyepiece is 10 degrees, then a 40X telescope system will have a 0.25 degree width field of view, whereas a 100X telescope system will only have 0.10 degree width field of view.

There are other things, such as objective and eyepiece diameters, which can reduce the actual field of view below the ideal, but the ideal is limited directly by $$M_{\alpha}$$.

• @MarkH Yep, my bad. Thanks. – Bill N Apr 20 at 14:35

It's mainly down to what they're good at - a lens is always going to produce a bit of aberration (both in colour and focus), so if you want a wide field of view you're going to get better results from a mirror, especially when you factor cost in to it. On the flipside, a high-magnification mirror is going to be trickier to maintain than a lens, any small error in alignment is going to ruin the image, so a lens is better.

You could have a high magnification reflector and a wide field refractor, but if you want the same results they're going to both be more hassle and more expensive than doing it the other way around.

• Thanks for the answer. I was looking more for an explanation as to why angular magnification relates to field of view – XXb8 Apr 20 at 14:30

A reflecting telescope uses a parabolic mirror. Its best focus is for objects on or near the axis of the parabola. The image produced by a spherical lens is not as good, but it is more likely to be tolerant of objects off axis.

• This doesn't explain the size of the field of view. – Bill N Apr 21 at 14:06