Telescope Reviews: New Member

'Stopping down' a telescope is an easy way to make a large-aperture 'fast' telescope perform like a small-aperture 'slow' telescope.

Speed refers to the ratio of focal length to aperture. Smaller f-ratios gather a lot more light for a given focal length and are called are 'fast', while larger f-ratios gather a lot less light for a given focal length and are called 'slow' - terminology from conventional photography which describes to how quickly a piece of film becomes exposed.

F-ratio has several other effects, too, but one is of particular concern in telescopes: 'faster' telescopes offer a wider field of view at the cost of focal length, which makes them more sensitive to the quality of their optics. That's because shorter focal-length optics bend the light path more, and thus amplify the effects of any flaws they may have. Shorter focal-length telescopes also require shorter focal-length eyepieces for a given magnifaction, so it's a double whammy at that. That's why people with high-quality 'fast' telescopes often have the very-highest quality eyepieces: they really need them in order to get the best performance out their instruments.

That's also why the highest-quality specialised planetary telescopes are often 'slow' designs, f/8 or greater. These telescopes have very long focal lengths for their aperture, and thus trade a reduced field-of-view for better optical quality at higher magnifications.

It's important to remember, though, that aperture still generally rules - a 'faster' telescope of the same optical quality and focal length will generally show more detail when conditions allow it, because its larger aperture both gathers more light and has greater resolving power. Since the limiting factor in most telescope designs is aperture per dollar, f/ratio 'speed' is generally used more as a measure of focal length in astronomy, as opposed to conventional photography where the focal length per dollar is the limiting factor and it's used to measure aperture.

How does all this relate to 'stopping down' a telescope? It comes from another photography term, f-stops, which are opaque masks put over a large aperture with a smaller hole therein, effectively 'slowing' the camera to a larger f/ratio. With 'fast' telescopes, one can create an aperture mask - a piece of cardboard, wood, metal, or other opaque material with a smaller hole in it - to put over the full aperture of the telescope and effectively reduce its aperture.

Basically, by reducing the large telescope's aperture while its focal length stays the same, one increases its f/ratio to match the performance of a equivalently-smaller-aperture 'slower' telescope of the same focal length.

People do this for two reasons:

Firstly, the 'slower' design is popularly misunderstood to offer increased resolution on brighter objects where light-gathering isn't a primary concern. This is comes from the misconception that 'slower' telescopes always offer better resolution at the expense of field-of-view, which is actually only true for a fixed aperture (where the 'slower' design increases the focal length and thus improves optical performance) rather than a fixed focal length (where the 'slower' design instead decreases aperture and actually reduces resolution).

Secondly, smaller apertures are less suceptible to the effects of 'seeing' - turbulent pockets of air high in the atmosphere, similar to a mirage on a hot summer day - and thus often offer what people consider a more aesthetically-pleasing view. These is because larger apertures can capture more variety across these fixed-size ripples of air and thus show more change across their area, while a smaller-aperture telescope generally only picks up a smaller portion of a single ripple and thus sees less change across its area. Some people argue that the larger-aperture image, while showing more distortion due to 'seeing', is actually showing at least as high a quality of view as the smaller-aperture telescope: the smaller telescope just obscures more atmospheric distortion due to its inherent reduced resolution.

In whatever case, many people with large-aperture 'fast' telescopes prefer high-magnifaction viewing of bright planets to be 'stopped-down' through aperture masks to simulate smaller-aperture 'slow' telescopes. Many other people argue that large-aperture 'fast' telescopes always give a better view than the 'stopped-down' image, regardless. I suggest you experiment and come to your own conclusion: it's largely a matter of personal aesthetics, similar to some audiophiles preferring the 'warm' sound of lower-performing vaccuum-tube amplifiers over higher-performing solid-state designs.

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