Astrophotography: real aperture vs f-number?

Question

While reading up on astrophotography, I discovered there seems to be a movement that believes real aperture (diameter of iris) is more important than f-number when talking about speed. How and where did this come from?

I have read one rebuttal but would be interested in hearing opinions. I guess you could attribute it to spreading the same light (a section of the image) over more photo-sites, or just a cryptic way of saying magnification is good, but this seems to have been also applied to wide angle shots.

I've also even read stuff about the f-number affecting the sky fog limit (as opposed to overall exposure).

Answer 1

In a camera, all parts of the image passes through all parts of the lens, so the aperture affects how much light hits each part of the image.

In a telescope, the incoming light is parallel, so each part of the image only passes through one point in the lens. The aperture only limits the image circle, it doesn't affect how much light hits each part of the image. So, the relation between aperture and focal length (f-number) is not relevant for the exposure.

The sky fog limit is mostly determined by how much stray light you get, and as the stray light is not parallel (as it comes from within the athmospehere) it's intensity is affected by the aperture. So, a smaller aperture would have some effect on the sky fog limit.

Answer 2

Consider, for a moment, pointing your camera at a wall that's completely even lit. Let's assume you start with a 50 mm lens with a 25 mm aperture (i.e., f/2). If you change to a 100 mm lens you're reducing the angle of view so you're collecting light from a smaller area -- so you're collecting less light. To be more specific, you're cutting the angle of view in half, which reduces the area to 1/4th as much, so you're collecting 1/4th as much light. To look at it from a slightly different viewpoint, the light from a given part of the input gets spread over quadruple the area on the sensor/film, so it only appears 1/4th as bright on any given part of the sensor/film.

Using a relatively aperture compensates for that so, for example, f/2 gives the same total amount of light entering the camera regardless of the combination of focal length and aperture size necessary to get to f/2.

Most astrophotography is a bit different though. In particular, when you're taking a picture of a star, doubling the focal length should not double the apparent size of the star. Other than the sun, all the stars¹ are far enough way that they should always show up as a point source. Doubling the focal length does not mean the star will be projected onto four times the area on the film/sensor. Rather the contrary, with the limits of sharpness of the optics, any focal length you use will still project the stars image as a point source.

I say "most" above, because this really applies only to stars. For the moon, nebulae, comets, and closer planets, you're typically magnifying to the point that the object in question projects as a disc on the sensor/film. As soon as that happens, you get back to the situation originally described: changing the focal length changes the apparent size of the object. A long focal length spreads the same light over more pixels, so you need to collect more light to compensate.

_{¹ Purely as a technicality, a few of the very largest telescopes theoretically have enough resolution to actually resolve a disc of a couple of extremely large, relatively nearby stars such as Betelgeuse. Even with them, this is still purely theoretical though -- the atmosphere is never still enough for them to achieve the necessary level of detail.}

If a 200 inch telescope were placed in orbit, outside the atmosphere, then we could actually see Betelgeuse as a disc rather than a point source. Even that's only possible because Betelgeuse is almost astoundingly huge and relatively nearby though. For most stars you'd need an orbiting telescope that was much larger still.

Answer 3

The f ratio on a telescope defines the angle of view it is capable of displaying with an eyepiece that focuses the entire image circle from the primary mirror (in a reflector) or the objective lens (in a refractor). The aperture of a telescope is the diameter of the primary mirror/objective lens. In practice the limiting factor when using an adapter to mount your camera to the telescope is usually the diameter of the T-mount adapter between the telescope and the camera that tends to choke off some of the light. During normal telescope viewing, to get higher magnification you replace the eyepiece that focuses the entire image circle with one that focuses the light from only a percentage of the image circle. You're still using the entire primary/objective, but you are only focusing the light that strikes it from the center of the field of view.

When you remove the eyepiece and insert a T-mount adapter what you are doing is allowing the point of focus to extend past the focusing tube and resolve on the cameras sensor plane. Focus is adjusted by racking the focuser in or out to change the distance between the primary/objective and the camera's sensor. Sometimes extension tubes might be needed to get the camera out far enough that the movement of the focusing rack can bring the light from the scope into focus.

What all of this means is that effective aperture is usually determined by the diameter of the T-mount adapter, rather than the f ratio of the telescope. In practice when using a DSLR on an astronomical telescope you will need to experiment a little with ISO and shutter speed to find the correct exposure values. There is no one "correct" exposure value. A lower exposure will reveal only the brightest stars, while a higher exposure will reveal dimmer ones as well. I generally use the focal length/600 rule to determine the maximum shutter speed that may be used without the stars motion relative to the Earth's surface becoming evident in an uncropped image, then go from there with the ISO until the dimmest magnitude I'd like to show in the image is just visible.