focal plane image size question?

Question

if i was to calculate the size of the image formed in the focal plane of a telescope, what would the units be?

Answer 1

The size of an image is generally specified either in pixels or in units of length^2 - like square mm or square inches.

Answer 2

Most telescopes provide an image size of 25 to 30mm in diameter. Some Telescopes made specifically for imaging with large format cameras can provide illuminated fields of up to 90mm or more.
If you are talking about the size of the prime focus image of a particular object like the moon for instance, this depends strictly on the focal length of the Telescope. The longer the focal length, the larger the prime focus image.

Adolph

Answer 3

The f number is a RATIO between the diameter of the aperture, divided into the focal length. It is used because it is a constant across any kind of lens, and therefore simplifies light metering. That's why the hole gets smaller when you go to a higher f number. f16 is an aperture 1/16th the size of the focal length, while f2 is 1/2. Comprende? The formula for calculating f number is to begin with LIGHT VALUE numbers, where LV1 is 1 sec at f1 with 100 ASA. When you halve or double the light passing through you change down or up 1 LV. Now imagine a light meter is a yardstick to measure light with, where 0 is no light, and each "inch" is a LV, doubling the amount of light intensity each time as you go up the scale. We already know that 1 LV = 1 unit of light, and 2 LV = 2 units, 3 LV = 4 units, 4 LV = 8 units, and so on in geometric progression. If you take the SQUARE ROOT of these geometric progression numbers, 1,2,4,8,16,32,etc. you get the f numbers in common usage. Why are these numbers used? Because when you DOUBLE the diameter of a pipe, you QUADRUPLE the flow capacity. Quadruple is 2 LV, not 1, so there are "in between" numbers like 2.8 and 5.6 to make steps of 1 LV each. And why have f stops at all? Because the smaller the aperture the more you can get in "acceptable" focus in you picture before and after the point the lens is focused upon. By opening up the lens you can get a faster shutter speed and maybe throw a distracting background out of focus, or you can stop down and get absolutely everything in tack-sharp focus from the near foreground to the mountains in the background. Ansel Adams used to be in a club called the "f 64 Club," where everybody was using the smallest aperture on 8x10 cameras to super max out the sharpness. That's also how "focus free" cameras work. They're set to what's called "hyperfocal" distance, using a small f stop, and the film is chosen so that the shutter speed in daylight is just fast enough to get a sharp image if the camera is held steady and nothing is moving too fast in the scene. Acceptable focus begins about six feet from the lens and just keeps on going. Wide angle lens too, to leave cropping room for people who don't know how to see a picture in the viewfinder. Also, wides are inherently deeper field lenses. Another tip: the shutter speed to use in broad daylight is the RECIPROCAL of the ASA at f 16. Use 1/100 sec with ASA 100 film. Got it?

Answer 4

I would think it would be a round image measured in inches or millimeters of the diameter.