![]() ![]() Let's compare image scales for our 8" SCT at f/10 to our 4" f/5 refractor. Image Scale (arcsec/pixel) = 205 x Pixel Size (microns) / Focal Length (mm) Longer focal lengths produce smaller image scales for a given pixel size. Image scale is determined by the focal length of the telescope combined with the size of the pixels in the camera used. It is a measure of the concentration of photons falling on a pixel for a given camera. Image scale is defined as the amount of sky in arcsec which is focused on an individual pixel. To understand this we must compare the image scales for these two telescopes. So how is it possible that the larger aperture telescope requires a longer exposure compared to a smaller telescope with a faster focal ratio? You can immediately see the benefit of shorter focal ratios for EAA. The faster the focal ratio the shorter the exposure no matter the size of the telescope aperture. A 20 second exposure is reduced to 5 sec. So with the f/5 refractor the exposure required is only 25% as long as at f/10. How much shorter? The exposure is reduced by the square of the ratio of the focal ratios: But the fact is that the f/5 refractor has the faster optical system and requires the shorter exposure to achieve a pleasing image. One might be inclined to think that because the 8" SCT has a larger aperture it would require a shorter exposure time to achieve the same image quality since it should capture more light with the bigger aperture. For comparison, a 4" (100mm) refractor with a focal length of 500mm has a focal ratio of f/5. It has a focal ratio given by the formula:įocal ratio = Focal Length / Aperture = 2000 / 200 = 10 An 8" SCT has a focal length of 2000mm and a 20 mm aperture. The focal length is the length of the path the light travels from the primary optical element (objective or mirror) of the telescope to the focal plane. The speed of the optics is determined by its focal ratio which is the focal length divided by the aperture. And, as discussed in the blog, " Choosing Mounts for EAA", shorter exposures make it practical to use Alt-Az mounts for EAA. It also means that a good, but not necessarily a precise polar alignment is required when using an EQ mount. ![]() Shorter exposures also put less demands on the mount's tracking accuracy which means a less expensive mount can be used. A fast system allows for shorter exposures and more immediate views. ![]() In fact, having the ability to work at two different focal ratios with the addition of a focal reducer provides greater flexibility in choosing targets of different size. To achieve this a fast telescope is preferred or at least one which can be made faster with the addition of a focal reducer. Even when using live stacking the objective is to enjoy the view in real time. This is not astrophotography where lots of sub frames are captured for post processing the next day to obtain a high quality image suitable for publication. EAA is all about viewing as much detail as possible in real time. ![]()
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