The focal length and field of view are handy information to have because many planetarium programs have a feature that allows a field-of-view overlay for framing shots of celestial targets. The easy way to do this is to shoot an image, and plate solve it to get the field of view, and then do the calculation. From that we can calculate the exact focal length and focal ratio. So how do we figure out what the exact focal length is?īasically, we take an image and determine the field of view. Field flatteners can also change the focal length a little bit.

However, when we use a focal reducer or barlow, the spacing will change the focal length. Now, that’s not much of a difference, and probably not enough to worry about. It turns out that my scope has a focal length of 825 mm. This would give a focal length of 819 millimeters. The focal length can even change a tiny bit based on temperature, but we can usually ignore that because it is so small.įor example, my Astro-Physics 130EDFGT refractor is marked on the lens cell as having an aperture of 130 millimeters, a focal ratio of f/6.3. Barlows and focal reducers in particular will vary in their magnification or reduction factor and change the focal length based on the spacing between the optical elements and the sensor. They can also vary from what you might expect if you are using focal reducers, Barlows or field flatteners. Many people are surprised that they are not exactly what is marked on the lens because of slight variations in manufacture. Ever wonder what the real focal length and focal ratio of your scope are?

Focal length on astrometry