Calculating An Eyepeice's Actual Field of View

[Mr Q]

Pick any EP and any star near the celestial equator.

Time (in seconds) that star takes to cross your field of view.

Take that time in seconds and divide by 240.

The answer is your actual field of view in degrees.

[cantab]

Shouldn't it be a star near the celestial equator, not necessarily near the ecliptic?

[Mr Q]

Shouldn't it be a star near the celestial equator, not necessarily near the ecliptic?

My error and corrected - thanks

[rory]

is this a more accurate calculation than , fov divided by magnification ? thanks.

[Josh Wilson]

Very helpful Mr Q. There was formula for finding the FOV specifically for Dobsonian and Newtonian telescopes but I can't remember of the top of my head what it was...

[John]

Very helpful Mr Q. There was formula for finding the FOV specifically for Dobsonian and Newtonian telescopes but I can't remember of the top of my head what it was...

Apparent field of view of eyepiece divided by the magnification = true field in degrees. This works for all scope types but can be an approximation in some cases. Especially if the eyepiece manufacturer is not being 100% accurate with the apparent field of the eyepiece concerned !

[Mr Q]

True, there are other ways of calculating the true FOV or actual FOV but from what I have found out, these calculations do not take in specific issues regarding the extreme edge problems of distortions and other issues.

My post takes in all of the minor issues of actual FOV by the fact that it does a real time measurement for any scope type, size, EP design/power, etc. And it's easy to calculate. In other words, once the star appears focused near the edge of the field (beyond this point fainter stars may not be detected), the timing begins and ends at the same point on the other side of the FOV. This is a practical, workable true FOV in degrees that's good enough for star hopping or general observing.

For a more detailed description of a practical FOV mentioned above, visit the site at TRUE FOV

[rory]

True, there are other ways of calculating the true FOV or actual FOV but from what I have found out, these calculations do not take in specific issues regarding the extreme edge problems of distortions and other issues.

My post takes in all of the minor issues of actual FOV by the fact that it does a real time measurement for any scope type, size, EP design/power, etc. And it's easy to calculate. In other words, once the star appears focused near the edge of the field (beyond this point fainter stars may not be detected), the timing begins and ends at the same point on the other side of the FOV. This is a practical, workable true FOV in degrees that's good enough for star hopping or general observing.

For a more detailed description of a practical FOV mentioned above, visit the site at TRUE FOV

thats answered my question, thanks mrQ, very helpful.

[Wednesday13]

Nice tip, MrQ, thanks! I will try it next time, just to check my "theoretical" calculations.

As for equations, you can find a lot of online calculators for various telescope properties, as well as math behind them. I created an Excel table, to compare various eyepieces, it was easier when I had them all in a list; I attache the table to this post, in case you need it; just enter focal length and apparent FOV of the eyepiece, and you'll get other parameters; of course, prior to that, change focal length and primary diameter of your scope. Sheet is protected to avoid unintentional changes (except cells that require input), but not password protected, so if you want to alter anything, just unprotect it.

Telescope.xlsx

[BinocularSky]

is this a more accurate calculation than , fov divided by magnification ? thanks.

Yes, because it's based on things you can actually measure, although the second line should be "Time (in seconds) that star takes to cross your field of view along a diameter" (If it crosses on any other chord, all bets are off). FoV is often incorrectly given and your calculation of magnification is dependent on the focal lengths of the eyepiece and the (effective) focal length of the telescope being known correctly. Either can be incorrectly given and, with catadioptric scopes that focus by moving the primary mirror (e.g. most SCTs and Maks), the focal length changes as you change the focus, so you won't have a clue what it is.

[Mr Q]

Thanks Steve for that important addition to the measurement. I automatically assumed everyone would time on a maximum diameter path of the star.

[BinocularSky]

I automatically assumed everyone would time on a maximum diameter path of the star.

I'm sure most would do that, but decades of bitter experience have taught me that there is always at least one....