# Thread: How is an eyepiece field stop calculated?

1. ## How is an eyepiece field stop calculated?

Sorry if this has been asked a gazillion times before, but the ocular plugin in Stellarium for eyepieces has a field for field stops. I'm not sure how relevant, or if leaving this field blank would skew the outcome. Could anyone tell me how this information is calculated?

Thanks...Rich

2. ## Re: How is an eyepiece field stop calculated?

Hello,
The eyepiece field stop is a physical characteristic of the eyepiece. It is not calculated it is measured.

Leaving that value blank in Stellarium and using only the eyepiece focal length will have no adverse effect on the magnification or field of view calculation.

Most manufacturers don't include the field stop measurement in their eyepiece specs.

Cheers,
JT

PS, if ever you have any question on telescopes and all the pieces Parts in regards to their Optics, refer to this site: http://www.telescope-optics.net/index.htm
Last edited by jaetea; 07-23-2018 at 10:48 PM.

3. ## Re: How is an eyepiece field stop calculated?

There are ways to work it out if you enjoy doing maths
This will give you a close approximation for eyepiece field stops.

FS = (AF x EPFL)/ 57.3 x [1 + (AF/TFL)]

FS = Field stop of eyepiece

AFOV = Apparent field of eyepiece

TFL = Telescope focal length
Last edited by Gabby76; 07-23-2018 at 10:53 PM.

4. ## Re: How is an eyepiece field stop calculated?

Field Stop needs to measured not calculated.
Some vendors list them, for others you might find it by googling (other people have measured it).

TFOV = AFOV/Magnification is the simple formula, quite close in most cases.

TFOVmax = 57.3* Field Stop/FL of telescope
is the limiting TFOV for a telescope and a particular field stop.

If the TFOV you calculate is larger than TFOVmax you will only see TFOVmax. So, the minimum of TFOV and TFOVmax is what you see. If you already have an eyepiece, put the eyepiece size 1.25" or 2" (in mm) as field stop in the Ocular Plugin, and during viewing check if it matches. If it does not, lower the field-stop value till the FOV in Stellarium matches what you see.

5. ## Re: How is an eyepiece field stop calculated?

They can be measured (timed) or calculated, one is just more accurate than the other but both will get you close enough for what we as amateurs do.

6. ## Re: How is an eyepiece field stop calculated?

Thanks everyone for clarifying field stop for me. If Stellarium doesnâ€™t need it, that works for me!

Rich

7. ## Re: How is an eyepiece field stop calculated?

Just a comment: often the given AFOV from a manufacturer is greater than the true AFOV (and hence the true FOV!) because the field stop is actually smaller than it "should" be. I suspect this is a bit like the stopping down that you see in cheap finder scopes, to reduce CA and give a sharper edge of field...
(In fact sometimes there is no true field stop (eg: my GSO 42mm wide-field), and the edge of the field is a bit fuzzy as a result.)
Only callipers and/or star drift testing will tell the truth; but even the latter can be difficult because of the fuzzy edge-of-field.

Rich: have a look at Al Nagler's article for lots of good info: https://www.skyandtelescope.com/astr...epiece-primer/ and more here: Tele Vue Optics: Advice

- Dean

8. ## Re: How is an eyepiece field stop calculated?

Great articles Dean, appreciate it

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