# Thread: Field of View

1. ## Field of View

How can I figure out the FOV of my EP's if I don't know?
Thanks.

2. Find a star on the celestial equator (Dec=0). With tracking turned off, time how long it takes to go from one side of the FOV to the other. Since the sky rotates at 15 minutes of arc per minute of time, the time will tell you how wide the field is.

If the star is off the equator, you will have to correct for its declination by dividing the result you get by the cosine of the declination. That's why it is easier to chose one on the equator.

3. ## The Following User Says Thank You to KathyNS For This Useful Post:

GWING (08-08-2011)

4. Keith,
Correct me if I'm wrong, but the method you stated will give the True Field of View and is not to be confused with the Apparent Field of View.
Doesn't the True Field of View change with the telescope the EP is inserted in?

GWING,
The Apparent Field of View is the FOV usually advertised with the EP.

5. ## The Following User Says Thank You to pederv For This Useful Post:

GWING (08-08-2011)

6. I've been considering eyepieces lately.

I used this a few times:

Telescope Field Of View And Power Calculator

The Select Eyepiece menu has a bunch of specific EPs (e.g. Meade Super Plossl 9.7mm), or you can just input the EP design and FL.

Edit: This is for when you want to know true FOV for different EPs on a hypothetical scope, such as your own.
Last edited by Widespread; 08-08-2011 at 10:29 PM.

7. ## The Following User Says Thank You to Widespread For This Useful Post:

GWING (08-09-2011)

8. Originally Posted by pederv
Keith,
Correct me if I'm wrong, but the method you stated will give the True Field of View and is not to be confused with the Apparent Field of View.
Doesn't the True Field of View change with the telescope the EP is inserted in?

GWING,
The Apparent Field of View is the FOV usually advertised with the EP.
You are correct, this will give the true field of view in the current telescope. You then multiply by the known magnification (calculated from the focal lengths) to get the apparent field of view, which will be constant for the eyepiece. If you know the advertised AFOV, there would be no need to do a measurement, so I assumed that the OP did not have that information.

9. ## The Following User Says Thank You to KathyNS For This Useful Post:

GWING (08-09-2011)

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