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# Maximum FOV in a 2" EP?

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### #1 TCW

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Posted 24 July 2014 - 06:53 PM

It seems that the max.(more or less) real FOV you can get for a given focal length is from a 40mm 70 degree EP. A 56 mm 52 degree EP gives virtually the same actual FOV.

Any other EP candidates that will give a larger actual FOV?

For my purposes I am using a 1 meter focal length for calculations.

### #2 StarStuff1

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Posted 24 July 2014 - 07:13 PM

I don't think so. You could remove the field stop from a wide field 40mm 2-in ep and get a little more fov but the edge will be a little messed up on faster f/ratio scopes.

About 14 or so years ago I modified the field stop on a 25mm plossel to give a 60° afov and a 1° tfov for an 8-in f/6 newt. Even tho the edge was not that great it made that ep a great star hopping device.

### #3 David Knisely

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Posted 24 July 2014 - 07:43 PM

It seems that the max.(more or less) real FOV you can get for a given focal length is from a 40mm 70 degree EP. A 56 mm 52 degree EP gives virtually the same actual FOV.

Any other EP candidates that will give a larger actual FOV?

For my purposes I am using a 1 meter focal length for calculations.

The largest field stop for a 2" eyepiece is around 46mm or so. The field stop equation gives the most accurate value for true field of view in a given eyepiece (better than the old AFOV/Mag formula):

TFOV = 57.3*EFSD/Fl, where EFSD is the eyepiece field stop diameter and Fl is the telescope's focal length.

This will mean that for a 1000mm focal length instrument, the maximum true field of view that can be obtained using a 2 inch barrel eyepiece without vignetting is about 2.64 degrees on the sky. This is what my University Optics 40mm Mk-70 Konig provides (a 46mm field stop and an apparent field of view of 68.8 degrees). My old 30mm WideScan III eyepiece has an apparent field of view of 84 degrees, but a field stop of only 44 mm. Thus, in a 1000mm focal length instrument, it will only produce a true field of view of 2.52 degrees (and it isn't that good of an eyepiece to begin with). Clear skies to you.

### #4 TCW

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Posted 24 July 2014 - 07:49 PM

Thanks. Field stop information is not always available. Usually only the focal length and FOV are given.

If I use the given 40mm focal length and 72 degree FOV that William Optics claims for their SWAN then I get a 2.88 degree actual FOV for a 1 meter focal length.

### #5 Lance1234

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Posted 24 July 2014 - 07:51 PM

Check out the 2014 Buyer's Guide (generously compiled by Don Pensack - thanks Don) in the "Best Of" posting on this forum. It lists essentially every eyepiece currently made. If you put in your scope focal length, etc. it will give you true fields for all the eyepieces.

### #6 David Knisely

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Posted 24 July 2014 - 09:09 PM

Thanks. Field stop information is not always available. Usually only the focal length and FOV are given.

If I use the given 40mm focal length and 72 degree FOV that William Optics claims for their SWAN then I get a 2.88 degree actual FOV for a 1 meter focal length.

The problem is that the old AFOV/Mag formula is often between five and 10 percent (or more) off of reality, so figures produced by that formula should be taken with a grain of salt. I reviewed the Swans several years ago in the CN REPORT section of Cloudynights:

http://www.cloudynig...gory_id=87&pr=1 (look at the very last article)

The 40mm Swan has a measured field stop diameter of 46mm and a measured apparent field of 70 degrees (not 72, as that figure is only for their 9mm, 15mm, 20mm, and 25mm models). Again, it is best to either use the field stop formula or actually measure the true field with the eyepiece in the telescope. With a 1 meter focal length, I'm afraid that you are still limited to a true field of about 2.64 degrees. Clear skies to you.

### #7 TCW

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Posted 24 July 2014 - 10:03 PM

The SWAN's seem to be the same as the Q70's and others. The claims for the SWAN's do seem to be out of line with the other versions sold under different names.

### #8 faackanders2

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Posted 25 July 2014 - 08:23 PM

It seems that the max.(more or less) real FOV you can get for a given focal length is from a 40mm 70 degree EP. A 56 mm 52 degree EP gives virtually the same actual FOV.

Any other EP candidates that will give a larger actual FOV?

For my purposes I am using a 1 meter focal length for calculations.

Need a shorter FL telescope, or a 3" eyepiece (harry siebert and possibly ES in future).

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