Eyepiece Apparent Field of View

WT · Post by WT » Mon May 19, 2008 2:16 pm

could like to ask anyone.. what does it mean " 82 degree " ?? from this eyepiece "William Optics 4mm 1.25" UWAN eyepiece 82 degree"

Post by **weixing** » Mon May 19, 2008 6:25 pm

Hi,

what does it mean " 82 degree "

The "82 degree" mean that this eyepiece got an apparent field of view (AFoV) of the 82 degree.

To get the actual field of view, divided the apparent field of view by the magnification.

For example, a 6" F5 scope (focal length is 150mm x 5 = 750mm) using this 4mm 82 degree AFoV eyepiece will give you:
magnification = scope focal length / eyepiece focal length = 750 / 4 = 188x
actual field of view = apparent field of view / magnification = 82 / 188 = 0.44 degree.

Have a nice day.

WT · Post by WT » Mon May 19, 2008 7:43 pm

Hi Weixing,

Thank for the calculation. But how's it look like at 0.44 degree???
Mine 4 mm eyepiece didnt state any AFoV??? why??

Post by **weixing** » Mon May 19, 2008 8:05 pm

Hi,

But how's it look like at 0.44 degree???

Err... What do you mean?? Look the same, but just cover a smaller part of the sky lor. Anyway, the full Moon is around 0.5 degree, so 0.44 degree is slightly smaller than the full Moon. Please note that a large AFoV eyepiece will cover a larger actual field of view compare to a smaller AFoV eyepiece if both have the same focal length. For example:
Using the 6" F5 scope (focal length is 150mm x 5 = 750mm):
1) 4mm 82 degree eyepiece Actual FoV = 82 / (750 / 4) = 0.44 degree
2) 4mm 50 degree eyepiece Actual FoV = 50 / (750 / 4) = 0.27 degree

Mine 4 mm eyepiece didnt state any AFoV??? why??

Some low cost, entry level eyepiece will not indicated the AFoV on the eyepiece. You can check whether it's indicate in the scope manual if the eyepiece come with the scope. Below are some of the AFoV for some common entry level eyepeiece type:
1) Kellner: around 50 degree,
2) SMA (Super Modified Achromat): around 40 degree to 52 degree
3) Plossl/Super Plossl: around 50 degree

Have a nice day.

WT · Post by WT » Mon May 19, 2008 8:24 pm

My manual didnt state any AFoV.. maybe low cost eyepiece.
That mean if i use 4mm 50 degree eyepiece to view the moon the image will be closer than 4mm 82 degree eyepiece??

Post by **weixing** » Mon May 19, 2008 8:39 pm

Hi,

That mean if i use 4mm 50 degree eyepiece to view the moon the image will be closer than 4mm 82 degree eyepiece??

No. The field of view is just smaller. To look closer, you need a higher magnification.

Have a nice day.

WT · Post by WT » Mon May 19, 2008 8:51 pm

o... ic. Oki thank a lot weixing
have a nice day

Post by **weixing** » Tue May 20, 2008 8:49 am

Hi,
I create a simple example of the view through the eyepiece with same magnification, but different apparent field of view.

Have a nice day.

WT · Post by WT » Fri Aug 08, 2008 8:31 pm

thank alot... now i know what u mean haha

WT · Post by WT » Fri Aug 08, 2008 8:32 pm

By the way, anyone knows how to clean a finderscope?