barlow FOV

[simon]

hi guys,

i was just wondering. can a barlow effect the FOV of an EP?

[Dave S]

As magnification increases FOV decreases. As a Barlow is a magnifier (yours is X2) then the answer is yes.

Cheers

Dave...

[simon]

thanks dave. so a 60 FOV barlowed x2 will give a 30 FOV or is it a bit more complicated than that?

[Spec-Chum]

Not strictly true, a Barlow doesn't magnify anything, it increases the focal length, doubles it in this case.

The reported fov of the ep will remain unchanged but the higher the mag the less you'll see in the same fov

[simon]

ahhh i see thanks Spec-Chum.

[Stu]

The Apparent Field of View remains the same eg 50° or 82° etc.

The Actual FOV is calculated by dividing the Apparent FOV by the magnification.

Using a Barlow will double (or triple etc, depending on the power of the barlow) the magnification so will halve the Actual FOV.

Stu

[simon]

if i use a 10mm ep with a FOV of 60 . i will see 60 FOV.

but if i x2 barlowed a 20mm with 60 FOV i would only see 30 FOV ?

[simon]

if i use a 10mm ep with a FOV of 60 . i will see 60 FOV.

but if i x2 barlowed a 20mm with 60 FOV i would only see 30 FOV ?

[Spec-Chum]

No, you'd still see 60 but at the same mag as the 10mm

[Spec-Chum]

I think you're confusing apparent and actual FOV. Stu described the difference above for you

[simon]

ok got it . thanks all.

[Stu]

To give an example.

Assuming a scope of focal length 1000 mm and a 20mm ep with apparent field of view of 60°, plus x2 barlow.

Without the barlow....

Magnification = 1000/20 = 50

Actual Field of View = 60/50 = 1.2°

With the barlow....

Magnification = 1000*2/20 = 100

Actual Field of View = 60/100 = 0.6°

In both cases, the apparent field of view remains 60°, but the actual amount of sky you see halves because of the doubled magnification.

If you were using a 20 mm ep with and 82° Apparent FOV, then you would see 1.64° and 0.82° respectively without and with the barlow showing the effect of ep's with wider fov, ie more sky with same magnification. Then you are into complex things like how good the ep performance is towards the edges etc but that is for another day :-)

Hope that helps.

Stu

[simon]

my head just exploded. lol.

only joking . completely understand.

thanks for the help. :-)

[John]

I don't want to complicate things further but some barlows do actually reduce, or vignette, the apparent field of view of ultra wide angle eyepieces. I've noticed this when using 16mm ultrawide eyepieces like 16mm Naglers and UWAN's with the Celestron Ultima 2x barlow lens - the the edges of the field of view became less distinctly defined and some field width was lost - I'd estimate down to around 70 or so degrees from the original 82.

This effect went away (as you might expect) when I moved to a 2 inch barlow lens.

[Stu]

Presumably that's related to the field stop in the 1.25" John? Is there any way to calculate it?

Stu

[John]

Presumably that's related to the field stop in the 1.25" John? Is there any way to calculate it?

Stu

Tele Vue give theirs in their eyepiece specs page:

http://www.televue.c..._page.asp?id=28

To deliver an 82 degree AFoV the UWAN / Nirvana will have the same field stop diameter as the 16mm Nagler, ie: 22.1mm or thereabouts.

There may be more to it than just the field stop size but I definitely recall the slight loss of field of view with these 16mm ultra-wides.