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FOV not as expected


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Newby here, looking for some help.

The FOV I am seeing through the eyepiece is not what FOV calculators (Stellarium, 12dstring.me.uk) indicate I should be seeing.

As an example, using a NexStar 8SE, with a Celestron X-Cell 25mm eyepiece I should easily be able to see the entire moon. I'm seeing maybe 2/3 of the moon. Even with my Luminos 15mm I should get the entire moon in the eyepiece but I'm not. I'm sure there is something I'm missing but I can't figure it out.

Any thoughts or help will be appreciated.

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Hi and welcome to SGL.

Maybe you are looking from further away then you should be?

There is something called eye relief for each eyepiece - distance that you should place your eyeball with respect to top most lens of eyepiece.

If you position your eye further away - you won't be seeing whole field of view offered by eyepiece.

Are you seeing field stop? That is edge of the FOV - dark part where there is no sky or object to be seen? It should be in the field of view when you position your eye properly.

By the way, eye relief for celestron x-cel 25mm is about 16mm - so you need to put your eye fairly close (but not as close as some other eyepieces - 16mm is quite decent eye relief).

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Thanks vlaiv, this is something I have wondered about. I'm still learning to be more stable when looking through the eyepiece.  More difficult than it would appear. I'll focus on this issue next time I'm out.

Yes Alan I am using a TeleVue Everbrite diagonal. I get the same result with the Celestron diagonal that came with the scope. 

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Hi Gocats, I'm a newbie too and have recently upgraded to an evo 8 so same scope as you. I had the same issue, not helped by wearing glasses.  I have a couple of BSTs and have found that winding the rubber eyepiece out helps get my eye the right distance from the eyepiece to get the full effect of the fov.

Hope that helps!

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I have a C8 SE, and with the stock 25mm Plossl eyepiece supplied the FOV is about half a degree, allowing the whole Moon to be visible.

The FOV also depends on the eyepiece design - some offer more FOV for a given focal length than others.

If you view with glasses this can also cut the field visible (I always remove my glasses to look theough an eyepiece).

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18 hours ago, Gocats said:

I'm still learning to be more stable when looking through the eyepiece.  More difficult than it would appear.

Are you observing sitting down?  I can't observe standing up at all.  I weave and bob about too much.

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2 hours ago, Louis D said:

Are you observing sitting down?  I can't observe standing up at all.  I weave and bob about too much.

Great suggestion! Never considered sitting. I will try next time I'm out.

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Calculations for the FOV are quite simple. The 8SE has a focal length of 1032mm, so, the 25mm gives x81. The X-Cel has a FOV of 60° therefore 60° / 81 = 0.74° actual FOV. Much larger than the moon's 0.52° :smile:

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8 minutes ago, Mr Spock said:

Calculations for the FOV are quite simple. The 8SE has a focal length of 1032mm, so, the 25mm gives x81. The X-Cel has a FOV of 60° therefore 60° / 81 = 0.74° actual FOV. Much larger than the moon's 0.52° :smile:

So Barlowed you'd be at 0.37 , so you'd see about 2/3 of the Moon give or take ... ?

Edited by Steve Ward
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