Field of view query

[AndyKeogh]

I am using the eyepiece calculator EP_Calc_v2 from this conference and tryng to understand how the view through an eyepiece changes with eyepieces oif differring FOV.

As an example, I have a 15mm Celestron eyepiece in a boxed set which I think has a FOV of 44 degrees. So with my C8 it gives a magnification of 135, an exit pupil of 1.5mm and a true field of view of 20 arc minutes.

If I were to use another 15mm eyepiece with say an 80 degree FOV then the calculator indicates that the magnification is still 135, the exit pupil is still 1.5mm but the true field of view is now 35 arc minutes.

I am wondering how the actual view will vary? With the 80 degree eyepiece will I see the whole 35 arc minutes in the view or will I see less and have to pan around to see the whole view?

Andy

[GazOC]

You'll see the whole 35 arc mins in one view. Its just like looking through a larger window....

Imagine if you were looking at a small island though a port hole on a ship. If the porthole was bigger the island would still be the same size but you'l be able to see more of the sea either side of it.

Hope this helps..

[John]

I enjoy using ultra-wide field eyepieces (80 degree aparrent field of view or more in my book) and don't find it difficult to see the edges of the field of view however they do tend to have shorter eye relief than their wide angle (60-70 degrees afov) cousins which can make it awkward if you wear glasses when observing.

John

[AndyKeogh]

Thanks for the replies, eye relief will be important if my wife and I both start to view with glasses as we both have different astigmatism prescriptions. At present we view without glasses.

Andy

[FLO]

Hope this chart helps Andy.

(The FOV of the 15mm Celestron Plossl is 52-degrees, you are thinking of the 32mm)

[GazOC]

I knew Steve would be along here with one of his FOV charts!!!

[FLO]

I think I'll change colour for the next one, my orange felt-tip is dry

[tinvek]

I think I'll change colour for the next one, my orange felt-tip is dry

if you do change, i'd suggest that black may not be ideal

[johninderby]

Here's another FOV chart.

John

[John]

Thanks John, but you have got me curious - Kohlers ????

Anyone manufacturing them or or are they rare one-offs ?

John

[AndyKeogh]

An interesting set of replies to my question and not quite what I had expected. I am left wondering what Televue mean by the spacewalk effect when looking through an Ethos.

I have a 38mm 70 degree eyepiece and when I look through that I have to pan around with my eye to see the whole field. I had assumed that it would be like that for the shorter focal length SWA eyepieces but noone has suggested this so far.

So far it appears from replies so far that the view size remains the same i.e. 1.5mm diameter but contains a wider view therefore the resolution must be higher.

So what is the spacewalk effect that Televue use in their advertising?

[johninderby]

Thanks John, but you have got me curious - Kohlers ????

Anyone manufacturing them or or are they rare one-offs ?

John

They were designed at Zeiss in the 1960s for use in military binoculars. Don't know why they have never been made commercially.The design doesn't look any more complex than some of the premium eyepieces available. Or did they have other problems perhaps?

John

[John]

Mmmm ....... 11 elements in 7 groups - it wouldn't be a budget eyepiece that's for sure !

Thanks John

John

[albedo0.39]

11 elements ! -It's a wonder the light can get through it (still, it doubles as a great weight training aid to keep you fit)