FOV

[dawson]

If two different eye pieces offer the same magnification, say 50x, but they have different fields of view, say 50* and 80*, would a large object, say a globular cluster, appear the same size in them both?

(I'm asking as i thought it would appear the same size but the 80* EP would just show more of the periphery, but the book i'm reading suggests the cluster would look smaller in the 80* EP)

James

[dawson]

I think the illustration which supports the text just isn't as I would have drawn it. The text suggests the magnification stays the same, but the FOV changes; the illustration suggests both the FOV and the magnification change.

 

This is why we need an EMS-authored book which would resolve all these issues for the reader.

 

 

James

 

[The book is "Observing the deep sky" by Darren Bushnall]

 

[philjay]

Try this http://www.csgnetwork.com/telefov.html

[Tweedledee]

If two different eye pieces offer the same magnification, say 50x, but they have different fields of view, say 50* and 80*, would a large object, say a globular cluster, appear the same size in them both?

(I'm asking as i thought it would appear the same size but the 80* EP would just show more of the periphery, but the book i'm reading suggests the cluster would look smaller in the 80* EP)

James

That is correct James.

However, the object will subtend a smaller proportion of the total field of view of the larger eyepiece thereby possibly creating the optical illusion to some observers that the object is not quite as big. But 50x is 50x in whatever eyepiece so it is the same size.

[Guest Tweedledum]

All correct above.

 

See link below, remember to click on to 'visual view'.

 

 

http://www.12dstring.me.uk/fov.htm

[dawson]

Thanks all.

Do you think the illustration is misleading?

James

[Guest Tweedledum]

Probably, but I would prefer to call it publishers licence, to try and show an effect. The image cannot be greater in size though.

[Tweedledee]

I have just had a look at my copy of the book. On my iPhone screen your image wasn't too easy to see.

I agree that the image could be misleading if you don't already fully understand what he is talking about. It would have been clearer if the nebula was exactly the same size in each image, but the circle marking the edge of the field was larger in the 80 degree one.

[dawson]

I'm glad i at least realised there was something amis. I think it would have be easy to make the right hand circle bigger and to keep the nebula the same size.