focal length vs Field of view

[neil groves]

Can someone clear up a little confusion for me please?

If I buy a 20mm 'standard' eye piece and a 20mm 70° eye piece, I will get the same size object in both eyepieces, the difference being I will have more space around the object with the 70°

version?

Neil.

[Moonshane]

yes, spot on.

 

[brantuk]

Yep - the standard ones only give you 50^o iirc

[Knighty2112]

I third that yes! 

[Mak the Night]

It depends what you mean by 'standard'. Most Plossls give you around 50° FOV, orthoscopics generally give around 42°. I have a Nagler with an 82° and a Panoptic with a 68° FOV. I believe it’s all to do with the configuration of the lenses inside the eyepiece. They can vary in size. I have a 9mm William Optics 72° SWAN and it is no bigger than a Plossl of the same size.

WO 9mm

TeleVue 19mm Panoptic, 16mm T5 Nagler

 

[cloudsweeper]

Same magnification, but you'd see a bigger chunk (angular spread) of sky.

Is the focal length 900mm?  That would give x45.  The true fields of vision would be about 1.11 and 1.56 degrees.  The Moon is about 0.5 degrees in angular width.

[Mak the Night]

Of course, there's always the TV calculator!

http://www.televue.com/engine/TV3b_page.asp?id=212&plain=TRUE

[neil groves]

18 minutes ago, cloudsweeper said:

Same magnification, but you'd see a bigger chunk (angular spread) of sky.

Is the focal length 900mm?  That would give x45.  The true fields of vision would be about 1.11 and 1.56 degrees.  The Moon is about 0.5 degrees in angular width.

Yes I have 900mm focal length to play with and it is spot on for M42, I am waiting for the chance to see this nebula at 45X wide view though, maybe tonight since the sky is clearing for once lol

Mak....I am actually referring to the amount of sky visible at one time through the eyepiece rather than the actual physical size of the eyepiece itself.

thanks guys

Neil.

[Mak the Night]

13 minutes ago, neil groves said:

Mak....I am actually referring to the amount of sky visible at one time through the eyepiece rather than the actual physical size of the eyepiece itself.

I don't think that they're totally unrelated though. This is what M42 approximately looks like through a 40mm Plossl at 32.5x on my 102mm Mak.

This is the FOV with the 72° of a Delos.

The Delos weighs 408 grammes as opposed to the 11 or 12 grammes of the Plossl. The physical size of the EP will affect the dynamics of how you perceive the FOV.

[jetstream]

The field stop diameter controls how much sky is seen (TFOV) for a given focal length. Apparent FOV is an interesting subject in itself.

[jetstream]

The widefield EP's usually have bigger dia field stops too...ie the 18mm ES 82 = 25mm, the 17mm Ethos 100 =30mm fs dia's

[neil groves]

how do I use the FOV thingy with stellarium....can someone give me some basic guidance here, I've never used stellarium for this, only for locating objects.

Neil.

[Mak the Night]

18 minutes ago, neil groves said:

how do I use the FOV thingy with stellarium....can someone give me some basic guidance here, I've never used stellarium for this, only for locating objects.

Neil.

You have to activate it in the plug-ins section IIRC.