2" barlows - Am I missing something?

[michaelmorris]

I'm trying to get my head around why one would need a short focal length eyepiece in a 2" fitting. This is based on my (quite possibly incorrect) assumptions about the benefits of using a 2" fitting.

From what I can work out, the advantage of a 2" fitting is that you can take advantage of a wide field of view on a long focal length eyepiece - for instance a 30mm 84 degree FOV model. If this is correct, would I also be correct in assuming that a 2" fitting short focal length eyepiece is therefore usually a bit pointless?

Have I missed something here?

If I am correct, then I struggle a bit to see the point of a 2" fitting 2x barlow. I presume there would be no advantage of a 10mm 84 deg FOV eyepiece in a 2" fitting over a similarly specified model with a 1.25" fitting.

Have I got this wrong?

If I was imaging with a DSLR and a 600mm focal length 80mm refractor and wanted to use a barlow, would I need a 2" barlow to avoid vignetting?

[michaelmorris]

Ooops - wrong forum.

Moderator - please could you move this to the Equipment help forum

Sorry folks.

[Merlin66]

I don't think you're missing anything in your logic.

You don't need a 2" barlow on short focal length eyepieces, but yes it may help with the DSLR.

[Ben Ritchie]

For visual, one advantage of using the 2" format is that it holds a big, expensive eyepiece (much) more securely. For example, the 13mm Ethos is a 1.25" eyepiece and barlows extremely well, but in a 1.25" barlow it's rather precarious - especially in a Dob or an EQ-mounted Newtonian where the eyepiece is parallel to the ground. Using the eyepiece in a 2" barlow is far more secure. So no optical advantage, but a very practical one. Personally i'd never use the 13E in anything other than a 2" diagonal, focuser or barlow.

Minor advantage for some is that you can pack a single barlow and use it for all your eyepieces; some 2" eyepieces are too large to barlow (the 31mm Nagler in a 2" barlow looks like a German WW2 hand-grenade and weighs about as much) but others like the 20T5 Nagler work very well.