How much will a 30mm 2" Aero vignette...?

[F15Rules]

Hi all,

I just acquired a nice SW Aero 30mm wide angle (68 deg) eyepiece with a 2" barrel.

I'm going to try this in my Baader T2 prism which has a 32mm clear aperture..I will use a 2" down to 1.25" adapter, which I think has around 27mm clear aperture at the 1.25" end, and since that will insert into the prism eyepiece holder, I'm assuming that that will reduce the 32mm clear aperture of the T2 prism to the 27mm of the eyepiece holder..My question is, how much will the Aero vignette, how much of the Aero's 68deg FOV will I get, and how is the answer arrived at? (OK, that's 3 questions!??).

Thanks in advance.

PS I do have a 2" diagonal and focuser, I just want to see if being able to just use 1.25" formats is less faffing about in the dark etc ( I only have one 2" only format Ep, the rest are 1.25" with a few with both 1.25 and 2" compatability).

Thanks again,

Dave

[Ruud]

It's hard to say how much vignetting you will see, bit I expect you will definitely see some.

A 30 mm 68° eyepiece without AMD has a field stop of 35.6 mm.
Usually SWAs have about 5% AMD or slightly more, in that case the field stop would be about 33.8 mm or a bit smaller.
With 10% AMD the field stop would be 32 mm.

If you get a 27 mm clear aperture from your 1.25" format parts, a 30 mm eyepiece, in the absence of AMD, can only show a 52° afov that is entirely free of vignetting.

 

 

 

 

[lenscap]

The  fov of a scope is ;     (field stop dia / focal length) X 57.3      degrees.

So for  a 30mm eyepiece with a 27 mm restriction in say a 1000mm scope you will get about 1.7 degrees compared to    about 2 degrees if it was unvignetted.

[Louis D]

Any vignetting is highly dependent on how far it is from the constriction to the eyepiece.  I use widest field 2" eyepieces on my 127 Mak with only a 27mm rear baffle diameter without any noticeable vignetting.  I do see odd reflections from bright objects once they pass the edge of the rear baffle.  However, what you're suggesting puts the constriction much closer to the eyepiece and will make vignetting much more noticeable.  The only way to find out is to try it and see if you can live with it.

[F15Rules]

Thanks Ruud and Lenscap?.

My scope (Tak FS128) has a focal length of 1040mm, so very close to Lenscaps' calculation.

It sounds as if I would only lose maybe c 10-12% of the native field vs using the full 2" barrel? I think I could live with that for the convenience, but once the skies clear I will try both formats for a direct comparison.

Thanks again for the help guys?.

Dave

[John]

I bought a 30mm Aero ED recently as well. I've not used it much as yet but I like the 40mm that I've had a while so I'm hopefull of decent results in my slower scopes. The Aero ED's (and their clones) seem pretty light for 2" eyepieces, which is an appeal with long refractors.

 

[Louis D]

2 hours ago, John said:

I bought a 30mm Aero ED recently as well. I've not used it much as yet but I like the 40mm that I've had a while so I'm hopefull of decent results in my slower scopes. The Aero ED's (and their clones) seem pretty light for 2" eyepieces, which is an appeal with long refractors.

 

There's a nice review of them by @John in this thread. ?

[John]

7 minutes ago, Louis D said:

There's a nice review of them by @John in this thread. ?

You know something, I'd forgotten that I'd done that one !