2" v 1.25"

[Sconesbie (Scott)]

Hi everyone.

I have 1.25" eyepieces and was wondering if having/using 2" is of any advantage?

Scope is 10" collapsible dob.


Regards
Scott

[vaztr (Andrew)]

Generally no,

Most 1.25" EPs can do the same job as the 2" EPs and for less cost. And 2" EPs can be awfully heavy which may be an issue in a Dob pointed low in the sky.

That being said, there's no reason not to go to 2" if that's what floats your boat

My 2c

Andrew

[astro744]

The maximum field stop diameter in a 1.25" barrel eyepiece is about 27mm. The field stop diameter determines the true field or how much sky you see. The apparent field of view (AFOV), 50, 68, 82, 100 etc is how big the 'window' is looking at that true field.

In 1.25" size a 24mm Panoptic, 32mm Plossl, and 40mm Plossl all have 27mm field stop diameters and therefore show the same amount of sky at different magnifications and exit pupils in any given 'scope.

If you want a wider true field you have to go 2" which will give you up to 46mm field stop diameter. Examples are 41mm panoptic and 55mm Plossl. There are other brands but I'm quoting Tele Vue because the specification data is readily available (and they're mighty fine eyepieces!).

True field of view (TFOV in degrees) = field stop diameter (FSD in mm) x 57.3 / focal length of telescope (f.l. in mm)

e.g. 1200mm focal length telescope with 27mm FSD eyepiece gives 27 x 57.3 / 1200 = 1.29 deg. TFOV.

e.g. 1200mm focal length telescope with 46mm FSD eyepiece gives 46 x 57.3 / 1200 = 2.2 deg. TFOV.

If you want to see all of the Pleiades for example you need over 1.5 degrees preferably over 2 degrees to frame it nicely and therefore need a 2" eyepiece in a 1200mm 'scope. If your 'scope is shorter in f.l. then you will get more TFOV but 2" will always give more no matter what the length is.

Note I've quoted maximum field stop diameters above. There is nothing stopping a manufacturer putting a 2" barrel on a short focal length eyepiece and there are examples of this. You can have a 12mm eyepiece with 82 degree field in a 1.25" barrel or 2" barrel and it will give identical views (12mm Type 4 Nagler is a dual barrel eyepiece). The eyepiece focal length and AFOV determine what FSD you end up with and what barrel is required. Note as the AFOV gets larger the maximum FSD gets a little smaller in proportion.

e.g. you can have 46mm FSD in a 41mm 68 deg Panoptic, only 42mm FSD in a 31mm 82 deg Nagler and only 36.2mm in a 21mm 100 deg. Ethos. Each of these is the longest focal length at the given AFOV in a 2" barrel whilst maintaining a high level of correction to the edge.

[Allan]

Absolutely, I use my 2" eyepieces a lot. You don't need many, perhaps just a couple of options that will provide what your 1.25" eyepieces can't do.

2" eyepieces come into their own when you want to frame large objects or multiple objects in the field of view. If you use narrow band or line filters the larger exit pupil and field of view make for some very engaging observing.

Do some exit pupil calculations and let that guide you. If you observe under dark skies a 2" widefield that gives a 5mm plus exit pupil is a real treat to use.