Advantages of 2" eyepieces?

[TheDecepticon]

I was wondering what advantage there is for using a 2" eyepiece over a 1.25"?

I have seen many that are both 2" and 1.25", so really that makes them a 1.25"?

I have an older 8"LX90 f10 and an 8" f5 dob I use eyepieces in. At the moment, the LX90 has no 2" visual back. I suppose what would suit one may not suit the other.

With the much wider field of the dob, do I really need a 2" eyepiece?

Any advice would be helpful.:shrug:

[Astro78]

A 2" EP is good at lower magnifications - you'll get brighter images towards the outside of the image.

[jeff65]

The diameter of the eyepiece field lens correlates to the field of view. A 2 inch eyepiece can have a larger field lens than a 1.25 inch eyepiece.

[ballaratdragons]

I have no technical answer as to which is better, but my own experience tells me I like using my 30mm and 32mm 2" EP's over my 1.25" EP's.
They just feel more comfortable to use.
Not that I hardly ever use EP's any more though :ashamed:

[UniPol]

TeleVue provides very useful information here (http://www.televue.com/engine/TV3_Page.asp?return=Advice&id=79)

[ColHut]

Almost nil really, and they are heavier to boot. The only advantage is that some eyepieces cannot be fitted into or onto a 1.25" body, and very much depending upon your particular scope a 1.25" eyepiece may restrict your true field of view compared to a 2" model by its smaller field stop.

If you look at the formulae on the Televue page your lx90 has a maximum TFOV of about 0.77 degrees with a 1.25" eyepieces (assuming a field stop of 27mm) and a maximum TFOV of about 1.36 degrees with a 2" eyepieces (assuming a field stop of 48mm).

For a plossl with a 50 degree AFOV these equate to x64 and a 31mm eyepiece, for 1.25" and x36 and a 55mm eyepiece for a 2” eyepiece.

For the 8” f5 dob, it has a maximum TFOV of about 1.52 degrees with a 1.25" eyepieces (assuming a field stop of 27mm) and a maximum TFOV of about 2.7 degrees with a 2" eyepieces (assuming a field stop of 48mm).

For a plossl with a 50 degree AFOV these equate to x32 and a 32mm eyepiece, for 1.25" and x18 and a 55mm eyepiece for a 2” eyepiece.

So there is not much point in getting a plossl with a 1.25" barrel bigger than 30-32mm without artificially restricting your TFOV in both cases.

If these were 100 degree Ethoi then there would be little point in getting an ethos with a 1.25" barrel bigger than 15mm without artificially restricting your TFOV, and you would only need a 27mm (!) Ethoi with a 2” barrel. Of course a 55mm eyepiece has an exit pupil of 5.5mm in the f10, and 11mm in the F5 which greatly restricts its utility in the latter. Choosing a wider AFOV lets you up the magnification and reduce the exit pupil size which might improve contrast. (As far as I know Televue are badly neglecting us all by not making a larger ethos.)

Of course these scopes, for a given eyepiece offer dramatically different views of the sky and magnification.

I hope this helps

:)
http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/1713776/page/260/view/collapsed/sb/2/o/all

[TheDecepticon]

Thanks, lots of good info.:thumbsup: