2" up-Grade, last silly question for a while!

[Caldwell 14]

I read recently that a 2 inch diagonal and eyepiece delivers 36% more light than a 1.25 set up. If this is the case does that mean that a 2 inch set up in my 4 inch refractor is the same as having a 5 inch (ish) refractor with a 1.25 diagonal and eyepiece?

[hunterknox]

No. Look down the barrel of an eyepiece. That's the area of light that gets through it regardless of the size of the diagonal.

The advantage of a 2" diagonal is that it allows you to use low power, wide field eyepieces, which are 2" only. At higher magnifications there's no advantage to a 2" eyepiece.

[John]

The light grasp of the scope won't change. I think the 36% figure might be referring to the additional field of view that a 2" eyepiece can deliver.

[umadog]

Each point along the focal plane sees all of the objective, so gets all of the light. A larger eyepiece provides a larger focal plane and so a wider view.

[rwilkey]

Hex, I use a 2" diagonal in a 4" refractor and it's great to have the extra FoV, a particular favourite is the SW PanaView 32mm, a great combination for DSO and anything else, especially open star clusters, pin-sharp and stunning.