I have a 2” OIII filter.
Sky magnitude 21.39 rural area at least 10 miles from nearest town.
Is there any targets worth using this range of eyepiece for or is it just a finders eyepiece for my
I was interested in a high quality Masuyama eyepiece in the 45 to 60mm range too expensive if there is not enough targets that would justify its purchase?
is that 21.39 mpsas from a chart or measured? Chart values are generally somewhat optimistic about how dark the skies are, generally they'll be somewhat brighter.
This is how I see it:
As others have said, matching the exit pupil to your dark adapted pupil diameter is useful, this maximizes the brightness of both objects and stars while retaining as much magnification as possible. If the exit pupil is larger than your eye, it has the same effect as putting a mask over the objective. It will dim stars and objects (nebulae etc) will be smaller.
For a finder, the goal is to maximize the true field of view but maximizing the brightness of objects is second to magnification since you're mostly looking for stars and smaller objects. For viewing large faint nebulae with a filter, maximizing the brightness and the field of view can be helpful.
As I see it, at F/7, both a ~40mm SWA (68 degree) eyepiece and a 55mm Plossl are possibilities. Both will maximize the TFOV . The 40mm SWA will provide a 5.8 mm exit pupil, the 55mm Plossl will provide a 7.9mm exit pupil.
For most observers and most situations, the 40mm SWA would probably be the better choice. Few observers can take advantage of a 7.9 mm exit pupil and for those than can, (I am one), the smaller 5.8 mm exit pupil with the greater magnification will almost always show more, be more useful. It's only for the largest, faintest objects with a filter that maximizing the exit pupil is an advantage.
A 5 inch F/7 has a 900 mm focal length. That means a 40 mm SWA and a 55 mm Plossl with both provide a 2.93 degree TFoV, the 40mm will provide 22.5x, the 55mm, 16.4x.
My thinking goes like this: Regardless of whether you decided to buy the 55mm, you'd want the 40mm SWA. In general, it would be the more useful eyepiece and for most observers would be the only one needed. Later, a 55mm could be added.
So, what objects? This time of year, I find the "Heart and Soul" Nebular region, the California Nebulae, the Flaming Star Nebulae are helped by large exit pupils with filters. The nebulosity in the Pleiades is best seen at a near maximum exit pupil without a filter. Barnard's loop is another.