Eddgie, what would you recommend for use in that scope?
While I think Denk may have slightly rolled down the actual power factors (these were the factors for the Denk 11, but the B27 has about 15mm longer light path so the powers should be slightly higher) let's go with the 1.3x because while you can always get higher powers with shorter length eyepieces, the #1 concern with most BV users is the small true field limit imposed by 1.25" eyepieces.
The D21 has a field stop size of 24.4mm (as I recall). With the 1.3x OCS, your effective focal length is going to be about .77 degree true field.
While it does not sound like much, using 24 Pans will open that up to .85 degrees. Again, that is a very small improvement, but as I said, the narrow true field is about the most common complaint you will ever hear about binoviewers. The Pans also have a slightly wider and more immersive apparent field of view.
The Panoptics also have excellent ergonomics. The Explore Scientific 24s have an even larger true field (just shy of .86 degree) but the ES eyepieces are quite fat at the top and have short eye relief, so even though they have one of the largest true fields, they also have the worst ergonomics so I don't recommend them.
At the top end, the D21s have a actual focal length of 21.6mm, so your high power would be about 200x while the 24 Pans will be only about 180x so the D21 would likely give a high power that had a bit smaller exit pupil and give a little better planetary reach. That being said, if you routinely use higher powers, you are going to be swapping out the eyepiece anyway.
The strengths of the D21s are that they are very light, and while they have a narrower apparent field, the have almost zero pincushion distortion. I personally don't mind pincushion distortion, but some people that like to pan around dislike it. Also, while the apparent field is bit smaller, it is actually easier to take in the entire field. With the Pans, you feel more like you have to move your eyes side to side or up and down to explore the whole field, but for some reason, with the field just seems to be there in front of you in the D21s and even a very slight left/right/up/down movement of the eye is required to feel like you are getting all of the field.
Not that you would be interested, but currently I use a pair of 25mm Celestron X-Cel eyepieces, and I actually love them. They have a narrower apparent field than either of the above choices but they are even better than the D21s when it comes to the ability to simply look in and feel as if you are enjoying the entire field without having to roam your eyes around. Apparent field simply does not seem so important when you are using both eyes as it does when using one eye, and if the D21s offer you the ability to take in most of the field with very little eye movement, the X-cels are even better. There is zero distortion and the apparent field is wide enough to still give some immersion, but you just seem to see all of the field rather than most of the field. The field stop is 26mm, so they offer almost as big a field as the 24mm pans, and they have excellent eye relief. They are also very light. Not that it looks like cost is a factor for you, but the 25mm X-Cel is my top recommendation for an inexpensive low power binoviewer eyepiece for use in refractors.
There are no bad choices among these. The 25mm X-cel is a surprisingly good low power binoviewer eyepiece at a great price and a good compromise between wide true field and natural view. The D-21s offer a bit wider apparent field that strikes a great balance between the X-cel and 24mm Pans, and the Pans offer the widest true field, the widest apparent field, and outstanding ergonomics, but at the penalty of some extra weight.
I will say it once again though: If you spend any time at all on the binoviewer forum, you will find that number one complaint interviewers get is the narrow true field. If you don't usually use your scope for larger objects and instead do a lot of doubles, galaxies, small clusters, planetary nebula, and planets, the D21s are probably the best choice here, but that small difference in true field that you get with the 24mm Pans while small, is still big enough, that combined with the extra apparent field and the bit bigger exit pupil, gives maybe the most immersive field of these.
I know that this is not actually advice on which of these is "best" and I don't like to really to make that kind of judgment as to what the "Best" choice because I can't really know what is most important to you, but I can tell you that any of these are great choices. Pick the characteristics that you think best suit your observing needs and preferences.
And whatever you get, please share your observing experiences when you have had a chance to use them.
Have an epic time!
