Telescope Reviews: 25mm ES 100* confirmed? sale date? preorder?

Mr. Black, et al., I think the matter is a bit more complicated than you may realize. You see, it is already well established that the ES-100 design is at least /very/ similar to that of the Ethos, at least according to x-ray analysis. Basically, these eyepieces work with a negative-power Smyth lens group /ahead/ of the field stop, which is internal, about half way to the next lens, which is a positive meniscus. These 4 lenses operate together as a powermate-like group with the field stop placed within the group after the third lens.

Look at the bottom of any of these eyepieces and you will see that there is no external field stop present like in a normal eyepiece, and if you bring your finger up to the field lens while looking through, you will not find any point where you can bring your finger into focus looking thorough the eyepiece! That is because the focal plane is well inside.

It is this group which is both essential to and really the basis for the performance of the eyepiece. Due to the negative power of the Smyth lens group and the placement of the focal plane and field stop inside where they must be, the aperture of the Smyth group must be /larger/ than the field stop aperture. If you look hard into the bottom of one of these eyepieces, you may be able to tell that.

The net result of all of this is that this places an upper limit to the maximum focal length of such an eyepiece to be around 22mm if you wish to not have any vignetting. So, unless Explore has reinvented optics (I don't expect that to be the case), the only other option I can see is to allow for some vignetting to take place.

There were a great many people hoping for a 100 degree Ethos from TeleVue that was long enough to take the place of the 31mm Nagler so that they could retire that eyepiece in favor of an all-Ethos line of eyepieces, and they were hoping for something like a 24-25mm for that purpose, but the practical limit of the design without making compromises was 21mm due to the confines of a 2" barrel, that is why they didn't make it any longer. In order to make a longer focal length, you would need to go to a larger barrel.

And I suspect that was a similar constraint in ES choosing 20mm for their very similar eyepiece. SO, either Explore has chosen to allow a considerable amount of vignetting to occur in the periphery, or they have departed greatly from the original design, which I do not see happening while also maintaining equal performance. Therefore, we do not really know what we are buying here and I myself would be a bit hesitant to invest in one blindly before others have had a chance to sample the eyepiece and evaluate it.

The one maxim that holds true for all of optics is that there are no free lunches: You do not gain in one area without giving something up somewhere else. For ES to have crossed the 22mm threshold still within the limits of a 2-inch barrel, they must have made some compromises elsewhere. ;^)

I would expect the eyepiece to likely have both visible vignetting (limb-darkening around the edges), not be a true 100-degree AFOV, and/or probably have a loss of eye relief (smaller eye lens) as a consequence, which is already at a minimum in the present ES design.

Regards,

Wayne