Short eyepiece for the Celestron NexStar 5"
Using the small Celestron NexStar 5" with a Tele Vue zoom 8-24 mm, I observed quickly the image quality degraded under 10 mm of focal, over to say 25x per inch aperture (x/") where the image is much dimmer even if wider, showing no more (maybe less) detail whatever the astronomical object observed, DSO, planets, the Moon or even the sunspots.
So I had to find another solution to complete that range of focals, to say between 10 and 6 mm, providing a power up to 35-45 x/".
On the other hand, I knew that I cannot use the shortest focals EP and reach +50x/" in my suburban area due first to the weather conditions (wet climate, a sky more overcast or of bad seeing than expected) but also to some light luminous pollution and an optic not as perfect as an high-end apochromat.
To read : How to select an eyepiece
Conditions to meet
First there is the budget constraint linked to my location; I do not want investing more than ~250$ for an eyepiece that I would only use a few hours each month, or even less.
In addition I excluded the heaviest eyepieces even if they give a splendid image; they are too cumbersome for my small and light scope and also too expensive to be used too rarely.
Then, knowing the excessive length (extension) of some Barlow or zoom (6" long for the excellent Antares Speers-Waler 5-8 mm, 80 degrees wide; over 5" using a Barlow), I didn't want to select a low power eyepiece and use it in conjunction with such a non-optimized combo.
I'm the first to confirm this is a very interesting solution but I found that barlowing an already long eyepiece (3-4" long) provided a too big extension on my NexStar (or other light scopes).
In fact I have found this optical combination is not always so well tightened to the focuser in many small scopes than using a single optics due to the fact this combo, once in place extended up to 7" or more, even if it is well balanced (or arms well locked on others). So I decided to buy a fixed eyepiece, of shorter focal length I should use in conjunction with a focal reducer/corrector for example.
For short, as many advanced observers I needed an eyepiece of the best optical quality, providing the best image, not too heavy (below 500 gr or 1 pound) which yields a wide apparent field (60 degrees of wider) and a good eye relief (³12 mm), enough for such a focal.
Competitors were numerous. Many could have been added to my list but some candidates were eliminated due to some"default" as I call them. Here are a few.
Some have residual colors (Vixen Lanthanum, Speers-Waler, Easyview), ghosting (Monocentric among others), not sharp edges (UWA, TV Plossl), a tight eye relief (Tak. 7.5 LE, 7 Nagler) or an apparent FOV below 60 degrees (Celestron Ultima and LV, Takahashi LE, many Plossl's). All these values are parameters I didn't want to meet in my future eyepiece.
To download : My Excel Eyepiece Sheet
Far from me to say that those eyepieces are not excellent for some scopes or in specific conditions of use. Some observers are not bored with a tight eye relief of around 8-10 mm, others appreciate a short field of 35-40 degrees wide, or never look at the edges of their eyepieces. The eyepieces listed above are very good models, providing images of excellent quality, bright and very sharp on-axis, much better than any low cost eyepiece.
Howver, they display some minors aberrations that I personaly don't appreciate, even if they are residual and probably hard to see in scopes of long f/ratio.
My opinion is quite simple : I prefer using an eyepiece in its optimal conditions, very comfortable and really "suited" to f/ratios used with my scope (so to my needs). As I discarded these kinds of eyepieces for "optical reasons" I only looked at their potential competitors. But even so, if some famous eyepieces as the 7 mm Nagler, 7.5 mm Takahashi LE ED and some enhanced Plössl's are disqualified for this run, they are still some famous competitors on my short list, which are (values for the NexStar, street prices) :
All Radian's are top-class eyepieces, sometimes considered as "the best in the world" but stay honest and objective. I tested them in the field around Christmas, so in winter.
Except for the eye relief, the 6 mm Radian yield a field of view less impressive than the 4.8 mm Nagler and the 8 mm Radian stays below the 9 mm Nagler, as I expected. On the other side, compared side by side in the field, Radian's present no difference in sharpness or field flatness against these small and old Nagler's.
Plus side, the very good eye relief of the Radian's and their large frontal lens are very comfortable, avoiding for example moisture on the frontal lens (excepting when return back home of course), very common in winter nights. They also produce a very clear image, quasi on par with my Plössl, a bit contrastier than with Nagler's 7 mm and lower focals.
Minus side these Radians's yield a 57.3 degrees of apparent FOV, short compared to super wides eyepieces from Meade, Tele Vue or Vixen, but quite large comparing to most eyepieces available in that range of focals (40-50°). Better, their price is not too high for their quality. At last comparing to the zoom, at daytime the pincushion is almost absent.
The 7 mm SMC XL Pentax is an excellent eyepiece even if I found it a bit too bulky, but it seems to have less contrast and surely less sharp edges than the Nagler's when panning a stars field as Cygnus. Its apparent FOV is however of 65 degrees.
Thus selecting the best eyepiece was not obvious at all.
After have tried them a few days on the sky, mainly looking at low contrast objects (Saturn's bands and C ring or trying to locate the Jupiter's red spot- not obvious -, cratelets near Triesnecker and inside Plato on the Moon - successful -, small faint galaxies and spirals arms of the brightest - not obvious neither -, bright HII nebula and even on sunspots's penumbra - very good -, I finally selected the 6 mm Radian from Tele Vue with a 0.63x focal reducer/corrector. Why ?
Some will may be argued against the use of reducer. But my problem is in my area I cannot go over 30 to 40x/", so on the NexStar the maximum theoretical power is limited to around 250x (5 mm EP or 50x/") but is practically limited to 130-150x for the reasons explain earlier. The 6 mm Radian with its reducer gives 131x (26x/") with a comfortable true field of 26', large and power enough to look at most of the astronomical objects in excellent conditions with full details and contrast.
Better, if the seeing is great I can still use this 6 mm straight, with a power of 208x, 42x/", very near the maximum, which still yields a true field of 16.5' with a comfortable 20 mm eye relief.
About the focal reducer, I selected this solution in place of barlowing a low power eyepiece because it is a small interface giving stunning results when enlarging the field of view, an interface much shorter than a Barlow which prevents a too long extension of the optical combo behind the diagonal.
At last this reducer flattens the field, may be not really useful on low f/ scopes using high-ends eyepeices but as my scope is not an astrograph known to be well corrected for this aberration and as it is part of the set... this is finally a solution I appreciate very much (although the barlowing as also its advantage as keeping the eye relief, etc).
The Radian comes first for the quality of its images, very similar to the 9 mm Nagler even if the true field is a bit shorter, even using the reducer (26.2' vs 35'), then for the power and resolution it provides (131x and 208x), not the maximum but the best power to reach the practical resolution of the NexStar in usual (average) seeings conditions, ideal to resolve binaries stars or globular clusters, at last for its generous eye relief and high contrast, this last being a bit superior to the Nagler and Pentax I tested.
My selection is probably closed now. I will not use it with a Barlow considering that the length of the optical combo is really too long this time (over 5"). Other parameters to take in account can be its excellent planetary potential (mainly on larger scopes or longer focals), its weight quite light, its medium size and its price.
Thinking to the future, this eyepiece works also fine on large scopes from 8 to 12".
This 6 mm Radian used in conjunction with a reducer/corrector fits exactly the same needs and perform as well on a 8" SCT (210 and 333x at 26 and 42x/"), the highest magnification being around 400x (5 mm Radian).
So now, with the confirmation in situ, I can tell you that this 6 mm Tele Vue Radian performs very well on both optimal and ordinary seeing conditions on SCT of various apertures. I think that it is also and not only a good but an excellent choice for a limited budget.