"Eyepiece philosophy"
Foreword
There cannot be absolute truth on something as subjective as the visual experience. Also, the information herein, assumes usage of a typical 3" to 12" consumer production scope. Furthermore, as a free-thinking spirit, I feel compelled to share my opinions, which differ in some points with what can be called internet dogma and popular edicts. This said, I will not "hide" the differences between those and my opinions in a covert propaganda war but highlight them and face them head-on with my own arguments. I believe the document that follows, by giving information in a usage context, will yield better knowledge of the eyepiece selection process, and what criteria to analyze. Whatever you come out thinking about it, if the information herein has been assimilated, and is of consequential use, I will consider my work to have been worthwhile, whatever your "eyepiece philosophy".
I have re-arranged the information within this document as to make it more cohesive and understandable. Information has been organized as follows :
- Different eyepiece limitation factors to be aware of.
- Collecting a coherent set. What to choose, and why.
- Edicts and dogma. Perceived flaws of popular simplistic ideas.
Different eyepiece limitation factors to be aware of.
0.92mm Exit Pupil.
Below that, unless for planetary and lunar, an eyepiece is too dim for any practical use. The Bell curve of light throughput takes a vertical dip at this point. anything below this, is completely unusable for DSO purposes. Moon and planetary are the only targets still workable below this point.
1mm Exit Pupil
Luminosity drop-off point for choosing DSO eyepieces. Below this threshold, things get dark and murky real fast. The Bell curve of light throughput seems to dip sharply from that point. Between 1 and 0.92, there is still usability for DSO purposes, but it gets real ugly, real fast.
2mm Exit pupil
Widely seen and recognized as THE sweet spot factor in a view. It is held as being an exit pupil where Contrast VS luminosity is at it's best to identify the smallest and faintest of fuzzies it is possible to view in a given instrument.
6mm exit Pupil.
Above this threshold, there is a very good chance that some of the light is lost outside the light gathering surface of the human eye. it is reckoned by most as being the average size of a night vision dilated pupil on the human eye. It is not an absolute measure, but is often treated as such to simplify and generalize knowledge used by amateurs to efficiently use visual astronomy equipement. A light cone larger than one's pupils will lead to loss of light. It is also important to know that on obstructed designs, a light cone larger than one's pupils will lead to an even more severe loss of light, contrast and resolution, as the obstruction circle's ratio to unobstructed light becomes even more important.
140X to 150X
Somewhere, between the two, there lies what can only be referred to as the "soup limit". Beyond the soup limit, atmospheric disturbances become conspicuous at the eyepiece. Albeit some keep on talking about the max theoretical limit, my real life experience, tells me otherwise. With many scopes, up to 300mm, in many geographical locations, some of which are up to 1200Km apart, at 150X, there is always a detectable "soup factor" even on the best of nights. Depending on seeing and conditions, it only gets worst. Perhaps people living in the Mohave desert have a different experience, but reading post on internet forums, from people all over the world, it quickly becomes apparent that most of the planet seems to fall in the 150X magnification limit before "soup".
The Basic set.
1- The finder.
Requirements: Wide-field, or UWA, if possible. Results in a ExPl of 2mm or very, very close to 2mm. Premium is recommended. This should achieve a near or better than 1° TFOV.
It's wide field helps find dim and/or unknown objects, some of which may be infinitely darker than expected, or at the very limits of an optical system. Having a premium eyepiece for this purpose averts that ever present and gnawing doubt that "perhaps a better eyepiece would've helped me find it..." Getting a premium item, such as the universally acclaimed Panoptic 24mm, or Nagler 20mm eyepiece will avoid what I can only call "thrift's remorse".
2 - The "max clean magnification" eyepiece.
Requirements: Exit pupil of approximately 1mm or more, no less than 0.92mm and capable of magnification not exceeding 150X. Premium is recommended.
This one will most likely end-up being your favorite planetary and DSO eyepiece. This is the one causes the most angst to newbies; as they often wonder if better quality would've afforded a slightly better view, sharper banding on Jupiter or a more defined Cassini. Albeit the finder causes doubt on whether they are at fault for not finding something, thus taking some of the doubts off of the eyepiece's faults, the "max clean magnification" eyepiece is usually used on known targets, thus the users rarely question their aptitudes to find, and instead, focus 100% of their attention on the view quality. One needs something known to be almost invariably "bad-seeing-proof". Some of the more aperture-equipped amongst our peers swear that the 1:1 measure of Aperture in millimeters to magnification power works. My own experience with an 8" newt, and a 12" newt, was otherwise. I denoted that 150X was still the "soup" limit. Many geographical areas have better skies than 150X, but it seems to be the upper limit for soup-less views in most of the world. Unless you live in New Mexico, Arizona or Nevada, chances are you sky also "soups-up" above 150X. If it doesn't soup up, you will want to use eyepiece #4, but should not go without this one either.
3 - The extra-low magnification eyepiece.
Requirements: Low magnification, results in a TFOV similar or wider than the "finder" eyepiece. ExPl should not exceed 6mm. Intermediate quality items can suffice, though most favor simpler designs from premium makers.
A complementary finder and a "best view" item. It's wide field capability mostly reside in it's low magnification factor. With a TFOV often is no wider than the finder eyepiece, some mistakenly think it as a useless duplication. This is a mistake, as this one's trump card is a bigger ExPl. One could compare a 24mm Panoptic and 32mm Plössl, and say "Same TFOV, needless duplication". I don't. Nor should anyone, for that matter. The ExPl numbers change. On my particular system, it means 33.5% more exit pupil. This is nothing to sneeze at. If something is to dim for my finder, if it isn't too small, chances one can spot the faint fuzzy on this eyepiece. You can test your aperture limit with it. Being a lot brighter, also, it can do better than the finder EP on dim, wide field nebulae using filters. That eyepiece can also give a better "wow" effect to newbies. Sidewalk astronomy will also favor this eyepiece for it's brightness. Eyes with no dark adaptation will see a lot better in it.
Optional add-ons. Some, or all, may be desirable. This, however depends on your equipment, and targets.
4- The non-planetary "max magnification for the instrument" eyepiece.
Reason to have it: You can go way beyond "max clean magnification" and yet still be around 1mm of ExPl.
This one's only physical limitation is ExPl. An exit Pupil of no less than 0.92mm, preferably of 1mm. This is as dim as a normal eye can use for DSOs like M57 and small globs. This one should also be a top-quality item, for the obvious reasons.
5 - The planetary set.
Reason to have them: You can go way beyond 0.92mm on the moon, and some of the planets, yet retain quite a bright view.
These have practically no limit even at 0.35mm of ExPl, The moon will show up bright enough for a view. If your scope, sky and target can deal with it, and it's good enough to work for you, anything goes. Low element count and simple designs make it possible for really good hardware to be found even from the cheaper brands.
6 - The hole fillers.
Reason to have them: Splitting doubles, filling gaps in the magnification scale, having a complete set. Whatever you can think of.
These are eyepieces that almost never get used, and when they do, they seldom get much time on the focuser. They are mostly used between your wide field finder and your "max "clean" magnification eyepiece" in FL. These can confirm how bad the sky is when your "max clean magnification" eyepiece gives you lousy views, or just look pretty in a case.
How many eyepieces becomes too many eyepieces?
There is no clear answer to this. How many telescopes you own or use, and how varying the respective specifications are may call for immense quantities of eyepieces, should you want a "complete kit" for every scope. Most will, at a certain point, rationalize. No need for a planetary eyepiece on a 80ED used for wide fields, when you have a 150mm refractor, 8" SCT or 12" Dob for that purpose... Common sense helps one sort this out. By the time one owns multiple scopes, one is usually well on the way to a coherent eyepiece collection anyways.
My take on the most popular eyepiece edicts.
Three, well selected, premium eyepieces is all you need.
I'm more inclined to believe that, out of a collection, 3~ 4 eyepieces should be premium, but a collection of eyepieces should, by no means, ever be so limited in scope. Even more importantly if the telescope is of large aperture. Being able to go to the atmosphere's allowed magnification limit is important to many observers, and that value is strongly variable in certain locations. The highest magnification useful for DSO, also, is often quite different from lunar and planetary maximums. Can I live with one extremely wide field, one "max magnification for the instrument" and one "max clean magnification" ? sure. but why stay so limited ?
Another popular edict.
Only premium eyepieces are ever be worth purchasing.
Many very wealthy, premium scope users have amongst their gear, some very humble Orthoscopics, Plössls and the likes from manufacturers as "lackluster" as GSO. An Antares Ortho is hardly a premium eyepiece, yet it seems to be enjoying qui a good reputation. What about Celestron's old Ultima series? Masuyama? Meade's 4K 8.8mm UWA? It's expensive, but it is not a Tak, Brandon, Zeiss, Televue or Pentax.... BTW, where should we peg the starting price for what we call "Premium"? Should I blow 350$ on an eyepiece that I only use to split doubles at low magnifications? Is it even beneficial? What about the magnification hole between my preferred DSO eyepiece, and my top-magnifying planetary eyepiece? That "filler eyepiece", well, it doesn't come out much. Doesn't it make sense to spend a little less on that particular eyepiece?
Originism - Did you say Japan?
China only makes junk.
There is an almost religious cult of admiration for Japanese stuff these days. Can I remind you all, specially the old timers, that in the eighties, Japan was the modern day equivalent of China? I have seen people saying that Taiwan stuff was so-so, and only slightly better than mainland China. Does a 24mm Televue Panoptic count as so-so ?!? The Panoptic 24mm eyepiece is made in Taiwan, yet people all over the world praise it for excellence.
China is a country, not a quality standard. Force too low a price on any business, in any part of the world, for a given product, and expediting and QC issues will arise, and you don't need an MBA to understand this. China is hungry. Companies will do anything to get the contract, and they deal with the impossibilities "later". Japan also had those issues to deal with at a certain point in it's past.. Stigma and condescendence towards emerging economies is very much part of our perceptions, never mind the fact that they are only obeying to orders from investors and customers here. Remember when a Toyota was called a "Japanese soapbox" by most North-Americans? Where is Toyota today ? First maker on the globe. They got there by being excellent, period. While a new Chevy still needed a screwdriver in the carb 15 days a year to start during winter in Canada and Alaska, The humble Datsun 210s, the Corollas, well, they just worked, even at -30°... So much so that at a certain point, some Japanese car owners became notoriously aggravating by annoying anyone having problems with a domestic car by shouting "You didn't get it yet ?". The real picture, in truth, is that some fantastic stuff is coming out of China right now. Many of todays thrift items outperform many of top shelf products from the pre 90's era.