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The SXN Eyepiece Saga

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SXN's picture
The SXN Eyepiece Saga

This is taken from a post on my blog, but since it has to do specifically with eyepieces and visual observing of variable stars I thought I'd share it here and maybe start some discussion.

Mike (SXN) Simonsen

If you are a visual observer, the eyepieces in your arsenal are as important as the optical tube or mount. One of my goals has always been to find the perfect combination of the fewest number of eyepieces to be able to handle all my typical observing requirements. I don't like to spend time changing eyepieces and refocusing. I want to observe, not focus and fiddle around. My search for the perfect combination of eyepieces for variable star observing has had episodes of experimentation and expense followed by long periods of satisfaction and observing action.

After learning the capabilities and limitations of my first set of standard Plossl eyepieces I turned to the TeleVue brand to get superior quality eyepieces that had longer eye relief and a wider true field of view.

Eye relief is simply the distance between your eye and the lens of the EP where you can comfortably and effectively view what you are looking at. If you've ever stuck your eyeball practically on top of the lens of a inexpensive, short focal length, short eye relief EP to see into the tiny peep hole, you know this is not how you want to spend two to four hours a night 100 times a year.

True FOV is the area of sky you can view in any particular eyepiece/telescope combination. When you are making variable star estimates, the larger the true field of view the better chance your comparison stars will fall in the FOV. Generally speaking, the brighter the variable and comparison stars, the further spread out they are on the sky. So an eyepiece that can show you a larger chunk of sky is very helpful.

You may notice I haven't talked at all about magnification yet. That's mostly because when you are looking at stars they always just look like points of light, unlike planets or the Moon, which get bigger and dimmer with more magnification. The real advantage of higher magnification for star viewing is it makes the sky background darker, so you can see fainter stars. This usually comes at the cost of FOV. Generally speaking, the higher the magnification, the smaller the FOV.

There is a practical limit to how much you can magnify celestial objects in any telescope. For short focal ratio telescopes 30x per inch of aperture is about right. For longer f ratios, like the f/10 Schmidt Cassegrains, 20-25x per inch of aperture is more realistic. It is rare for any telescope to be able to use more than 400x ever. I've only experienced a dozen nights where I thought 350x was reasonable.

That never stopped me from buying a bunch of eyepieces that would deliver more magnification! I've had everything from 5mm (600x!!) to 10mm (300x) and eventually sold or traded them all away because they never came out of the eyepiece box. My 7mm Nagler was a great eyepiece, but I only used it a few times in ten years, mostly to view Mars, Jupiter and Saturn on those exceptionally rare nights of almost perfect seeing.

As you can imagine, sorting through all these factors has taken some time. For almost a decade my two "go to" EPs have been 12mm and 17mm Nagler Type 4's. The 12mm gave me .33 degrees of sky at 250x, which was perfect for the 12" LX200 telescope. It's about as much magnification as you can really use on an average to good night, and 1/3 of a degree is a large enough piece of sky to cover a typical chart I would use to observe faint cataclysmic variables. The 17mm Nagler had an awesomely comfortable eye relief. I could observe all night long with that eyepiece- I loved it. It delivered .46 degrees of sky at a generous magnification of 176x. I could see almost as faint in the 17mm as the 12mm on average nights, but the lower magnification delivered more pleasant star images in fair to poor seeing. It also weighs 5 pounds! Think about that before you buy one. You may need to add counter weights if you use it. 

Another thing I loved about these eyepieces was the adjustable click stop eyeguard. They are a handy light shield built right into the housing of the EP to position your eye and block any stray light from getting in to mar the view. Every EP should have this.

For observing Mira stars I still needed something with a much lower magnification and wider FOV. These stars can range from the very faintest I can see, 14th to 15th magnitude, all the way up to 7th magnitude. I needed something to cover the mid and top ranges of these stars. I eventually settled on mounting an 80mm short tube refractor on the LX200, but this was only useful down to about mag 10.5 or so. The real answer to this issue was the TeleVue 55mm Plossl. It gives me almost a full degree of sky (.92 degrees) at 55x. I can see down to 13th magnitude easily, and observe stars as bright as 9th magnitude while still having some useful comparison stars in the FOV. Anything brighter is perfect for the 80mm refractor and its generous FOV.

I probably could have gone on forever, blissfully, with this combination of EPs, but at the Texas Star Party in 2011, I was able to borrow a 13mm Ethos EP from a friend and using it in my telescope was a life altering experience. I couldn't believe how much larger the FOV was. It's ridiculous! You have to move your head around to take it all in. And the crisp, flat, pinpoint star images it produces are stunning, even compared to the near perfection of the Nagler type EPs I'd come to love. I had to own a set. 

But I really had no desire to give up my 12mm and 17mm Naglers. There really was nothing wrong with them and after trying dozens of EPs I had stuck with these for a decade. I have to admit, they were sort of like old friends. After 50,000 variable star observations we'd grown close. So I thought I would try to get a little more oomph out of my telescope by getting the 10mm Ethos. This would give me 300x and the same 1/3 degree of sky the 12mm Nagler did. Hopefully, I could use it as my new go to EP for faint CVs because the added magnification would give me a darker sky background. And for those really good nights I bought the 8mm Ethos. A whopping 375x with still a 1/4 degree FOV. What could be better? I could have the best of both worlds.

I settled in for a few months with the new set of EPs and found myself, as before, using the 10mm once in a while and the 8mm almost never. I don't know why I thought the Ethos EPs were going to make the sky conditions better. They are just eyepieces, not atmospheric stabilization devices! I was back to using the old-faithful 12mm and 17mm Naglers in no time. It finally dawned on me what an idiot I was after the final night of the Cherry Springs Star Party when I realized I'd never even taken either of the new Ethos EPs out of the case. I had $1350.00 invested in two EPs I wasn't using.

So I took the plunge. I traded in the Naglers and the Ethos EPs and came home from Cherry Springs with just three EPs in the case. A new 13mm Ethos, a new 17mm Ethos and the trusty TeleVue 55mm Plossl. It didn't take me long to get used to the idea of using these fantastic new eyepieces as my trusty companions at the telescope. They are both simply stunning performers. 

The weather has been warm and dry and unusually clear at night for summer. I have had some of the best viewing in the 12" I can recall in the last four or five years. I have been able to see as faint as 16th magnitude on several nights with the new EPs.

I've had several nights of very exceptional seeing where I thought to myself, "You know, Self, we could probably use at least one more eyepiece, maybe a 10mm to give us 300x for nights like this." I know, it's a disease. But this time I told myself I'm not going to spend $600.00 or more for an EP I will only use on special occasions. Besides, the situations I want 300x for don't generally require a large FOV. I don't need a Nagler or Ethos EP to see extremely faint cataclysmic variables, or to view Saturn or Jupiter at high power.

So I decided to try out a TeleVue Delos 10mm. It still has a generous apparent FOV of 72 degrees, which yields .24 degrees true FOV and 300x in my 3000mm focal length telescope. I've only had it long enough to use at the telescope for one night, but I think we have a winner. It still cost $370.00, but that is a little more than half what a 10mm Ethos would cost, and I just can't do that again.

The Delos has a new kind of adjustable eyeguard that slides into place and twist locks once you have determined the perfect place for it in relation to the lens. This is almost a requirement for this EP. The Delos is very sensitive to eye placement. If you position your eye pupil too far in or too far out you experience edge darkening and field loss. This is not a star party eyepiece for the general public. It takes a pretty experienced observer to get the most out of this complex combination of elements.  But it is a very good quality EP with no ghosting and it delivers great definition on those really good nights at 300x. I think I'm done buying EPs for a long time...or at least until Al Nagler comes up with a new exotic glass and lens combination.

lmk's picture
Eyepieces for faster scopes

This is a good topic for sure! I think Mike SXN has made some very valid points. But, I think some of it is from the perspective of the longer f/ratio scopes, which is typical for the ubiquitous commercial (Meade/Celestron) SCT which seem to dominate the amateur market.

I use fast Newtonians for my visual observing. Why? Several reasons: 1. You can make them yourself (just try figuring a Schmidt corrector plate for the first time ;) 2. They provide the max aperture for the minimal size/weight. 3. When used with a Paracorr they provide just as high quality wide field resolution as any scope out there. (including those megabuck A-P APO Flouride finderscopes ;)

For the fast scopes, my preference is the short FL Ethos eyepieces. My fav is the 3.7mm 110 degree afov Ethos SX. Why? As Mike said, the higher the magnification the darker the background, so faintest stars show up better. So, it logically follows you want to use the widest apparent field, shortest FL eyepiece to give the maximum true field, with the highest power. The 3.7mm Ethos gives me 500x with a half-Moon's width true field in my 20" Newtonian. This is a truly spectacular view! I feel sorry for the CCD guys who look at just a B&W computer screen:(

Of course the Ethos are expensive eyepieces, you would need around $400-600 on the used market to get them, but if you are a serious observer, the cost is well worth it. You just need to spend it once, so consider it "amortized" over your remaining visual observing life. A second choice would be the short FL Naglers. I like the 3.5mm the best. Around $200 on the used market.

Regarding maximum power, Mike says 350x or above is hardly ever used. I find that is not the case at all. I regularly use 500x, maybe 90% of the time. If this power is not usable, you need to look at two things: 1. Is your seeing particularly bad? 2. The optical quality of your telescope. I know from experience the commercial SCT's can be very lacking in optical quality. I have star tested many of them, a few are pretty good, most are marginally usable, some are complete disasters. Whats lacking is consistency in the production. The same problem exists with the import Newtonians too.

If you want to get the full enjoyment and performance for a given high power eyepiece/aperture, you need to get a premium class optic primary. Some names that come to mind are Zambuto, RF Royce. I know personally each of these manufacturer's makes a nearly perfect optic, and do so consistently. They guarantee 50-60x per inch without image degradation, and I can concur with that. You will only have seeing left to deal with when using this class of primary mirror!

So, this is my view on eyepieces, and the associated requirement for a premium class optical system to reap the full benefits. Remember, using marginal optics is like cutting your seeing in half all of the time!

Mike LMK

potterrb's picture
Eyepieces for Faster Scopes

Mike LMK,

My primary scope is an 8" f4, and I have to concur with your thoughts on eyepieces.  I favor the Explore Scientific EPs (very close performance to TeleVue at 1/2 the price).  I have a 24mm ES82 and a 11mm ES82, providing 2.5 deg, and 1.1 deg respectively at 33x and 73x power; the 11mm barlowed provides a 0.55 deg FOV at 146x.  These provide excellent views and the FOVs work very well for with the traditional B, D, and E chart sizes from AAVSO.  I have a 9mm ES100 on order that will replace my 11mm ES82, providing the same FOV but at 89x and 178x barlowed.  The only thing better would have been an 8mm TV Ethos, but I personally have trouble justifying the price, even used.  So far, I haven't gone below the E charts with limiting magnitude 13.5.  In my neck of the woods the sky glow doesn't allow me to see too much below 12.5 anyways with my current scope.

I also agree about the optics...that will be my next upgrade, not EPs.


lmk's picture
Explore Scientifics

Hi Brian,
Yes, I have the ES 9mm 100deg too. Optically it is very good, quite similar to the Ethos 8mm. However, it is inferior in 2 important respects (you get what you pay for?):
1. It weighs substantially more than the Ethos, causes some balance problems in my 8" scope 2. It tends to fog up the lenses a lot worse than the Ethos.
Not sure why it fogs so easily and densely than the Ethos? It is supposed to be "Nitrogen purged" which seems like it would help from moisture standpoint, but obviously it worsens the condensation on the outside for some reason! This makes them pretty useless for me except under very dry conditions. Not sure of the same fogging problem exists for the 82 degree series?
Mike LMK

potterrb's picture
RE: Explore Scientifics

Hi Mike,

So far I have not had the fogging issue.  I have been out a couple times with the 24mm 82deg and 9mm 100deg where my plastic-covered charts have dewed up and no problems with the EPs, but I'll let you know if that changes.

As for weight, I am on a homemade dob mount and balance it with the 9mm in the focuser.  The 24mm is a bit lighter, and the 9mm with barlow is heavier so it pretty much works itself out.


paw's picture
eyepieces, and more

After years of fiddling around I've ended up with a set of Vixen Lanthanum eyepieces (not the wide angle). So I have 5,9, 15, 25, and 40mm, all small and light, very high quality, mid-price, all par focal, and I don't have to take my glasses on and off. (I use a 16" F4.5 newt.)

I got fed up with other brands saying they were 'high eye relief' and not being so. Indeed, with the wide field things I have to get my eye ball in contact with the lens! The ethos are beautiful views, but useless to me, wearing glasses and VSOing. (And we wonder why our observations are different. How can they not be? We are not machines.)

On another note: Being 'awe-struck' is good for us! (I only heard about this research second hand, so no reference.) I've been telling my wife for years that quite the opposite from being tired from working at night, I feel happier. I suffer from depression, and it helps that, big time. I am still awe-struck every time I'm at the telescope.


lmk's picture
Best eyepiece value?

Forgot to mention, I think the Meade Ultrawide 82 deg series may be the best value considering they perform just about like the Nagler's but at half the price! I used the 4.7mm and 6.7mm UWA's for a long time, and really loved them, until selling to fund my Ethos! Otherwise, I would definitely have kept them.

Mike LMK

pox's picture
My best eyepiece is a

My best eyepiece is a not-terribly expensive 25mm Plossl, bought about 20 years ago when I got my 36cm newt. I've seen down to mag 14.5 with this, despite it being kept in my coat pocket. I have no idea who made it, and to be honest, in the words of Catherine Tate (has she made it over to the USA yet?) "ain't bovvered".

A few years ago I splashed out on a Swan Optics wide-field 7mm eyepiece which has proved pretty good ('splashing out' for me equals about £60 or $100 - that's my limit) but while on the subject of optics something else that's important is collimation, especially with fast newts. I find that, since I am now back to carrying the tube to the mount 'en plein air', collimation needs to be done more often and more thoroughly, especially when using high power.

Don't neglect the finder for variables either! I've followed R Vir down to magnitude 10.6 with a 7x50 finder.

potterrb's picture
Best EP


I try not to spend too much on EPs, have a line of UO orthos purchased used for planetary, and the Explore Scientifics are the most I've laid out.  Unfortunately with a very fast scope the EPs are important.  Still, I stay away from Televues because I want to stay married!

You must have very dark skies to be able to see down to 10.6 with a 50mm finder!  I have Canon 15x50 binocs and from my suburban skies I typically can only measure down to about mag 9.  And that's with two eyes!


PYG's picture
Fast scope oculars

I use a 20 inch f4 Newtonian, and last Xmas Santa dropped me a 6mm TV Delos down the chimney. This is a superb eyepiece and quickly replaced my Nagler 7mm as the eyepiece I depend on to get me deep.  At x340 it gives me sharpness, very wide FOV for a 6mm and a flat field (I don't use a Paracorr or similar as I paid extra for better quality optics in the scope). With this eyepiece I've been through mag 17 a few times. I love it to bits.  For medium power I use a 10mm Ethos, which again is an excellent eyepiece.   My skies are orange with pollution, but these two eyepieces give me the best views I could hope for in a fast light bucket.  I have a box of expensive Naglers and other TeleView eyepieces collected over the years, but these two eyepieces get 99% of my sky time.


Gary (PYG)

lmk's picture
Radians are well corrected

I dont want to forget to mention the Televue Radians. They are an excellent quality eyepiece, lightweight and relatively inexpensive. ~$150 used. I use the 3mm for all of my telescope star testing since it gives about the most neutral eyepiece correction which I have found, even down to f/3.3 ! (Many older design like Plossl's tend to show undercorrection, which doesnt exist in the primary, in the faster scopes.)

The only thing I dont particularly like about them is their narrow 60deg apparent field, it feels quite "claustrophobic". But they are excellent for optical testing and planetary use.

Also, other than a bit wider field, I do not see why Al Nagler made the new Delos line which essentially covers the same Radian range and performance, but at twice the price!

Mike LMK

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