February 27, 2015 – The 10mm Luminos Eyepiece and EOFB

Right at the same time as I got the Ultrablock, I also received my new Celestron Luminos 10mm eyepiece.  I had ordered it in early February, using up the last of my Amazon gift card.  For some reason, there was a two-week delay in getting it out to me, which is pretty unusual for Amazon.  But last week, it came in.  And fortunately, the day it came in, it was relatively clear out.

Now the Luminos is a big, heavy eyepiece.  It is an 82-degree apparent field of view (AFOV) design, known as an “ultra-wide”, as opposed to the 52-degree AFOV common in Plossls.  It uses a large number of lens elements in order to take the incoming light and provide that extremely wide field of view – 8 elements, versus 4 in the Plossl, so it weighs three-quarters of a pound.  Holy hand grenade!

The Luminos line of EPs had gotten particularly bad reviews on Cloudy Nights because it was supposed to suffer from extremely bad “edge-of-field brightness” (EOFB).  This means that as you look through the EP, there is a ring around the very outside of the FOV that is significantly brighter than the rest of the field.

However, there was a definite split in the scuttlebutt on this EP – the worst complaints came from people who had bought the longer focal length EPs – the 19, 23, and 31mm EPs, which were all 2-inch EPs anyway, which I can’t use.  Meanwhile, those who had bought the shorter focal lengths, the 15, 10, and 7mm EPs, were pooh-poohing this whole EOFB problem.

Me, I was curious about the EP because of the 82-degree AFOV; I wanted to see what that was like.  And I also wanted to get an EP that would still give a relatively wide FOV in my ST-80 at a “higher” power; when I use my own shorter focal length EPs in the ST-80, and I get the power up to 40-50x, I’m getting the same one degree-ish TFOV as I get in the Mak with the 32mm Plossl.  I wanted something more.  And because for whatever reason, Amazon had dropped the regular price on these by over a third, I thought, let’s give it a try.  (The scuttlebutt on the “whatever reason” is that the EP was living up to its unfortunate “Luminos” name with the EOFB, and is about to be discontinued and replaced by Celestron, so that they were trying to move their back stock out.)

In making my decision as to which focal length to get, the 15mm Luminos was duplicative of my15mm Paradigm, so I didn’t want that.  The 7mm was just a bit too high of a magnification for the Mak at 220x (above the magical 200x barrier), so that if I got it, I would only get to use it on nights of very good seeing, which have been rare this winter.  But what really sold me was that by my calculations, the 10mm would be just about the highest magnification I could achieve and STILL be able to see the entire moon at one go.  I thought that was as good a reason as any, so, why not?  Not only that, but the 10mm would still give me just over 2 degrees TFOV in the ST-80 at 40x.  Nice.  Of course, it would duplicate my 10mm Orion Sirius plossl, but eh, I could live with that.

It was between the 10mm Luminos and another EP, an 11mm Olivon 80-degree AFOV EP, which would also have let me see the entire moon at once at high power, and which also would have given me over 2 degrees TFOV in the ST-80.  The Olivon had been selected by Astronomy magazine as a Star Product in 2013, so I thought that was a pretty good recommendation for it.  However, someone on Cloudy Nights with far more knowledge than I have had told me that the Luminos was a better-corrected EP, meaning that it would work much better in my relatively fast focal ratio f/5 ST-80.  Because of Amazon’s Luminos sale, I got mine for just 62 bucks, which I think is practically a steal for what is otherwise supposed to be a top-quality EP.

Was it, though?  Maybe it actually WAS a piece of junk.  Maybe the EOFB was terrible.  Fortunately, with Amazon’s generous return policy, I could take a chance and find out for myself.  It arrived in a nice padded box, and it is a beautiful-looking eyepiece, that’s for sure.  And it has an internal mechanism where you turn the rubber grip to raise the eyeshield that was pretty neat, too.  I would have prefered to save a couple bucks and have just a standard roll-up eyeshield instead, but I had to admit, it was one cool piece.

I was able to get a clear night immediately – screw you, astronomer’s curse! – and went straight for three of my very favorite targets:  the moon, Jupiter, and the Great Orion Nebula.  Unfortunately, the crescent moon wasn’t quite fitting into the FOV as I had hoped it would – it was about 90% of the moon, but not the whole thing.  But the field of view was enormous, and yes, it was immersive – sort of like the difference between looking out a window and looking through a straw.  I’m exaggerating with comparison, of course, but I really had to look around the FOV to take everything in.  I feel more “there” with this EP, rather than just looking through a telescope at something.  And it was sharp right to the very edge, to boot.  Pretty impressive.

As for why it wasn’t showing me the whole moon, I had read that the 1540mm focal length given by Orion for their version of my Synta-made Maksutov was more correct than Celestron’s 1500mm specification.  But I had also read that those focal length specs DON’T take into account the use of diagonals, which would somehow work to make the focal length longer, and therefore, the magnification greater and the TFOV smaller.  Don’t ask me how the use of a diagonal effects the focal length, but I am assured by the wise men at Cloudy Nights that it does.  I find this weird, because the Maks are provided with diagonals, so you would think that they would already be taken into consideration when the focal length specs are calculated.  Either that, or maybe it wasn’t actually a full 82 degrees.

Next up, Jupiter.  The light haze of clouds meant that the seeing was pretty stable.  The 4 moons were spaced widely that night, but the Luminos was able to get all 4 easily in the same FOV at 154x.  Not that that matters, much, however on Jupiter, because when I’m looking at Jupiter at high power, I’m just looking at Jupiter – it doesn’t need to be properly framed in the sky or anything.  It was the detail on the cloud tops that I was interested in checking out.  But eh, the view through the Luminos wasn’t any sharper or any better than my lowly 10mm Orion Sirius Plossl.  Sure it was sharp, it was fine, but no better.

I pretty much knew that that would be the case even before I bought it.  I’ve come to learn that premium EP or no, almost every EP today will show you a very clear, detailed view, whether that EP costs 30 bucks or 100.  My 8mm TeleVue Plossl, which is supposed to be amongst the pinnacle of supreme optics, wonderful green lettering and all, is just the same as all my other EPs, no better, no worse.  And actually, I think my very lowly 9mm Celestron Kellner EP, the one that came stock with the scope, is a smidge better than all of these high-falutin’ EPs.

Finally, I turned to the Great Orion Nebula, that showcase object of the winter sky, to finally test out this whole EOFB business.

And it was there.  Not much, not in any way distracting, but there it was.

Now, some of the theorizing over at Cloudy Nights was that it was caused not by any light that was coming from the object, through the telescope, and into the EP, but from stray ambient light sources entering from the top of the EP.  So I was observing with my eyepatch over my non-observing eye, so I could keep it open, and with a full hood over my head so that the only thing I could see was the eyepiece, and the Great Orion Nebula through it.  I could see plenty of detail.  The nebula looked terrific in the Luminos at 154x, surrounded by a lot of sky.  There was no stray light entering my eye at all.  I was observing like this for a full 15 minutes straight to let my eyes dark-adapt.

And there was a very slight ring of brightness when I was looking right at the nebula.  BUT – as soon as I looked directly at that ring, the ring just disappeared.  Gone.  Poof!  It was strange.  I could make it come and go at will, just by looking at it and looking away.  HOWEVER, it wasn’t distracting at all.  Certainly nowhere near enough to want to return the EP.  It took up, at most, less than 10% of the entire FOV – maybe more like 5%.  I only really noticed it at all because of all the bad press these Luminos EPs have gotten.  But it was there.

I can’t even begin to guess why this was happening.  But, what the hell, I’ll guess anyway – I would think that it’s the interaction of the eyepiece coating with the cornea, that somehow light from the EP is bouncing off my eye and back onto the eye lens, which has not been properly coated, and bouncing off of the eye lens back into my eye somehow.

Of course, I’m totally making that up, and have absolutely zero basis for saying that because I don’t have any idea of what I’m talking about.  This explanation doesn’t even make sense, because the exit pupil of the 10mm on my scope is only 0.8mm, so that there couldn’t even be any light that was not directly entering my eye pupil to be able to bounce off of my eye and back onto the lens, and then back into my eye again.

But it was there, I tell ya.

So, all-in-all, I do like the Luminos.  But what I like most about was the cheap price I bought it at to get those ultra-wide field views it gives me.  I don’t know if it would be quite worth it to pay over $100 for this kind of a view at other focal lengths, but I always love a bargain.

Next up – Astrophotography from Manhattan?  Surely you jest!  And stop calling me Shirley.


2 thoughts on “February 27, 2015 – The 10mm Luminos Eyepiece and EOFB

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