December 15, 2016: The 24mm Explore Scientific 68-Degree Eyepiece

I ordered a 24mm 68 from High Point Scientific on April 30 – the last day of the $99 NEAF sale, a very nice one-third savings off of the normal $149 price.  I had a $100 Visa gift card that was just sitting around, burning a hole in my pocket, and I’ve heard very good things about these eyepieces.  The 24/68 is supposed to be able to give me a true field of view a smidge wider than my 32mm Plossl, and with the giftcard, the EP would be free, so why not?  My first premium eyepiece, as opposed to my, ahem, 10mm Celestron Luminos.

Back in April, High Point Scientific listed it as being out of stock when I ordered, but they honored the sale price, and promised to deliver once they received a shipment.  At first they told me that they expected to receive the shipment in June/July; then that slipped to August; and then December.  7 months?  Really?  What’s the delay?  Why aren’t there any of these EPs available?  To answer these questions, someone suggested I contact Scott Powers, the President of ES.  Well, there he was, right on Facebook, so I instant messaged back and forth with him.

He told me that because the astronomy industry is so small when compared to the giant camera industry (Canon, Nikon, GoPro, etc.), the lead times to get their stuff manufactured is very long.  Everyone places their orders and gets in line.  Astronomical glass is always on the very bottom of the totem pole as compared to manufacturing for cameras.  They might only do a production run of only about 1000 eyepieces a year, and that comes after all the other glass is made for all of the big consumer camera companies.

He then said that even once the manufacturer gets around to making the astronomy company’s order, then grinding, polishing, and coating of the optics has to occur, while the eyepiece barrels are simultaneously being machined, anodized, painted, and laser etched. After that, the eyepieces go through “the tedious process of hand assembly, gas purging, and many QA inspections before they are packaged and shipped to the ES facilities in Arkansas.” Then upon their arrival at ES, they again scrutinize the eyepieces for performance testing. This whole process itself takes about 60 days, start to finish.  He concluded by saying that “there is no mass production of astronomy products for the serious hobbyist.”

Well, after 7 months of waiting, the mailman finally delivered:


And of course, the astronomer’s curse kicked in.  It was cloudy.

While the 24/68 is a definitely a large EP, a Holy Hand Grenade, it isn’t quite as large as my 10mm Celestron Luminos.  But it’s close.  However, I suspect most of that bulk on the Luminos is related to the mechanism that raises and lowers the eye guard, as opposed to being actual glass.

After being clouded out for a good week or so, I finally got it in the scope and out under the sky for a look at the moon.  I was eager to see how well it would work, not only in my Mak, but in the ST-80 as well.  It will give me a wider field of view than a 32mm Plossl (4.08 degrees vs. 3.92), and it’ll give me that wider field at a nicely higher magnification to boot (17x vs. 12.5x).  And it’ll give me a significantly wider field of view than the 25mm Orion Sirius Plossl  (3.25 degrees) that lives in the ST-80 now.  Even in the Mak, my calculations show that it’ll be 1.06 degrees and 64x with the ES 24 vs. 1.02 and 48x with the 32mm Plossl. So in both scopes, it’ll give me larger fields of view at higher magnification, which means – theoretically, at least – that it’ll spread the background light pollution/skyglow out a bit further as well, giving a darker background and increasing contrast.  Me likey.

So far, with the clouds, I’d only gotten to test it out on the moon; first at 4 days, then 10, and finally at 15.  The moon looks nice and big and bright and crisp and detailed.  But not a whole heckuva lot different from the 32mm Plossl – the moon always looks really good at lower magnifications.  The ES showed it a bit bigger than the 32.

Finally, after almost three weeks, I got to test it out on the Pleiades, to see both how nice they looked, and to see how much of a smidgen more of them I can see than I can with the 32mm Plossl.  I started with the ST-80, and the sisters were sure looking good.  The entire group easily fit the field, of course, as they always do at low magnification, and with plenty of dark sky surrounding them to give a really aesthetically pleasing view, at a nice 17x magnification.  I couldn’t really tell the difference in the size of the field of view in the ST-80 between the 32 and 24, but another advantage of the 24 is the smaller exit pupil.

As your eyes age, the maximum amount your eye pupil can dilate becomes less and less with each passing decade.  Kids and teenagers generally have a maximum dilated entrance pupil of about 7mm (really young kids can have up to 8mm).  People in their 20s and 30s generally have about 6mm, while us middle-agers in our 40s and 50s generally can only open up to about 5mm.  Older than that, and you’re talking about 4mm.

It therefore becomes important to have an exit pupil in your eyepiece that matches the size of the entrance pupil in your eye, because a larger exit pupil simply means that light, and therefore, aperture, is wasted.  Those photons are splashing off of your iris instead of passing through your pupil on their way to your retina, with the result that you may as well be using a smaller telescope.  In the ST-80, the ES 24mm gives you a 4.8mm exit pupil vs. 6.4mm for the 32mm.  This is perfect for someone with middle-aged eyes like me – and more importantly, no light is being wasted.

On to the Mak – where, because of its long focal length, a large exit pupil is never a concern.  Like the 32, the 24 can’t show the entire Pleiades cluster in one view – the group is about 1.2 degrees across; more like 1.3 degrees to squeeze them in and have just a little dark sky around them to set them off.  But the 24 did show a noticeably larger amount of sky.  The standard calculation of field of view is done by dividing the apparent field of view of the eyepiece by the magnification.  So, the 32mm = 49/48 = 1.02 degrees, and the ES 24 = 68/64 = 1.06 degrees.  But the 24 showed even more than that.

With the 24, I was able to see from Atlas straight through to Electra and a bit beyond, to a faint 9.1 magnitude star.  Both Atlas and 9.1 were at the very edge of opposite sides of the field of view in the ES 24; and indeed, this was just a bit larger than I could see at the same view (swapping the EPs quickly back and forth) with the 32mm.  With the 32mm, I could get one or the other star in the field, but not both, and definitely by a noticeable amount.

Stellarium has a feature called Angle Measure.  With it, I measured the distance from the center-to-center between Atlas and 9.1 as being 1.09 degrees.  In my spreadsheet, that is more than the maximum true field of view than the supposed 1540mm focal length of my scope would allow.  That number is from the Orion flavor of this same scope, which I figured would be more accurate than the 1500mm that Celestron claims – that for whatever silly reason, Celestron was rounding down.  But when I change the focal length in my spreadsheet to 1500mm – the focal length Celestron actually lists for the scope – and, voila, the numbers come out perfectly.  The spreadsheet calculates that the ES should – and does – provide me with a 1.09 degree true field of view. So, mission accomplished there!

And the view was nice, of course.  Sharp, pinpoint stars.  By the time I turned the scope to the Orion Nebula, a high haze had creeped in.  But even with a 97% moon glaring away right nearby, and even through the haze, I could still see all four stars in the Trapezium directly, glaring away in different colors, like they always do.  Not much to see in the nebula with all of that going on, though.

All-in-all, my “first light” impressions of the ES 24mm are all positive.  This is a heckuva honey of an eyepiece.  Well, with the exception of the weight, that is – it sure is heavy.  But otherwise, it’s sharp, it’s clear, and it’s certainly wide.  Al Nagler was definitely right all those years ago when he designed the original 68-degree Panoptic.  His goal at this particular width was that a 68-degree eyepiece is the widest possible width, showing you the widest field of view, without your having to move your head around to take it all in, unlike other 82-degree widefield eyepieces.  Like Goldilocks, 68 degrees is just right.

Instead of the 24 earning a permanent place living in the ST-80, where I thought it might reside to give me a much wider view than the 25mm Plossl that lives there now, it looks like the 24mm might displace my 32mm Plossl as my finder eyepiece and reside permanently in the Mak.  We’ll see!  But either way, I’m going to be using this eyepiece a lot.  


2 thoughts on “December 15, 2016: The 24mm Explore Scientific 68-Degree Eyepiece

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