30mm 100° Explore Scientific Eyepiece and 3" 99% Dialectric Diagonal Review

By: Vlad Fedosov


Is it worth spending $1600+ to use a single eyepiece? That is the $1600+ question! I resisted the temptation to pick up the hand grenade of hand grenades: the Explore Scientific(ES) 30mm 100° field of view(FOV) eyepiece for quite some time. For those not in the known this is a 3”(yes not 2″ but 3”) format eyepiece that weighs in at 5.18lb and requires a 3” telescope focuser and diagonal. Part of what made me hold out on getting this monster was that I did not have a scope with a 3” focuser. When I picked up a TEC 140 APO that has a Feather Touch 3.5” focuser I could not resist the king of eyepieces any longer and had to pick it up! Let’s dive right in and see what this eyepiece is all about and if you should consider the extremely high price of entry…

I’ll start off by going over the financial part of the equation of acquiring one of these. The first thing you need to have is a scope that has a focuser large enough to support the 3” eyepiece form factor. This is quite rare, and from what I know only high end larger ES refractors, and a few ultra-premium brands have 3” focusers from the factory. Besides that, you can also either use a 3.5” Feather Touch focuser, or the very rare Moonlite 3.6” focuser(witch I’m working on mating to my Meade 178ED).


Assuming you have the telescope focuser part figured out you will then need to purchase the following: the eyepiece MSRP is $999(sometimes on sale for $799). The 3” ES 99% Dielectric diagonal MSRP is: $489.99(sometimes on sale for $399). In my case, I also needed to buy a 3” visual back for my Feather Touch 3.5” focuser that MSRP’s for $189. This all adds up to a pretty penny even if you manage to snag it all on sale!

 Once I acquired all the parts, I started to put everything together. The 3” visual back was the first thing that I installed on the 3.5” Feather Touch focuser that on my TEC 140. The procedure is very simple, you loosen the set screws that hold the 2” visual back and unscrew the visual back from the focuser drawtube. Installation of the 3” unit is the reverse of that. Quite simple! As with all Feather Touch products that I have owned it all just works very well and is of excellent, excellent quality!

Next up was unpacking and trying out the 3” ES diagonal. It comes in one of the nice flip top ES boxes that their premium eyepieces are packaged in. Upon taking it out of the box I noticed an immediate issue! The entire eyepiece caller was totally loose on my unit right out of the box. Not very encouraging for a premium product to come out of the box being brand new and already having an issue. I contacted ES and they just recommended tightening all 6 of the set screws. They said that the screws probably got loose during shipping(that’s a fair assessment but what I would say to that is that I don’t see their telescopes or other accessories arriving with a bunch of loose screws…). Simple enough, I proceeded to do that. Well not so simple! One of the threads was 100% stripped. The screw would neither tighten or back out… Not super impressed at this point. I contacted ES again and they told me to send in the diagonal for replacement. They did provide me with a prepaid return label. Cool stuff, not a big deal. I sent the diagonal in and got a confirmation email that they did receive it in a few days. This was around winter time with cloudy and rainy skies so I kind of forgot about the diagonal expecting to receive the replacement any day. About 3 weeks later I realized that its probably been kind of too long and decided to call ES and get a status update. The rep went and found my diagonal in the back and nothing had been done with it… Yikes… In any case, after a bit of a chat, he agreed to expedite the process and send me a replacement.

Having received the replacement I was happy to see that this one did not seem to have the set screws loose! I decided to not touch them at all and see if they would stay put. Well as you can see in the picture below one of them did back out on its own the first night I used the diagonal! I did tighten them all down and so far so good but I have a feeling that I’m going to need to back them all out and put a drop of simi-permanent Lock-Tite in them. This is overall not that great of a design in my opinion. This eyepiece is about twice the weight of the largest 2” units and really, really needs a solid diagonal with the ES unit does not honestly feel up to the task. I hope that long term that I’m wrong and that it works great. I will post an update here in a year or two. The good news is that optically I did not see any issues with either of the diagonals that I had. Both are on par with the other 2″ 99% Dielectric diagonals that I have used.

On to the actual eyepiece!!! So I will say that I bought the eyepiece used but in excellent shape. I have read some reports of people having issued with weird-shaped stars with this one in the outer FOV so I was a bit scared… Much to my delight, this thing produces a wonderful field from edge to edge(ok just like any other hyper-wide FOV eyepiece the last 20% of the field does have a bit of comma but that’s expected)! I remember feeling like a little kid on Christmas the first time I had a look through this thing with the TEC 140 form my backyard.

The bulk of my testing of the eyepiece was from a very dark sky site in central Oregon using the TEC 140 APO over two moonless nights. The eyepiece produces 32.7x, with a 3.06° real FOV and a 4.29mm exit pupil with this setup. That is a very, very good overall setup to do some moderately wide field work from a truly dark sky! So what’s the experience like? Well to sum it up, pretty much any object that I looked at was one of the best and most memorable views that I have ever had of it!!! Yes, this even includes comparing it to my former 18” dob!

Now to kind of slow down and explain my above statement. The view that you get with the 30mm 100° is not more detailed than the higher power view of an 18” dob. But you do get such a new and refreshing perspective that it almost seems better. This is kind of hard to describe in words, just like most of the best things in life. The object being viewed is framed to where you can get a great perspective of what’s around it. The contrast with this setup is through the roof(especially when at a truly dark sky). The FOV is truly breathtaking with this eyepiece. I do own a few of the other ES 100° eyepieces(see review) and somehow this one seems even larger in the immersion effect of the FOV. Even though it was only produced 32x I saw detail in objects that I would not figure possible! Maybe the skies made it so special, maybe the TEC 140 made it special, but the view was out of this world!

A few notes that I made during the night:

Rosette Nebula(Caldwell 49):

This thing just pups into the view with the 30mm. It is of course not nearly as bright as you see on astrophotos but quite impressive on how well it shows up. The general structure of the nebula is quite evident as a dim glow. The open cluster inside is marvelous to look at!

Double Cluster(NGC 869 and NGC 884)

One of my favorite objects. This was perfectly framed with this eyepiece! The contrast between the stars is quite nice! Every star was a nice little pinpoint in the two clusters with many, many surrounding dimmer stars!

M81 and M82

This pair was exceptional! Even though they are quite small at this magnification I was surprised that both the galaxies revealed structural detail. The view was really special since you really got a good sense of these just floating in space!

M97 and M108

This was probably the most special view! I was surprised that the eyes in M97 actually popped into view at such a low power! M108 showed some hints of the spiral structure as well. The overall framing of this pair was just out of this world. It was kind of like taking the really detailed view that you get with a large dob, shrinking it down a bit in scale and then expanding your total FOV to where it’s almost like you are in space floating and looking at these objects! Very memorable!!!

So let’s move on to the few negatives that I saw about the eyepiece. The biggest one really is the weight. This thing is soooooo heavy that you are really are stuck with using only it as your eyepiece for the night. I mean you can put a different eyepiece in the diagonal witch does include a 2” stepdown but your balance is so far off that it’s really not safe to do so. It’s not practical to rebalance the scope every time you want to switch eyepieces(at least to me). The other thing is that it is quite difficult is to change the orientation of the eyepiece to make it more comfortable to view when pointing to different parts of the sky. This is true with both just trying to rotate the diagonal or the focuser. Both need to be snugged down so tight that this thing holds in place that when you try to loosen it and keep it from rotating down you end up moving the scope and probably blowing your GOTO alignment.

The other thing that can be a downside if you like eye guards and taking in the entire FOV is that it is nearly impossible to get close enough to the eyepiece to see the entire FOV without removing the rubber eye guard completely. Yes, I’m talking about removing the eyecup entirely and not just folding it down. It makes quite a big difference in how close you can get to the eyepiece. If you are someone the enjoys using the eye guard and kind of moving your head around to see the FOV this is really no different from the other ES 100° eyepieces. The last thing that I will mention that I feel is a bit of a potential downside is that the lens on the bottom of the eyepiece is fairly close to the bottom of the barrel. I can see how an unfortunate move in the middle of the night could damage it. I wish it was more recessed.


In conclusion, would I recommend this eyepiece? In all honesty, if you are seriously considering this monster of a piece of glass you are likely someone that has the discretionary funds for it and already has a scope that will support the 3” format. To me, this thing is like the ultimate luxury visual observing accessory. I would say just do it! The view that the eyepiece provides is truly spectacular! The reaction that people have when they see this eyepiece in real life is worth the price of admission alone😉


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