Thursday, September 25, 2014

The SR Eyepiece Reconsidered

The SR eyepieces for this comparison.
Left to right, the Meade tall .965" 4mm "SR", the Meade
.965" short 4mm SR, and the Celestron 1 1/4":4mm (5mm) SR.
Recently, I came across a rather harsh thorough review of the Celestron FirstScope 76mm table top Newtonian. While I did not necessarily agree with all of the author's findings, one item stood out; he rated the 4mm SR included in the telescope rather highly. 
SR eyepieces have not been generally appreciated in my experience. They have very poor eye relief, the images are frequently dark and murky, the view is narrow. Most of my SR eyepieces have ended up either in the spares box or the parts bin. To be honest, based upon my experience with smaller SR eyepieces, I simply did a cursory test of the Celestron 4mm model, and it too ended up in spares.
Perhaps I should have tried it again. Most of my astronomy has been solar, deep sky (clusters primarily), and lunar, with a smattering of planetary work. I tend to use optically friendlier eyepieces, in my case a number of Kellners and a Plossl or two. Higher magnifications have seldom been used. The SR eyepieces have been there if I needed them. I seldom have.
Let us look for a moment at what an SR eyepiece technically is. I've come across several definitions for "SR"; "symmetrical Ramsden", "super Ramsden", "achromatic Ramsden", and simply "symmetrical". The classic Ramsden design consists of two plano convex lenses. For the unfamiliar, plano convex are flat on one side, convex on the other. In the Ramsden design, the flat sides face "out", with the field lens (forward) larger in diameter and with a longer focal length than the eye lens (rear), with a space between them. There is a specific formula for this, but for now we'll simply deal with the design's geometry. 
As it turns out, while most of those definitions are probably incorrect; they are certainly "symmetrical" in some sense, but have very little else in common with the "Ramsden" design aside from having two elements. 
To that end, I decided to take apart each of my SR eyepieces to study their construction. By the way, this is not something I recommend doing. It is very easy to to get dust on the lenses, and when dealing with such small focal lengths the result is dreadful.
The 1 1/4" Celestron 4mm SR, which actually yields a focal length closer to 5mm (as initially confirmed in the linked article), appears to have two symmetrical convex lenses of similar diameter.

The field lens looks identical to the eye lens, but may have a slightly longer focal length. Beyond that, this eyepiece has more in common with what was once called the continental variant of the even older Huygens eyepiece Both lenses are double convex. The only exception is how similar the lenses are to one another. 
How then does the 1 1/4" Celestron 4mm (5mm) SR compare in design to a smaller .965" version?
Currently I have two that are still intact and in use, and both were imported by Meade at various times. Externally, they look different, with one being longer.

I decided to open the shorter one up first. Again, I was greeted with two lenses, but much smaller in diameter than those found in the newer Celestron 1 1/4" design. They can only be described as tiny, around 3.5mm in diameter a piece. Like the Celestron lenses, both were double convex, and again the field lens apparently had a slightly longer focal length. But again, they were extremely small. This will explain much, as you will read later.

How about the taller .965" Meade 4mm SR? One thing that stands out immediately is the diameter of the field lens opening. It is close to twice the diameter of the one found on the smaller SR. In construction, however, there is a big difference. When opened, there is another lens.

Ahead of the field lens is a double concave. In effect, this forward lens serves as a built in Barlow, increasing the magnification. Otherwise, the construction of this SR is almost the same, though using larger lenses than the ones found in the shorter .965" SR. The spacing between the field and eye lense, however, is rather close, being more akin to simplified symmetrical eyepiece.
Optically, how do they compare?
For the test, I used my 60mm f/7.5 refractor "Bianca" and chose the top of a nearby pine as a target. 
As expected, the Celestron model provided a nice, bright image that was fairly crisp, though typical with the design the eye relief was a little poor. The shorter Meade SR, with its small lenses, had a narrower view and poorer eye relief. The view was adequate, with a slight discoloration at the edges. For stellar work, this is fine, provided the target is centered.
I was not quite sure how the taller faux SR would compare, as I had never really used it. I expected it to be somewhat darker, as smaller, simpler Barlow lenses tend to produce that. Not surprisingly, it was indeed darker and narrower than the Celestron version, though the eye relief was better. It makes one wonder how would this eyepiece behave without the Barlow? Would it create an 8mm eyepiece of some sort?
This little test proved that sometimes judging an eyepiece by its designation is tricky. SR eyepieces, regardless of what SR really means, are a mixed bag. The newer, cheaper, smaller .965" are iffy at best, while the larger 1 1/4" Celestron model is a fairly decent performer. The only real test is with the individual. The older .965" standard is fading, and chances are likely that the only time you'll encounter them is on older equipment, or very inexpensive equipment. It is probably best to err on the side of caution and go with the better performer. In this case, that would be the Celestron. Just remember, it is closer to 5mm than 4mm.

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