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Home > Articles > Other Articles > Equipment/Optics > Comments on low cost, long focal length 2” widefield eyepieces

Comments on low cost, long focal length 2” widefield eyepieces
By David Russell - 8/2/2009

As I teenager in the 1980’s I purchased Vernonscope Brandons of 32mm, 24mm, and 16mm FL and added a 2.4x Dakin Barlow to use with a 10.1” Coulter dobsonian. When I purchased a 4” unitron refractor I added a 2” 48mm Brandon and while never purchasing one I frequently had the chance to use the 2” 32mm 60 degree University Optics Konig.

The telescopes eventually had to go to help pay for college expenses but I kept the Brandons. In the 1990’s I purchased several different scopes and did a lot of observing for which the Brandons – with their high contrast and sharpness – served me extremely well.

For some foolish reason, in the late 1990’s I let the 48mm Brandon go when I sold the refractor I had been using it with. During the last 10 years I have done very limited observing due to the busy life associated with having a young family, financial obligations that prevented me from replacing a scope I was unhappy with, coaching basketball, and the diversion of conducting independent astrophysics research and publishing papers in astrophysics journals.

I give you this background because this spring I decided that it was time to purchase a new scope and get serious about observing again. One of the challenges I have faced is finding a long fl 2” widefield eyepiece that satisfies my expectations. The commentary on the low cost long fl wide field (hereafter LCLFWF) market that follows is not that of an inexperienced observer, but rather a fairly experienced observer that is new to the current eyepiece market – as I’ve only had extensive experience with Brandons and the older recently discontinued 32mm 60 degree U.O. Konig. The comments that follow are intended to be constructive rather than critical.

I picked out a 120mm f/8.3 Celestron XLT 120mm achromat as a scope in my price range and decided to try a LCLFWF eyepiece because 48mm Brandons are waitlisted new and hard to come by used. (Anybody reading this that has a neglected 48mm Brandon they would be willing to part with please contact me!)

The LCLFWF I tried first was the 38mm 70 degree AFOV. When it arrived I dropped it in my star diagonal, trained it on the Milky Way at 3:00 am and the views were very sharp and contrasty --- in the inner 75% of the field diameter. But the outer 25% … am I on the Millenium Falcon as it streaks into hyperspace?

I found the outer 25% of the diameter of the field of the 38mm 70 deg eyepiece to be unusable due to significant aberrations. “Can I live with this?” I began to ask myself. About this time I discovered the existence of Astromart and Cloudynights and I began to read the reviews of the various LCLFWF eyepieces. Based upon the reviews I decided to try a 32mm 72 degree field eyepiece – which is a clone of the 38mm 70 degree eyepiece. The performance in the center of the field was excellent – very sharp and high contrast. But as with the 38mm, the edge performance was the Falcon going into hyperspace – in this case the outer 30% of the diameter.

Now this issue is a matter of personal preference. How much field distortion can you stand before it becomes unacceptable to you? For me the answer is the outer 10% to maybe 15% of the field diameter can suffer from some noticeable aberration. Now some might look at the situation and say “Wait, if you have a 72 degree field with a long FL eyepiece, you still have a lot of usable field if only the inner 70% is sharp.”

But the problem is that the “outer 30%” in this case is the estimated diameter at which aberrations become disruptive to observing. The actual FOV is an area, not a diameter. A 32mm 72 degree EP has a 2.30 degree field in my scope or a radius of 1.15 degrees. Since the area of a circle is Pi x r^2, the actually number of deg^2 in the FOV is 4.15. The inner 70% of the radius is sufficiently sharp or 0.805 deg radius. This gives a sharp area of 2.03 deg^2. In other words, 51% of the sky area covered in that 32mm 72 degree EP is unusable due to distortions! This was simply not acceptable to me.

When I tried a 42mm 65 degree eyepiece and found it no better, read more reviews detailing similarly poor edge performance in all the other LCLFWF EP on the market, I decided the current LCLFWF market did not offer anything I could use for my wide field viewing pleasure.

Before I move to what I think the resolution to this problem can be, let me say that the problem I’m describing is a problem a lot of people encounter, but is not a problem for everybody. Some people don’t mind the poor edge performance because they only use these LCLFWF eyepieces as finders and do not actually spend significant time observing with that eyepiece. Others find that the % of the field that is severely distorted is reduced with a longer FL scope.

I am a person that spent a lot of time observing with my 48mm Brandon. In addition to the high contrast Brandons are known for, the 48mm Brandon was razor sharp to the edge. In comparison to that standard, I do not find a “wide field” eyepiece an enjoyable observing experience when 40-50% of the field area suffers from significant distracting aberrations.

The cause of this problem is that manufacturers are trying to satisfy the huge demand for wide FOV eyepieces at a low cost that people can afford. But the reality is that a well corrected, high contrast, sharp to the edge 70 to 100 degree AFOV eyepiece must be designed – and a high price tag is going to come with that.

However, any eyepiece can be a “wide field”. All the manufacturer has to do is remove the field stop. The 48mm Brandon has a large field stop that cuts the field to 50-52 degrees. If Vernonscope removed the field stop they could have a 65-70 degree “wide field” eyepiece, but it wouldn’t be razor sharp at the edge like it is with the field stop because the optical design is not a wide field design.
But this is what is going on with the LCLFWF market today – eyepieces that don’t have the optical design needed to perform well at a 70 degree field are nonetheless being sold as “widefield” eyepieces. The 70 degree fields are achieved simply by not inserting a large field stop. And this problem is not all the manufacturers fault because this is what the consumers are demanding. Unfortunately, a LCLFWF 70 degree eyepiece is a promise that cannot be delivered.

For those such as myself that are looking for a nice long focal length wide field EP but find it unsatisfactory to observe with an eyepiece for which 25-30% of the radius of the field is distorted by severe aberration, there are no options on the market – but there used to be.
Enter my old friend the discontinued 2” 32mm 60 degree field University Optics Konig (also sold by Orion Telescopes for a time as the “Optiluxe”). As I came to the realization that the LCLFWF eyepieces available were not for me, one of the last U.O. 32mm Konigs made appeared on the used market and so I snapped it up.

If you look at the barrel of one of these Konigs you will see that it has a 7mm field stop. This cuts the AFOV to a nice 60 degrees which is not a “widefield” but is still generous. It had been a long time since I had looked through my old friend so with anticipation I put my “new “ used 32mm Konig into my star diagonal, trained it on the Milky Way, peered in and .. . YES … Now we’re talking! The Millenium Falcon is back in real space. The 32mm Konig is not sharp to the very edge, but it is sharp until about the last 10% of the radius and the aberrations in the outer 10% are not as severe as the modern LCLFWF options.

But what is also interesting is that the 32mm 60 degree Konig actually has more usable field than the 32mm 72 degree eyepiece has. With my scope the 32mm Konig gives a 1.92 degree field which results in a total area of 2.89 deg^2 – recall that the 32mm 72 degree eyepiece had 4.15 deg^2. But with only 10% of the 32mm Konig’s field suffering from aberration, the usable field is 2.32 deg^2 compared with only 2.03 deg^2 for the 32mm 72 degree eyepiece. In % terms 19.7% of the field area is lost to aberration as compared with 40-50% for the LCLFWF with 70+ degrees AFOV. And the aberrations that are seen with the 32mm Konig are not as severe.

Sharp to the edge wide field views are possible, but it is an optical engineering feat that comes with a much higher price tag. I recently purchased a 40mm Pentax XW which has a 70 degree field. This eyepiece is sharp to the edge – but that is what it was designed to do. It was not made into a 70 degree field eyepiece by taking a design not meant to be 70 degrees and removing the field stop.

I write all this to come to two main points – one for the consumer and one for the manufacturers.

Point #1 is to the consumers. If you are looking to get a low cost, 2” long FL widefield eyepiece you should understand that it will not be sharp to the edge. You are not getting 70 degrees of sharp performance. Perhaps as much as 50% of the field area will be unusable due to aberrations. You should know this going into the purchase. Evaluate whether that will be acceptable to you based upon your purpose for the eyepiece. If your only purpose is to use the EP as a quick finder, then these should be fine, but if you want to spend time observing and taking in a wide field, then you may be disappointed depending upon your scope and your tolerance for aberrations. Be prepared for this possibility if you decide to purchase one of these eyepieces.

If the current LCLFWF will not satisfy your needs, then your options might be to (1) look for a used market 2”32mm 60 degree UO Konig (or Orion optiluxe) or (2) save for mid-cost eyepieces offered by Baader or Seibert that are reviewed to be good to the outer 10% of the field (but by all means check one out before you buy if possible) or (3) save for a high cost premium wide field EP that will be sharp to the edge or (4) hope that someone in the EP manufacturing field takes seriously my suggestion below to manufacturers.

Point#2 is for the manufacturers. The LCLFWF eyepieces you offer provide very nice, high constrast, sharp images in the center of the field. Some of your consumers do not mind the outer field aberrations, but many of them do. For those that do mind the aberrations a very plausible low cost option could be offered.

My suggestion to you is this – manufacture a field stop that has standard 2” threads so that it can be threaded into the barrel of the LCLFWF eyepieces to cut the field down to 60 degrees. The availability of such a field stop would be a low cost way for observers who were not satisfied with the performance of these EP at 70 degrees to get a better overall viewing experience. They could simply purchase the field stop and thread it into the barrel of their LCLFWF eyepiece.

I will say that in my opinion, the 38mm and 32mm wide fields I purchased would be great eyepieces if field stopped to 60 degrees. They would be a sharp, high contrast option that would satisfy my needs as an observer. But I and - from reading comments on discussion forums – many others find the overreaching 70 degree fields of these eyepieces unsatisfactory.

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