Astrobuffet 1rpd ST80 30mm

Most people believe that eyepieces that provide wide views together with pinpoint star images (in the whole field) and good light throughput are only the expensive products of the most famous brands. Well, my opinion is a little different.

The optical performance of many cheaper eyepieces depend on the focal ratio and the focal plane curvature of the telescope we use them in. In simpler words: there are some cheap eyepieces that provide "Nagler-like" performance if we use them in the right telescope, namely in large and slow refractors.

A very good example of these eyepieces is the 1rpd ST80. It is a 30mm focal length EP with (naturally) 2" barrel size and a really Nagler-like 80 degree apparent field of view. It is marketed by www.astrobuffet.com and this eyepiece costs only 95 dollars new, but you can occasionally buy it for 60-80 dollars on the used market. Practically the same EP is sold under the "BW Optik Ultrawide" name, I have one of both and I can not find any differences between them (except the labels).


On the dining room table

When I first took a similar EP in my hands, it was a memorable experience! This thing is VERY BIG!!! Its eye lens is bigger than the objective lenses of my 8*30 binos. Somewhere I did read, that a real "refractor-phile" can be recognized by the fact, that the lenses on the opposite ends of his/her telescope have an aperture ratio which is less than two. Well, with a 60mm APO, this eyepiece could easily fulfill this condition! :-)

The eyepiece is well built and it is labelled to be made in China. Probably the low price is in tight connection with the place of manufacturing, but in this case the build quality is excellent. The lenses have nice green multicoatings, but the coatings are not exactly as dark as on my 13mm Nagler Type 6. Even if the coatings are not in the TeleVue class, they are really good. The eyepiece contains only 5 lenses, and every air-to-glass surface has multi coatings: you can not see any "white reflections" in the optics. Compared to a Nagler, the bit less effective coatings paired with the less number of glass elements promise rather similar light throughput, and a "white paper test" proves, that there is only negligible (if any) difference in this respect.

The eyepiece also features standard filter threads, but the safety undercut (to prevent the EP from falling out of the telescope) is missing. So, taking into account the mass of this piece, you have to make sure to safely tighten the set screw every time you use it!

Under the sky

The first time when I tried this EP was a real disappointment, just as I expected. The view through my 80/500 achromatic refractor was really unsatisfying. The image was sharp only in the center area of the FOV, the stars in the outer 50% of the field were badly distorted. This was exactly what I expected, because a Fraunhofer achromat has a rather severe curvature of its field: the radius of curvature is about 37% of the focal length, which means, that in the case of this 500mm focal length achromat, the field curvature radius is about 185mm. Furthermore, this EP has a field stop which is larger than 40mm in diameter, in this large area, the strong curvature of the field means that the stars in the edge of the field will be badly defocused - just as I saw it.

But I did not want to regularly use this EP in the small refractor. With only 500mm focal length, I can get satisfying wide views in the small scope with standard 1.25" eyepieces. My real goal with the 1rpd was to use it in my 100/1200 achromatic refractor, and also in my 150/1800 folded achromat. These scopes have F/12 focal ratios, which means that their light cones are much "slimmer", and this way, the same amount of defocusing means that the star's image is much less distorted. Furthermore the longer focal lengths mean longer field curvature radiuses, respectively 450mm and 670mm, which looks much better compared to the small refractor. So, theoretically the peformance in these larger scopes should be much better.

When I tested the EP in the 100/1200 refractor, I was shocked. The EP provided a performance that I can call only "excellent". And in my 150/1800 refractor, the performance is even still better, it is VERY close to that of my Nagler Type 6, but there are less geometrical distortions. The field curvature becomes totally invisible, and the stars are sharp almost across the whole field, I could say they are pinpoints in about 95% of the FOV, only at the very edge are they a very little softer.

This EP (just like Naglers) is rather sensitive to the position of the observer's eye, but it reacts to the wrong eye position in a totally different manner: it does not produce the "kidney bean" effect of Naglers, instead, if you place your eye too near to the eye lens, the edge of the FOV becomes somehow softer. So, in practice, when I notice that the edge is soft, I move my head a few millimeters away from the EP, and this "cleans up" the image nicely. Whether or not you prefer this behaviour or the "kidney beaning" of Naglers depends on your taste, I can definitely live together with both.

After the first shock of enthusiasm, I started observing with the 1rpd, and what I saw was really nice.

Maybe the most beautiful object I have seen with this EP was M31 with the 150/1800 folded refractor. The EP provided 60x magnification in this telescope, and this looked just right taking into account the level of light pollution of my observing place, where I can usually see mag 5.5 stars with the naked eye. I have seen this galaxy thousands of times in the past 25 years, but this view exceeded all the previous ones: the huge galaxy filled the 1.33 degree field together with its two, totally different companions. These 3 objects together in the same field were nothing less than spectacular! M57 was also nice, but there the extra field added by the wide field EP did not mean so much, as the object itself was much smaller. On the other hand M33, which is really big, benefited a lot from the extra field. M13 was also very nice, but here I prefer higher mags to resolve the cluster into individual stars. The double cluster was again breathtaking. Here, the little pinpoints that filled the field looked like many little diamonds on black velvet from edge to edge. This EP really excels on deep sky objects, no question about it!

I did not test it on planets very thoroughly, as a 30mm EP is not designed for planetary observing, though I took a look at Jupiter with it. The planet was nice, but at only 60x magnification, only the two main belts and the four moons were visible.

All in all, the EP provides excellent, really Nagler-like views, but unlike Naglers, this EP is really sensitive to the scope we use it in. In large and/or slow refractors it really excels in every respect. In these scopes (with little field curvature and "slim" light cone) the EP rivals the performance of the 5x-10x more expensive high-end eyepieces. But in smaller and/or faster scopes, the performance can not be compared to real Naglers. E.g. in my newly aquired 100/800 TMB triplet APO, the curvature of the field is much stronger, so the edge of the field is a little distorted, I could say the center 75% of the field is practically perfect, which is still not bad, especially when taking the price into account.

A friend of mine has tested a similar EP in an AstroPhysics Starfire 206 EDT in a public observatory, and he said, that the eyepiece provided definitely better views in the big AP compared to the 100/800 TMB, but that scope also had longer focal length (and roughly similar focal ratio). Though I do not think that for someone who can afford an AP 206 it could be a problem to buy a 31mm Nagler, but if this would ever happen, he/she could still buy an 1rpd for only 95 dollars. :-)

On the other hand, the EP was also tested in a 10" F/5 Dobson, and the performance was really poor, only the center half of the field was enjoyable. This was not surely the problem of the EP, the coma of the Newtonian might be the real source of the problem, but we can still state, that the EP is not suitable to be used in fast scopes.

Many people claim on Internet, that large refractors are not only good for planets, but these are also excellent deep sky instruments, and I believe they are absolutely right. But these refractors also have another important advantage: these large and slow scopes perform excellently with much cheaper eyepieces compared to fast reflectors where you MUST buy a real Nagler (or a similar expensive EP) to get a similarly nice wide view.

So, I believe it is very important to select EPs you purchase by taking into account the "personal" properties of your telescope(s). If you choose the right EP, you can get very similar performance to those super-duper high end EPs, and all this for the fragment of the price.

Gyulai Pal

Pros

- 80 degree FOV
- low price
- excellent performance in big and/or slow scopes
- pinpoint star images almost to the very edge of FOV
- good multicoatings on all surfaces

Cons

- poor performance in fast scopes and/or with strong focal plane curvature
- sensitive to correct eye position
- big and heavy (might have to rebalance scope)
- no safety undercut on barrel (might fall out from the telescope)

Rating

-- End of review