GPU 127/1200 Triplet APO

GPU 127/1200 Triplet APO refractor review

The other day I was browsing the ads on the web page of the Hungarian Astronomy Club, I was looking for a good quality planetary EP for my 100/800 TMB APO. Suddenly, a very strange ad caught my attention: "For sale is a GPU 127/1200 Super Planetary Triplet APO lens, optionally in a nice OTA". I did not know what to think: is this a lens from a manufacturer I do not even know? What is this "GPU" thing? I simply had to look after this!

So, I contacted the guy (Mr. Gyulai) referenced in the ad, and he informed me, that he is the optical designer of the lenses, and he is just starting up the production of these lenses right here, in Hungary, only a few kilometers from my home! I did not even know what to think: was he joking? Or is this serious? Triplet APO lenses prouced in this little country?

So, we agreed that I would visit him as soon as the sky clears up. He told me that I could read online some specifications and a few words about the design of his lenses, at http://gpuoptical.fw.hu, but he warned me not to expect a web page with fancy graphics and animations, as the page is in a "very preliminary" phase at the moment. He was right, the web page was a text only document without any graphics (now it is a bit improved, but still very far from being "acceptable"), but the text convinced me: this guy (and his colleagues) looked like people who have exact knowledge about what they were doing: their design- and market philosophy seemed really interesting to me. They wanted to make scopes that are extremely error-resistant, and this way give excellent performance under the stars. They call this "field optimized" design (as opposed to "laboratory-optimized" designs that give nice measurement data but do not perform so brightly at the field). So, at first, this philosophy looked good to me, but later I had some doubts: maybe they are only trying to hide the low optical quality of their products with this "cover-philosophy"? Still, the simulation data of their lenses (spot diagrams, Strehl ratios etc.) looked very similar to the best I have ever seen on other sites (what a pity, most vendors do not publish any detailed technical info about their products). Their lenses are oil spaced triplet designs, which is somehow inferior optically to air spaced lenses, but they say, the thermal properties of their lenses are much better. Well, I definitely had to see it for myself!

So, when the sky cleared a week later I visited Mr. Gyulai (who was nice and helpful) at the headquarters of their company. I also personally met the optician there, who actually builds the lenses. We went to the back-garden where a huge equatorial mount was permanently installed. It looked a bit over-sized for a 127mm scope, but later it was proven, that the setup was really rock stable.

It was a memorable experience when I met their APO scope personally: it looked like a strange mix of high quality optical work and some "garage-tuning". The lens was uncoated, and the cell of the lens was not anodized. They informed me, that the lens was not fully ready yet, but we could test it immediately, if I liked it, they could have the lens coated and the cell anodized within a week. They also told me, that the OTA we would use was NOT the final one (that I could order for the lens), and this was something that made me really happy: this OTA was not something that I would ever like to have in my living room.

So, finally we took the scope outside, attached it to the mount, and looked at some DSOs with low mags, waiting for the scope to cool down. They told me, that their lens was fully aplanatic (corrected for spherical aberration and coma) and they seemed to be right: the stars looked perfect little pinpoints at the field stop of a 31mm Nagler. (This was also my first personal encounter with a Nagler of this size, but that is another story…:-). The double-cluster was a fantastic view, similarly to Andromeda galaxy and its companions. It was interesting to see, that the two uncoated air-to-glass surfaces seemed to have very little effect on the light throughput of the scope, the 127mm uncoated lens still gave visibly and impressively brighter images compared to my 100/800 TMB. The told me they would coat the lens with broadband multi coatings before delivering it, and showed me a smaller example lens with identical coatings. Well, that multi-coating on the small demo-lens was really impressive. I hope they can reproduce it on their commercial lenses in larger size!

After some time, they started to increase the magnification, but I believed the scope did not have enough time to cool down: we were in the winter, and it was freezing! My TMB is stored in an unheated room, still, I have to wait somehow longer for the lens to stabilize thermally and for the undercorrection to go away. After this period, my TMB gives a practically perfect star test, but without sufficient time given to the scope, the star test shows serious undercorrection. I expected this much (27%) larger lens to take much longer time to stabilize. But I was wrong: after about 20-25 minutes, its image seemed similar in quality to my TMB after the same time, but the two lenses had different types of problems: the GPU showed more turbulence in the image, with less undercorrection, but the final image was somehow similar. I shared my opinion with Mr. Gyulai, who told me it was natural: the larger oil spaced lens taken from a warm room takes more time to cool down. But he assured me, that in a side-by-side test, his lens would probably beat my smaller air spaced triplet in cooldown time. It seemed reasonable: taking out his scope from a heated room, it provided similar performance as my 100/800 after taking out of an unheated room. Well, it was impressive!

After half of an hour, the performance of the GPU triplet started to visibly exceed my 100mm scope. Even on bright stars I could not detect any false color at 50x/inch relative magnification (250x), not in focus, and not in the extra- or intrafocal images. The Fresnel rings were pure white, everywhere. The star test showed only a little undercorrection for some more time, but after about an hour it completely disappeared and I could not detect any aberration in the star test anymore. Slowly, the increased aperture over my regular scope started to dominate: I believed I could see fine surface details on Mars more easily, the contrast was somehow better, due to the increased aperture (and remember, the lens was uncoated!). I wonder how much the image will further improve when the lens will be coated with multicoatings... The atmosphere was rather steady that night, and the view of Mars exceeded all my expectations. In the meantime, Saturn has raised well over the Horizon, and this was the final shock for me: the planet and its ring system was defined in such an excellent clarity, that I had never experienced before, with any telescope I tried in my life. The banding on the surface of the planet, the Cassini division and the little moons surrounding the planet, all these were simply beautiful. So, I decided to buy this scope, even it it is coming from an unknown manufacturer, and this way, probably it would be more difficult to sell it later then selling a used scope from an already well known brand.

Now I think there is really significant difference between a 4" high-end APO and an 5" one, at least if the optical quality is similar. And in this case I could not detect any realy difference my scope and this GPU, except aperture. Even if we could compare them side by side, I believe the difference in optical quality would be negligible, if detectable at all. This larger lens shows similar star test and similarly color-free views as my 100mm high-end APO, even if more aperture gathers more atmospheric turbulence. This is impressive IMHO!

Now I am sure, that "aperture rulez", but usually it takes a lot of money to buy it, especially in the case of APOs. But this lens actually costs about the same as the money I can get for my 4" OTA by selling it used, and even if I buy the larger lens in a custom ordered GPU OTA, the little extra I have to pay is really reasonable if we take into account the extra performance.

GPU Optical is a small and unknown company at this moment, so they have to sell their products at a very reasonable price in order to gain their place on the market. This is a good opportunity for many amateur astronomers with limited budget, to buy a good triplet APO scope for a bargain price. If Mr. Gyulai can keep this level of optical quality in the future, then I guess GPU products will be widely regarded very soon. As GPU optics are produced in Hungary and are all hand-figured, this seems really possible to me.

Mr. Gyulai has informed me, that they will soon produce even larger lenses (e.g. 155/1200) and after seeing the difference between the performance of 4" and 5" aperture, I guess I will be among the first customers to try and possibly order a larger scope from him.

Andrea Langermann