"I never once mentioned price as a determining factor of anything. Why do you guys keep bringing it up? Unless you think an inexpensive telescope cannot be good? Trust me, they can - the Synta ED80 is THE perfect example of this."
I'm an engineer. I build, tinker, and....measure. In my audio hobby, measurement is a very handy, almost mandatory, tool set for me to have for evaluating what I'm hearing subjectively and has saved me countless days in setting up and/or, designing and building, a speaker.
So it's natural for me to bring the same attitude and curiosity to my astro hobby. I've, accordingly, developed some capacity to do some meaningful optical testing. I've got DPAC and outdoor star testing (with both real and artificial stars) and I'm experimenting with indoor star testing for exactly the reasons Paul mentions, it's repeatable, and controllable, and at will.
My testing and subjective real world visual observations with these ST120 samples were/are simply meant to compliment Ed's very nice subjective overview and evaluation of his sample (actually, it would have been nice to have his sample for testing). I also though it a little unusual that ED made no mention of what he saw optically with his sample, like some mention of a star test, which he almost always dose during his evaluations. So, I was curious, one was available new from Orion, it was cheap, and I had it in my grubby hands just a few days after ordering it. I can tell you right now that if Ed had mentioned something about optical quality, I probably would not have started down this path with the 120ST. Honestly, however, I'm rather glad Ed didn't mention optical quality as now I have two samples of what I consider to be great lower power scope.
Now, I typically view first then test later. This is the exact review "model" used by some of my audio rags, for example Stereophile. The reviewer gives an extensive, subjective evaluation of the product which is then passed off to the measurement guy for a complete set of standard measurements for that type of product. The measurement suites are identical for that product type (amp, speaker, preamp..) regardless of the price of the product. Price commentary usually comes in only at the front and end of the subjective and measurement evaluations.
But, dang, it was just sitting there, all nice and "cute", smiling at me with that nicely curved front element, my DPAC set up was unoccupied, and the weather was lousy, so........ I flipped my preferred process. No big deal.
Now, an update for the two samples. I finally had good weather to take the second, used sample out for star testing and planetary views of Jupiter, Saturn and Mars. It was a chilly evening, very clear with rather rapidly falling temperatures. The second sample had been outside for a couple of hours before I started the star testing on Vega. I stared at 120x, this time with a deep green filter . I went no further. It looked terrible. Sharply defined inside of focus rings but complete mush outside of focus with a very "hot" bright center. At focus, the airy disk was "fat" and bright and the center stayed that way, hot, as I moved outside of focus. This seemed at odds with a brief indoor star test and MUCH worse that the new sample (but still no visible astigmatism). I could focus on Jupiter but the image was very soft with ill defined belts. Really!? So I took it inside and brought out the new, first sample to let it cool. After about a half hour indoors, I looked carefully at the second sample to see if maybe it had been tinkered with and judging by the two small dabs of "nail polish" between the objective retaining ring and cell, it had not. But that did not stop me from excavating my spanner wrench and removing the retaining ring (it was rather tight) and "O" ring under it, then dropping the cell down from around the lens. This lens has a spacer ring between the elements rather than 3 tabs. Everything looked just fine really but there were no orientation marks on the lens edges. I picked up the front element and yes, it was not backwards. So, I put it all back together, tapping the lens cell while screwing the retaining ring back on until it and the "O" ring provided a little resistance. Shaking the cell, I could hear a faint rattle. I then screwed the lens cell back on the tube and took it outside. So it was "tweaked" a little.
By this time, the new, first sample had cooled down a bit and it performed just like it had before, very nice really, up to ~180X with the bino-viewer system and green filter, no astigmatism of coma and a rather "good" star test considering its center zone. With the 55mm mask, the test was "perfect" and sans green filter, the airy disk was quite white. Ok, nothing different there from before.
So I swapped out the new unit for the "tweaked" used one, reinstalled the Denk viewer system, slewed over to (actually up to) Vega and had a look at 120X with the green filter. It was notably "better" than before. A lot better. The center was still "hot" but I could now actually see the first few outer diffraction rings outside of focus though the inner rings were smooshed together. But the scope was still cooling a little. After about a half hour more it had settled out, still looking notably better than "before". Stopped to 55mm sans filter, the airy disk had a slight red tint to it, unlike the white of the new sample, but the patterns look "good". The new sample however was still giving the superior star test and sharper views of Saturn and, especially, Mars (Jupiter has slid behind a tree).
Both samples were however, complete dynamite on the Pleiades with both my Edmund 28mm Plossls and APM 24mm UFF, both in mono and bino-vision. The two samples were indistinguishable from each other. No surprise there.
This all leads me to conclude that the retaining on the older, used sample was just too tight for the falling ambient temperatures that night. But not for the new sample...and I'm resisting the urge to "tweak" that sample too. Subsequent DPAC testing shows the used sample behaves the same as it did before the "tweak". Go figure.
The fate of the used sample is a good one as it will be refinished white then installed on the 8.75" F12 achromat as a low power, wide FOV "finder" scope, a mission it is extremely well suited for. I'm keeping the new sample as I find it to be a two trick pony. The first trick is being a truly excellent low power scope that is actually rather good at higher power and full aperture. The second trick is that at 80mm aperture, it is an excellent 80mm F7.5 achromat, capable of sharp high power views (for an 80mm aperture anyway).
Edited by Jeff B, 19 October 2020 - 01:25 PM.