今天看到的最新消息：yahoo讨论组上的一位资深玩家对常见的各种三寸折射镜给出了一个大致排名，供同好参考：
从消色散能力上来看：TMB 的三片f 7.5萤石镜当之无愧排老大，其次是：tmb 80 f6 fluorite triplet > TAK FS 78 fluorite doublet > Televue 76 > Orion 80ED > 景德 triplet ED APO > Televue Oracle (75mm/f7.4) > (景德semi-apo/ TV Pronoto/TV ranger) >普通折射镜。
如果TMB的三片萤石镜打十分的话，我估计最近升级的景德80ED/APO最多打6分，不可能再多了。以下是作者原文：
Color correction is obviously much better than the MII doublet was, looks like a solid ED-class performer. Not zero color like the much more expensive zero-color fluorite triplets, but at half the price, something of a bargain. Most importantly, the star test is very good indeed, very round fresnel rings with ruddy-fringed rings inside of focus, essentially perfect correction of spherical abberation. In focus, the airy disk is hard and round, with a single dim red diffraction ring, on moderate brightness stars. I do not see how you could ask for a better result at this price. Cosmetic quality of the glass is very clean indeed. It does not look like the cell is collimatable like the doublet, then again mine doesn't need it like the doublet did. I haven't had a chance to do critical comparisons yet, but it looks like this is William's answer to the Orion ED80.
On A-B nighttime star tests, initial impression, I would estimate the color error about 2x the color of the TV76, primarily a little excess red. I once compared the Orion ED80 to the TV76 and the former had a little more red excess than the latter, lets say 1.5x. A Tak FS78 beat them both, again in the red, but not by much, just to give you an idea of the level of performance we are talking about here. All these scopes have many times less color than an achromat. Vignetting is not an issue, either at the foam ring baffles, which can be scooted against friction if necessary, nor at the focuser body or extender tube. Next comparo: I estimate the TV Ranger (Pronto) has a good 4x the color of the MII/EDT. The color halo around stars is quite obvious on the Ranger, with a actinic mixture of red and blue. Switching back to the MII/EDT is a dramatic improvement, the airy disk changes from yellow to off white, and the halo goes from a bright violet 3+ diffraction rings to 1 dim red. The MII/EDT color error reminds me of the original TV Genesis I once owned, and also the TV Oracle, the under-appreciated and short-lived 75mm F7.4 triplet. The colorful rings of the Ranger are due purely to the color defocus, as it has a superb star test otherwise.
My tally from most color to least:
Achromat >Ranger/Pronto/Megrez doublet > TV Oracle >MegrezII EDTriplet >Orion ED80 APO >TV76 > FS78 fluorite doublet > 80 TMB F6 fluorite triplet > 80 TMB F7.5 |
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