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Post by mikey cee on Feb 20, 2014 19:16:00 GMT -5
Mike or Ales either one of you. How are the strehl ratings made? What is meant by "average" strehl? Is it the same as "polychromatic" strehl? I'm dumber than a rock so you needn't worry about being "sucked in" as they say over on Cloudy Nights for an ambush.  Mike |
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Post by astromanuk on Feb 21, 2014 3:53:57 GMT -5
Interesting question Mike,
I would think that given the high numbers it is monochromatic strehl and probably measured in the green. I wouldn't be at all surprised if it is the theoretical average strehl that is quoted. I am looking forward to finding out as well.
From my experience although theoretical measures are a useful guideline the best test of any system is what it delivers through the eyepiece. Like you I have had decades of using great refractors but have never been one to chase the numbers.
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Post by mistervista on Feb 21, 2014 13:32:12 GMT -5
Due to the quality of the glass necessary for a reasonably priced achromat a 6" f/15 will rarely have polychromatic strehl better than .81-.83. However, I like the use of a fringe killer filter to correct CA by 50% but CA fringing or not, such an MTF drop does effect contrast on bright objects and will not increase the polychromatic strehl factor. Still, planets can look so good in such a refractor that it's really a testament to how good the .8+ strehl performance really is. Look at the tight outer ring of this Istar 150 F10 airy disk vs a Synta 150 F8 ... b ![]() ig diff Biff. Attachments:
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Post by Mike on Mar 13, 2014 13:05:11 GMT -5
I'm leaving this to Ales as he is more involved in the bench testing of the lenses. I star test and this is what I rely on more than what the test sheet shows. Not saying that bench testing is not necessary as it is very much to keep QC but nothing reassures me like a good concentric circle. Wow! Big Jim B. over on CN would give birth if he saw that Istar Synta comparison. This is a nice comparison. Did you do this yourself?
Mike
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Mike
