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  1. #9
    Join Date
    Nov 2005
    Oregon, USA


    I think the point Bill makes about his system not being par-focal for different wavelengths is quite key - if the focal length is different for different filtered wavelengths, then the focal ratio will also be different and thus the slope.

    So, the theory stating that the thickness of the filter doesn't matter is true, but the non-convergence of the different wavelength light is a different matter.

    Still, having said that, I'd imagine the effect of that slope delta will be lost in the noise of measuring best focus in seeing effects.

    I'd suggest averaging them out and using that
    The interesting thing here is that this system is one that is essentially identical (except for aperture, which varies with the user) to that being used by many if not most of the top imagers for long focal length use. I have never seen or heard anyone mention or discuss this issue despite having used such systems (OGS then RCOS) for over 10 years. It seems to be pretty much ignored. That may indeed be because the curves generally gathered and used may be "good enough" given the other variables such as tracking, seeing, etc.

    Having said that, the quest in this hobby is a quest for excellence and perfection so the question remains. Where, exactly is the significant (FocuseMax) digit when it comes to what is or is not usable for perfect focus with a given configuration? This seems to be a pretty important piece of information that is seemingly either unknown or at least not well known. The reason for oddness in the numbers can be debated and could have numerous causes, but it seems to me still important to be able to look at a good set of curves and say "curve A is effectively the same as curve B" or "I guess they are different enough to use a different curve for the different filter/lens, configuration, etc."

    It should be possible to calculate if one knows the workings of Focusmax and the step size at the focal plane of the system (easy to find out) since the depth of focus of an F9 system is (rough number) about .009 inch. If a curve was enough different from the "ideal for that configuration" curve that using that "non-ideal" curve might put one outside that range this would be, it seems to me, cause to use a more appropriate curve. While it may well be that under most seeing conditions one would never see the difference, I would hate to think that I was missing a few tenths of an arcsec when the seeing gives me one of those all-too-rare perfect nights!

    In fact, I have seen a couple nights here at home recently with raw 800 second exposure FWHM in a few images as low as 1.7 arcsec. Could that have "really" been 1.6 or 1.5?

    That is the problem. The better it gets, the more one wonders if one could do just a bit better....
    Last edited by Bill McLaughlin; Sep 15, 2010 at 05:19.
    Bill McLaughlin


    For every complex problem there is an answer
    that is clear, simple, and wrong.

    H. L. Mencken


    My Website: Nightskypictures

    My Observatory: Raptor Ridge



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