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  1. #21


    Quote Originally Posted by Bob Denny View Post
    Tom Krajci has some interesting data on this. I'll ask him to post it here.
    The attachments may interest you. I used FocusMax V curve tool to make some 'CFZ runs'...where I look at the HFD plot right around best focus. It's a chance to compare theory to reality for the size of your CFZ.

    I found that for a doublet refractor (with a fluorite element, and two element field flattener))...the CFZ curves for B and I are asymmetric...probably because of spherochromatic aberration.

    An SCT/focal reducer...shows roughly identical CFZ curves in B V R and I.

    Most of my rigs here show classic secondary spectrum when you plot the focus offsets versus bandpass central wavelength...except for one scope...a Meade...of course!

    I recommend you do some plate solves of your images through various filters (R G B, luminance, and no filter)...and I bet you'll find that the image scale (focal length) differs by a small amount.

    We'll see if my hunch is right....
    Attached Images Attached Images
    Tom Krajci
    Cloudcroft, New Mexico

    Center for Backyard Astrophysics (CBA) CBA New Mexico

    American Association of Variable Star
    Observers (AAVSO): KTC

  2. #22
    Join Date
    Nov 2005
    Oregon, USA


    Interesting data! I will have a look at some images for scale. I have the RCOS w and w/o the flattener as well as the AP155 (now sold but lots of data) and the FSQ. Should be interesting.

    This is related to my post from last night when I was running the focus offsets script:

    Offsets Post

    Wherein I wounder why we are using one Vcurves to generate multiple offsets....

    Thoughts on that would be appreciated.
    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

  3. #23
    Join Date
    Oct 2005
    Mesa, AZ


    More interesting would be if a statistically valid sample of focus runs with one VCurve for all filters produces consistently poorer focus compared to using the VCurves generated from each filter. Measuring the focus is half the battle, as the HFD measurement itself is noisy. A better way would be to do a multi-star PSF fit then use the FWHM from that, using an image taken immediately after a focus run. If the differences are "small" then the seeing could dominate. But then it does so often... so what is the answer anyway :-)
    -- Bob



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