Loading ...
Sorry, an error occurred while loading the content.

Re: [William-Optics] Re: New Megrez: filter??

Expand Messages
  • gary
    Generally you get that false color on high contrast images using a non-APO scope. I m not sure daytime viewing warrants the badder filter for visual use. I
    Message 1 of 6 , Sep 30, 2004
    • 0 Attachment
      Generally you get that false color on high contrast images using a
      non-APO scope. I'm not sure daytime viewing warrants the badder filter
      for visual use. I think for daytime terrestrial photography,
      band-limiting is useful, so the badder as a minimum is a good idea. I
      have them both, and I use APO scopes. As viewing conditions get worse,
      the extra cutoff of the William minus-V comes in handy.

      http://www.lazygranch.com/images/tiktips/ttrraw.jpg no filter
      http://www.lazygranch.com/images/tiktips/ttrmv.jpg William minus violet

      You can see the yellow cast with the minus-violet filter, but you can
      also see the ability to correct the color is dramatically improved with
      the minus-violet filter. Even without color correction, it is obvious
      the filtered image is better.

      Note that the light fall-off at the sides is due to the 2 inch focuser
      on the scope. I generally don't do prime focus shots on that set-up. I'm
      going to do some comparisons using eyepiece projection (raw, William
      minus-v, and the Baader UVIR). At the time, I didn't own a 1.25 William
      minus violet.

      Be sure to compare the histograms. For the raw image, the film was
      basically metered on the blue channel, with red and green under-exposed.
      Slide film has little latitude, so that kind of image is difficult to
      compensate because R and G are very underexposed. Look at the RGB
      composite histogram, and you can see the blue "spike" dominating. The
      William minus violet was probably metered more by the low frequency
      light (mostly red). You can sort of see the tail of the red histogram
      showing up in the RGB composite histogram.

      I'm not sure what your goal is, but one filter probably won't meet all
      your needs if the goal is to restore a natural look to the image. I wish
      one of these band limiting filter companies would study the terrestrial
      market and come up with a range of filters, much like they do with
      warming filters.

      One interesting thing about the William filter is the color cast looks
      different depending on the scene, This is probably because the cutoff is
      sharp. I find at about 30 miles, the sky starts to look green. Also
      note that with the William minus-V, the R, G, and B channels all had
      useful information in these photographs done at 15 miles. If you are
      just filtering the UV, I suspect the image will look too blue for long
      distances (say in excess of 5 miles). If you are birding or perhaps
      trying to read markings like a plane tail number 2 miles away, the
      neutral Baader should be all you need.

      I personally believe that filtering IR is not useful in long distance
      terrestrial photography. In fact, the reason I joined this list was to
      find out if anyone knew if the minus-violet filter effected IR. Those
      who have done IR photography have probably experienced the "wood" effect
      where green foliage starts to look white. I have never had that happen
      over long distance terrestrial photography. What little IR there is does
      have useful information.

      gordogower2000 wrote:

      > --- In William-Optics@yahoogroups.com, gary <yahoogroups@l...> wrote:
      >>The baader UVIR has no cast. The William minus-V has a yellow cast.
      >>I'm not sure what your daytime use is, but I use the William minus-
      > V
      >>explicitly to increase contrast by removing the haze over long
      > distance
      >>terrestrial viewing. If you do photography under these conditions,
      > Thanks, Gary.
      > I am purely a visual observer and will use the Megrez in daytime for
      > scenery, wildlife, etc. Thanks for the info on the Baader.
      > Gordon G
      > Yahoo! Groups Links
    Your message has been successfully submitted and would be delivered to recipients shortly.