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Subject: Re: [M]: Optec filter slider
From: JohnLX200
Reply To: mapug@shore.net
Date: Tue May 26 01:32:38 1998

In a message dated 98-05-25 15:10:18 EDT, you write:

<< I have added to my web site a long description, with pictures, of the
Optec filter slider. It is under Attachments/filters/Optec. I have also
added some photos to the article on focal reducers that show the
Lumicon, Meade and Optec reducers in some detail.

Finally, I have just read a fine book, called Pluto and Charon. It is
reviewed briefly in my bibliography section under Bibliography/History

Happy holiday and clear skies. Doc G >>

Doc,

Thanks for posting this practical guide to filters and tricolor! I was
looking for information just like this. I have a question and comments:

Has anyone else on the list used different filter combinations with good
results?

Are the "filter thickness" values you give the actual thickness of the glass?
I'd think that's the most important measurement, but that the glass location
relative to the shoulder at the end of the threads might also matter, in
addition to thickness from that shoulder to the other end of the metal for
mechanical fit reasons in some cases.

I'd be happy to measure my 48mm filters and privately email you the results to
add to your excellent page. I have perhaps a dozen or more types of new 48mm
Hoyas floating around at home here as I'm finishing accumulating a few hundred
new filters (a few for personal use and the rest for resale to astronomical
friends: anyone interested please email me privately) due to having found a
real bargain source which I've cleaned out of this quantity.

I think your discussion of the 80A as giving good aesthetic results with short
exposure times was extremely interesting. I'd done a little thinking about
this myself, and came to the conclusion that a good "tricolor" setup, now that
most astrophotographers have Photoshop or other good software for combining
the multiple images any way they'd like, would be to use something like
80A/(either "no exposure", "no filter" or some type of light or medium
green)/85B and then experiment as to exposure ratios and how to combine the
two or three images using various fudge factors summing them. The exposure
ratios should be closer to 1:1:1 (or maybe 1:0:1) than with cutoff filters.

The 80 series moves the color temp upward with shallow bandpass
characteristics, the 85 series moves it downward an equivalent amount with
shallow bandpass, "no filter" leaves it alone, and hopefully there's a green
which when added in some proportion will smooth out the frequency response of
the sum. To move the color temp, these filters have nice, bell-shaped
response curves centered on different wavelengths, somewhat like bandpass
filters but not nearly so steep at the sides and a bit less flat on top.

While it's not as rigorous-sounding an approach as using steep, perfectly
matched bandpass filters to do a true separation, it throws away fewer
photons, and would very likely give more accurate color rendition to the eye
if my photographic instincts are correct. The reason for this is that a
response curve shaped like a bell or sine wave is more forgiving than square
waves which don't have their cuts at the exact same frequency or aren't
perfectly square (none do and none are!!) It's much like the speaker cross-
over design problem before and after arbitrarily steep electronic crossovers
became available. At least with electronics, you can twiddle with the
parameters more easily than you can by looking for exactly the right color
glass. Murphy's law tells us that the downward spikes due to mismatch on
steep bandpass filters will fall at some important emission line of our
subject, and the upward spikes at the emission line of our town's streetlights
;-)

The frequency response curve of this approach might not be as flat at most
frequencies as using bandpass filters, but its lack of horrible atrocities at
crossover frequencies is a major advantage. It also makes the ratios of the
three exposure times less critical to maintaining a good overall response
curve.

Thanks for posting the info, Doc! If you want to try out my theory and
publish the results, and in appreciation for your contributions to MAPUG, I'd
be glad to give you a matched pair of 80A/85B or 80B/85 48mm Hoyas to add to
your collection.

John Hopper
JohnLX200@aol.com
MAPUG admin/48mm filter hoarder