Mike Jones wrote in message
...
HAVRILIAK wrote:

"The optimal focal ratio for the use of this filter is between 1:3.5
and
1:6. The range of application is from 1:2.8 up to 1:15"

Many filters are interference filters and therefore depend on
reflections
that cause reinforcement and destruction of the wave front.

True statement.

Coming in in
perpendicular leads to no reflections, thefore no-nothing.

False statement. Eyepiece filters are designed to be perpendicular to the
optical axis. The design spectral bandpass(es) change transmission and
wavelength with increasing angle of incidence, thus setting a lower bound
on
focal ratio. Thus even for objects exactly at the center of the FOV, at
low
focal ratios there will be out-of-band light leakage at the image that
reduces
contrast. The in-band normal incidence transmission of the filters is
always
less than unity, in some cases significantly so, and thereby limits the
upper
usable focal ratio only because of reduced image brightness. (Same point
Brian
Tung just now made).

See for yourself. Look at a light through an Ha or O-III filter and tilt
it back
and forth. At normal incidence it performs as intended. As you tilt it,
the
color transmission shifts and the filter does not give the desired
spectral
performance.


Mike,

More precisely it`s about the edge rays and how much the filter shifts to a
lower central wavelength as the AOI increases. In addition to the CWL
shifting lower, the filter bandwidth gets wider and loses some TX%. Fast
scopes have a very steep cone angle and therefore "potentially" could affect
the filter`s CWL and bandwidth more so than slower scopes.

Not so sure about the center being affected though, as it`s the one place
where at least some of the ray bundle is nearly perfectly perpendicular to
the optical axis.

This is from a similar thread "Filters and Fast scopes?" on SAA a couple of
years ago;

Here`s some spectral scans http://users.erols.com/dgmoptics/filters.htm of a
nebular filter I designed using TF Calc Thin Film software. The scans are at
0, 10, and 20 AOI. The filter is not as narrow as an OIII but it does
demonstrate that with nebular filters anything less than about a 10 AOI
doesn`t effect the filter very much. Now the OIII has narrower bandwidth but
is still not a especially narrow filter as far as interference filters go
and is therefore not effected as much by AOI.

Dan McShane




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