focal ratio and filters

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

Pierre Vandevennne wrote:
I was browsing Astronomik's site, learning about filters and found many
comments such as this one

"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"

How can a filter be better suited for one focal ration than for another?

If the filter is primarily for imaging/photography, the filter may be
too dim to be used with slow lenses/mirrors. In visual use, the apparent
surface brightness of an object is governed by the aperture and the
magnification; in photography, it is governed by the focal ratio.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

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

HAVRILIAK wrote:
Many filters are interference filters and therefore depend on reflections
that cause reinforcement and destruction of the wave front. Coming in in
perpendicular leads to no reflections, thefore no-nothing.

I don't believe that's the right explanation. If it were, then if you
were to look straight through an Orion UltraBlock, for example, it would
look perfectly clear. That's obviously not the case.

Another observation is that f/3.5 (the short end of the "optimum range")
is awfully fast for a visual scope. That lends additional credence to
the idea that the filters mentioned in the brief excerpt are intended to
be used photographically, and that an f/10 scope is simply too slow to
use that filter with.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

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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Brian Tung nous a écrit :

HAVRILIAK wrote:
Many filters are interference filters and therefore depend on
reflections that cause reinforcement and destruction of the wave
front. Coming in in perpendicular leads to no reflections, thefore
no-nothing.

I don't believe that's the right explanation. If it were, then if you
were to look straight through an Orion UltraBlock, for example, it
would look perfectly clear. That's obviously not the case.

But these filters *are* interference filters. And their effect comes from
interferences between the direct rays and those which are reflected by the
optical surfaces, including the surfaces of the coating.
When you tilt them, you change the length of the optical path between the
surfaces of the filter (including coating), so you change the interference
effect.

--
Norbert. (no X for the answer)
======================================
knowing the universe - stellar and galaxies evolution
http://nrumiano.free.fr
images of the sky http://images.ciel.free.fr
======================================

Norbert wrote:
But these filters *are* interference filters. And their effect comes from
interferences between the direct rays and those which are reflected by the
optical surfaces, including the surfaces of the coating.
When you tilt them, you change the length of the optical path between the
surfaces of the filter (including coating), so you change the interference
effect.

Do you have a pointer to the page in question, by any chance?

I'm not saying that what Havriliak is saying is incorrect--I'm just
saying it isn't the reason that ratios from f/3.5 to f/6 are optimal.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Pierre Vandevennne wrote:
I found this page interesting - it did answer some of my questions

http://micro.magnet.fsu.edu/primer/j...nce/index.html

No--I'm not looking for pages on how interference filters work. (By the
way, I do like the site you cited; it's just not what I wanted to see at
the moment.) I'm looking for the page from which you originally quoted
Astronomics as saying that the filters work best from f/3.5 to f/6.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Dan McShane wrote:

snip

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


Dan,
Thanks for the more detailed response. I have access to and occasionally use TF
Calc and FilmStar for film design and performance trades, but not for nebular
filter design. Not having the exact filter stack prescriptions for various Ha,
O-III, etc. filters, I couldn't specifically analyze their AOI performance, so
thanks for the more precise value for maximum permissible incidence angle.

My comment about performance at the FOV center was that at sufficiently low
focal ratios, ray angles at the rim of the entrance pupil could be incident on
the filter at sufficient angles to shift the passband and reduce image contrast
even at FOV center. The cone half angle U for a given focal ratio is ASIN(1 /
(2f/#) ). This gives

f/# ASIN( 1 / 2(f/#) )
f/2 14.48º
f/3 9.59º
f/3.5 8.21º
f/6 4.78º
f/15 1.91º

You said that up to 10º AOI no significant passband shifts occur, which means
that the filters could give good central FOV performance down to about f/3.
This agrees well with the specifications from Astronomix. Perhaps Astronomix
was being conservative in their minimum focal ratio (maximum AOI) by limiting
the lower focal ratio to f/3.5.

Another factor limiting lower focal ratios with filters is their introduction of
overcorrected spherical and chromatic aberration. A perfectly plane-parallel
plate in a converging cone shifts the focus by a wavelength-dependent factor of
(thickness)(n-1)/n. The index "n" rises with decreasing wavelength, therefore
the filter introduces overcorrected chromatic aberration. The spherical
aberration of a plano plate is given in "Modern Optical Engineering" by W.J.
Smith as

LA' = (t/n) [ 1 - ((ncosU) / SQR(n²-sin²U)) ]

where U is the maximum cone angle given above as ASIN(1 / 2(f/#)). A quick
ZEMAX simulation shows that for a 12.5" aperture f/3 light cone used with a
0.15" thick plane-parallel plate of BK7 over F-C spectrum, at best visually
weighted RMS focus the filter introduces about 0.3 wavelengths polychromatic P-V
OPD of overcorrected spherical and chromatic aberration. At f/3.5 this values
reduces to about 0.23 wavelengths P-V, and at f/6 gives 0.08 wavelengths P-V.
Thinning the plate to 0.1" thick at f/3.5 gives about 0.15 wavelengths P-V OPD.
Best-focus RMS wavefront errors for all these values are roughly 1/3 to 1/4 the
P-V values, so except at f/3 the filters introduce little visual image
degradation or Strehl impact. I don't know how thick these filters are, but the
lower focal ratio limit of f/3.5 given by Astronomix still appears credible. Of
course all this analysis assumes the filter faces are perfectly flat and the
internal index of refraction is perfectly homogeneous, which of course isn't the
case.

Thanks as well to Pierre for initiating this interesting thread.
Mike