Re: wide-field beam expander?

Hi Jamie,

I hate these tit-for-tat exchanges in usenet since they are usually center
around misunderstandings, misinterpretations, unstated assumptions, taking
statements too literally or perhaps not literally enough. An then, since
they are archived forever, one feels obliged to defend one's professional
reputation. I will just respond to a couple of things you brought up and
then that will be the end of my contribution to this thread. You are
welcome to respond in kind.

"Jamie" <jacarter3@xxxxxxxxxx> wrote in message
news:1113942998.775144.296170@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> While it is true that the spot size is dependent on the ray bundle
> diameter for field aberrations, it may not be true that using approx.
> 1/10 of the pupil will result in 1/10 the of the full pupil spot size

True. I do not think I ever argued or implied that spot size was exactly
proportional to size of the pupil.

> and this assumes that the spot at the scanner is _exactly_ centered in
> the eye-piece pupil.

Even if the beam uses a portion of the pupil off-axis, the wavefront error
over any 1/3 dia. subaperture is USUALLY much better than over the entire
aperture in most telescopes.


> I have not encountered any f-theta lenses, from quality vendors, that
> do not approach or exhibit diffraction limited performance over their
> entire angular range. My experience is that a quality f-thetat lens
> _will_ provide diffraction or near diffraction limited performance over
> the full angular range of the lens capability. After-all, the intent is
> to produced as many resolvable spots diameters over the scan extent as
> possible. This true of image scannnners and laser machining scanners.
> Most commercial f-theta lenses will specify the calibrated focal
> length, the scan acceptance angular range and the expected spot size
> for a properly formated bean distribution at the entrance pupil as well
> as the number of resolvable spots that can be created. This is
> fundamentally limited by the space band-width product of the lens and
> then degraded by the quality of the design and its fabrication. Also
> note that the long focal length f-theta lens will probably use only a
> fraction of its angular range.

It is true that quality vendors will produce quality lenses, and I take your
point that any lens of this type probably has specs published for it. My
original point was that you can find either rifle scopes or f-theta lens
that meet the demands of this application and those that do not. Do you
think its really true that all the cheap, plastic f-theta lenses being made
now are diffraction limited? Frankly I do not know first hand, I just
assumed the one in my $39 dollar flat bed scanner is not the same as one
made by Rodenstock (now Linos?).

>
> The good news is that you will be able to find specifications for the
> lenses (for a single wavelength collimated laser) that will be
> difficult if not impossible to get with the rifle scope. So the rifle
> scope represents a lot more "buy and try" development than the f-theta
> lens approach will.

This is a good point, and is the reason you would not use the rifle scope
solution for a commercial or military project. However, I understood the
O.P.'s application was a one-of-a kind apparatus for a civilian research
project where budget is usually a primary concern, (in fact, I generally
assume anyone who asks for important advice on usenet is trying to get by
with limited funds). So cost was one of the chief reasons I suggested a
rifle scope. Could you get two high-quality telecentric f-theta lenses
(they have to be telecentric to be put back-to-back) including one
long-focal-length, telecentric lens with a 10mm diam. pupil for a little
more than $1000 (the cost of a high-end rifle scope)? If there is a source
for such things, then that is the solution.

Regards,

--
Adam Norton

Norton Engineered Optics
Optical design and systems engineering for Silicon Valley and beyond.
http://home.ix.netcom.com/~anorton/

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