Using Achromat in Spectroscope

I have an achromatic refractor lying around going unused. I am
considering using it to build a spectroscope. Assume this is a
classical Fraunhofer objective with a standard amount of color.

Lets assume I can deliver a light beam through an optical fiber
just above the center of the focal plane. I am using the fiber
as a delivery device and as the 'slit'.

This light beam diverges and encounters the objective where it
becomes colimated. The colimated beam is reflected and dispersed
off a difraction grating operating in extreme litrow configuration
(that is; as close to right back from whece it came as possible).
The dispersed beam is focused by the objective and arrives just
below the center of the focal plane as a spectrum.

Will the longitudinal color error and/or the field curvature end
up requiring a field flatener so that the spectrum can be imaged
by a CCD camera with the whole spectrum in (reasonable) focus?

Will the CCD be required to be tilted to achieve good focus across
the array?

Will the distortion of the objective cause the spectrum to be imaged
as an arc on the CCD? Will the distortion be 1X or 2X Fraunhofer
distortion?

Thanks in advance

Mitch

Mitch Alsup wrote:

snip

Will the longitudinal color error and/or the field curvature end
up requiring a field flatener so that the spectrum can be imaged
by a CCD camera with the whole spectrum in (reasonable) focus?

MIJ: Maybe, maybe not - you haven't described your lens, fiber, grating,
desired spectral band and CCD array parameters in enough detail for
anyone to give you more than a qualitative answer. Your achromat will
definitely have a wavelength-dependent focal surface radius of
curvature, curved toward the lens. If it is f/15, though, and your CCD
array is small enough, you might get a nice sharp spectrum across the
CCD anyway. You will likely have to re-focus the CCD as you rotate the
grating, especially as you get into the ultraviolet where best focus is
rapidly moving toward the lens.



Will the CCD be required to be tilted to achieve good focus across
the array?

Will the distortion of the objective cause the spectrum to be imaged
as an arc on the CCD? Will the distortion be 1X or 2X Fraunhofer
distortion?

Thanks in advance

Mitch

MIJ: All the questions you ask can be best answered by just setting up
the system and seeing for yourself. Feed the fiber with a mercury or
low-pressure sodium light to give you several spectral calibration
lines. Let us know how it works.

Mike Jones wrote:

You will likely have to re-focus the CCD as you rotate the
grating, especially as you get into the ultraviolet where best focus is
rapidly moving toward the lens.


Sorry - caught myself after I hit SEND - your achromat will have a minimum
EFL somewhere in the green. The EFL will increase toward the red and blue,
and REALLY increase into the UV.

The slit makes the real focus pretty broad so there's nothing to worry
about. The spectrum is working off the focus of the slit, not the focus of
the lens.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

The scope is apparently using a subdiameter corrector lens/barlow to obtain
the F6 light cone and this means that there are a whole bunch of names that
can be used for the deisgn. The spherical primary tends to indicate
something along the line of the Jones-Bird derivitives. The literature for
scope designs is rife with subdiameter corrector lenses for scopes. There
has even been a number of designs propounded for the Hale 200" scope over
time and I really don't know if any of them got built but some had some very
nice looking specs.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

Mike Jones wrote in message ...
Mike Jones wrote:

You will likely have to re-focus the CCD as you rotate the
grating, especially as you get into the ultraviolet where best focus is
rapidly moving toward the lens.


Sorry - caught myself after I hit SEND - your achromat will have a minimum
EFL somewhere in the green. The EFL will increase toward the red and blue,
and REALLY increase into the UV.

This is what I was thinking, that the change in EFL[wavelength] will
work in the opposite direction as the field curvature of the objective
canceling (e.g. flattening) some of it out.

Thanks

Mitch