Optical design of steering mirror

Daigo Tomono¹

November 19, 2003

Abstract:

Steering mirror on the pupil image is proposed as an alternative way to select FOVs, In this report, sample optical designs are described to show the feasibility of such a field selector.

Postscript version (steering_mirror.ps.gz), source tar ball of the report (steering_mirror.tar.gz), and Zemax files are also available.

Introduction

Steering mirror on the pupil image is proposed as an alternative way to select FOVs, In this report, sample optical designs are described to show the feasibility of such a field selector. The steering mirror concept divides the patrol field into smaller cells of fields with typical size of 1 arcmin or 0.5 arcmin. Each field cell has a field lens which images the entrance pupil onto the steering mirror. The steering mirror steers and re-images a small portion of the field cell. Following it, a re-imaging optics is located to image the entrance pupil again on the cold stop and adjust the image location to be supplied into the image slicer units.

In the consideration, field cell size of 1 arcmin is critical because current design of the steering mechanics needs about 35 mm (corresponding to about 1 arcmin on a telescope with focal length of 120 m). If the field size is grater than 1 arcmin, the steering mirror can be directly situated under the field lens. Otherwise, we have to have extra flat mirrors for spaces needed.

In the following sections, requirements are summarized, and sample designs for filed cell sizes of 1 arcmin and 0.5 arcmin are shown.

Requirements

The overall requirements given by the scientific requirements and the telescope are summarized in Table 1. We have to assume the linear size of a spatial element on the pseudo slit to define locations of the pseudo slits. Figures 1 and 2 show configuration of filed lenses on telescope focal plane.

**Table 1:** Overall requirements
Telescope	VLT Nasmyth focus
Focal ratio	15
Focal length	120m
Field cell size	1 or 0.5arcmin (35 or 17.5mm) side-to-side
IFU FOV	2.8arcsec (1.68mm)
Spatial element	0.2arcsec (0.12mm, 14 $\times$ 14 elements)
Spatial element on the pseudo slit	0.2 - 0.3mm

In this report, spatial magnification of the steering mirror optics (from the telescope FOV to the slice input port) is optimized to be about unity. We require steering mirror optics to re-image the field with spot PSF less than 0.15arcsec FWHM and re-image the entrance pupil onto the cold stop with spot PSF FWHM less than 1% of its diameter.

**Figure 1:** Patrol field of view with 1 arcmin field cells as viewed from the telescope. Pseudo slits are assumed to be 150 mm from the optical axis of the telescope. Entrance ports of the image slicers are assumed to be at the middle of the pseudo slit-let in x-y plane. Minimum and maximum offset distance from the field cell center to the image slicer entrance ports are 46 mm and 88 mm, respectively.
$\includegraphics[width=0.5\textwidth]{fixed_focalplane.eps}$

**Figure 2:** Patrol field of view with 0.5 arcmin field cells and 1.5 arcmin field cells as viewed from the telescope. Pseudo slits are assumed to be 140 mm from the optical axis of the telescope. Small hexagons at the center shows the 0.5 arcmin FOV field lenses, while the other hexagons show the 1.5 arcmin FOV field lenses. One of the three spectrographs is assumed to be supplied with the smaller field lenses. Minimum and maximum offset distance from the field cell center to the image slicer entrance ports are 127 mm and 168 mm, respectively.
$\includegraphics[width=0.5\textwidth]{small_focalplane.eps}$

Paraxial calculations

Figure 3 shows the configuration of optical elements of the steering mirror. Observing field can be selected changing direction of the steering mirror. The steering mirror is placed on the entrance pupil image formed by the field lens so that change of its direction does not change location of entrance pupil in the following optics. Because the inclination of the steering mirror avoids us from locating cold stop on it. Thus, a re-imaging optics has to be inserted after the steering mirror to place a cold stop.

**Figure 3:** Paraxial configuration of steering mirror optics.
$\includegraphics[width=0.5\textwidth]{paraxial.eps}$

Because the optics has to be optimized on spatial magnification, location of the cold stop, and telecentricity of the output beam. Because Zemax can normally optimize only on image plane, we have made a separate software to optimize on multiple image planes of the field and entrance pupil which can be downloaded². In the following sections, results for paraxial calculation and realistic optimization with Zemax are shown for one-arcmin and half-arcmin filed steering mirror optics.

One-arcmin FOV

Steering mirror optics with one-arcmin FOV is designed assuming 30mm offset between the optical axes of the field lens and the output beam. Distance on the z-axis between the field lens and the output focal plane is set to be 260mm. Table 2 shows the result of paraxial optimization.

**Table 2:** Result of paraxial optimization
$\begin{table}\begin{center} {\footnotesize \begin{verbatim}...$

Starting from the paraxial parameters, a Zemax model³is optimized on spatial magnification, image quality on cold stop (only at 2.2 $\mu$ m), image quality on output focal plane, and telecentricity of the output beam.

Figure 4 shows the resulting layout and spot diagrams on the re-imaged field. Table 3 shows qualities of the image of the field, entrance pupil, and telecentricity of the output beam. The requirements desscribed in § 2 are marginally satisfied.

**Figure 4:** Left panel shows the optical layout. The telescope images the stellar field onto the field lens at the left. The field lens images the entrance pupil onto the steering mirror at the right. The light is reflected to the two small flat mirrors and goes through the re-imaging optics upwards. Between the two lenses, entrance pupil image is again formed. Right panel shows the spot diagrams at field center in the J, H, and K-band from left to right. Circles show 0.15 arcsec diameter with a telescope with 120-m focal length.

**Table 3:** Image quality
	1.2 $\mu$ m	1.6 $\mu$ m	2.2 $\mu$ m
Magnification	0.92	0.93	0.94
Image spot rms	0.15''	0.13''	0.14 ''
Cold stop edge spot rms/geometric radius	1.1%	1.4%	2.0%
Telecentricity	3.6%	3.1%	2.4%

Half-arcmin FOV

Steering mirror optics with half-arcmin FOV is desgined assuming 150mm offset between the optical axes of the field lens and the output beam.

**Table 4:** Result of paraxial optimization
$\begin{table}\begin{center} {\footnotesize \begin{verbatim}...$

Starting from the paraxial parameters, a Zemax model⁴is optimized on spatial magnification, image quality on cold stop (only at 2.2 $\mu$ m), image quality on output focal plane, and telecentricity of the output beam.

Figure 4 shows the resulting layout and spot diagrams on the re-imaged field. Table 3 shows qualities of the image of the field, entrance pupil, and telecentricity of the output beam. Despite the larger offset than the one-arcmin FOV optics, image qualities are better. Overall length of the optics is larger for the half-arcmin desgign.

**Figure 5:** Left panel shows the optical layout. The telescope images the stellar field onto the field lens at the left. The field lens images the entrance pupil onto the steering mirror at the right. The light is reflected to the two small flat mirrors and goes through the re-imaging optics upwards. Between the two lenses, entrance pupil image is again formed. Right panel shows the spot diagrams at field center in the J, H, and K-band from left to right. Circles show 0.15 arcsec diameter with a telescope with 120-m focal length.

**Table 5:** Image quality
	1.2 $\mu$ m	1.6 $\mu$ m	2.2 $\mu$ m
Magnification	1.00	1.01	1.00
Image spot rms	0.05''	0.01''	0.09 ''
Cold stop edge spot rms/geometric radius	1.7%	1.2%	0.3%
Telecentricity	0.3%	0.4%	1.1%

File locations

Original source of this document is at gin-an:Presen03/031106_SteeringMirror/. Optical design is performed on siroan:Design03/SteeringMirror/030512_Paraxial, siroan:Design03/SteeringMirror/030731_Multiband, and siroan:Design03/SteeringMirror/031022_ElipticalMirror.

Following Zemax files demonstarate field selection with steering mirror being moved: 0813a1.zmx for one-arcmin optics, and 1031b2.zmx for half-arcmin optics.

Revision history

Nov 19, 2003

First release

About this document ...

Optical design of steering mirror

This document was generated using the LaTeX2HTML translator Version 2K.1beta (1.52)

The command line arguments were:
latex2html -dir html -mkdir -image_type png -local_icons -split 0 -up_url .. -up_title 'Parent directry' steering_optics.tex

The translation was initiated by Daigo Tomono on 2003-11-19

Footnotes

... Tomono ¹: tomono at mpe.mpg.de
... downloaded ²: lenscalc.tar.gz
... model ³: 0811a4.zmx
... model ⁴: 1031b1.zmx

Up: Parent directry

Daigo Tomono 2003-11-19