# Presentation: 190595

## SDSS 2.5m and 3.5m OPTICSSteve Kent, 1/2/04

**A. Principles of Telescopes**
**Reference: D. Schroeder, *****Astronomical Optics***
**B. Features of 2.5m design **

**Correctors (2)**

## Two descriptions

**Physical Optics**
**Diffraction (wave properties)**

**Geometric Optics**
**Ray Tracing**

## Fundamental Laws ofGeometric Optics

**θ****1**

**θ****2**

**Snell's Law**
**   ****(Lenses)**

**n****1****sin ****θ****1**** = n****2****sin θ****2**

**n****1**

**n****2**

**Mirror**

**θ****i**

**θ****o**

**θ****i**** = θ****o**

## Simple Lens

## Simple Designs

**Newtonian (parabolic mirror)**

**          ****Cassegrain**
**(hyperbolic secondary)**

**Perfect images on-axis**

## Aberrations

**Spherical**
**On-axis**
**Symptom: Light from edge of mirror has different focus from center**
**Cause: Spherical mirror or lens**
**Best focus - circle of least confusion**
**Cure: Parabolic Primary**

***Aberrations are reduced by adjusting surface shapes, element***
***spacings, other free parameters in a design.***

## Aberrations

**Coma**
**Off-axis, limits useful field of view**
**Symptom: Light from edge of mirror has offset focus from center**
**Cause: Parabolic Primary**
**Cure: Hyperbolic Primary & Secondary mirror (Ritchey-Chretien)**

## Aberrations

**Astigmatism**
**Off-axis, limits useful field of view**
**Symptom: Light from top, bottom of mirror have offset focus from left, right**
**Cause:**
**Cure: Corrector lens or extra mirror**

## Additional Aberrations

**Distortion**
**Can't map a sphere onto a plane**
**Field Curvature**
**Longitudinal Chromatic Aberration (lenses only)**
**Latitudinal Chromatic Aberration (lenses only)**

## APO Telescope Designs

**2.5 M Features:**
**1. Two**** mirrors, conics + 6th**
**    ****and 8th order (modified**
**    ****RC)**
**2. Two Schmidt-type**
**    ****correctors for imaging;**
**    ****Schmidt plus meniscus**
**    ****for spectroscopy**
**3. Nearly flat focal plane;**
**    ****very low distortion.**
**4. Focal Length 12269 mm**
**    ****Focal ratio f/4.9**
**   ****Scale 16.6 arcsec/mm**
**   ****FOV 3 degrees diameter**

**3.5 M Features:**
**1. Two**** mirrors, conics plus**
**    ****flat tertiary (RC)**
**2. Focal Length 35360 mm**
**    ****Focal ratio f/10.1**
**   ****Scale 5.8 arcsec/mm**
**   ****FOV 24 arcmin (240 mm)**
**         ****diameter**

## Fundamental of PhysicalOptics

**Exit pupil is image of**
**primary mirror as seen**
**at the focal plane.**

**Image intensity at focal**
**plane is Fourier transform**
**of wavefront error ****φ**
**integrated over exit pupil.**

**Perfect exit pupil (****φ=0):**
**Intensity = *****J******1******(v)/v *****where***** v=πr/(fλ)***

**Airy disk: 1st dark ring at r=1.22****λ/D**
**	****	****SNAP: 0.063" for B filter**** **

## 3.5m Diffraction

**Exit Pupil**

**Wavefront error: r = 283 mm**

**Image**
**(closeup)**

**Image**
**(wide field)**

## PSF (Point Spread Function)factors

**Optical Design (aberrations and diffraction)**
**Fabrication errors**
**Surface roughness**
**Optics cleanliness**
**Collimation**
**Atmospheric seeing (ground only)**
**Filter characteristics (ghosting)**
**Detector characteristics**

## SDSS 2.5m Focal Plane(kent003.dat)

## Optical Design Software

**Windows**
**Zemax**
**Oslo (Free, useful version available!)**
**Code V**
**Linux/Unix**
**trace (snap.fnal.gov => calibrations)**