LRIS-ADC Homepage

Phillips' Figures for LRIS ADC

New materials:

DRAFT Delta-CoDR Optics Talk (PPT)

  • Differential distortion study (10feb03) and figures for text.
  • Model B Performance Supplement (31jan03)
  • Improved Throughput Estimates requested by Review Committee (20jan03)
  • Performance Results requested by Review Committee (21jan03)
  • Improved Throughput Estimates requested by Review Committee (20jan03)
  • Ghosting Discussion requested by Review Committee (15jan03)
  • Report discussing focal-plane tilt (08jan03; PDF) and ghosting (17jan03)
  • Figure showing the above problem schematically (06jan03)
  • Spot diagrams for various wavelengths (0.32--1.1 micron), all at 7 arcmin, and with phi = 0, phi = 90, phi = 180. These demonstrate the PSF does not change strongly with wavelength, even in the UV. This is as expected, since the aberrations will be dominated by the difference in index between fused silica (on average) and air, rather than by the relatively small change in index as a function of wavelength. (18jan03)

    Old material:

    Optics Talk (10:00 am)

    Software Talk (11:20 am)

  • Atmospheric Dispersion vs Z
  • Atmospheric Dispersion vs Airmass
  • ADC-corrected Dispersion vs Z
  • Alternative (rejected) ADC Designs
  • Effects of Tilted Input Beams
  • Linear ADC, Conceptual
  • Linear ADC, Parameters to Vary
  • Spot Diagrams
  • Distortion with ADC

    Click on pictures for PS versions.



    click for PS Atmospheric Dispersion (arcsec) at Keck, relative to reference position of 5000 A light. Note how badly the dispersion increases after about 60-deg zenith distance, nearly doubling at Z = 72-deg.

    The refraction values in this figure were calculated using the Starlink library refraction routines, with parameters for Mauna Kea.



    click for PS Atmospheric Dispersion (arcsec) at Keck, relative to reference position of 5000 A light. Now plotted vs. airmass. Past Z = 60, the airmass changes very rapidly with increasing zenith distance, so that the airmass is more than doubled by Z = 72-deg.



    click for PS Atmospheric Dispersion (arcsec) with the ADC correction. The current ADC design has only 700mm of travel, corresponding to full correction up to Z = 55-deg. (The correction was calcuated using the Phillips-developed IRAF software for producing ADC spot diagrams.)



    click for PS Different ADC design, rejected due to difficulties. On the left is the crossed prism pair (Risley prism), which produces large and variable tilt to the input beam. An Amici prism is a pair of prisms of different glasses (eg crown and flint), combined to give zero-deviation at some design wavelength but still disperse light. A pair of Amici prisms can be crossed as with the Risley prism set (right) to produce zero-deviation of the input beam. However, it is difficult or impossible to find glasses with both the needed dispersive properties and high tranmission over the range 0.3 to 1.1 micron.



    click for PS Schematic showing conceptually the effects of a tilted beam entering a slit (or arbitrary point in the telescope focal plane). The effects are twofold: (1) the pupil is displaced on the grating; and (2) the beam now runs through the instrument at non-design angles, leading to greater aberrations. Thus, any angular deviation of incoming beams must be small with respect to the beam opening angle.



    click for PS Conceptual diagram of Linear ADC design. The second prism exactly cancels the effects of the first prism, leaving only an offset in the positions of the rays (and thus the images in each color). Note that while this design is zero-deviation in angle, there is a large offset in the position of the reference ray (green).



    click for PS Diagram showing the free parameters for exploring improvements in image quality. In the Nelson and Mast study (general Keck ADC), the prism angle of 5-degrees was assumed, and values of alpha, beta and epsilon coarsely tested. Best results were found with the inner surfaces parallel and perpendicular to the optical axis.



    click for PS Spot diagram for current ADC design, full 700mm extension, at 7 arcmin off-axis. For scale, the box is 1-arcsec in Y. This figure is for PA of the prisms at 0-degree wrt the field. Other diagrams are available:
  • No ADC
  • ADC, zero-separation (null position)
  • ADC, 350 mm separation, PA=0
  • ADC, 700 mm separation, PA=0
  • ADC, 700 mm separation, PA=90
  • ADC, 700 mm separation, PA=180



    • click for PS Plate-scale changes introduced by the ADC. As a quick test, the distance between chief rays and centroids for spots located at 4 arcmin and 10 arcmin off-axis were calculated using spot diagrams (above). As seen in this figure, the maximum deviation from "No ADC" is about +/- 0.1 arcsec over 360 arcmin. NB: This does not include any differential-refraction effects from the atmosphere!



    Last modified: 27 nov 02

    Andrew C. Phillips / Lick Observatory

    phillips@ucolick.org