ASTR633 Astrophysical Techniques Course slides Chapter 4: Optical - - PowerPoint PPT Presentation

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ASTR633 Astrophysical Techniques Course slides Chapter 4: Optical - - PowerPoint PPT Presentation

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ASTR633 Astrophysical Techniques Course slides Chapter 4: Optical and Infrared Imaging; Astrometry Optical camera schematic Figure 4.1 Field correcting optics Figure 4.2 Refraction n i sin Z i = n ( i 1) sinZ ( i 1) => P 0 = 760

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ASTR633 Astrophysical Techniques Course slides

Chapter 4: Optical and Infrared Imaging; Astrometry

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Figure 4.1

Optical camera schematic

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Figure 4.2

Field correcting optics

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Refraction

ni sin Zi = n(i−1)sinZ(i−1)

=>

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P0 = 760 mm, T0 = 288 K

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Chromatic Aberration

Kaczmarczik et al. 2009, AJ, 138, 19

Note scale!!!

Affects images and spectroscopy

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Differential Chromatic Aberration

Kaczmarczik et al. 2009, AJ, 138, 19

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Figure 4.3

Near-infrared camera

For more details, talk to Klaus Hodapp, or take his instrumentation course!

Multiple mirrors fold light path and enable cold optics and small filters but also lead to vignetting and ghost images

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Figure 4.4

Nyquist sampling

=> want pixel angular size on sky < λ/2D

aliasing

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Why image?(!)

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How to take good data

Repetition Consistency

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Calibration

i.e., how to make a uniform image so that a star would have the same value if it were moved to a different part of the image

  • Multiple dithered exposures
  • Bias frame (or “offset”)
  • Dark frame
  • Flat field (possibly created from data => “response”)
  • Pre-process to remove read-out artifacts
  • Median filter to remove cosmic rays

Note that the book assumes response created from data and therefore has the same integration time. If you use a separate flat field, scale counts appropriately by integration time.

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Coordinate Systems: horizon

a.k.a. “alt-az”

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Coordinate Systems: horizon

a.k.a. “alt-az” Advantages:

  • Things like to be built perpendicular to local gravity.

E.g. often used as reference frame to describe where telescopes are physically pointed.

  • Stars are only visible with altitude > 0°

Disadvantages:

  • Coordinates of stars depend on observing location,

because the Earth is really a sphere, not a plane.

  • Even at one specific site, the position of a star varies

with time as the Earth rotates.

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Coordinate Systems: equatorial

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Coordinate Systems: equatorial

Spherical trigonometry gives the conversion between alt-az and HA-δ, given your latitude, φ North Star = Hokupa’a in Hawaiian = “star that never moves”

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The book that almost killed my high school interest in astronomy…

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Coordinate Systems: equatorial

https://en.wikipedia.org/wiki/Right_ascension

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Coordinate Systems: Ecliptic

https://en.wikipedia.org/wiki/Ecliptic_coordinate_system

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Coordinate Systems: Galactic

https://commons.wikimedia.org/w/index.php?curid=20028939

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Coordinate Transformations

http://docs.astropy.org/en/stable/coordinates/

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Time

Solar and sidereal time differ by

  • ne day over one year (i.e. stars

rise 4 minutes earlier each day). Local Sidereal Time, LST = 0h when the Vernal Equinox is on the observer’s meridian

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Time

HA = LST - RA

Negative hour angles mean the source is rising, positive => setting Qu: What is the RA of a transiting source?

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Practical matters for the Observer

At the telescope:

  • LST (software) clock tells the RA of objects that are transiting. This provides the

quick initial guide to what is observable at that moment.

  • The δ of the target and its hour angle (= LST – RA) determines the altitude of the
  • target. At HA=0, the target is transiting and at maximum altitude.
  • It is preferable to observe targets when they are high in the sky, i.e. high altitude =

small zenith angle = low airmass, due to better image quality and less atmospheric

  • absorption. Altitude > 30° (airmass < 2.0) is the typical practical limit.
  • At Mauna Kea (latitude = 20°), this means targets with δ> -40° can be observed. Of

course, you can go to more southern targets if you are desperate.

  • The observing “window” (duration above 30° altitude) depends on δ. Targets with δ

similar to your latitude have the longest window.

  • Remember the factor of 15 conversion between RA(sec of time) and RA (arcsec).
  • Remember the cos δ term when computing the RA offset from 1 position to another.
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Precession

https://en.wikipedia.org/wiki/Precession

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Nutation

https://en.wikipedia.org/wiki/Nutation

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FITS headers

RA(i) = CRVAL1 + (i — CRPIX1) * CDELT1 i = 1, 2, 3,… NAXIS1

More generally, CD1_1, etc provide a matrix transformation between pixel (i,j) and sky position (x,y) — see Griesen et al. 2002

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Movement within the ICRS

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Stellar Aberration

http://cseligman.com/text/history/bradley.htm

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Stellar Aberration

https://commons.wikimedia.org/wiki/File:Stellar_aberration.JPG

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Stellar Aberration

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Parallax and Aberration

http://cseligman.com/text/history/bradley.htm