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ASTR-3030 Methods & Instrumentation

ASTR-3030 Methods & Instrumentation. Day 4. Announcements. Read Chapter 2 & 3 Homework Set 1: Due Thursday Sept. 15. Observing Project Design. What are the types of targets? (bright/faint, moving/stationary …) comets, asteroids, planets, stars, variable stars, clusters, galaxies

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ASTR-3030 Methods & Instrumentation

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  1. ASTR-3030Methods & Instrumentation Day 4

  2. Announcements Read Chapter 2 & 3 Homework Set 1: Due Thursday Sept. 15

  3. Observing Project Design What are the types of targets? (bright/faint, moving/stationary …) comets, asteroids, planets, stars, variable stars, clusters, galaxies Type of program (photometry, spectroscopy, polarimetry) If photometric, do you have a backup plan? If differential, are there adequate comparison stars in the FOV? Wavelength regime (UVis, NIR, MIR, FIR, sub-mm, microwave) water vapor could be a concern Time of year (when does it transit at midnight?) Position of the Moon? Time of twilights? Airmass constraints? Targets – west to east. Build finder charts

  4. APSU Observatory Log book – if you open the door, fill it out. Who? Doing what? How long (open/close times)? Sky conditions (estimate cloud cover?) Any problems (also let Dr. Smith/Buckner know) Lat: 36:33.7041 N Long: 87:20.5071 W Alt: 573 ft (174.6504m)

  5. APSU Observatory Procedures manual being built – feel free to contribute Y’all will be the beta testers anyway. Working on polar alignment Iterative process (good class project (or subset) – a couple of nights) Pointing Model (T-Point map) Also a good class project (good fraction of a night) Install CCD and rebalance (Another good class project)

  6. General Optics Stuff • Univ. Virginia site Examples from book

  7. Cassegrain Systems • The radii of curvature of the primary and secondary mirrors, respectively, in a two-mirror Cassegrain configuration are • and • where • F is the effective focal length of the system, • B is the back focal length (the distance from the secondary to the focus), and • D is the distance between the two mirrors. • If, instead of B and D, the known quantities are the focal length of the primary mirror, f1, and the distance to the focus behind the primary mirror, b, then D = f1(F − b) / (F + f1) and B = D + b.

  8. Ritchey – Chrétien Optics For a Ritchey–Chrétien system, the conic constantsK1 and K2 of the two mirrors are chosen so as to eliminate third-order spherical aberration and coma; the solution is and where M = F / f1 = (F − B) / D is the secondary magnification. Note that K1 and K2 are less than − 1 (since M > 1), so both mirrors are hyperbolic. (The primary mirror is typically quite close to being parabolic, however.) The hyperbolic curvatures are difficult to test, especially with equipment typically available to amateur telescope makers or laboratory-scale fabricators; thus, older telescope layouts predominate in these applications. However, professional optics fabricators and large research groups test their mirrors with interferometers. A Ritchey–Chrétien then requires minimal additional equipment, typically a small optical device called a null corrector that makes the hyperbolic primary look spherical for the interferometric test. On the Hubble Space Telescope, this device was positioned incorrectly (due to an unnoticed paint flake being lodged in the test mounting) leading to the error in the Hubble primary mirror.

  9. Ritchey – Chrétien Examples • The 10.4 m Gran Telescopio Canarias at Roque de los Muchachos Observatory • The two 10 m telescopes of the Keck Observatory • The four 8.2 m telescopes comprising the Very Large Telescope in Chile • The 8.2 m Subaru telescope at Mauna Kea Observatory • The two 8 m telescopes comprising the Gemini Observatory • The 3.9 m Anglo-Australian Telescope at Siding Spring Observatory (Australia) • The 3.5 m Calar Alto Observatory telescope at mount Calar Alto (Spain) • The 3.5 m Herschel Space Observatory currently operating in orbit at the L2 point 1.5 million km from Earth • The 3.5 m WIYN Observatory at Kitt Peak National Observatory • The 2.5 m Sloan Digital Sky Survey telescope (modified design) at Apache Point Observatory, New Mexico, U.S.A. • The 2.4 m Hubble Space Telescope currently in orbit around the Earth • The second RCT was a 102 cm (40 in) instrument constructed by Ritchey for the United States Naval Observatory; that telescope is still in operation at the Naval Observatory Flagstaff Station.

  10. USNO-FS 40-inch

  11. General Optics • Focal Length – property of the optics surface. • Radius of curvature (mirror) • Radii of curvatures and index of refraction (lens) • Focal Ratio (f/#) = F.L. / Diameter of “primary” • Smaller number is “faster” system • APSU telescope: • Primary mirror: f/6.2, 0.5m • Focal arrangement: Ritchey – Chrétien

  12. Plate Scale & Image Size • WIYN 0.9m telescope: • Primary mirror: f/7.5, 0.9m • FL = (0.9m)*(7.5) = 6.75m = 6750 mm • S2KB CCD detector (SITe chip) • (2048)X(2048) pixels; 21m square • Plate scale = 206265 (arc-sec/radian)/FL(mm) • = 206265 ̋/6750 mm = 30.5578 ̋̋̋/mm = 0.0305578 ̋/m • 21 m pixels = 0.6417 arc-sec/pixel • = 1314.23 arc-sec/chip side = 21.9 arc-min • Advertised as 20 arc-min  VERIFY your scale size!

  13. Gain & Read Noise in Detectors • WIYN 0.9m telescope – S2KB CCD • Read Noise: 14-15 e-/RMS • Gain = 2.5 e-/ADU • Linearity,0.1%,e-: ~210,000 • Linearity,1.0%,e-: ~230,000 • Column Spillover, e-: ~240,000 • Internal Radiation Event Rate, Events/HR: ~2700 • Dark Current, e-/hr/pix: ~5-10

  14. Gain Calculation • Gain = # electrons per pixel / # counts per pixel • A simple method to calculate the system gain is shown below: • Collect a bias image (zero-integration dark image) and label it "bias". • Collect two even-illumination images and label them "flat1" and "flat2". • Calculate a difference image: diff = flat2 - flat1. • Calculate the standard deviation of the central 100 x 100 pixels in the difference image. • Calculate the variance by squaring the standard deviation and dividing by 2 (variance adds per image, so the variance of the difference image is the sum of the variance of flat1 and flat2). • Calculate a bias-corrected image by subtracting the bias from one of the flat images and label it corr: corr = flat1 - bias. • Obtain the mean illumination level by calculating the mean of the central 100 x 100 region of the corr image. • The mean divided by the variance equals the gain: gain = mean /variance.

  15. Read Noise Calculation • The gain and read-out noise values were determined using the IRAF CL script stisgain which derives these parameters from a pair of flat field frames and a pair of bias frames as follows: • 1. Create a “difference flat” and a “difference bias” for each CCDGAIN setting, e.g., • flatdiff = flat1 - flat2 • biasdiff = bias1 - bias2 • 2. Then calculate the gain and read-out noise as follows: • gain = ( (mean(flat1) + mean(flat2) - mean(bias1) - mean(bias2) ) /((sigma(flatdiff))2 - (sigma(biasdiff))2) • read-out noise = gain * sigma(biasdiff) / sqrt(2) • where the gain is given in electrons per ADU and the read-out noise in electrons. Pairs of • bias frames and flat field frames are used to render the effects of non-flat bias frames and/or flat field frames negligible. Statistics for the bias- and flat field frames were derived in regions where the bias-and flat field frames are reasonably flat and free of dust ‘motes’ (i.e., dust on the CCD window).

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