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Telescope Performance

Telescope Performance. 2007 Visiting Committee Meeting Phil Perillat. Talk Outline. Pointing Specs and how we point Current pointing Errors Pointing related issues. Telescope Gain and T SYS Current gain for lband and cband Alfa T SYS and calibration diode stability.

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Telescope Performance

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  1. Telescope Performance 2007 Visiting Committee Meeting Phil Perillat NAIC Visiting Committee Meeting · February 19-21, 2007

  2. Talk Outline • Pointing • Specs and how we point • Current pointing Errors • Pointing related issues. • Telescope Gain and TSYS • Current gain for lband and cband • Alfa TSYS and calibration diode stability. More info: http://www.naic.edu/~phil/ NAIC Visiting Committee Meeting · February 19-21, 2007

  3. Pointing specs • Motion: • Radius of curvature:870’, paraxial surface:435’ • 1/8” motion in paraxial surface = 5 arcsec error. • 1 lambda motion along focus = 3 db gain loss. • 5 deg F moves the platform 1 inch vertically. • 24 kips lowers platform 1” (td’s left alone). • Assuming tiedowns not returned to original tension. • Receiver beamwidths: • Lband 200 asecs, Xband: 30 asecs NAIC Visiting Committee Meeting · February 19-21, 2007

  4. How we point • Tiedowns and distomats. • Remove temperature variability. • Tiedowns have 24” stroke, 120 kips max tension per tiedown. • Pointing model. • Removes repeatable errors caused by deflections and misalignments. • New model takes 10 days (nights) tracking 20 sources rise to set. NAIC Visiting Committee Meeting · February 19-21, 2007

  5. Pointing errors • Historical/current pointing errors from previous models NAIC Visiting Committee Meeting · February 19-21, 2007

  6. Pointing issues • 2005 distomat failures • Failing 10 to 20% of the time • Platform height adjustment needs all 6 distomats. • New type of distomat window solved the problem. • Azimuth encoder failures. • Corrosion inside encoders causing failures. • Switched to encoders with better seals (after overcoming rfi problems). • People who do the actual work: • Jorge Rodriguez & Hector Camacho NAIC Visiting Committee Meeting · February 19-21, 2007

  7. Tiedown cables losing tension • This can occur when: • Azimuth arm points at tower, dome at high za, and temp > 80F. • Incorrect platform tilt causes a pointing error. • Example 1: 2004 calibration run shows size of error • 10am, za=16.5, az=155, temp. 84 F. Errs > 1 arcmin. • Example 2: 13sep06 ALFA meridian nodding. • 9am, 83F. Tension loss occurred on at least 7 days of this project. • Solution to cable tension loss: • Tertiary mirror position controlled by 5 motors (in progress) • Normal tertiary position (where pointing model made). • Alternate position with longer optical path for warm weather. • Increases dynamic range of cable tension. NAIC Visiting Committee Meeting · February 19-21, 2007

  8. Recent System Performance • Data taken Dec06 through Feb07. • Lband (1400 Mhz) and Cband (5000 Mhz) • Gain (K/Jy) • SEFD (Jy) System Equivalent Flux Density • When comparing two receivers, SEFD is not biased by errors in the calibration diode values. • TSYS (K) • BeamWidths (Arcseconds) NAIC Visiting Committee Meeting · February 19-21, 2007

  9. ALFA TSYS 2005 to 2007 • ALFA spectral line data archived online and accessible via idl routines. • ALFALFA cal measurements 2005 thru 2007 used to check ALFA TSYS and calibration diode stability. • Data taken in drift scan mode at the meridian. • 10,000+ measurements (1 per 10 minute strip) • TSYS Vs. sample • Normalized TSYS Vs. sample • TSYS Vs. RA (right ascension) • TSYSA/TSYSB ratio Vs. sample NAIC Visiting Committee Meeting · February 19-21, 2007

  10. Summary • STATUS • Current pointing of 7” is still useable. • Lband SEFD=2.5 Jy. • Cband SEFD=2.9 Jy. • GBT webPage@5Ghz SEFD: 25K/1.75Jy=14.3 Jy. • ALFA TSYS (and calibration diode values) have not been jumping by large amounts. • TO COMPLETE: • Make new model after painting. • Tertiary control will help solve the pointing problem when the tiedown cables go loose. NAIC Visiting Committee Meeting · February 19-21, 2007

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