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

2. NAIC Visiting Committee Meeting · February 19-21, 2007 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. More info: http://www.naic.edu/~phil/

3. NAIC Visiting Committee Meeting · February 19-21, 2007 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 Pointing specs

4. NAIC Visiting Committee Meeting · February 19-21, 2007 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.

5. NAIC Visiting Committee Meeting · February 19-21, 2007 Pointing errors Date Az Err (arcsec) Za Err (arcsec) Total Err (arcsec) 19983.822.374.49 19995.43.16.2 20005.683.456.65 20035.383.486.41 20044.202.414.84 Dec06 to Feb07 4.95.17.1 Historical/current pointing errors from previous models

6. NAIC Visiting Committee Meeting · February 19-21, 2007 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

7. NAIC Visiting Committee Meeting · February 19-21, 2007 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.

8. NAIC Visiting Committee Meeting · February 19-21, 2007 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. – T SYS (K) –BeamWidths (Arcseconds)

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

10. NAIC Visiting Committee Meeting · February 19-21, 2007 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 T SYS (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.