Increased Thermal Background for the post-NCS NICMOS TIPS – June 19, 2003 Megan Sosey NICMOS

Cycle 11 Calibration Plan  Parallel observations in NIC3:F222m, NIC2: F222m & F237M were crafted to re-measure the thermal background for cycles 11 and beyond -See proposals 9269 and 9702  Accounting for DQE increase, the average thermal background was predicted to be approximatly 20% higher than in Cycle 7

So, Where’s the extra signal?  NO correlation was found between the variation in the thermal background and –telescope pointing –sun angle –prime instrument –time since SAA passage –or spacecraft orientation  The T-1-1 temperature sensor shows the detectors are stable under operation of the NCS and do not change by more than 0.1K on long time scales

Telescope Warm Season Commanded Set- point change T-1-1 Temperature Sensor vs. Time 9/10/ /30/2003

Thermal Stability of the NICMOS Enclosure and Telescope Assembly  NICMOS elements which affect the TB: re-imaging mirror (RM), FOM, PAM, filters, FDA(imaging mirrors), cold mask, bend mirrors, baffles, pupil  HST optics which affect the TB: primary mirror, secondary mirror, spiders,pads – these play significant roles in the TB because of their high temperatures (~290K)

Representation of Cold Mask Misalignment See Robberto, Proc. SPIE v4013, p.386 and Krist, NICMOS- ISR for more details on the cold mask alignment

Rough Optical Path Diagram for the NICMOS Fore-Optics

Thermal stability of the HST optics and aft shroud  The primary and secondary have remained thermally stable, introducing no extra thermal emission - however, the aft shroud has increased approximately 10K since Cycle 7  There are multiple thermistor sensors (thermally sensitive resistors) in the aft shroud which are used to monitor temperature - TAFTBULK is a weighted average of the internal sensors located on the aft bulkhead -TASINAFB is an effective sink measurement from all the internal sensors, including some of the ones from the TAFTBULK average

Thermal Environment of the HST Aft Shroud around NICMOS

Aft Temperature Comparison of HST and NICMOS naftbtmp NIC3, F222m HST AFT TASINAFB TAFTBULK

C7 & C11 NICMOS Aft Temps

KeyDescrip. Avg. C7 Temp Avg. C11 Temp Delta T nfob2tmpFore-optical brkt C-0.48 C+5.34 C ndosftmpDewar Fore-7.27 C-1.21 C+6.06 C ntrs3tmpTruss Aft1.22 C7.52 C+6.3 C ndosatmpDewar Aft-1.60 C5.30 C+7.24 C naftbtmpAft NIC Encl.8.5 C17.5 C+9.0 C taftbulkHST Aft Shroud -17 C-7 C+10.0 C NICMOS Temperature Variations

 The NIC fore-optics swing in temperature in the same manner as the aft end, but with a smaller amplitude

Direct plot of thermal background vs fore-optics temperature

Does this explain the increase?  The previous plot indicates that a 2K variation in temperature, translates to a 1.5DN change in the thermal background  An increase of 5K between Cycle 7 and Cycle 11 yields an additional 3DN of thermal background signal! - this makes up the difference between the predicted and observed thermal background

Simple blackbody comparison of HST and NICMOS optical surfaces using NIC2 Optical Parameters

Comparing to the NICMOS Thermal Background Code Estimates

Conclusions The overall temperature environment of NICMOS and the operating temperature of the detectors has changed since cycle7 resulting in: Increase of ~17K in detector temperature General temp. increase in the aft HST Increased temperatures in the aft NICMOS Increased thermal background in long wavelength, or extra wide, camera 2 and camera 3 filters Thermal background exposures are still affective in removing the excess signal