8218( 6) - 07/21/99 09:38 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8218 Version: 6 Check-in Date: 21-Jul-1999 09:37:30 1.Proposal Title: Exploring Triton in the Act of Global Change ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Alan Stern Southwest Research Institute CoI: Dr. Marc Buie Lowell Observatory N CoI: Dr. Bonnie Buratti JPL N CoI: Dr. Joel Parker Southwest Research Institute N CoI: Dr. John Stansberry Lowell Observatory N CoI: Dr. Larry Trafton McDonald Observatory N ------------------------------------------------------------------------------------ 5. Abstract Triton appears to be undergoing the start of a long-awaited, and perhaps fundamental, global seasonal change. As a result of this change, seasonally deposited ices should sublime into the atmosphere, revealing extensive areas of the more permanent underlying surface, most likely with different photometric properties. We propose a 3-orbit Cycle 8 program consisting of STIS mid-UV spectroscopy and VIS/mid-UV imaging in order to better quantify the changes now underway, and to provide a critical benchmark for Triton's photometric state before still more time elapses. HST is required for this work owing to Triton's small (0.13") apparent diameter and the need for UV observations. ------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 2] Observations Description ------------------------ Our goal is to provide a snapshot of Triton's UV properties early during the global change, in 1999. We propose to accomplish this by obtaining STIS mid-UV spectra and VIS/mid-UV images at 3 epochs as Triton revolves synchronously around Neptune (period, 5.88 days). The 3 observations do not have to all be carried out in a single Triton orbit, but it is necessary to space them relatively closely in time (within 24 days of each other) to minimize phase angle differences and other time-base related issues. An additional scheduling constraint is determined by the range of observable Sun-Triton-Earth phase angles so as to match the phase angle of the 1995 HST/FOC imaging observations of Triton. At each of the 3 requested observation epochs, we will obtain a STIS mid-UV spectrum of Triton, and visible and UV STIS images of Triton (each of the image observations will be a set of four dithered images). Each observing epoch will require 1 orbit. We have chosen STIS owing to its solar-blind UV capability, PSF sampling, and its UV spectroscopic capabilities. We have used the STIS Exposure Time Calculator (ETC) to calculate the S/N and exposure times for the spectra and images using the following parameters: the spectrum of Triton has a slightly redder color slope than the sun, so we model Triton with a G5V Kurucz spectrum; Triton has a total magnitude of V~13.5, and the corresponding surface brightness assuming a diameter of 0.13 arcsec is V~8.8/arcsec^2; average zodiacal light and earthshine backgrounds; E(B-V)=0. Also note that according to the STIS Instrument Handbook, the S/N calculations are per resolution element, which is 2 pixels for spectroscopy and 2x2 pixels for imaging. We have verified that these spectroscopic and image observations will not exceed any detector count rate limits. Also the observations will be constrained to relatively large separations between Triton and Neptune so as to keep Neptune and most Neptune-scattered light out of the frame. The STIS spectroscopic observations will be performe d using the NUV-MAMA, grating G230L, and the 52x0.5" slit. The STIS spectroscopy ETC estimates that in a 1150 sec exposure on Triton, S/N ~ 12, 36, and 40 can be obtained around 2200 A, 2700 A, and 3000 A, respectively. The spectral scale for the G230L is 1.58 A/pixel, so each 2-pixel resolution element in the S/N calculation corresponds to 3.2 A. When summed into 6 A binning, which is more than sufficient for studying surface reflectance (and which was used for the FOS Triton data analysis we published in 1995), one predicts S/N values of 16, 50, and 55, around 2200 A, 2700 A, and 3000 A, respectively. Such precision is capable of detecting 2-10 percent changes in UV reflectance relative to the 1992-1993 FOS data. Now consider the imagery. On each orbit we propose to obtain one NUV-MAMA image and one visible-CCD image. The NUV-MAMA image will be obtained with the 2700 A continuum (F25CN270: 2525-2875 A) filter. This filter bandpass is nicely comparable to the FOC mid-UV images obtained by HST in 1995 using the in-series filter combination F275W+F278M. The 2700 A NUV-MAMA images will exhibit a FOV of 25x25" and have a plate scale of 0.025", and will closely match the 0.03" Rayleigh criterion resolution limit of HST at 2700 A. However, we will perform a four-point dither for this observation to smooth out flatfield variations and improve the sampling. Although this filter has some non-trivial red leak, ETC and SYNPHOT calculations show that only 1 count in 10 on Triton will come from Lambda>3000 A. The objective here is to obtain a crude map of Triton at 2700 A that is concurrent with the MUV spectroscopy. This map will allow us to search for changes relative to both the 1995 HST/FOC images, and (though with somewhat less confidence) relative to the Voyager 2 low-resolution images obtained globally about a week prior to encounter at 3500 A (Voyager's shortest imaging wavelength). The other Triton image we propose to obtain on each orb it will consist of a CCD image with the blue-band [OII] filter (F28X50 OII: 3700-3780 A), which we have chosen to isolate the blue part of Triton's reflectance. This image will have a 28x50" FOV and a 0.05"/pixel plate scale. We will perform a four-point dither for this observation to smooth out flatfield variations and improve the sampling. The primary purposes of the visible-CCD image are to: (i) provide a visible-wavelength reference image to compare to the UV image and (ii) to provide concurrent visible-wavelength photometry for comparison to the UV imaging and spectroscopy derived photometry we also plan to obtain. Exposure time calculations using the STIS ETC reveal that the CCD/F28X50OII image will obtain S/N=80 (per 2x2 pixel resolution element) in 50 sec of exposure time on Triton. We will perform a 4-point dither of the CCD, each dithered image will have a 50 sec exposure time. Similarly, the STIS ETC estimates that the NUV- MAMA/F25CN270 image will yield S/N~36 (per 2x2 pixel resolution element) on Triton in 150 sec of exposure t ime. We will perform a 4-point dither of the MAMA, each dithered image will have a 150 sec exposure time, so the combined image will have S/N values around 70. No special calibrations are required to analyze the proposed dataset. In this Phase II proposal we have maximized the integration times and minimized STIS buffer bumps to optimize the efficiency of each orbit. The 3 visits each require 1 orbit, for a total request of 3 orbits. Real Time Justification ----------------------- There are no Real Time constraints. We do impose two scheduling constraints: [1] The 3 visits do not have to all be carried out in a single Triton orbit, but it is necessary to space them relatively closely in time (within 24 days of each other) to minimize phase angle differences and other time-base related issues; [2] the range of observable Sun-Triton-Earth phase angles must be close to the phase angle of the 1995 HST/FOC imaging observations of Triton. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 4] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 1 | TARGET NAME: TRITON-000 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Triton ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=NEPTUNE | STD=TRITON | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | CML OF TRITON FROM EARTH BETWEEN 335 | 345, SEP OF TRITON NEPTUNE FROM EARTH GT | 11" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 13.5+/-0.2 | The separation constraint is to TYPE=G2V | minize scattered light contamination F-CONT(2700)=1.5+/-0.3e-15 | from Neptune. The CML constraint is F-CONT(2000)=1.7+/-0.5e-16 | to allow us to match the longitudes | of the HST/FOC observations of | Triton that were made by Flynn et | al. in 1995. | ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 2 | TARGET NAME: TRITON-120 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Triton ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=NEPTUNE | STD=TRITON | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | CML OF TRITON FROM EARTH BETWEEN 95 105, | SEP OF TRITON NEPTUNE FROM EARTH GT 11" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 13.5+/-0.2 | The separation constraint is to TYPE=G2V | minize scattered light contamination F-CONT(2700)=1.5+/-0.3e-15 | from Neptune. The CML constraint is F-CONT(2000)=1.7+/-0.5e-16 | to allow us to match the longitudes | of the HST/FOC observations of | Triton that were made by Flynn et | al. in 1995. | ------------------------------------------------------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 5] TARGET LIST Solar System Targets ------------------------------------------------------------------------------------------------------------------------------------ TARGET NUMBER: 3 | TARGET NAME: TRITON-240 ------------------------------------------------------------------------------------------------------------------------------------ TARGET DESCRIPTION: SATELLITE Triton ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 1 | TARGET POSITION LEVEL 2 | STD=NEPTUNE | STD=TRITON | ------------------------------------------------------------------------------------------------------------------------------------ TARGET POSITION LEVEL 3 | WINDOWS | | CML OF TRITON FROM EARTH BETWEEN 215 | 225, SEP OF TRITON NEPTUNE FROM EARTH GT | 11" | ------------------------------------------------------------------------------------------------------------------------------------ DATA | COMMENTS | V = 13.5+/-0.2 | The separation constraint is to TYPE=G2V | minize scattered light contamination F-CONT(2700)=1.5+/-0.3e-15 | from Neptune. The CML constraint is F-CONT(2000)=1.7+/-0.5e-16 | to allow us to match the longitudes | of the HST/FOC observations of | Triton that were made by Flynn et | al. in 1995. | ------------------------------------------------------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 6] Visit: 01 Visit Priority: Visit Requirements: GROUP 01, 02, 03 WITHIN 24 D BETWEEN 13-Aug-1999 AND 18-Sep-1999 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 TRITON-000 STIS/N ACCUM 52X0.5 G230L 2376 1 1153 S EXPAND UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 TRITON-000 STIS/C ACCUM F28X50O MIRROR PATTERN=SPIRAL-DITH,NUM-POS=4, 1 50 S CD II STEP-SIZE=0.175,CR-SPLIT=NO, SIZEAXIS2=900 Comments: The dither pattern is to effectively sample each 0.050 arcsecond pixel by a factor of 2x2. The step size is selected to be exactly a factor of 7/2 of a pixel size, so that the dither will be sampling different pixels in each step. We are reading a subarray to minimize the overhead due to STIS buffer dumps. ------------------------------------------------------------------------------------------------------------------------------------ 30 TRITON-000 STIS/N ACCUM F25CN27 MIRROR PATTERN=SPIRAL-DITH,NUM-POS=4, 1 150 S UV-MAM 0 STEP-SIZE=0.084 A Comments: The dither pattern is to effectively sample each 0.024 arcsecond pixel by a factor of 2x2. The step size is selected to be exactly a factor of 7/2 of a pixel size, so that the dither will be sampling different pixels in each step. ------------------------------------------------------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 7] Visit: 02 Visit Priority: Visit Requirements: GROUP 01, 02, 03 WITHIN 24 D BETWEEN 13-Aug-1999 AND 18-Sep-1999 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 TRITON-120 STIS/N ACCUM 52X0.5 G230L 2376 1 1153 S EXPAND UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 TRITON-120 STIS/C ACCUM F28X50O MIRROR PATTERN=SPIRAL-DITH,NUM-POS=4, 1 50 S CD II STEP-SIZE=0.175,CR-SPLIT=NO, SIZEAXIS2=900 Comments: The dither pattern is to effectively sample each 0.050 arcsecond pixel by a factor of 2x2. The step size is selected to be exactly a factor of 7/2 of a pixel size, so that the dither will be sampling different pixels in each step. We are reading a subarray to minimize the overhead due to STIS buffer dumps. ------------------------------------------------------------------------------------------------------------------------------------ 30 TRITON-120 STIS/N ACCUM F25CN27 MIRROR PATTERN=SPIRAL-DITH,NUM-POS=4, 1 150 S UV-MAM 0 STEP-SIZE=0.084 A Comments: The dither pattern is to effectively sample each 0.024 arcsecond pixel by a factor of 2x2. The step size is selected to be exactly a factor of 7/2 of a pixel size, so that the dither will be sampling different pixels in each step. ------------------------------------------------------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 8] Visit: 03 Visit Priority: Visit Requirements: GROUP 01, 02, 03 WITHIN 24 D BETWEEN 13-Aug-1999 AND 18-Sep-1999 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 TRITON-240 STIS/N ACCUM 52X0.5 G230L 2376 1 1153 S EXPAND UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 20 TRITON-240 STIS/C ACCUM F28X50O MIRROR PATTERN=SPIRAL-DITH,NUM-POS=4, 1 50 S CD II STEP-SIZE=0.175,CR-SPLIT=NO, SIZEAXIS2=900 Comments: The dither pattern is to effectively sample each 0.050 arcsecond pixel by a factor of 2x2. The step size is selected to be exactly a factor of 7/2 of a pixel size, so that the dither will be sampling different pixels in each step. We are reading a subarray to minimize the overhead due to STIS buffer dumps. ------------------------------------------------------------------------------------------------------------------------------------ 30 TRITON-240 STIS/N ACCUM F25CN27 MIRROR PATTERN=SPIRAL-DITH,NUM-POS=4, 1 150 S UV-MAM 0 STEP-SIZE=0.084 A Comments: The dither pattern is to effectively sample each 0.024 arcsecond pixel by a factor of 2x2. The step size is selected to be exactly a factor of 7/2 of a pixel size, so that the dither will be sampling different pixels in each step. ------------------------------------------------------------------------------------------------------------------------------------ 8218( 6) - 07/21/99 09:38 - [ 9] Summary Form for Proposal 8218 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 52X0.5 F28X50OII F25CN270 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/NUV-MAMA STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters PATTERN=SPIRAL-DITH NUM-POS=4 STEP-SIZE=0.175 CR-SPLIT=NO SIZEAXIS2=900 STEP-SIZE=0.084 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 01, 02, 03 WITHIN 24 D BETWEEN 13-Aug-1999 AND 18-Sep-1999 EXPAND ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G230L MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names TRITON-000 TRITON-120 TRITON-240 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 ------------------------------------------------------------------------------------------------------------------------------------