Proposal_Information Title: Mapping the UV Surface Albedo Distribution of Triton Proposal_Category: GO Scientific_Category: Solar System Cycle: 5 Investigators PI_name: Brian C. Flynn PI_Institution: Southwest Research Institute CoI_Name: S. Alan Stern CoI_Institution: Southwest Research Institute CoI_Name: Laurence Trafton CoI_Institution: University of Texas at Austin CoI_Name: John Stansberry CoI_Institution: NASA Ames Research Center CoI_Name: Keith Noll CoI_Institution: Space Telescope Science Institute Abstract: ! Free format text (please update) We propose to measure the albedo distribution of Triton at three rotational epochs in two key ultraviolet and visible bandpasses using the FOC. The unique, high-resolution and UV capabilities of HST will allow us to resolve Triton's disk and to map a new spectral region never before imaged on Triton. Our scientific objectives are to: (1) Place further compositional constraints on Triton's surface by imaging farther into the UV than did Voyager 2; (2) obtain a baseline HST measurement for comparison with future HST albedo maps as seasonal change brings about volatile transport; (3) provide context for FOS spectroscopy of Triton; and (4) enhance confidence in albedo maps obtained from similar FOC imaging of Pluto and other small bodies in the solar system through a comparison of HST and Voyager imaging of Triton. Questions ! Free format text (please update) Observing_Description: We are proposing to obtain FOC images of Triton using the f/96 chain with two filter combinations: F278M+F275W, and F410M+F4ND. These filters were chosen to bracket a well-known UV absorption edge between ~eq3000-3500 Angstrom\ for non-icy materials. The F410M filter overlaps the 4100 Angstrom\ imaging of Voyager 2. Triton has never been mapped at wavelengths below 3500 Angstrom. The FOC obtains its maximum spatial resolution near 3000 Angstrom; this resolution of 0.020-0.034'' (Paresce, pers. comm.) is higher than is possible with WFPC2 and is the best obtainable on HST. It corresponds to ~eq400-700 km per resolution element on Triton's 2700 km diameter disk. To isolate UV light with the F278M bandpass, we will simultaneously employ F275W to supress red leaks. For the blue F410M image configuration, a neutral density filter is required to supress N_max and thus operate the FOC detector in the linear regime. Each image will be exposed to obtain a S/N of ~eq30-40 per resolution element. This will permit the detection of relative albedo contrasts of ~eq8-10 percent pixel-to-pixel, or ~eq5-7 percent between resolution elements, at the two-sigma level. We estimate that this can be accomplished in three orbits per pair of F278M+F275W and F410M+F4ND images. However, because we were granted 8 orbits, one visit will consist of two orbits and will therefore have a shortened UV image. Three pairs of images are requested in three visits on Triton. Each set will consist of a UV and a blue image. These image pairs will be spaced about Triton in rotational phase (Triton rotates in 5.88 days). It is not necessary to obtain all 3 image sets in a single rotation. We request 3 evenly-spaced rotational epochs in order to obtain good overlap, i.e., 60 degrees of longitudinal overlap for two adjacent rotational epochs. For all of the observations, the 512*512 aperture will be combined with an FOC IMAGE mode with 0.014'' pixels, to give a field of view (FOV) of 7.2'' across. This FOV is large compared to Triton's diameter (0.12''). The FOV will also allow us keep Neptune well off the detector, because the Triton-Neptune separation ranges from ~eq12-17'' at mean opposition. We will acquire Triton in the FOC FOV using blind pointing, which the Cycle 5 FOC Handbook describes as the standard acquisition mode for all imaging observations with a FOV>3''. During exposures, HST will be held in Fine Lock mode. Fine Lock provides a pointing stability of 0.005'', which is several times smaller than the optical resolution. Real_Time_Justification: There no special dark time, time-critical, CVZ, or TOO observations required for this program. No special calibrations are needed. However, to achieve the desired goals, each of the 3 requested rotational epochs must be timed to within +/-30 degrees of the specified Triton central meridian longitude, corresponding to an ~eq24 hour window centered on each rotational phase. However, these observations may be obtained out of sequence and at any apparition of the specified rotational phases while the illumination phase angle of Triton (or, equivalently, Neptune) is between 1.70-2.00 degrees (quadrature), when it is changing most slowly. Each desired rotational phase window recurs every 5.88 days. The three requested visits must occur within two Triton rotations (12 days), so that the illumination phase angle varies by only a few percent between visits. None. Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Solar_System_Targets Target_Number: 1 Target_Name: TRITON-000 Description: SATELLITE TRITON Level_1: STD = NEPTUNE Level_2: STD = TRITON Window: CML OF TRITON FROM EARTH BETWEEN -30 30 PHASE OF NEPTUNE FROM EARTH BETWEEN 1.70 2.00 Flux: V=13.47 B-V=0.72 U-B=0.29 SIZE=0.128 Comments: IMAGE TRITON AT CML OF 0 DEG. FLUX PARAMETERS ARE FOR MEAN OPPOSITION. Target_Number: 2 Target_Name: TRITON-120 Description: SATELLITE TRITON Level_1: STD = NEPTUNE Level_2: STD = TRITON Window: CML OF TRITON FROM EARTH BETWEEN 90 150 PHASE OF NEPTUNE FROM EARTH BETWEEN 1.70 2.00 Flux: V=13.47 B-V=0.72 U-B=0.29 SIZE=0.128 Comments: IMAGE TRITON AT CML OF 120 DEG. FLUX PARAMETERS ARE FOR MEAN OPPOSITION. Target_Number: 3 Target_Name: TRITON-240 Description: SATELLITE TRITON Level_1: STD = NEPTUNE Level_2: STD = TRITON Window: CML OF TRITON FROM EARTH BETWEEN 210 270 PHASE OF NEPTUNE FROM EARTH BETWEEN 1.70 2.00 Flux: V=13.47 B-V=0.72 U-B=0.29 SIZE=0.128 Comments: IMAGE TRITON AT CML OF 240 DEG. FLUX PARAMETERS ARE FOR MEAN OPPOSITION. Visits Visit_Number: 1 Visit_Requirements: GROUP 1-3 WITHIN 12D BETWEEN 20-SEP-95 AND 11-NOV-95 Visit_Comments: ALL 3 VISITS MUST BE DONE WITHIN 2 TRITON ROTATIONS BETWEEN 22-MAR-96 AND 16-MAY-96 is also OK. Exposure_Number: 01 Target_Name: TRITON-000 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 1539S Special_Requirements: SEQ 1-2 NON-INT Comments: BLUE IMAGE Exposure_Number: 02 Target_Name: TRITON-000 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Number_of_Iterations: 1 Time_Per_Exposure: 373S Special_Requirements: END ORBIT EXPAND Comments: BLUE IMAGE Exposure_Number: 03 Target_Name: TRITON-000 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Number_of_Iterations: 1 Time_Per_Exposure: 1350S Special_Requirements: SEQ 3-4 NON-INT Comments: BLUE IMAGE Exposure_Number: 04 Target_Name: TRITON-000 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 959S Special_Requirements: EXPAND END ORBIT Comments: UV IMAGE Exposure_Number: 05 Target_Name: TRITON-000 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Number_of_Iterations: 1 Time_Per_Exposure: 1601S Special_Requirements: SEQ 5-6 NON-INT Comments: UV IMAGE Exposure_Number: 06 Target_Name: TRITON-000 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Number_of_Iterations: 1 Time_Per_Exposure: 826S Special_Requirements: EXPAND END ORBIT Comments: UV IMAGE Visit_Number: 2 Visit_Requirements: BETWEEN 20-SEP-95 AND 11-NOV-95 Visit_Comments: ALL 3 VISITS MUST BE DONE WITHIN 2 TRITON ROTATIONS BETWEEN 22-MAR-96 AND 16-MAY-96 is also OK. Exposure_Number: 01 Target_Name: TRITON-120 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 1539S Special_Requirements: SEQ 1-2 NON-INT Comments: BLUE IMAGE Exposure_Number: 02 Target_Name: TRITON-120 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Number_of_Iterations: 1 Time_Per_Exposure: 373S Special_Requirements: END ORBIT EXPAND Comments: BLUE IMAGE Exposure_Number: 03 Target_Name: TRITON-120 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Number_of_Iterations: 1 Time_Per_Exposure: 1350S Special_Requirements: SEQ 3-4 NON-INT Comments: BLUE IMAGE Exposure_Number: 04 Target_Name: TRITON-120 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 959S Special_Requirements: END ORBIT EXPAND Comments: UV IMAGE Exposure_Number: 05 Target_Name: TRITON-120 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Number_of_Iterations: 1 Time_Per_Exposure: 1601S Special_Requirements: SEQ 5-6 NON-INT Comments: UV IMAGE Exposure_Number: 06 Target_Name: TRITON-120 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Number_of_Iterations: 1 Time_Per_Exposure: 826S Special_Requirements: END ORBIT EXPAND Comments: UV IMAGE Visit_Number: 3 Visit_Requirements: BETWEEN 20-SEP-95 AND 11-NOV-95 Visit_Comments: ALL 3 VISITS MUST BE DONE WITHIN 2 TRITON ROTATIONS BETWEEN 22-MAR-96 AND 16-MAY-96 is also OK. Exposure_Number: 01 Target_Name: TRITON-240 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 1539S Special_Requirements: SEQ 1-2 NON-INT Comments: BLUE IMAGE Exposure_Number: 02 Target_Name: TRITON-240 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F4ND Number_of_Iterations: 1 Time_Per_Exposure: 373S Special_Requirements: END ORBIT EXPAND Comments: BLUE IMAGE Exposure_Number: 03 Target_Name: TRITON-240 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F410M,F1ND,F2ND Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 631S Special_Requirements: SEQ 3-4 NON-INT Comments: BLUE IMAGE. NEUTRAL DENSITY CHANGED FROM F4ND TO F1ND+F2ND TO INCREASE COUNT RATE AND SHORTEN EXPOSURE TIME FOR THIS BLUE EXPOSURE. COUNT RATE WILL BE IN NON-LINEAR RANGE, BUT ALBEDO CONTRASTS WILL BE PRESERVED. Exposure_Number: 04 Target_Name: TRITON-240 Config: FOC/96 Opmode: IMAGE Aperture: 512X512 Sp_Element: F278M,F275W Optional_Parameters: CHECK-FILTER=YES Number_of_Iterations: 1 Time_Per_Exposure: 1580S Special_Requirements: END ORBIT EXPAND Comments: UV IMAGE Data_Distribution Medium: 8MM Blocking_Factor: 10 Ship_To: PI_Address Ship_Via: UPS ! UPS (2-day) or OVERNIGHT