Ultimate Scattered Light Levels, 24 um

Principal: James Muzerolle
Deputy: Karl Gordon
Data Monkey(s):
Priority: Normal
Downlink Priority: Normal
Analysis Time: 48 hours
Last Updated:

Objective

This task will measure two effects of scattered light on the 24 µm array. One, the amount of light scattered into the FOV when the array is pointing towards the dark position(s). Two, the amount of light scattered into the FOV from a star located off the array near the edge of the FOV.

Description

Part 1: The importance of scattered light in the dark measurements of the MIPS 24 µm array can be measured by pointing SIRTF towards a very bright extended region while performing the standard 24 µm dark measurements. Usually, the dark measurements are taken while SIRTF points at a dark region of the sky. By pointing at a bright region, the amount of scatterd light will be measured by differencing the results of this task with those obtained from the standard dark task. A second set of exposures will be taken of the standard dark sky position in photometry mode with 70 µm as the primary array. This will provide an additional constraint on the actual scattered light component in the darks.

Part 2: Scattered light due to bright sources located just off the edge of the array FOV can be measured by placing a very bright point source near the edge of the FOV and taking standard DCEs. This task will search for reflections in the detector plan which cause glints to be seen when bright stars are beyond, but near the edge of the FOV (ie., in the projected FOV, but not in the detector FOV). Such glints are possible because the lyot stop is larger than the 24 µm array FOV.

A saturating star will be placed so that it is 13 (radial extent of the airy ring in the 24 µm PSF) arcsec off the edge of the array FOV for the standard photometry AOR. The fact that the standard photometry AOR puts the star 64 arcsec (crossscan) and 76 arcsec (scan) from the center of the array is needed to do this task properly.

The saturating star choosen in TY Dra with a 24 µm IRAS flux of 44 Jy.

Data Collected

The part 1 dataset will include the standard 24 µm dark IER, except pointing towards a very bright region of the sky, as well as a standard photometry AOR pointing at the nominal dark sky position.

The part 2 dataset generated for this task is a standard AOR.

Data Reformatting Requirements

Array Data Desired:

24 µm

Data Reformatting Option:

Special Instructions:

Task Dependencies

Calibration Dependencies

Output and Deliverable Products

The first output of this task are measurements (or upper limits) on the scattered light level in the 24 µm dark position. (Also note that the dark sky position photometry also includes 70 and 160 µm data, which should be used for MIPS-341 and 342).

The second output of this task is the location of any glints near the edge of the 24 µn;m array FOV.

Data Analysis

All the datasets will be run through mips_sloper & mips_caler. The dark sky position images should also be coadded with mips_enhancer. These will then be compared to IOC dark images in order to assess the absolute level of scattered light in the darks. The resulting images for part 2 will be examined for glint artifacts. Any glints found will be noted and the locations where the source was when the glints appeared will be listed.

Software Requirements

Actions Following Analysis

Using the dark sky position image and ground-test darks (where scattered light is negligible), estimate the amount and structure of the scattered light component in the IOC darks. Is it possible to remove it effectively? Does it change with time? The level of the scattered light in the darks in Part 1 will also put limits on where SIRTF should be pointed when performing routine dark calibrations.

If glints are found then the observer handbook will need to be updated.

Failure Modes and Responses

If this task fails, the reason of the failure needs to be determined and corrected. This task would then need to be rerun.

Additional Notes