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Space Telescope Imaging Spectrograph
Phase I: A DB white dwarf as a fundamental calibrator for G130M

STIS Cycle-18 Calibration Phase-1 Template

Last Revised:  20 Oct. 2010
Proposal ID:   12418
Title:         A DB white dwarf as a fundamental calibrator for G130M
PI:	           Massa
Co-I(s):       Bolin, Chayer

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# Orbit estimates are TOTAL for all of cycle-18; assume 72 weeks) 

Total Prime Orbits:             1
External Parallel Orbits:       0
Internals or no-impact orbits:  0
Comments on orbit estimate:     (1 STIS orbit)

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# This section should be written with a GO audience in mind -- use 
# Calibration plans in WFPC-2 handbook as a guide.

Purpose:                        
DB wds are ideal stars for FUV calibration.  Unlike the DA wds which are 
typically used, DBs do not have strong photospheric Lyman absorption 
lines and their flatter SEDs provide a more uniform S/N when recorded by 
the COS UV detectors.  This proposal requests the observations needed to 
derive a calibration-quality model for the DB wd WD0308-566.
To establish WD0308-566 as a COS/STIS calibration standard, which means 
that we must obtain the observational coverage needed to produce a good 
model.  The modeling requires S/N coverage of the UV and optical.  
Optical coverage is essential, because the property that makes these stars 
ideal UV flux standards (a lack of features in the UV), makes it 
impossible to determine their physical parameters from UV observations 
alone.  Consequently, we need optical spectra to model the optical He I 
lines which are sensitive to both temperature and gravity.

Description:                    (1-2 paragraphs)
We would like to adopt the DB wd WD0308-566 as a calibration standard.  
The FUV continua of 15 - 25kK DB white dwarfs can be ideal for calibration 
because they are smooth and, due to the absence of atmospheric hydrogen, 
they lack the strong photospheric Ly alpha seen in DA wds, which makes 
them superior to DA wds for calibrating the Ly alpha region for two 
reasons.  First, the absence of strong photospheric absorption means that 
a uniform, high S/N spectrum can be obtained throughout the Ly alpha 
region.  Second, even the best DA models are somewhat uncertain near Ly 
alpha since the SEDs are strongly affected by the extremely strong Ly 
alpha Stark wings and it is unclear how well current broadening theory 
applies to the extremely high pressure environments of wd atmospheres.  
In addition, the DB wds are considerably cooler than DAs, so their SEDs 
are flatter in the UV.  Given the response of the COS detectors, this 
results in a more uniform S/N over a given grating setting.  LDS749B is 
a good example of a DB wd standard.  Its UV spectrum is relatively 
featureless, lacks stellar Ly alpha and has a relatively flat UV 
continuum.  Unfortunately, LDS749B is quite faint, so high S/N noise 
COS observations require long exposure times.  WD308-566 is between 10 - 15
times brighter than LDS749B throughout the UV, and would make an ideal COS 
G130M calibration target for the following reasons:
1. It is faint enough to be observed in TTAG mode (essential for 
     calibration sources) and between 1.25 and 2 times brighter over the 
     G130M range than the brightest DA wd standard (WD1657+343) that can 
     be observed by the G130M in TTAG mode.
2. It lacks stellar Ly alpha and its relatively flat continuum, produces  
     count rates that vary less than a factor of 2 over the G130M range.
3. This increased in brightness could result in enormous time savings for 
     flat field monitoring or additional flux calibrations.  
4. It is bright enough to serve as a precision STIS/COS cross-calibration 
     source.  
5. It would be a useful calibration target if the new G130M settings are 
     adopted, since the FUV spectra of DA wds contain the confluence of 
     the upper Lyman lines, making their spectra complex and problematic 
     for flux calibration.

WD0308-566 has been monitored for variability in the optical and is 
stable.  It also has low resolution IUE and a FUSE spectra available, so 
we are certain that it does not have significant atmospheric metal content 
(like a few DBs) which could make its continuum too busy to be a good 
calibration source.  Therefore, we request 2 orbits to observe WD-308-566: 
1 COS orbit to obtain high S/N G130M spectra, and; 1 STIS orbit to obtain 
the UV data needed to cross-calibrate COS and STIS and the optical data 
needed to determine the high quality model fit needed to adopt it as a 
calibration standard.  

(if at all possible, a third COS visit should be made to determine the 
ensure the stability of its UV SED.)   

WD1657  -- COS.A352179 -- 5 min exposure
WD0308  -- COS.A352178 -- 5 min


Accuracy:                       1%
Comments on Accuracy:           Poisson, over a 1 angstrom bin
Products:                       A model fit that can be used as a new
                                fundamental UV calibration source.


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# Link to Cycle 18 Science
# Use the GO + GTO Phase 1 exposure  listing and derived statistics
# posted on WWW.

Fraction of science programs supported by this calibration:    50%

List categories of science which can't be executed until after this proposal:
why:  All programs which require G130M flux calibration.

List categories of science which are best performed contemporaneous with 
this proposal:  Any program needed absolute flux calibration.  
why:


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# THE REST OF THIS FORM IS FOR INTERNAL USE
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Observation summary: 
	(e.g. Take spectra of G191B2B at each supported central wavelength 
	 of each grating with the CCD and the 52x0.5 slit.) 

# Target information (copy for each target)

Target Name.............................:	
RA......................................:		
Dec.....................................:		
Flux (and units)........................:	
Visibility window (if known or relevant):
Comment on choice of target:

# Exposure list plan at the Phase I level. Be as complete as reasonable. 
# If possible information should be complete enough to allow easy 
# submission of a Phase 2 from this form. However, if you have to leave
# some of the fields blank below, or want to provide the information
# in another format, that is acceptable. (Make sure you do the BOP checking
# though, as this is a phase-1 issue.)

Table of exposures (copy for each exposure as appropriate/desired):

Target or lamp..........................: e.g. G191B2B, or HITM
Type of Acquisition.....................: e.g. ACQ-PEAK
Detector for observation................: e.g. CCD
Operating mode .........................: e.g. ACCUM or TIMETAG
Spectral Element........................: e.g. G430L
Aperture................................: e.g. 50x0.5
Central Wavelength......................: e.g. 4300
Exposure Time...........................: e.g. 300s, crsplit=2 
Number of Iterations ...................: e.g. 1
BOP Predicted Local Count Rate for MAMA.:
BOP Predicted Global Count Rate for MAMA:
ETC PID IDs for Count Rates.............:
Special Requirements or options.........: (e.g. GAIN=4; LOW-SKY)
Comments: (e.g. if monitoring, specify frequency) 


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# Special needs of this proposal?

Scheduling: (e.g. Apr 2009, ~1yr after last performed in SMOV, or monthly)

Prerequisites: (e.g., specific SMOV proposal; command development?)

PDB update?               (Specify table name if known e.g. SIAF).
On-board table update?    (Specify table name if known e.g. CCDBadPixelTable)

# Special Requirements:
Real time?                Y/N
Special commanding?       Y/N
Quick Data turnaround?    Y/N
Special timing required?  Y/N
Other?                    Y/N
Describe (& justify) special requirements:


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# Analysis Plan

Expected FTE weeks for analysis:                                0
Required turnaround on analysis (weeks):                        0
Data volume (Mb needed for analysis):                           0
Special software needed for analysis? (Describe):

Description of analysis Plans:	(1-2 paragraphs)