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   ACS SBC Bright Object Limit Revisions

As discussed in the ACS Instrument Handbook for Cycle 17,
Sections 4.4.2 and 5.5.2, during 2007 it was found that the
SBC sensitivity to near-UV and optical light (the "red leak")
is significantly greater than previously recognized (or
alternatively, it has increased with time).  Since the
release of the current Handbook, further analysis has shown
that the effect is somewhat less than specified there, but
still much larger than previously recognized.  Further
details will be provided by Boffi et al. in ACS ISR 2008-02.
***
(UPDATE NOTE: This will not be in an ISR, but is shown here 
and on the ACS Bulletin Board with subsequent posts, and 
will be updated for the next version of the ACS Instrument 
Handbook as well.) Also, see more details about "brightest
pixe"l calculations and "IUE Stellar Spectra used for determining 
the KM III star bright object Vmag limits" in the last postings 
in the ACS Bulletin Board on this topic. The second pdf file 
there is the best-formatted source to look at or print for 
yourself. (Click on the link for this subject under "What's new 
on the Bulletin Board" on the top-level ACS page to get to this.)
***
A revised sensitivity curve has been delivered to the
calibration database, and since November 2007, that has been
reflected in the ETC.  Hence, the V-magnitude screening
limits in Table 7.4, Section 7.2.2 of the Handbook are now
fainter for stars later than type F0 V.  Revised values
are given in the following table.

Revised bright-limit V-band magnitudes for observations
with the SBC filters and prisms (no reddening).
----------------------------------------------------------
Spec F122M F115LP F125LP F140LP F150LP F165LP PR110L PR130L
---- ----- ------ ------ ------ ------ ------ ------ ------
F0 V  4.8   10.1   10.0   10.0   10.0    9.9    8.3    8.3
F2 V  4.0    9.3    9.3    9.2    9.2    9.1    7.6    7.5
F5 V  2.2    7.7    7.6    7.6    7.6    7.5    5.9    5.9
F8 V  0.9    6.8    6.8    6.7    6.7    6.7    4.7    4.7
G2 V  2.2    6.3    6.0    6.0    5.9    5.8    3.2    3.1
G8 V  ...    5.6    5.6    5.4    5.2    4.8    2.2    2.1
K2 V  ...    5.7    5.7    5.2    5.0    4.3    3.2    3.0
KM III ...   4.5    4.5    4.2    4.0    3.6    2.7    2.3
----------------------------------------------------------
All values have been calculated with the ETC to satisfy
the local countrate screening limit of 50 counts/second/pixel
(Table 7.3), and all other input sources are also as
specified in the Handbook, except that the KM III values
are now the faintest for each spectral element among IUE
data for 8 objects with spectral types from G8 through M5.

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