Subject: comments on the SWG document
From: Gary Bernstein
Submitted: 07 Jun 2004 10:39:55 -0400
Message number: 211
(previous: 207,
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Following are my comments/questions on the draft. Congratulations,
Michael, for bringing it all together. You may have to distribute some
of these questions to the relevant authors. Following the technical
questions are some simple typos, etc.
Gary
--------------------------------------------------------------
Technical questions:
p11 & Table 1: is the 4-meter really losing half its light to loss at
mirrors and corrector surfaces?
p17: Add at end of section 4.1: "This is emphasized by the recent
discovery (Brown et al 2004) of the $R\approx22$ object Sedna near
its 80~AU perihelion. Sedna may be the sentinel member of a new
dynamical class, the inner Oort cloud, with hundreds of members
potentially within the reach of LSST."
p40: "such as only LSST could produce" -> "such as LSST could produce"
Pan-Starrs, SNAP could also usefully detect bispectrum.
p41: "[a shear floor of 0.0001..." - as I understand this paper, no
systematic-error contribution was included in the analysis, but a
back-of-the-envelope estimate is that systematic shear power of $l^2
C_l/2\pi<10^{-4}$ would be a subdominant error.
p47: The argument for resolving 85 galaxies per square arcmin with
LSST images is not valid. It states that 70 galaxies per arcmin^2
were detected without crowding in the HDF simulation; this does not
say that these galaxies were usefully resolved. Also there is the
implication that 200x10s of 6.9-meter LSST data will resolve all the
galaxies that are unresolved in a 3600s 6.5-meter Magellan image with
the same seeing. Why should this be true?
p47: Last paragraph of 4.7.3 repeats text from previous paragraph.
It should probably also say "factor of five lower than that obtained
at favorable angular scales with older 4-meter telescopes"
p48: The figure of 60-70 resolved galaxies per square arcminute
appears again bottom of page.
p49: "must be stable on arcminute scales at the 1\% level during an
individual exposure"
-> "must be stable on arcminute scales at the 0.1\% level over the
timescale of several exposures."
Here's why: If "shear floor" on stack of 200 images needs to be
0.0001 in a stack of 100x2x10s exposures, and we reduce by sqrt(100)
the systematic shear by averaging over the 100 epochs, then
systematic power must be 0.001 or lower in each epoch. Must be
stable over several exposures if we are to combine the info from many
exposures to diagnose & remove this systematic that is below the
star-separation scale.
here and elsewhere: value for "shear floor" is noted variously as 1,
2, or 3 parts in 10^4 throughout this section, should all be the
same.
p50: Change 0.01 to 0.001 in "requirements on PSF and pixel size" as
per above note.
p51: "Astrometric error tails can be clipped" - not sure what this
means, can we remove it?
p52: SNAP update: "optimistic scenarios for SNAP envision a weak
lensing survey of 1000 deg^2"
-> "baseline scenario for SNAP envisions a 1-year weak lensing survey
of 1000 deg^2, with larger surveys possible in an extended mission."
p54: In 8.1.1 it would be useful to know, of the 250k Ia's
per year will be discovered, how many will have light curves that
extend enough below maximum light to be cosmologically useful.
p54: I presume that the rough estimates of 10,000 low-z Ia's giving
1% estimate of w assumes that there are no systematic errors from
lack of spectroscopy, e.g. core-collapse contamination, etc.? Can
this be stated here?
p55: I haven't heard how SNe are useful for weak lensing. True, if
they are reliable standard candles with 0.15-mag dispersion, then
each offers a measure of local convergence accurate to 0.15. But for
z~0.5 sources, the expected convergence variance is a few tenths of
percent, so the S/N per event is about 1/50, and there are 1000x as
many galaxies as SNe, so it's hard to see how it would help.
p56: SNAP police: the limits on $\Omega_m$ and $w_0$ in the Kim paper
are for a model with time-variable $w$. The limits on $w$ for
LSST SNe proposed on top of p58 are for w' fixed at zero. The
comparison should assume similar circumstances.
p58: "five filters, returning to the same field every three or four
nights as specified in the universal cadence..." The later sections
point out that the Appendix-A cadence would not suffice because only
2, rather than all 5, filters are used at each 3-day visit. This
discrepancy should likely be mentioned in this section as well as the
later chapters.
p70: In last paragraph of discussion of short v long exposures, it
should be noted that long exposures from multiple telescopes provide
an equally useful guard against lensing systematics and robustness
for transient detection, so the direction of the 8.4m/PanSTARRS
argument here is also unclear.
p82: Did the simulations of scheduling conform to the ecliptic
latitude & solar elongation pattern proposed in the NEA chapter?
------------------------------------------------------------------
Typos, etc:
p2: "essentially a complete sample" -> "an essentially complete sample"
p5: "v~24" - do you want lower-case "v" filter?
p8, 2nd-to-last paragraph: "hazard...is...about 60 per year" - are the
units here deaths?
p11: "tumble fast" -> "tumble quickly"
p24ff: do we capitalize "Celestial Sphere"?
p31: There are some units and parentheses missing from the estimate
of \sigma on 8th line.
p46: use "sample variance" instead of "cosmic error"
p47: "in space in time" -> "in space and time"
p49: "ABmag" is in math font here and elsewhere in the document.
p51: text is garbled in paragraph on VISTA.
p54: "peak redshift of z~0.7" -> "maximum redshift of z~0.7"
p54: "three or filters" should be ???
p56: "Ni" is in math font.
p56: "14,000 SNe per field in 10 years" - is this assuming that we
spend 20 minutes per night on this single field for 10 years?
p69: "ACAC" -> "UCAC"
p77: caption of fig23, are these really "diffraction PSFs" - look
like ray traces to me.
Throughout the 8.4m appendix there are typos like "8.4-m 8.4m"
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