Subject: Re: *Complete* draft of the LSST SWG document

From: Nick Kaiser

Submitted: Thu, 10 Jun 2004 07:25:05 -1000 (HST)

Message number: 223 (previous: 222, next: 224 up: Index)

>
> *  The LINEAR telescopes have no way of finding
> specifically asteroids whose orbits take them
> close to the earth; this distinction is made
> later when orbits can be calculated

They can (by looking along our orbit), but for their detection limit this
is not a huge effect, except for very large objects.  For Pan-STARRS with
its 300m goal, this is a big enhancement, and even more so for a
hypothetical future larger instrument.

> *  The definition of entendue could be confusing,
> due to ambiguity of what is meant by "solid angle
> subtended by the instrument" (what instrument?).

It is both the telescope collecting area times the field of view and the
detector area times the solid angle of the beam entering the detector.

> *  The Congress never mandated 90% or any other
> completeness for PHA surveys, but only for NEAs
> (which is also the way the NASA Spaceguard Goal
> is expressed)

That was a marvellous achievement!  However, it seems clear to me that
their intent was asteroid collision hazard and not NEA science.  I think
this is an important distinction, as these goals lead to different
observing strategies.

> *  As in my comment on the abstract, I am not
> sure the same term (Pan-STARRS) should be used
> for both the 4-telescope Hawaii system and a
> possible 15-20 telescope system. This can lead to
> confusion.

The DMT proposal to NSF refers to "multi-STARRS", I think.



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check we have used the same simulator modified to simulate Magellan, Subaru, and the DLS (4m) data and the results compare favorably to actual data which was also run through the same pipeline. The key is LSST's ability to go to low surface brightness for each patch (low f/#). In such a stack in selected seeing averaging 0.7" FWHM LSST should usefully resolve 60 galaxies per sq.arcmin or a bit more, in V and R separately in 10 years. < 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. There are separate specs for precision in the processed stack and stability during an exposure. Your argument is correct if applied to the ultimate precision, but the spec in question is rather the stability during an exposure. The reason stability on the timescale of an exposure and on arcminute scales is important is because we use stars in the field to correct for PSF ellipticity, and that must be suitably stable on those spatial and timescales. Right now in our 4m data it is stable at the 10% level and we are requiring conservatively 1% stability for LSST. We currently do better than a factor of 10 rounding of PSFs, per exposure, and I would expect that would be even better by the time of LSST. < here and elsewhere: value for "shear floor" is noted variously as 1, < 2, or 3 parts in 104 throughout this section, should all be the < same. I agree this should be consistent. We require 2 parts in 10^4. < p50: Change 0.01 to 0.001 in "requirements on PSF and pixel size" as < per above note. The stability requirement should remain 0.01, but we might as well add that we require a precision after PSF correction of 0.001 in each exposure. (See above answer) < p51: "Astrometric error tails can be clipped" - not sure what this < means, can we remove it? This refers to the ability to de-weight or cull exposures which have large errors due to wind shake etc. This is enabled by the short exposure times. < p52: SNAP update: "optimistic scenarios for SNAP envision a weak < lensing survey of 1000 deg2" < -> "baseline scenario for SNAP envisions a 1-year weak lensing survey < of 1000 deg2, with larger surveys possible in an extended mission." Regarding precision reached on cosmological parameters, the sqrt of the number of usefully resolved source galaxies gives the statistical limit. That is a function of both area covered and depth, and is proportional to etendue. i.e. one can trade depth for area. With SNAP's etendue a 1-year survey over 1000 sq.deg will not go as faint as a 1-year survey over 300 sq.deg and will not go nearly as faint as LSST's planned survey. < 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. I agree. < 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. I agree. < 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. In Abi Saha's operations simulation he starts a patch emphasizing 2 filters but cycles through all 5. < 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. I agree. While it is always helpful to have short exposures (because of the non-Gaussian nature of the errors), it is the total number of good exposures that matters. That is proportional to etendue. p82: Did the simulations of scheduling conform to the ecliptic latitude & solar elongation pattern proposed in the NEA chapter? I will check. LSST LSST LSST LSST LSST Mailing List Server LSST LSST LSST LSST LSST LSST LSST LSST This is message 223 in the lsst-general archive, URL LSST http://www.astro.princeton.edu/~dss/LSST/lsst-general/msg.223.html LSST http://www.astro.princeton.edu/cgi-bin/LSSTmailinglists.pl/show_subscription?list=lsst-general LSST The index is at http://www.astro.princeton.edu/~dss/LSST/lsst-general/INDEX.html LSST To join/leave the list, send mail to lsst-request@astro.princeton.edu LSST To post a message, mail it to lsst-general@astro.princeton.edu LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST