Subject: another entry for Scoville search file
From: Greg Aldering <[email-masked]>
To: [email-masked]
Date: Sun, 27 Apr 2003 17:42:15 -0700 (PDT)

>From [email-masked]  Sun Apr 27 17:37:48 2003
Date: Sun, 27 Apr 2003 17:37:47 -0700 (PDT)
From: Greg Aldering <[email-masked]>
To: [email-masked]
Cc: [email-masked]
Subject: Scoville search: ETC results

Hi Saul,

Ok, now I have done a little exploring with the ACS ETC.

First I looked at the V-band sensitivity, where you already know where
to look based on an I-band detection. Here are some cases, calculated
for average zodiacal light and average Earthshine:

  Exptime  CR-SPLIT   Filter  V mag giving
                              optimal S/N ~ 4
                              in subtraction
  ---------------------------------------------------------------
   1020       1       F606W    28.0
    900       2       F606W    27.8
   1020       1       F475W    27.4
    900       2       F475W    27.2
  ---------------------------------------------------------------

Note that F606W is aka "Broad V", and F475W is aka "SDSS g". The
exposure times were chosen to match the results of the RPS2 and CR
experiments reported in the previous email.

Here one can see that getting F606W rather than F475W is a big deal
for us in terms of implementing the V-band rejection test.

Next I looked at the sensitivity of the search. I have assumed equal
depth for the reference and the search, which is the most
straightforward, a probably the optimal, case.  My understanding is
that there is no discussion about the filter, and that F814W will be
used. In my mind, there is little room for discussion about the number
of CR-SPLITS; the search needs at least three CR-SPLITS, and if one
tries for 4 CR-SPLITs then time has to come from the V-band due to the
buffer readout penalty (RPS2 confirms the penalty).

  Exptime  CR-SPLIT   Filter  I mag giving
                              optimal S/N ~ 5
                              in subtraction
  ---------------------------------------------------------------
   1030       3       F814W      26.6
  ---------------------------------------------------------------

Note that previously I had gotten a much brighter detection limit of
I=25.3 for S/N=6. I can't reproduce that calculation now - probably
something on the ETC was set wrong. I am able to confirm that the
minimum deposition limit of 500e- is reached by a I=24.8 object in 343
seconds (3x343=1030). So, one can work between 24.8 and 26.6 and
get some help in rejecting CR's using the minimum deposition threshold.

I=26.6 is enough to get a s=1 SN at maximum light at z = 1.68
according to a table I have from Peter. Note this requires searching to
S/N ~ 5, which is the lowest threshold I can imagine for this search
given the contamination issues. Since there will be almost 2 billion
pixels to search, there will be dozens of spurious groups of pixels at
this threshold.  But I'm sure Adam will say he searched down such a S/N
with GOODS.  (Although note that the number of GOODS pixels in the
search images was significantly less than it will be for the Scoville
search.) Considering control time, I would guess that the search
sensitivity will peak around z~1.4.

Also, recall that I suggested that maybe it would be wise to search
half the fields to twice the depth? Well, in F814W you would get only
another 0.15 further in redshift, because the SN flux is dropping so
fast. I don't think this added depth will add back enough SNe to
offset the lost area (although it would make the search much cleaner).

Note that for the subset of CDFS fields that overlap with GOODS, we
might want to try something different - don't know what.

Note that CDFS is only about 30 degrees off the ecliptic. So, we
probably want to avoid observing when the field is opposite the Sun,
since at that time the zodiacal light will increase (known as the
Gegenshein). In relative units, at ecliptic latitude of 30 the zodi can
be as dark as 86 units when the field is 45 degrees in ecliptic
longitude from opposition. When the field ecliptic longitude is at
opposition, the brightness is 105 units. This would be a 20% efficiency
hit. Given this constraint, one wants to wait until Jan 1, 2004 for the
mid-point of the references and search.

Note that the zodi for CDFS is still 50% brighter than for a
HDF-N/EGS/SNAP-like field.  For sky-dominated observations such as our,
this 50% translates directly into a 50% loss in observing efficiency.
With a better field choice, HST could have been saved 200 orbits off a
600 orbit program! It also means our 60 orbits is only worth 40 orbits
relative to the EGS.

That's all I have thought to explore so far.

- Greg