NOTES FOR WIRE DATA ANALYSTS

WHAT ARE THE PREDICTED CONFUSION AND INSTRUMENTAL NOISE ON A WIRE COADD?

 I used a program (written by Perry and modified by Cong) to predict the confusion noise at 25 microns for various evolution models.  (Data for 12 microns to be added later.)

Instrumental noise is calculated by combining READNOISE and SKY BACKGROUND NOISE in quadrature.
(Refer to  webpages on  Analysis of Noise on WIRE Coadd Frames, and How to Change Gain nd Readnoise When Data are Represented in ADU per sec.)
 
 
 
Table 1. Noise Parameters for 25 micron Coadds 
EVOLUTION MODEL INSTRUMENTAL NOISE per pixel, 
on a single coadd frame 
ADU/sec		 CONFUSION NOISE per pixel, 
ADU/sec 
No Evolution * 0.001168
Mod. Density D ~ (1+z)2.5 * 0.004367
Mod. Luminosity  L ~ (1+z)2.5 0.0676 0.005183
Strong Luminosity  L ~ (1+z)3.5 * 0.01114
 
* The instrumental noise will vary with the square root of the sky background.
I do not yet have data on expected sky background for other models.
For the purposes of calculation, one could simply use the value from moderate luminosity evolution.

HOW MANY FRAMES ARE NEEDED FOR MEDIUM, DEEP AND ULTRADEEP WIRE SURVEYS?

 
First let's review the survey definitions,  taken from the WIRE Science Plan.
 
 
 
Table 2. Survey Definitions
Survey Type Definition
Medium Instrumental noise is three times the confusion noise
Deep Instrumental noise is equal to the confusion noise
Ultradeep Instrumental noise is half the confusion noise
 
 

The integration times needed to reach these levels will depend on the evolution model, and can be determined from the values of instrumental and confusion noise given in TABLE 1, above.
Here is a table to indicate how many 56-sec frames (or 8-segment coadds) are needed for each survey type (calculated for 25 microns).
 
 
 
Table 3. Number of Frames/Segments Needed for Various Survey Types
Evolution Model Medium Survey Deep Survey Ultradeep Survey
No Evolution 373/47 3361/420 13445/1681
Mod. Density 27/3.4 240/30 960/120
Mod. Luminosity 19/2.4 170/21.3 681/85
Strong Luminosity 4/0.5 37/4.6 148/18.5
 

GETTING DAOPHOT TO CALCULATE A GOOD 'FIND THRESHOLD'

DAOPHOT determines the SIGMA, the noise in a pixel, from four input parameters: GAIN, READNOISE, NAVG, and NADD.
NAVG is the number of frames being coadded, and NADD is the number of subframes added to make a frame.

The formula used is

SIGMA2 = SKY/(NAVG*GAIN) + (NADD/NAVG)*READNOISE2
 

But the true noise in a pixel should include the confusion noise:

SIGMA2 = SKY/(NAVG*GAIN) + (NADD/NAVG)*READNOISE2 + CONFUSION2

In order to make DAOPHOT account for confusion noise in its calculation of the pixel noise,
we can determine an EFFECTIVE READNOISE R' so that when R' is input in place of READNOISE,
DAOPHOT calculates the noise correctly:
 

Thus  we want (NADD/NAVG)*(R')2 =  (NADD/NAVG)*READNOISE2 + CONFUSION2

This gives R' = sqrt[READNOISE2 +(NAVG/NADD)*CONFUSION2]

Below are tabulated the values for 25 micron R' for various evolution models.
 
 
Table 4. Effective 25 micron Readnoise Parameter for Input to DAOPHOT
Evolution Model R', Medium Survey R', Deep Survey R', Ultradeep Survey
No Evolution 0.0218 0.0277 0.0418
Mod. Density 0.0121 0.0209 0.0376
Mod. Luminosity 0.0092 0.0194 0.0368
Strong Luminosity 0.0080 0.0188 0.0365
 

WHAT ARE THE FIVE-SIGMA FLUX LIMITS FOR 25 MICRON WIRE COADDS?

An important characteristic of the source extractor is its performance at the 5-sigma flux limit, i.e. the flux at which the signal-to-noise ratio is 5.  This is also the limit at which the photometry is 20% or better.

Confusion noise is combined with instrumental noise to get sigma, from which the 5-sigma limits are determined.
 
 
Table 5. Five-Sigma Limits in log[flux(Jy)] for 25 Micron Coadds 
EVOLUTION MODEL MEDIUM SURVEY  DEEP SURVEY  ULTRADEEP SURVEY  CONFUSION LIMIT
No Evolution -3.54 -3.89 -3.99 -4.04
Mod. Density  -2.97 -3.31 -3.42 -3.47
Mod. Luminosity  -2.89 -3.24 -3.34 -3.39
Strong Luminosity  -2.55 -2.91 -3.01 -3.06
 
 

last updated 9/23/98, by Joe Catanzarite