Background

There are several sources of offsets in the IRCamera.

Dark Current

The dark current of the chip was investigated separately. The at chip is not getting as cold as it should be so the dark current is believed to be about 1300 e -/sec.

Thermal Background

The chip looks out at the telescope an stuff that might be in thermal equilibrium near 300K. 

Integrated photon flux from lambda=0 from 300K Black Body 

Lambda    Photon Number Flux
microns   photons/m^2/sec/steradian
 0.900     1.2e+02
 1.000     2.0e+04
 1.100     1.3e+06
 1.200     4.1e+07
 1.300     7.5e+08
 1.400     9.1e+09
 1.500     7.8e+10
 1.600     5.1e+11
 1.700     2.6e+12
 1.800     1.1e+13
 1.900     4.1e+13
 2.000     1.3e+14
 2.100     3.8e+14
 2.200     9.7e+14
 2.300     2.3e+15
 2.400     5.1e+15
 2.500     1.0e+16
 2.600     2.0e+16
 2.700     3.7e+16

Integrating this over the filter bandpasses we have the following: 

Filter  range        photonflux
        microns      photons/m^2/steradian
J       1.16-1.32    1.3e+09
H       1.48-1.77    7.8e+12
Ks      2.0 - 2.3    2.2e+15
none        - 2.55   1.5e+16
The Mark I camera does not have a cold stop and the chip looks out at the world with a 39 degree half angle. Using this value of the solid angle and correcting for the cosine factor we get 1.24 steradian. The detector pixel is 40 microns square. 
Calculated Thermal Background Fluxes
Filter   calc flux       obs flux 
          p/pix/sec     e/pix/sec 
J          3            0.8e4     300DN/sec  large light leak 
H          1.5e+4       1.9e4     600DN/sec 
Ks         4.0e+6       1.7e6     300ms fill 500,000 Full well
none       3.0e+7       1.7e7     30 ms fill 500,000 Full well

The thermal background could be reduced considerably by using a cooled stop. The telescope focal ratio is F:8 so that the steradian factor should be .012 or about 100 times less than the present value.

Sky Background

On Jun09 looking at H band images taken at UT=5:07 when the sun was 19 degrees below the horizon. The sky value of the mean decreased by 1744 DN/sec while the dark value taken with mirror and dome closed decreased by 633 DN/sec. Therefore the sky contribution was 657DN/sec. The pixel scale is 1.32 arc-seconds. Therefore the brightness is 377DN/sec/arc-sec2. Using the derived zero point calibration of Mzp=19.0 at H, we get the sky brightness as an amazing 12.6M.

On Jul29 at J band the chip had background value of 266DN/sec when looking with mirror closed. When looking out at sky this value increased to 556DN/sec.

Gemini Comparisons

The Gemini Signal to noise 
Filter     B               B
        mag/arc-sec2     e-/s/pix
J          16.1           1200
H          14.3           5000
K          13.0          10100
The background at K intrigues me because I calculate for a 40 micron square pixel illuminated by an F:4.0 beam (which is the final aperture of the camera) a rate of 170,000 e-/pix/sec. The observed is 17 times less; it is hard to believe that the emissivity of the 3-m telescope is only 10%.

[ Mark I Index | IRCam Index ]

Last Revised September 15, 2000