BACKGROUND
There are several sources of offsets in the IRCamera.
- Thermal background from inside the camera
- Thermal background from the telescope
- Dark current in the chip
- Sky background
- Light leaks
INTERNAL THERMAL BACKGROUND
The thermal background comes from emission of different parts
of the IR camera at differing angles from the central ray.
The following table is based on the fact that the chip holder
intercepts rays that are greater than 12 degrees from the axis
and the Thirty inch telescope secondary is 11 inches in diameter.
Note: there is no internal cold pupil stop on the
secondary obscuration.
The solid angle has the cosine factor included.
| Internal Thermal Background Contributions |
|---|
| Factor | Angle | Solid Angle | Temperature | Photon Flux |
|---|
| | Deg | ster | K | p/pixel/sec |
|---|
| Chip | 12.0-180. | 3.0 x 10 0 | 75.0 | 5 x 10 -18 |
| Optics | 3.6 - 12.0 | 1.2 x 10 -1 | 155.0 | 3 x 10 -2 |
| Filter | 0 - 3.6 | 1.2 x 10 -2 | 190.0 | 4 x 10 0 |
| Telescope | 0 - 1.3 | 1.6 x 10 -3 | 300.0 | |
| Sky | 1.3 - 3.6 | 1.1 x 10 -2 | varies | |
It can be seen that the interior of the camera is plenty cold (with the possible
exception of the filter wheel), so that the thermal background is
neglible.
EXTERNAL THERMAL BACKGROUND
The contribution to the background from the sky and telescope depend on the
filter used.
The terms below are the following:
- Tel - is telescope secondary alone at 300K
- Both - is entire primary as if covered by mirror cover at 300K
- Mark I - is the old Mark I camera for reference
| External Thermal Background Contributions |
|---|
| photons/pixel/sec |
|---|
| Factor | J | H | Kshort | Open |
|---|
| Tel | 4.0 x 10 -3 | 2.3 x 10 1 | 5.8 x 10 3 | 3.8 x 10 4 |
| Both | 3.0 x 10 -2 | 1.7 x 10 2 | 4.4 x 10 4 | 2.9 x 10 5 |
| Mark I | 3.0 x 10 0 | 1.7 x 10 4 | 4.4 x 10 6 | 2.9 x 10 7 |
OBSERVED BACKGROUND
Using the
conversion factor of
35 e - per DN.
The telescope temperature was 15.3C.
| Observed Background |
|---|
| e -/pixel/sec |
|---|
| Setup | blank | J | H | Kshort | Open |
|---|
| Cover | 2.5 x 10 3 | 2.5 x 10 3 | 2.6 x 103 | 4.0 x 10 4 | 2.5 x 10 5 |
| Sky | | 3.2 x 10 3 | 5.8 x 103 | 2.2 x 10 4 | 2.1 x 10 5 |
There is an interesting indication that the background is slightly higher
with the blank than the J filter (on the order of 10 e-/sec).
Remember that the blank is not anodized.
Note also that for the Kshort and Open filters the background went
up when the dark slide was closed; welcome to the infrared.
DARK CURRENT
It is difficult to sort out dark current from other forms of
background.
However, we have the following data:
- When chip went from 74 -> 86K the background rose by 350 e-/sec.
- Chip Temp=108 background went up to 6860
- Mark I determined log10(e/sec)=-1.6 + .0525*T(K)
These discrepant results, lead to a compromise using
the formula log10(e/sec)=-2.0 + .0525*T(K).
Which in the range of interest gives the following table.
| Estimated Dark Current |
|---|
| Tchip | Rate |
|---|
| K | e -/sec |
|---|
| 70.0 | 4.7 x 10 1 |
| 75.0 | 8.7 x 10 1 |
| 80.0 | 1.6 x 10 2 |
| 85.0 | 2.9 x 10 2 |
| 90.0 | 5.3 x 10 2 |
| 95.0 | 9.8 x 10 2 |
LIGHT LEAKS
It is very difficult to estimate the magnitude of the light leaks.
However, the following gaps are known.
- two .0625 inch notches on chip holder drawing 34/2
- .0625 inch gap around chip snout
- .0625 inch gap around filter wheel snout
You can see the notches on the
this photograph.
By taking the ratio of the area of the hole to the square of
the effective distance from the chip, we can estimate
the solid angle of room temperature radiation that might
be arriving.
The photon flux from 300K Black body arriving shortward of
2.55 microns is 2.3 x 10 7photons/pixel/sec/steradian.
| Light Leaks |
|---|
| Leak | area | R | Solid Angle | Photon Flux |
|---|
| cm 2 | cm | ster | p/pixel/sec |
|---|
| Notches | 0.05 | 10 | 5 x 10 -4 | 1.0 x 10 4 |
| Chip gap | 1.9 | 50 | 8 x 10 -4 | 2.0 x 10 4 |
| Filter gap | 1.9 | 50 | 8 x 10 -4 | 2.0 x 10 4 |
It is impossible to estimate is the efficiency of
these leaks as they involve multiple
bounces on an anodized surface.
DISCUSSION
The background appears to be limited by light leaks, with the
most likely culprit being the lead entrances into the
chip case.
The background levels of the warm background in the
open, Kshort, and possibly even the H filter agrees
with the expected values.
Last Revised November 2, 2000