ARCHIVED FAQs: Legacy Calibration

(Note that these are OLD archived FAQs from August 2000)

Q:   How many stars (or other astronomical objects) are used to calibrate the Spitzer science instruments?

A:   The number of astronomical sources that will be used to calibrate each instrument is still being considered by each instrument team.

For IRAC:
There are two sets of calibration stars: the ecliptic set is spread along the ecliptic plane, the CVZ set is near the ecliptic poles. The number of stars in the ecliptic set is about 2 every 30 degrees so about 24 stars. The CVZ set would be about 6 stars. These numbers are only illustrative and have not been finalized.

Q:   How many stars (or other astronomical objects) are used to cross-calibrate the Spitzer science instruments?

A:   We are still in the process of selecting cross-calibration stars. Cross-calibrator candidates lie in the Constant Viewing Zones near the ecliptic poles. The stars under consideration are 6th visual magnitude A dwarfs, 6th visual magnitude K giants, and 6-7 visual magnitude solar-type stars. The number of cross-calibrators will be narrowed further after assessing early Spitzer observations of them. A few red objects have been observed to check that the cross-calibration derived from stars applies to red objects.

Q:   What are the relative and absolute calibration uncertainties for the Spitzer science instruments?

A:   There is a requirement that the absolute calibration of all three science instruments is consistent to within 10%. This requirement refers to observations made in the best circumstances, and will likely not be achieved before the second year of operations.

For IRAC :
There is a requirement for 2% relative photometry uncertainty and 10% absolute photometry uncertainty, interpreted as rms error among a group of sources. This requirement refers to observations made in the best circumstances, and will likely not be achieved before the second year of operations.

For MIPS:
The ultimate absolute accuracy of the photometry that can be derived from MIPS data is expected to be 4%. The relative accuracy of the data from MIPS is expected to reach an ultimate accuracy better than 4%. However, observers should expect the initial relative and absolute accuracy to be about 10% and 20% to 30% respectively, with considerable improvement expected by the end of the first year.

For IRS:
There is a requirement that IRS achieve a relative radiometric accuracy of 5%. This requirement refers to observations made in the best circumstances, and will likely not be achieved before the second year of operations.

Q:   What is the error budget for the absolute and relative calibration uncertainties for the Spitzer science instruments?

A:  
For IRAC:
The error budget (for the relative error) breaks down officially as follows: 1% polarization, 0.5% bandwidth effect (SOM 6.3.3.2.6), 1.3% warm electronics stability, 1% array effects (flat-field, photometry extraction, detector read/dark/bias noise).

No breakdown of source of errors is available for IRS or MIPS at this time.

Go back to FAQ page.