Calibration data are taken using Requirements. The use of the CALIBRATION Special Requirement is described below.

CALIBRATION CALIBRATION TYPE LED STATE

The CALIBRATION special requirement is motivated by the need to enable certain modes of observation that are considered only desirable for calibration observations.  Some such modes activate internal LEDs or block outside light, and as such can be detrimental to general astrophysical observations.  As a consequence, the CALIBRATION special requirement is only available when Allow Restricted is selected (see Program Information).

The CALIBRATION special requirement is applied individually to selected observation specifications within a Pass Plan. 

This requirement specifies that the selected observation specification is used for calibration, using the specified CALIBRATION TYPE and LED STATE .  The CALIBRATION TYPE chosen determines whether the DARK element is selected in the Element Wheel, thus blocking external light, and whether the internal light source is turned on with the appropriate specifications indicated in LED STATE .  Details are given in Table 2 below.

CALIBRATION   and ENGINEERING special requirements are not allowed on the same observation.

LED Lamps

There are 12 LED lamps (2 pairs at each of 6 different wavelength bands, arranged in two banks, see Table of LED Banks). Each LED is named according to its bank number followed by its band number. For example the Band 4 LED in Bank 2 is "LED24".

Calibration Type

Depending on the CALIBRATION TYPE , certain parameters of the observation specification are set to required values and cannot be changed (see Table of Required Observation Specification parameters).

Calibration types are: Dark Imaging, Dark Spectroscopy, Internal Flat, CRNL Direct Illumination, CRNL LOLO. CALIBRATION TYPE is required.

Calibration Types Dark Imaging and Dark Spectroscopy are used to obtain dark images to generate dark calibration reference frames.  These images will be taken with the DARK element in the Element Wheel and with LEDs turned on.  No dither shall be specified, although a repeat count is allowed.  These observations will typically use the diagnostic MultiAccum table and use guide windows of sizes appropriate to Imaging (16x16) and Spectroscopy (32x16), respectively.  The position of the guide window needs to be randomized.

Calibration types Internal Flat and CRNL Direct Illumination are used to obtain frames illuminated by the internal light sources.  The DARK element of the Element Wheel is in place.   LED STATE  specifies how many exposures are taken and at which LED wavelength and illumination level.  Typically, Internal Flats will be obtained with a bright LED setting suitable to collect about 50k electrons per pixel in a 50s exposure.  CRNL Direct Illumination data will be obtained with a combination of two LEDs at the same wavelength, turned on at different settings; a sequence of several dozen such combinations will be taken in order to calibrate the nonlinearity of the detector using the Combinatorial Flux Addition method.  More information under LED STATE .

Calibration Type CRNL LOLO is used to obtain an image of the sky superimposed with light from the internal source.  Pointing, OPTICAL ELEMENT , and DITHER are defined as with any other exposure.  An LED associated with the selected optical element needs to be turned on at the desired intensity; LED light is back-reflected by the cold mask on the filter and added to the scene light.   Note that there is no LED associated with the optical element F213.

LED State

LED STATE specifies the state of active LEDs for the observation.

LED STATE is available and required when CALIBRATION TYPE is Internal Flat, CRNL Direct Illumination, or CRNL LOLO.  The use of the LED is different for each of these cases.

For Internal Flat observations, one LED is turned on for one of the six bands at a high flux level, suitable to collect 50K electrons per pixel per exposure.  (A typical exposure may be around 50s.)  Multiple exposures are obtained until at least 10^6 electrons per pixel are collected in that band.  The process is then repeated for the other 5 bands.  Two LEDs for the same band can be turned on if needed to achieve the desired luminosity.  The DARK element is in use throughout, and the light is diffused by the back of the DARK element.

For CRNL Direct Illumination observations, a sequence of observations is taken with both LEDs in the same band turned on.  During the sequence, the intensity setting of both LEDs is cycled through several values, resulting in images with multiple levels of illumination.  The Combinatorial Flux Addition method is then used to determine any deviation from linearity in the detector response (after correction for classic nonlinearity).  This procedure can constrain non-linearity over a broad range of impinging fluxes and total count levels.  The procedure is then repeated for other bands as needed.  As for the Internal Flat observations, the LED light is diffused by the back of the Dark element, which is in place throughout.  Note that the exposure length may need to change depending on the LED setting; at low intensity, long exposures are needed to collect enough counts, while at high intensity, short exposures are needed to avoid saturation.  Using the number of elements in the sequence to determine number of exposures is only useful for sequences of exposures reaching similar level of total lamp flux.

For CRNL LOLO, the telescope is pointed at a carefully chosen external scene, with the desired filter, pointing, and dithering options.  The CRNL LOLO observations are characterized by turning on one LED corresponding to the optical element chosen; its light is diffused by the cold mask in the filter, and added to the signal received by the detector.  This provides the essence of the LED-On/LED-Off (LOLO) process; the change in signal in a source when a smooth illumination (from the internal LED) is added provides constraints on the detector linearity at that signal level.  Typically we expect the observing sequence to be repeated with LED off, LED on at low setting, and LED on at higher setting. Note that there is no LED suitable for use with F213.

LED State entry format

Enter specially-formatted lines of text to specify LED STATE , with one line per exposure. For each exposure, LED STATE specifies the Bank Number, Wavelength Band, and the selected flux value for the LEDs. The format is:

<exposure number>, LEDxy = nnn.nnn, LEDxy = nnn.nnn 

where

x is the LED bank number (1 or 2)
y is the LED wavelength band number (1-6).
nnn.nnn is the flux value for the LED . Allowed range is 0.0 to 65535.0  Units are electrons/pixel/second. For values less than 1, a zero is required before the decimal point.

Examples:

One exposure and one LED: 

1, LED21=0.222

Two exposures, one LED per exposure:

1, LED14=18
2, LED14=128

One exposure with two LEDs:

1, LED14=222.2,LED24=10388.5

Additional rules: 

• Up to 100 exposures are allowed.
• A maximum of two LEDs are allowed per exposure.
• If two LEDs are used for an exposure, the LEDs cannot both be from the same bank.
• An LED may be used only once in an exposure.

LED Bands for LOLO Calibration

For LOLO calibrations, APT provides a warning if a specified LED Band is not consistent with the Optical Element for the observation. The Table of LED Bands Consistent with Optical Elements for LOLO Calibration shows the rules for choosing LEDs consistent with the Optical Element. 

For additional questions not answered in this article, please contact the Roman Help Desk at STScI.