1. Image Calibration

Commands: wisp-calibrate, wisp-stack-to-master, wisp-stack-to-master-flat

Important Parameters

The most important parameters to set for the calibration are:

• exposure-start-utc or exposure-start-jd

• exposure-seconds

• saturation-threshold

• fnum

• image-area: If your camera generates images with overscan or other areas that are not part of the image.

• split-channels or raw-hdu: If using a color detector

• gain: If known. Used for accurate error estimates.

Description

Before raw images are used, they need to be calibrated. The sequence of steps is:

calibrate the raw bias images

Accomplished using wisp-calibrate command with no master files specified with raw bias images as input.

generate master bias image

Use wisp-stack-to-master command with the calibrated bias images as input.

calibrate raw dark images using the master bias

wisp-calibrate with the master-bias option set to the master bias file generated in the step above.

generate master dark image

Just like generating master bias but using the calibrated dark images.

calibrate the raw flat images

wisp-calibrate with the master-bias option set to the master bias file and the master-dark option set to the master dark.

generate master flat image(s)

wisp-stack-to-master-flat command. This is different from how master bias and dark are created, because AutoWISP is designed to allow using sky flats, which may be affected by clouds, or the sky is not perfectly uniformly bright.

calibrate raw object images

wisp-calibrate with master-bias, master-dark, and master-flat all specified.

No Calibration Data?

In case calibration data is not available, only the last of these steps needs to be performed, with no masters specified (see below). Even though in this case the pixels values will not be corrected for any of the effects described above, this step is still needed. It will add change the data format (float point instead of integer), add required information in the headher, split the different colors if using a color camera etc.

Overscan Corrections

In many instances, the imaging device provides extra areas that attempt to measure bias level and dark current, e.g. by continuing to read pixels past the physical number of pixels in the device, thus measuring the bias or by having an area of pixels which are somehow shielded from light, thus measuring the dark level in real time. Such corrections can be supierior to the master frames in that they measure the instantaneous bias and dark level, which may vary over time due to for example the temperature of the detector varying. However, bias level and dark current in particular can vary from pixel to pixel, which is not captured by these real-time areas. Hence, the best strategy is a combination of both, and is different for different detectors.

AutoWISP allows (but does not require) such areas to be used to estimate some smooth function of image position to subtract from each raw image, and then the masters are applied to the result. This works mathematically, because the masters will also have their values corrected for the bias and dark measured by these areas from the individual frames that were used to construct them. In this scheme, the master frames are used only to capture the pixel to pixel differences in bias and dark current. We refer to these areas as “overscan”, although that term really means only one type of such area.

Overscan area(s) can be specified using the overscans option of the wisp-calibrate command.