WEBCAMS UNDER THE SPOTLIGHT
MEASURING THE KEY PERFORMANCE PARAMETERS OF A MODIFIED WEBCAM
This presentation was given at the 2006 Conference of the Quickcam and Unconventional Imaging Astronomy Group(QCUIAG 2006) It can be downloaded as an acrobat pdf file (2 parts to keep file sizes below 2MB)
If a camera is going to be used for scientific measurements, it is important to know the linearity, gain and noise characteristics. Understanding these parameters can also help in getting the best images.
Unfortunately, while most astro-camera manufacturers publish these figures, we have little knowledge about these important values for webcam based imagers.
This presentation describes how these have been measured using simple equipment, for a monochrome SC3 long exposure and RAW firmware modified Vesta webcam.
Examples are given of how these results can be applied to get the best out of the camera for both imaging and scientific applications, including:
Correcting the camera output for linearity errors.
Using the measured noise figures to predict the result of stacking many short exposures.
The presentation finishes off with some examples of photometry, spectroscopy and spectroheliography using webcam based imagers
Notes and links
These results were obtained for my Philips Vesta webcam which has been modified for long exposure and the original colour CCD replaced by an ICX424AL monochrome CCD. This is known as an SC3 modification. More information can be found here. Details of the specific camera used can be found elsewhere on my website.
Additionally the camera firmware has been modified to allow the raw individual pixel values to be accessed without the Bayer colour encoding process. The modification also allows the lowest levels of brightness to be accessed and the sharpening algorithm to be disabled. More information on the RAW firmware modification can be found here. There is also a Yahoo group (TWIRG) for development these firmware modifications
Other models of cameras will most likely perform differently. I leave it to individuals to discover the quirks of their particular camera.. If you find anything interesting please let me know or report it on the QCUIAG group.
The calculations and Excel spreadsheet used to predict the effect on signal to noise of stacking many images can be found on my website here
The gain and noise parameter estimates depend on seeing the true noise variation in the images. The Vesta Webcam firmware performs some image compression to allow the video to be streamed over a USB1.0 connection. It was suggested during the conference that this might affect the measurement of gain and noise. The tests were performed with the camera set to 640x480 resolution and the minimum frame rate of 5fps. It is not clear if any residual compression is affecting the results but the effect of under estimating the noise in the images would be to give a higher figure for the gain (e/ADU) It appears however that as this figure is then used in the calculation of the camera noise parameters, the end result would be unaffected. This remains to be tested in practise.
K3CCDTools was used for image capture and stacking.
WcCtrl was used to record the camera settings numerically
Teleauto was used to analyse the statistics of the images and for photometry.
ImageTOOLSca was used calibrating astronomical images for bias, darks and flats. It also includes a routine for converting webcam avi to sets of 16bit FITs files including the gamma correction described in the presentation.
The spreadsheet to calculate the effect of stacking images on signal to noise can be downloaded here.
Visual Spec was used for spectroscopy
Astrosnap (version 2.2 Beta ) was used for producing the spectroheliograph images
Various functions of IRIS were also used
Photometry on my website
Spectroscopy on my website
Star Analyser low resolution diffraction grating
LHIRES III high resolution spectrograph, now commercially available here