Comparing Xbox Webcams with and without the IR FIlter

Comparing Xbox Webcams with and without the IR FIlter

While on a camping trip to Croatia this year, I took two Xbox Live Vision webcams with me. In February 2013, I had installed one on the roof of my parents house in Melbourne, Australia without the infrared filter, so during this trip I wanted to see how much of a difference the IR filter makes to the sensitivity of the webcam. 

I used SharpCap to take 10 second images of several regions of sky with both webcams. I had removed one of the infrared filter with a swiss army knife. 

Raw Images

Figure 1: Raw 10 second exposures of Cygnus an Lyra with and without the IR filter in place

With the IR filter removed, the wavelength range available to which the CCD is sensitive is almost doubled (from 400nm-700nm to 400nm-900nm). Consequently more stars are visible. However, as the sky background emits at all wavelengths, the gain in sensitivity is barely noticeable. Other object which emit more in the infrared regime are also visible - for example the tree in the bottom left corner.

Reduced Images

 Figure 2: The same images after the dark current had been removed.

The sensitivity increase is much more noticeable once the dark current is removed. The amount of stars visible has almost doubled. This step was done using Registax 6 and a simple dark subtraction.

Annotated Images

Figure 3: The constellations Cygnus and Lyra and information about several of the stars

The annotated stars in the left panel are visible in both images, whereas the star named in the right image is only visible once the IR filter has been removed. This is because the star is an M class dwarf whose SED peaks in the IR and drops off sharply in the green and blue bands. Almost all of the stars visible with the IR filter are blue stars, more massive than F class. In contrast, without the IR filter the there is a much more even spread over spectral types. The images taken through an IR filter are limited to around 4th magnitude stars, while the removal of the IR filter, the limit reaches 6th magnitude for late type stars and 5th magnitude for early type stars.

Source Extraction on Images

Figure 4: Comparison of the amount of stars found by the extraction algorithm in the Astrometry.net program

The amount of stars extracted from an image also increase after the IR filter was removed. This both increases the amount of stars for the XERO catalogue and helps the programme Astrometry.net find a better WCS match for the coordinated of the image.

In summary, by removing the IR filter from the Xbox Live Vision webcam, the webcam moves from a curiosity to a usable CCD for taking images of the night sky. Images taken with the webcam are on the same sensitivity level as the human eye, and comparable to the BRITE satellites. The downside is that Red filter photometry is no longer roughly comparable to the Bessel R filter.