Installation[edit | edit source]
|Organization||Brown University Physics|
|Location||Providence, Rhode Island, US|
|Altitude||226 feet (69 m)|
|Weather||See the Clear Sky Chart|
|Established||October 21, 1891|
|Open Tuesday nights, weather permitting.|
The Night Sky Live CONCAM installations were very expensive which caused some observatories to seek lower cost alternatives. This projects describes one such low cost alternative installed at the Ladd Observatory.
The camera is located in an urban environment where there are many street lights and other sources of light pollution. The camera is very sensitive and will produce overexposed images of the full moon. The camera can not be used during the daytime for this reason. It is important to mount the camera in a location where there is no direct artificial light which could wash out the details of dim astronomical sources such as meteors. The camera was mounted adjacent to the observatory dome, but great care was taken to prevent the dome from appearing in the field of view of the camera. Light from the sky (street light scattered from clouds) would reflect off the dome and degrade the image quality. This would also cause problems with the auto-contrast adjustment feature of the camera control software.
The camera system used here is a commercial off-the-shelf product manufactured by Santa Barbara Instrument Group. It includes a weatherproof housing with a window that is heated to prevent condensation. The window is an optical filter that transmits light from 630 - 1000 nm (red to near infrared) and is used to block light pollution. Inside is an ST-402ME digital imaging camera which uses a monochrome 16 bit per pixel Kodak KAF-0402ME CCD chip with 9 micron pixel size. The camera body is mated to a Computar fisheye lens that gives a wide-angle view of the sky. It has a 2.6mm focal length and a 1.6 focal ratio. The field of view is about 140 x 90 degrees. The camera described here is no longer manufactured; it has been replaced by the SBIG AllSky 340 Cameras. In 2017 the list price for this new model was US$ 2,500.
The camera is connected to a personal computer running Microsoft Windows with the CCDSoft camera control software from Software Bisque. A Visual Basic script turns the camera on at dusk and begins taking images every 10 seconds until dawn. The raw images are saved in the Flexible Image Transport System (FITS) format for later analysis. During a typical night this results in some 3,000 to 4,000 images, depending on the length of the night which varies by season. The total amount of uncompressed data is about 2 to 3 GB per night. Every 5 minutes the latest image is auto-contrast adjusted to compensate for a wide range of sky brichtness conditions. It is then converted to a compressed JPEG file which is immediately uploaded to a web server along with a text file containing the time-stamp and other information about the image. An Internet bot called Mu301Bot then copies one image per night to Wikiversity using the pywikibot Python library and scripts.
Maintenance of the camera is minimal. The top cover of the weather resistant enclosure has a slight tilt. When it rains the drops slide off to remove dust and pollen from the window that the camera views the sky through. No hardware repairs have been needed. The software occasionally must be restarted due to memory leaks or lack of disk space. The quality of the images has slowly degraded over the course of a decade. A number of defective pixels which are brighter than others can now be seen in the images.