Pan-STARRS Site Evaluation : SHABAR
Lunar Shadow Band Ranging
Shadow band ranging uses an array of photodiodes to measure the auto-correlation function of the irradiance from extended sources to probe the contribution to seeing from the lower levels of the atmpsphere.Since it is this boundary layer seeing that discriminates Haleakala and Mauna Kea, these studies are critical for site selection. Measurements of the low altitude seeing can also be used to evaluate the gain in performance from orthogonal transfer CCD charge shifting that can partially compensate for seeing arising at altitudes of order 100m.
Scintillation of extended objects was analysed in Roddier '81, and the idea of using this as a probe of the seeing profile was first suggested by Seykora and elaborated on by Beckers.
Solar SHABAR has been used effectively in the ATST site testing program ( ATST Site Evaluation Report , see also Beckers' SPIE 2002 paper ). The SHABAR can be seen in operation at Haleakal here .
The idea of performing night-time seeing profilometry seems to have originated in an unpublished paper by Hickson and Lanzetta (see the ATST report for details of their analysis). Hickson constructed a SHABAR using avalanche photodiodes and obtained the first measurements of the lunar SHABAR autocorrelation function.
Jeff Kuhn is building a SHABAR device for Pan-STARRS. The data from this device will be analysed by a new inversion algorithm developed by Kaiser.