Evaluation of resolution and periodic errors of a flatbed scanner used for digitizing spectroscopic photographic plates.

Abstract

We evaluated the use of a commercial flatbed scanner for digitizing photographic plates used for spectroscopy. The scanner has a bed size of 420mm by 310mm and a pixel size of about 0.0106mm. Our tests show that the closest line pairs that can be resolved with the scanner are 0.024mm apart, only slightly larger than the Nyquist resolution of 0.021mm expected from the 0.0106mm pixel size. We measured periodic errors in the scanner using both a calibrated length scale and a photographic plate. We find no noticeable periodic errors in the direction parallel to the linear detector in the scanner, but we find errors with an amplitude of 0.03 to 0.05mm in the direction perpendicular to the detector. We conclude that large periodic errors in measurements of spectroscopic plates using flatbed scanners can be eliminated by placing the plate along the short side of the scanner and scanning the plates with the dispersion direction parallel to the linear detector.