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

We evaluated the use of a commercial flatbed scanner for digitizing photographic plates used for spectroscopy. The scanner has a bed size of 420 mm by 310 mm and a pixel size of about 0.0106 mm. Our tests show that the closest line pairs that can be resolved with the scanner are 0.024 mm apart, only slightly larger than the Nyquist resolution of 0.021 mm expected from the 0.0106 mm 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 0.05 mm 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.