*01/06/2025
With the recent release of the monochrome IMX585 sensor, now available from several manufacturers (ZWO, Player One, Altair, etc.), amateur spectroscopy has taken a significant leap forward in the near-ultraviolet and near-infrared domains. This sensor offers remarkable sensitivity, opening up new possibilities for experimentation. I wanted to try observing reflectance spectra of bright asteroids using a small 150 mm Newtonian telescope paired with the Star’Ex low-resolution spectrograph. Similar work was successfully conducted in 2023 by Valérie Desnoux and Christian Buil at the T60 telescope of Pic du Midi (see here).
For these observations, a Star’Ex low-resolution spectrograph equipped with a 300 lines/mm grating blazed in the visible range was used. Although the Star’Ex is not optimized for infrared, I wanted to explore that wavelength range. I simply added an IR pass filter (650 nm) as an order-sorting filter. The setup was mounted on a Skywatcher 150PDS Newtonian (150/750) and a ZWO AM5 harmonic mount.
The star Eta UMa, whose reference spectrum is available in the CALSPEC database, was used for wavelength calibration and instrumental response. A flat field was carefully acquired on a table using a 4700 K halogen lamp.
Acquisitions were then made on asteroid (4) Vesta, a solar analog star HD126053 (V=6.3 - G1V), and asteroid (3) Juno, all at similar altitudes in the sky to minimize atmospheric effects. The reflectance spectrum was obtained by dividing each asteroid’s spectrum by that of the reference solar analog star.
The results are interesting! The IMX585 CMOS sensor proved to be very sensitive, providing usable flux despite the optical limitations of the spectrograph. However, the spectral resolution varies significantly with wavelength, degrading the line sharpness at the spectrum's edges. Despite this, comparison with reference spectra (via the M4AST service) remains satisfactory: Vesta shows a characteristic "V"-shaped absorption around 1 micron (caused by pyroxenes), typical of V-type spectral objects while Juno exhibits a flatter spectrum, consistent with an S-type. Noise is, however, quite high beyond 900 nm, particularly for Juno. Weather conditions limited the number of exposures on this magnitude 10.26 object.
All of this makes me want to purchase an optics kit optimized for infrared use with my Star’Ex, and to redo cleaner observations on other asteroid types, with longer exposure times per object to improve the signal-to-noise ratio and the overall quality of the reflectance spectra. 🙂