RR Aql flux spectrum as a function of wavelength from 2.4 μm to 40 μm (top). The lines represent the flux spectra from ISO-SWS observations covering wavelengths from 2.5 μm to  ~240 μm (solid red thick line), IRAS-LRS observations covering  ~7.7 μm to  ~23 μm (dashed thin black line), and the mean of our N-band MIDI measurements (solid thin green line). Here, the vertical bars span the maximum and minimum values measured. The diamonds denote 2MASS measurements at 1.25 μm, 1.65 μm, and 2.2 μm. The solid black line indicates our atmosphere and dust shell model as explained in Sect. 5.2. The bottom plot shows an enlarged segment of the plot in the MIDI wavelength range of 8–13 μm.