Freeform surfaces have been introduced in the optical design of a two-mirror Cassegrain telescope for Earth observation purposes. The application requires two field positions at the image plane with minimum aberrations where two micro spectrometers (HAMAMATSU C12880MA) shall be placed. These micro spectrometers are small and compact devices that incorporate the slit, a concave diffractive element and the CMOS sensor in the same component. Although these are very small components their position at the image plane are slightly deviated from the optical axis, where it is well known minimum aberrations occurs for classical optical systems. Freeform surfaces are introduced to correct those aberrations, mainly astigmatism and coma, for the specific fields determined by the positions of the micro spectrometers. Nodal Aberration Theory (NAT) will be used to determine the mirrors surface deformation as a set of ϕ-polynomials calculating, at the same time, the effects of such deformations on the total aberrations