I've been reading on the subject and constantly find that in astronomy Echelle spectrometers are used.
I've also found that greater order separation favors long slit spectroscopy. Why is this?
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I am not quite sure I "get" your question. But I think what you are looking for is that in an echelle spectrograph, you operate at very high order numbers - typically 20-50 - and that the spectral orders would ordinarily completely overlap.
In order to separate the orders you pass the dispersed light through a "cross-disperser", often a prism or set of prisms, that disperse the light in the perpendicular direction to the echelle dispersion.
The result on your detector is an "echellogram", consisting of a set of short spectrum intervals of increasing wavelength, that are separated in the vertical direction.
In order to provide a clean separation of the orders in the vertical direction, and prevent them overlapping with each other, you have to restrict the length of the entrance slit using a "dekker". Thus you cannot do "long slit" echelle spectroscopy unless you have a large order separation in the vertical direction. This would require a very dispersive cross-disperser element, but would have the disadvantage that the wavelength range you could fit onto your detector was consequently smaller.
The picture below shows an echellogram for the bHROS echelle spectrograph on the Gemini S telescope. The cross-disperser is a set of fused silica prisms. These provide an order separation of at least 3mm. The detector is represented by the grey rectangle - you can basically position that to record any part of the echellogram. The width of each of the echellogram orders (order number labelled on the left hand side) is governed by the length of the slit. You cannot make the slit length too large or the orders will overlap with the orders above and below.
