Collimating mount
| Case (a) : telescope f-ratio > collimator f-ratio | OK. Still, a smaller etalon can be used. |
| Case (b) : telescope f-ratio = collimator f-ratio | Optimal. |
| Case (c) : telescope f-ratio < collimator f-ratio | NOK. The telescope aperture is vigneted. Equivalent to having a small aperture telescope. |