Keywords: class-spectral-axis-v12

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## Signal and image frequency axes in the rest frame

The discussion of Section is still valid for both the signal and image frequency axes. Eq. can easily be rewritten

| (20)

and

| (21)

with

| (22)

is simply the tuning frequency entered by the user. However, must be computed. The easiest way is to define the side band separation as

| (23)

It is straightfoward to show that it is constant and equal to

| (24)

In the rest (source) frame, the following relations hold

| (25)

The side band separation in the rest frame is defined as

| (26)

It is then straightforward to show that

| (27)

and

| (28)

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