The Cases Where Fast Mode is Useful

When You Should Use Fast Mode


This is due to the fact that Cosmic Ray Glitches can cause a change in the responsivity and one would like to move across as many detector elements as possible before that happens.

What is Fast Mode, Anyway?

Fast mode is essentially a breaking of the first of the three LWS rules: 1) All integrations (ramps) at any grating position (wavelength) shall be done three times before moving to the next grating position, no matter how strong the source, and all Fabry-Perot settings (wavelengths) shall be done three times before moving on to the next setting; 2) The minimum number of sweeps of the grating is 3 or "sweeps" of the FP LWS04 is three (for LWS04 it is 1); 3) Since the minimum amount of information (called a "frame") that can be down-linked from the LWS in one AOT is a fixed record size in bytes, a frame shall be filled (optimally) before ending the AOT.

Each of the above rules can cause the realized S/N to surpass the requested S/N and cause the observation to take longer than naively anticipated by the observer. The reason for the first two rules is to insure the highest possible data quality: by taking three or four integrations (ramps) at a grating or FP setting, spurious results can be eliminated by polling. (Since FAST MODE is now required, this polling possible no longer exists). By sweeping the grating and FP back and forth three times, memory and directional effects can be identified and removed (these are known, at a low level, to exist). Finally a frame is filled with the maximum amount of data, this being preferable to nothing.

A Fast Scan is one where the first rule is dropped, with the intention of taking the fastest grating or FP scan as possible and doing one and only one integration (ramp) per step. Thus on a single sweep or the grating of FP, there will be no indication as to the reality (repeatability) of a measurement. Only by comparing the three or more results of sweeps of the grating of FP, will a polling be possible. This illuminates the danger of using fast scanning - of not having sufficient statistics to tell if a blip is real or not.

Note that the second and third rules are never dropped.

Does Fast Scanning Save Time?

One must understand the material in the previous section to understand the answer. Fast scanning can (in addition to its purpose of enhancing the quality of the resulting data) cause a speed-up in particular cases. These are cases where the signal is very strong and therefore very few ramps are required to achieve the requisite S/N. Otherwise, fast scanning will take longer.