Reflectometers

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The aim of the reflectometer project is to allow users of the reflectometers to have a common and easy experience to set the position of items on the beam line. The setup is fairly complex because of there are multiple items that need to be coordinated to make a measurement. This page should give an overview of what is going on and the user cases that are needed.

Related Pages:

• Setup and science behind the experiment

Components in a mode

Table of our understanding for each mode what is in the beam and what is tracking the beam. States are:

• O: Out of the beam
• I: In the beam but not tracking it
• T: In the beam and tracking it

Mode	S1	Polariser	S2	sample stack	ideal sample point	S3	analyser	S4	Detector
NR	T	O	T	I	T	T	O	T	T
polarised	T	T	T	I	T	T	O	T	T

In sim language

Mode | S1 Height above beam | Polariser angle and height above

Calculating Height of an Item Above Straight Through Beam

The height of an item along the beam can be calculated based of the incoming angle. This diagram shows how this works.

Items to Document

Theta beam path calculation

The theta beam line parameter and its associated component is special because it depends on other item(s) in the beamline. All other components are independent. For read-back, the "angle" of the component is set based on the incoming beam and the angle from the point where the incoming beam hits the sample axis to where a specified component is. For example, on CRISP it is the angle to the detector. This means if the detectors moves it must signal back to the Theta component that it has moved. TODO Write done what happens when the given component is not on the beam path. In disabled mode, the incoming beam is no longer altered and this means changing theta would have no effect on the component it is pointing at, e.g. changing Theta would not alter the position of the detector. To fix this (and a question will be asked about this) there is a special route to force an incoming beam path to be set. This should allow the component defining theta to move when theta is changed.

Questions

Answered

1. Is the ideal sample point x position fixes?
   • Yes. The beam coming in will hit any pre-sample active components and the height of the sample point is the point at which the beam reaches this x coordinate.
2. I think each mode of operation sets up fixed rules and relations and this is what we need to capture.
3. We need to calculate both the positions based on composite parameters and composite parameters based on the positions of the components, is this right?
   • Yes, it would be strange not to have this readback
   1. What happens if the beam having hit the super mirror does not hit the sample axis?
      • apart from pathological cases, beam going straight up or backwards, there is always a point that the sample axis is hit
   2. What happens if the detector is not looking at the sample point?
      • Nothing, but a warning in this case would be useful.
4. Does the beam always go forwards?
   • Yes, on reflectometers the beam always goes forward.
5. Are there any corrections to the idealised model, at that level?
   • No
6. What does an analyser do to the beam? Is it an active component? How does it affect the measurement of theta?
   • Yes it is an active component on some beam lines (depends on type). It doesn't effect the measurement of theta; although if you are calculating the readback the outgoing beam goes to the analyser not the detector.
7. How are slits measured (along the arc or along there length)?
   • In the end it doesn't really matter but they are measured along their length.
8. Room coordinates are these ok as we defined them?
   • Maybe, they should be the same as Mantid default.

Unanswered

1. Currently, the theta readback displays the angle to the detector but if the detector is set to be above the beam the readback is still to the detector and so the theta sp and readback do not agree. What should we do:
   1. Don't allow displacement relative to the beam to be set on components which interface with Theta
   2. Use the difference between the beam intercept and the position for the setpoint and add this to the read back position
   3. Do nothing this is the correct behaviour and is telling the instrument scientist they are not measuring what they think they are
2. Will you want to do individual moves while another move is going on?
3. Do we need to reflect SP of motors into Reflectometry server
4. Can we have the dance script.
5. Does it matter whether you can change parameters on components parked out of the beam? (e.g. SM Angle in NR Mode)
6. CRISP: What is the position for S4? (found it in mantid IDF, but values differ from those in VI - assumption is VI is correct)
7. Coordinates: according to IDFs, ZYX on INTER, SURF, OFFSPEC is XZY on CRISP, POLREF. We use the former. Is this a problem?