Remove the Linear from the SANS reduction

[Gesner Passos]

Executing the Reduction on SANS you get the following warning message:

CalculateTransmission started
Linear is deprecated. Use Fit instead.
CalculateTransmission successful, Duration 0.11 seconds

[Gesner Passos]

It will be moved to the next release, because moving to Fit will introduce differences to the resulting workspaces. They will make the system tests to fail. So, it requires some time...

[Karl Palmen]

Linear is scheduled to be removed in release 2.7 according to ticket #7072

[Gesner Passos]

It was found that some requirements from Linear were different from Fit. After some discussion, it was agreed that implementing #7182 allows us to use Fit algorithm.

But, the error from the Linear and Fit using function LinearBackground are different, the reason is that Linear, calculates the errors as:

err = sqrt(s0² + x²s1² + 2s01²)

while fit calculates the erros as:

err = sqrt(s0² + x²s1²)

Where s0² is the covariance of the first parameter(c0), s1² is the covariance of the second parameter(c1), and s01² is the covariance between these two parameters. For the liner equation:

y = c0 + c1 x

The fit does the simplification of considering the parameters independent, which is acceptable for the generic approach that fit offers. So, changing from Linear to Fit will cause us to update the systemtests that used to use Linear mainly because the errors are different right now.

[Gesner Passos]

Update nexus files to be processed with new CalculateTransmission

re #6981

Changeset: ​https://github.com/mantidproject/systemtests/commit/bcf6282b116d1dce28f9b2e67b2aca2493770689

[Gesner Passos]

CalculateTransmission: substitute Linear by Fit

re #6981

Changeset: 120bc055459e512050f5f2080595e662b5e3b7e3

[Gesner Passos]

Tester:

This ticket is composed by changes inside the systemtests and the mantid code (branches: feature/6981_from_linear_to_fit and feature/6981_remove_linear_calculate_transmission)

The aim is to remove the Linear child algorithm from CalculateTransmission. First of all, by inspection of the code you can see that Linear was removed.

The second step is to evaluate if the change from Linear to Fit is acceptable. This you have to do analyzing the comment:5. Are you happy with the difference in the error calculation introduced by the Fit algorithm? - Look: I'm giving the tester the responsibility to evaluate this issue because I do not feel quite comfortable in judging this issue.

The third step is to check that the execution of scripts that call CalculateTransmission will continue to work as expected. Reductions on SANS instruments use this algorithm.

Below you have a script that executes the reduction for SNS SANS instruments.

from reduction_workflow.instruments.sans.sns_command_interface import *
config = ConfigService.Instance()
config["facilityName"]='SNS'
EQSANS(False)
AppendDataFile("EQSANS_1466_event.nxs")
SolidAngle()
UseConfig(False)
UseConfigTOFTailsCutoff(False)
CombineTransmissionFits(True)
UseConfigMask(False)
SetBeamCenter(96.29, 126.15)
TotalChargeNormalization(normalize_to_beam=False)
DirectBeamTransmission("EQSANS_1466_event.nxs", "EQSANS_4061_event.nxs", beam_radius=3)
ThetaDependentTransmission(True)
Reduce1D()  
# Scale up to match correct scaling.
Scale(InputWorkspace="EQSANS_1466_event_Iq", Factor=2777.81, 
          Operation='Multiply', OutputWorkspace="EQSANS_1466_event_Iq")

For the sans instruments in ISIS, you can follow the instruction at: ​http://trac.mantidproject.org/mantid/ticket/7314#comment:4 .The reduction will occur nicely, and, as you expect, there is no more the warning message:

Linear is deprecated. Use Fit instead.

Finally, the system tests were changed in order to cope with the changes using Fit algorithm. Here, the first step is to evaluate the files that have changed. But, specially, to accept that the update on the EQSANSTransAPIv2.py file is acceptable ( ​https://github.com/mantidproject/systemtests/commit/bcf6282b116d1dce28f9b2e67b2aca2493770689)

Besides, all the systemtests that calls the CalculateTransmission must be passing on develop branch, before you can pass this ticket.

If you are happy with everything, than you can merge to the master both systemtests and mantid code.

[Mathieu Doucet]

Test that everything still works correctly. I also checked that the new reference files are close to the originals. The changes are tiny and orders of magnitude below the signal. So although I'm not a fan of changing reference data to accommodate code changes I'm going to pass the ticket :)

[Stuart Campbell]

This ticket has been transferred to github issue ​7827