Astropy units need a normalisation component

[jajmitchell]

Describe the feature

Currently the ThermalEmission, ThickTarget and ThinTarget models have hidden normalisation factors which aren't reflected in the units of the relevant parameters.

ThermalEmission - 1e49 cm**-3
ThickTarget - 1e35 electrons s**-1
ThinTarget - 1e55 cm**-2 s**-1

These units are currently reported without the normalisation factors. This is necessary as fitting isn't possible over many decades, therefore the factors are currently hidden. We need to find a way of having the input and output units reflect these factors without breaking the fitting.

Proposed solution

No response

[samaloney]

Potentially could use astropy parameters internal_unit might have to define custom units for the factors for this to work.

[rh2082]

Potentially could also look at the getter and setter parameters as a way of doing this

[KriSun95]

Didn't realise this issue was open, but have a look at #291 . I've defined new Astropy units to handle this. In my search, I independently tried both of your suggestions.

• I could not get the internal_unit to actually do anything. Additionally, it's the external unit that needs to be scaled since that's the units the fitters need to pass to the model. There isn't an issue with the internal units being the full, expanded units.
• I tried the getter and setter but it was getting complicated and I think there is a bug in Astropy preventing this being done for parameters with units. I ultimately gave up. Essentially, there is a check (yip! another Astropy check just breaking stuff unnecessarily) the getter and setter only have either 1 or 2 inputs (in Parameter()._create_value_wrapper); however, when the parameter has a unit then the following happens: value=self._getter(self._internal_value, self.internal_unit, self.unit).value. That's right, the getter gets passed 3 inputs. There wasn't a way of getting around this as far as I could see since the getter and setter aren't allowed to use arbitrary number of args as inputs like *args.