[Bug]: Exponential model doesn't work if first time element is masked

[erinmmay]

FAQ check

• Yes, I checked the FAQ and my question has not been addressed.

Instrument

Light curve fitting (Stages 4-6)

What happened?

The exponential model subtracts off the first time element, but if that is masked it results in a fully masked array and the exponential model doesn't "work". Just need to change the -time[0] to -time.data[0]. I'll open a PR shortly.

Error traceback output

No response

What operating system are you using?

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What version of Python are you running?

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What Python packages do you have installed?

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