bias input readony

        const applyBias = () => {

            const isBias = imagetypeEl.value === 'Bias';

            exptimeEl.disabled = isBias;

            if (isBias) exptimeEl.value = '0';

            // readonly, not disabled: a disabled input is excluded from the

            // form's values (readForm() would find exptime missing), while

            // readonly still reads back normally and just blocks editing.

            exptimeEl.readOnly = isBias;

            if (isBias) {

                exptimeEl.tabIndex = -1;

                exptimeEl.value = '0';

            } else {

                exptimeEl.removeAttribute('tabindex');

            }

        };

        imagetypeEl.addEventListener('change', applyBias);

        applyBias();