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modified ETCs so a sub exposure time is not required #44

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modified ETCs so a sub exposure time is not required #44

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17 changes: 14 additions & 3 deletions gunagala/imager.py
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Original file line number Diff line number Diff line change
Expand Up @@ -511,7 +511,7 @@ def extended_source_etc(self, surface_brightness, filter_name, snr_target, sub_e
snr_target : astropy.units.Quantity
The desired signal to noise ratio, dimensionless unscaled units
sub_exp_time : astropy.units.Quantity
length of individual sub-exposures
length of individual sub-exposures, or None if no sub-exposures are needed
calc_type : {'per pixel', 'per arcsecond squared'}
Calculation type, either signal to noise ratio per pixel or
signal to noise ratio per arcsecond^2. Default is 'per pixel'
Expand Down Expand Up @@ -563,7 +563,12 @@ def extended_source_etc(self, surface_brightness, filter_name, snr_target, sub_e
# Measuring the sky background itself.
rate = self.sky_rate[filter_name]

sub_exp_time = ensure_unit(sub_exp_time, u.second)
if sub_exp_time is None:
no_sub_exp_time = True
sub_exp_time = 1 * u.second
else:
sub_exp_time = ensure_unit(sub_exp_time, u.second)
no_sub_exp_time = False

# If required total exposure time is much greater than the length of a sub-exposure then
# all noise sources (including read noise) are proportional to t^0.5 and we can use a
Expand All @@ -582,6 +587,9 @@ def extended_source_etc(self, surface_brightness, filter_name, snr_target, sub_e
noise_squared_rate = noise_squared_rate.to(u.electron**2 / (u.pixel**2 * u.second))
total_exp_time = (snr_target**2 * noise_squared_rate / rate**2).to(u.second)

if no_sub_exp_time:
sub_exp_time = total_exp_time

# Now just round up to the next integer number of sub-exposures, being careful because the total_exp_time
# and/or sub_exp_time could be Quantity arrays instead of scalars. The simplified expression above is exact
# for integer numbers of sub exposures and signal to noise ratio monotonically increases with exposure time
Expand Down Expand Up @@ -1061,7 +1069,7 @@ def point_source_etc(self, brightness, filter_name, snr_target, sub_exp_time, sa
snr_target : astropy.units.Quantity
The desired signal to noise ratio, dimensionless unscaled units
sub_exp_time : astropy.units.Quantity
length of individual sub-exposures
length of individual sub-exposures, or None if no sub-exposures are needed
saturation_check : bool, optional
If `True` will set the exposure time to zero where the
electrons per pixel in a single sub-exposure exceed the
Expand All @@ -1087,6 +1095,9 @@ def point_source_etc(self, brightness, filter_name, snr_target, sub_exp_time, sa
total_exp_time = self.extended_source_etc(rate / self.psf.n_pix, filter_name, snr_target, sub_exp_time,
saturation_check=False, binning=self.psf.n_pix / u.pixel)

if sub_exp_time is None:
sub_exp_time = total_exp_time

# Saturation check. For point sources need to know maximum fraction of total electrons that will end up
# in a single pixel, this is available as psf.peak. Can use this to calculate maximum electrons per pixel
# in a single sub exposure, and check against saturation_level.
Expand Down