Exposure Time Calculator

1. The model

For the BANZAI-reduced LCO instruments (MuSCAT3, MuSCAT4, Sinistro, QHY600, SBIG) the peak pixel of a star is its total flux times the fraction of that flux landing in the brightest pixel:

Exposure time follows by dividing the target peak (a fraction of the saturation level) by this rate. "Calculate peak counts" multiplies it by the exposure time instead.

2. Where the numbers come from

Every BANZAI e91 frame records its own photometric zeropoint (L1ZP), image FWHM (L1FWHM), saturation level (MAXLIN) and a catalog of every detected star. Calibration samples up to 800 archive frames per instrument and fits, per band and readout mode:

• ZP: the 84th percentile of the zeropoint corrected to zenith — a good-transparency night, so a clear night never gives a brighter star than predicted;
• A: the peak-to-flux ratio of bright unsaturated stars times FWHM² (about 0.9–1.0 in focus; defocused donuts have their own value);
• c: how fast the image grows with defocus. The saturation estimate uses a narrow-donut slope (16th percentile), never wider than the typical image;
• the saturation level: the lowest MAXLIN seen in the last year, since cameras of one instrument differ (QHY600 units span 42,000–47,400 e⁻) and Sinistro's 2×2-binned mode saturates near twice the unbinned level;
• the default seeing: the 16th percentile of in-focus FWHM. Enter your own seeing to override it.

Until an instrument is calibrated in this database, a fit made from the muscat-db archive in October 2026 is used. SBIG frames carry no zeropoint, so its zeropoints are LCO's own exposure-time-calculator values. Checked against frames held out of the fit, the model predicts the measured in-focus peak with a median error within 0.1 dex and a 16the model predicts the measured in-focus peak to within about ±0.15 dex (median within 0.1 dex) for every LCO instrument.ndash;84% spread within about the model predicts the measured in-focus peak to within about ±0.15 dex (median within 0.1 dex) for every LCO instrument.plusmn;0.2 dex for every LCO instrument with zeropoints in its headers.

3. Airmass and focus

Extinction k (mag per airmass; 0.15 for g, 0.09 for r, ...) dims the star as airmass grows. Defocus widens the image by c arcsec per mm, lowering the peak as 1/FWHM². Where the archive holds no defocused frames (QHY600), defocus is assumed not to lower the peak, which errs toward a shorter, safe exposure.

4. Narrowband filters

A narrowband filter without its own fit uses its broadband parent, fainter by 2.5 × log10(narrow bandwidth / broadband bandwidth) mag.

5. Saturation and the recommended exposure

The peak is compared with the saturation level of the chosen band and readout mode to get the % of full well (green/orange/red below 70%/100%). The recommended exposure is the minimum across all requested bands and stars, since MuSCAT exposes every band and every star in the field at once.

6. muscat and muscat2

These raw-frame instruments carry no zeropoint, so they keep the older coefficient model: log10(peak) = coef − 0.4 × mag_eff + log10(exptime / 60), calibrated from frames in this database where available, otherwise the MuSCAT3 table scaled by aperture and pixel area. That fallback has not been validated against measured peaks.