HSC Exposure Time Calculator

Brightness: = mag (AB)
Point source: Seeing: arcsec (use circular aperture of given diameter)
Extended source: Solid angle: arcsec 2 (Use mag per arcsec 2 in "Brightness")
Moon: days after new moon with moon distance: degrees
Photometry: Aperture = arcseconds in diameter
Sky count: ADU per frame at maximum.

for S/N ratio =
for exposure time = seconds
in total
for each of frames

About the ETC

This Exposure Time Calculator (ETC) is developed mainly for HSC queue mode PIs. It assumes that total noise arises from photon shot noise by sky flux in various moon phase and distance condition, and detector properties such as dark current and read-out noise. Additional sources of noise, such as poor flat fielding, incorrect sky determination, and confusion, could result in worse sensitivity than one estimated by this calculator.

What's New

  • Bug fix Jan. 5, 2021
    • A bug was found in the code for estimation of sky level with moon separation less than 65 deg.
    • hsc_etc.cgi was updated on January 4, 2021.
  • Display Saturation magnitude Jul. 27, 2016
    • ETC now output magnitudes corresponding to the saturation level of 45,000 ADU in a frame.
  • HSC-r2 filter installed Jun. 30, 2016
    • To improve the ring-shaped flat pattern on HSC-r band, new r band filter named "HSC-r2" has been installed.
    • In the HSC observation run at and after S16B (Aug. 2016), HSC-r2 band will be used as "r" band. In this ETC, canculations with old "HSC-r" band will be also available after S16B.
  • HSC-i2 filter installed Feb. 5, 2016
    • To improve the ring-shaped flat pattern on HSC-i band, new i band filter named "HSC-i2" has been installed.
    • The HSC-i2 filter has been used in the actual observations since S16A (Feb. 2016) except the small number of test frames taken before the semester. For archive images taken in S15B and before, HSC-i should be used for the calculation.

The ETC returns signal to noise ratio (S/N) or exposure time (T) based on the input parameter such as object magnitude, filter, and object type, in addition to the condition parameters of seeing, transparency, phase and distance of moon.

  1. Brightness: Select a filter and input the target magnitude in AB mag. For extended sources, put the brightness in AB mag per square arcsec.
  2. Object type: Select source type from Point source and Extended source. For point source, also input the assumed seeing in arcsec. For extended source, input the solid angle of the target.
  3. Conditions: Input observational condition such as Transparency, phase of Moon and Moon distance. In addition, specify photometric aperture in arcsec. This ETC returns the same results for the moon distance larger than 65 degree.

If you would like to calculate T, just input assumed S/N and click "Calculate exposure time" button. The ETC will calculate T with limiting magnitude and expected sky background count. If sky count exceeds the value given through "Sky count" for one frame, the ETC suggests the minimum frame number to achieve the input S/N with less sky counts than given value on each frames.

If you would like to calculate S/N, you have some way to calculate it. If you select "in total", the ETC will return simple S/N for the input T. If sky count exceeds the value given through "Sky count", the ETC suggests the minimum frame number to realize less sky counts than given value on each frames. If you select "for each of N frames", the ETC calculates the total S/N after N frame stacking, in addition to S/N for each frame.

S/N ratio v.s. Exposure time in HSC-z band with target AB magnitude 20.0 to 26.0 mag

S/N variation according to the phase of Moon and Moon distance in HSC-z band. S/N is normalized by the value without any moon effect.

Core script last updated : FF, 22 Jul 2016

Web IF last update : SK, 26 Jul 2016

Bug fix : TT, 05 Jan 2021

Filter list last update : TT, 27 Jan 2021

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