LOCI mounted on LT, with SPRAT in background.
Image ©2025 Chloé Miossec.

LOCI

Introduction

The Liverpool Optical Compact Imager (LOCI) is a backup instrument for IO:O. It is cooled with an onboard peltier cooler, in contrast to IO:O's external crytotiger that is connected to the instrument by coolant pipes.

Status

ONLINE

Specifications

  • Andor Ikon-L 936 Camera
  • 2048 x 2048 E2V CCD
  • Operating temperature 203K
  • 13.5 x 13.5 micron pixels giving 27.6 x 27.6 mm detector
  • BV sensor (not deep depletion)
  • Peak counts ~65000
  • Built-in iris shutter
    • NB due to differential exposure caused by finite speed of iris petals,
      we recommend a minimum exposure time of 5 seconds
  • Filter Wheel
  • Optional foreoptics (currently not installed)
    • Edmund Optics 300mm focal length achromatic doublet
    • VIS-NIR coating
    • lens tube 3 inch (76 mm) diameter
  • Pixel scale (no foreoptics) = 0.2765" per 2x2 binned pixel
  • Field of view (no foreoptics) = 4.7 x 4.7 arcmin

  • Readout Speed = 2 seconds with 2x2 binning
  • Gain = 1.87 e/ADU
  • Read noise = 7.5 electrons
  • Mean bias level = 300
  • Median dark current = 1 count (~2e) per 100s at -70C

Pipeline

A basic data pipeline is in place which carries out debiasing and flatfielding. No correction for fringing in the i' and (particulary) z' bands is made.

Filters and Photometry


LOCI filter transmission curves and detector QE.

LOCI offers a set of u'g'r'i'z' filters from Baader Planetarium. Instrumental zero points and colour transformations from LOCI to the SDSS system of Smith et al. 2002, AJ, 123, 2121 (https://www-star.fnal.gov) have been derived for standard star observations immediately following the July 2026 wash of the LT primary mirror (2026-07-10 – 2026-08-17).

\[ u = ZP_u + u_{\rm inst} + E_u({\rm airmass}) + A_u (u_{\rm cat} - g_{\rm cat}) \] \[ g = ZP_g + g_{\rm inst} + E_g({\rm airmass}) + A_g (g_{\rm cat} - r_{\rm cat}) \] \[ r = ZP_r + r_{\rm inst} + E_r({\rm airmass}) + A_r (r_{\rm cat} - i_{\rm cat}) \] \[ i = ZP_i + i_{\rm inst} + E_i({\rm airmass}) + A_i (r_{\rm cat} - i_{\rm cat}) \] \[ z = ZP_z + z_{\rm inst} + E_z({\rm airmass}) + A_z (i_{\rm cat} - z_{\rm cat}) \]

where:

 
\( {\rm f_{inst}} \)
=
instrumental magnitude in filter f, calculated as \( {\rm -2.5 * log_{10}(photoelectrons)} \)
 
\( {\rm f_{cat}} \)
=
catalogue magnitude in filter f from https://www-star.fnal.gov/
 
\( {\rm ZP_f} \)
=
One-photo-electron per second zero-point for filter f, above the atmosphere (airmass=0)
 
\( {\rm E_f} \)
=
Airmass extinction correction (magnitudes per airmass)
 
\( {\rm A_f} \)
=
colour term in filter f

Calibration coefficients derived from observations 2026-07-10 – 2026-08-17:

colour term fits
Derivation of LOCI colour terms and zero points, July and August 2026.
filter
f		 zero point
\( {\rm ZP_f} \)		 colour term
\( {\rm A_f} \)		 colour
 		 extinction
\( {\rm E_f} \)
\(u\) 21.30 ± 0.05 +0.035 ± 0.033 \(u-g\) -0.55
\(g\) 24.79 ± 0.02 +0.098 ± 0.011 \(g-r\) -0.21
\(r\) 24.69 ± 0.02 -0.021 ± 0.010 \(r-i\) -0.16
\(i\) 24.26 ± 0.01 -0.019 ± 0.010 \(r-i\) -0.11
\(z\) 22.99 ± 0.02 -0.173 ± 0.022 \(i-z\) -0.06