Use the atmospheric dispersion corrector with the Mosaic II camera

The Earth's atmosphere disperses the light from stars significantly when observing away from zenith. Blanco has ADC prisms that can be configured via a rotation to counter this effect nearly completely, thereby greatly reducing the elongation of the image introduced by the atmosphere. The telescope operators will enable the ADC for you.

Old hands with the Mosaic II camera are suspicious of the ADC, especially since it was damaged in the late 90's. It is believed that PSF's are better behaved if the ADC is not used at all (i.e. placed in its "nominal" position and left there) or set to the appropriate position at the beginning of an exposure then turned off. This is not the case however. Do not fear to use the ADC.

Observations:

    Consecutive images were taken on the same field. One of these images was taken with the ADC on, and the other was taken with the ADC set to the appropriate position at the beginning of the exposure and then turned off.

    Test 1:

  • Exposure time: 10 min
    Filter: V
    Airmass ADC on: 2.2
    Airmass ADC off: 1.8

    From forty stars relatively close and spaced evenly around the center, the average fwhm:
    ADC on = 5.23 ± 0.05          →  1.41" ± 0.01" (error in the mean)
    ADC off = 5.04 ± 0.06          →  1.36" ± 0.02" (error in the mean)

  • Δ airmass: +0.4
    Δ fwhm: +0.19 (0.05")     (Δ = ADCon - ADCoff)

    Test 2:

  • Exposure time: 10 min
    Filter: V

    Airmass ADC on: 2.0
    Airmass ADC off: 2.2

    From forty stars relatively close and spaced evenly around the center: average fwhm:
    ADC on = 4.73 ± 0.08          →  1.28" ± 0.02" (error in the mean)
    ADC off = 5.31 ± 0.08          →  1.43" ± 0.02" (error in the mean)

  • Δ airmass: -0.2
    Δ fwhm: -0.60 (0.16")     (Δ = ADCon - ADCoff)

    Test 3:

  • Exposure time: 10 min
    Filter: R

    Airmass ADC on: 2.2,
    Airmass ADC off: 2.4

    From forty stars relatively close and spaced evenly around the center: average fwhm:
    ADC on = 5.12 ± 0.07          →  1.38" ± 0.02" (error in the mean)
    ADC off = 5.77 ± 0.07          →  1.56" ± 0.02" (error in the mean)

  • Δ airmass: -0.2
    Δ fwhm: -0.65 (0.18")     (Δ = ADCon - ADCoff)

Conclusions:

  • At high airmass and with broadband filters, the ADC makes a clear improvement
  • In terms of affecting your PSF, airmass is a larger player than the ADC

Back To Mosaic II

Page created: 23 March 2004 by Tim Abbott
updated: April 2010 by Andrea Kunder

Staff Contacts:
Alistair Walker: awalker@noao.edu
Chris Smith: csmith@noao.edu
Andrea Kunder: akunder@ctio.noao.edu
Tim Abbott: tabbott@noao.edu