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Atmospheric Dispersion Corrector (ADC)

Commissioning of the ADCs for GMOS-N and GMOS-S have not yet been scheduled. Observers should not currently plan on using the ADC with GMOS.

Introduction

The GMOS ADC corrects for the effects of atmospheric refraction that would otherwise cause serious wavelength dependent slit-loss problems (see Fillipenko 1982, PASP, 94, 715). Thus, use of the ADC is critical for studies of continuum features and absorption and emission line ratios with narrow slits. The ADC does not cause significant degradation of image quality for zenith distances less than 60 degrees, but it does induce some cylindrical distortion that must be accounted for when very narrow slits are used.

Basic design and performance

Image Quality

The lens added to the rear of the ADC approximately doubles the radius of curvature of the focal surface. Ray tracing for the 0.39 - 0.59 micron wavelength region demonstrates that the performance at zenith distance 60° is virtually as good as that at the zenith, and that all the light is contained within 0.2arcsec. Table 1 compares the performance in terms of encircled energy diameter (EED) to the Pre-Slit Error Budget specifications. The values given are for an optimum focus chosen to give good performance on-axis as well as at 3.5 arcmin off-axis at the specified wavelengths at the zenith. The image quality is excellent, although the on-axis values exceed the specification by ~0.01 arcsec at wavelengths longer than 0.5 microns, Note that the 85% EEDs are considerably better than the specification on average. A different focus could be chosen to obtain on-axis images within the error budget, but then the off-axis images degrade.

**Table 1: Expected ADC image quality performance versus specifications (in parentheses)**
Location in Field	Encircled Energy	0.37 microns	0.55 microns	1.0 microns
On-axis	50% EED	0.07" (0.07")	0.07" (0.07")	0.07" (0.07")
On-axis	85% EED	0.12" (0.18")	0.13" (0.15")	0.14" (0.13")
Off-axis	50% EED	0.09" (0.09")	0.08" (0.07")	0.08" (0.06")
Off-axis	85% EED	0.14" (0.20")	0.13" (0.18")	0.14" (0.15")

In summary, the current ADC design meets the formal image quality specifications to within 0.01 arcsec. In practice, however, GMOS will not be used at monochromatic wavelengths at the zenith. Typically, it will be used at zenith distances of 0 to 45° over wavelength ranges spanning 400nm - 1000nm. Calculations have been made to show the performance for two configurations that will likely be used: (A) the wavelength range from 0.37 to 0.50 microns, which includes many important stellar and nebular lines at rest at a zenith distance of 30° and (B) the range 0.45 to 0.9 microns which is often used in redshift surveys, at a zenith distance of 20°. The 50% EEDs for images in example A range from 0.04 to 0.09 arcsec over the full GMOS field, the 85% EEDs range from 0.08 to 0.15 arcsec. The 50% EEDs for images over the whole field in example B range from 0.04 arcsec to 0.08 arcsec, the 85% EEDs range from 0.11 arcsec to 0.15 arcsec. Thus, the zenith image quality goals are very nearly met or substantially exceeded in these two typical full-field, average zenith distance examples.

Distortion

The ADC introduces some cylindrical distortion over the field which is potentially a problem when attempting to meet the very strict requirements necessary to achieve the highest velocity accuracies. One of the basic GMOS goals is to measure radial velocities to an accuracy of 2 km/s which implies that objects must be centered in slits to within a few microns. In practice, this means that the variation of the distortion introduced by the ADC ideally must be small or at least precisely predictable so that slit masks can be accurately made for a specified zenith distance/ADC configuration. Even in the absence of the ADC, differential refraction over the field produces some distortion.

Table 2 shows the results of calculations of the distortion due to the ADC and the atmosphere and the usable diameter of the field given the required accuracies. For "low-accuracy" it is assumed that the distortion must not exceed 0.5 pixels (~40 mas). "High-accuracy" means that the distortion must not exceed 0.1 pixels (~8 mas). For GMOS 1 pixel subtends 0.072 arcsec or 72 mas.

**Table 2:Distortion due to the ADC and the atmosphere**
Distortion			Usable Diameter of Field
			High accuracy		Low accuracy
ZD (deg)	Both(mas)	ATM (mas)	Both (%)	ATM (%)	Both (%)	ATM(%)
0	0	0	100	100	100	100
15	0	1	100	100	100	100
30	15	7	50	100	100	100
45	60	20	13	40	67	100
52	95	33	8	24	42	100
60	170	60	5	14	24	66
Both=ADC+ATM		ATM=atmosphere only

The offsets within 30° are similar in magnitude with and without the ADC, but beyond 45° , the offsets become large and clearly any high precision velocity work should be carried out much closer to the zenith, if possible. To achieve the highest possible accuracies, the masks will have to be designed for a specific ADC configuration. Of course, fixing the ADC can produce wavelength-dependent positional shifts of several hundred milliarcsec, depending on ZD, due to the atmospheric refraction that the ADC is designed to correct. Alternatively, or possibly in addition, images taken before and after the spectroscopic exposures will have to be secured to measure the locations of objects within the slits to the required precision to calculate velocity offsets.

In summary, although the ADC basically delivers zenith-quality images at zenith distances up to 60 °, it also introduces some variable field distortion that will limit the zenith distances for which precision velocities can be acquired over the entire field, limit the field over which precision velocities can be measured, and/or require that masks be made for specific zenith distance configurations.

In original form; Bryan Miller
Last update August 19, 2002; Inger Jørgensen