Abstract

Differential image motion monitors (DIMMs) have become the industry standard for astronomical site characterization. The calibration of DIMMs is generally considered to be routine, but we show that particular care must be paid to this issue if high-accuracy measurements are to be achieved. In a side by side comparison of several DIMMs, we demonstrate that with proper care we can achieve an agreement between the seeing measurements of two DIMMS operating under the same conditions to better than ±0.02 arc sec.

© 2007 Optical Society of America

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References

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  1. M. Sarazin and F. Roddier, "The ESO differential image motion monitor," Astron. Astrophys. 227, 294-300 (1990).
  2. A. Tokovinin, "From differential image motion to seeing," Publ. Astron. Soc. Pac. 114, 1156-1166 (2002).
    [CrossRef]
  3. V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
    [CrossRef]
  4. A. Tokovinin, "Influence of defocus on DIMM," MASS profiler report, available at http://www.ctio.noao.edu/atokovin/profiler/archive.html (2004).
  5. R. J. Noll, "Zernike polynomials and atmospheric turbulence," J. Opt. Soc. Am. 66, 207-211 (1976).
    [CrossRef]
  6. These data are available at http://www.ctio.noao.edu/telescopes/dimm/dimm.html (2002).

2004 (1)

A. Tokovinin, "Influence of defocus on DIMM," MASS profiler report, available at http://www.ctio.noao.edu/atokovin/profiler/archive.html (2004).

2003 (1)

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

2002 (1)

A. Tokovinin, "From differential image motion to seeing," Publ. Astron. Soc. Pac. 114, 1156-1166 (2002).
[CrossRef]

1990 (1)

M. Sarazin and F. Roddier, "The ESO differential image motion monitor," Astron. Astrophys. 227, 294-300 (1990).

1976 (1)

Kornilov, V.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Noll, R. J.

Potanin, S. F.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Roddier, F.

M. Sarazin and F. Roddier, "The ESO differential image motion monitor," Astron. Astrophys. 227, 294-300 (1990).

Sarazin, M.

M. Sarazin and F. Roddier, "The ESO differential image motion monitor," Astron. Astrophys. 227, 294-300 (1990).

Sarazin, M. S.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Shatsky, N.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Tokovinin, A.

A. Tokovinin, "Influence of defocus on DIMM," MASS profiler report, available at http://www.ctio.noao.edu/atokovin/profiler/archive.html (2004).

A. Tokovinin, "From differential image motion to seeing," Publ. Astron. Soc. Pac. 114, 1156-1166 (2002).
[CrossRef]

Tokovinin, A. A.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Vozyakova, O.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Zaitsev, A.

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Astron. Astrophys. (1)

M. Sarazin and F. Roddier, "The ESO differential image motion monitor," Astron. Astrophys. 227, 294-300 (1990).

J. Opt. Soc. Am. (1)

Proc. SPIE (1)

V. Kornilov, A. A. Tokovinin, O. Vozyakova, A. Zaitsev, N. Shatsky, S. F. Potanin, and M. S. Sarazin, "MASS: a monitor of the vertical turbulence distribution," Proc. SPIE 4839, 837-845 (2003).
[CrossRef]

Publ. Astron. Soc. Pac. (1)

A. Tokovinin, "From differential image motion to seeing," Publ. Astron. Soc. Pac. 114, 1156-1166 (2002).
[CrossRef]

Other (2)

A. Tokovinin, "Influence of defocus on DIMM," MASS profiler report, available at http://www.ctio.noao.edu/atokovin/profiler/archive.html (2004).

These data are available at http://www.ctio.noao.edu/telescopes/dimm/dimm.html (2002).

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Figures (10)

Fig. 1
Fig. 1

Examples of DIMM measurements taken during two individual nights (left: 29 August 2004; right: 19 September 2004). Seeing measurements and Strehl ratios are binned in 15  min intervals in order to reduce scatter. The bottom plots show the average Strehl ratios (mean Strehl ratio of two DIMM images) of the DIMM data. We can see a significant difference between the two DIMMs on 29 August, when the T2 Strehl ratios were low. On 19 September, after the optical alignment of T2 was improved, T2 Strehls were higher and good agreement in seeing measurements was found.

Fig. 2
Fig. 2

Plot of the measured seeing difference between T2 and T3 (T2–T3) versus the minimum Strehl ratio for the period from 13 August to 26 October 2004. The error bars are the standard deviation of the seeing difference for respective Strehl ratio bins. As the minimum Strehl ratio decreases, an increasing seeing difference between the two DIMMs is found.

Fig. 3
Fig. 3

Seeing values versus mean Strehl ratio for T2 (left panel) and T3 (right panel) from the 2004 Tololo comparison campaign. All data are shown, including periods when the optical alignment was not good. Data points are divided into three discrete regions due to different levels of optical alignment. Region C corresponds to the period of good optical alignment of T2. Also shown are the theoretical and simulated Strehl limit curves for different values of S 0 and f (theoretical curve) and S optics (simulated curve). See the legends in the plots for the values used for each curve.

Fig. 4
Fig. 4

Five curves with symbols (bending to the right for low seeing values) show seeing versus Strehl ratios for simulated DIMM data with different levels of optical aberration as shown in the legend. The other five curves are the theoretical curves calculated from Eq. (5) that best fit the simulated curves. The respective values of S 0 and f are also indicated in the legend. Increasing values of S 0 correspond to shifting the curves to the right. The background points are those of T2 and are the same as in Fig. 3.

Fig. 5
Fig. 5

Scatterplot of the measured seeing values for T2 and T3 for the period between 15 September and 5 October 2004.

Fig. 6
Fig. 6

Difference between the two DIMM measurements (T2–T3) for the period between 15 September and 5 October 2004. The data are folded onto a 24 h UT day simply to spread out the points. The error bars show the standard deviation of seeing difference in respective UT bins. The scatter is mostly caused by local turbulence effects. The mean systematic difference is 0.017 ± 0.001 arc sec, in which ± 0.001   arc   sec is the standard deviation of the seeing difference.

Fig. 7
Fig. 7

Data for 16 September: on the left we plot T2 versus T3 DIMM measurements for all data points taken during this night. In the right image we plot the T2 measurement versus the measurement from the same DIMM taken 3 min later. The larger scatter in the right plot indicates that turbulence differences along the respective line of sight at the time of measurement is the dominant cause of the scatter in this and the two previous figures.

Fig. 8
Fig. 8

Difference between the seeing measured by T2 and T3 during the Tololo campaign. All data points, including those for which the optical alignment was known to be bad, were used. Equation (5) with S 0 = 0.4 and f = 2.14 was then applied to identify corrupted data points, which were excluded from the points shown here. The dotted line shows the best fit horizontal line to the data, corresponding to the mean of the data set, while the crosses are the median values for the respective data bins. The error bars indicate the rms scatter in the data bins.

Fig. 9
Fig. 9

Data for 23 September: The wind speed and direction (right plot) changed suddenly between 2:00 and 4:00 UT causing large systematic differences between the seeing measured by T2 and T3 (left plot) during this period. The error bars show the standard deviation of seeing difference in respective UT bins. The average difference for the entire night is very small (3 mas).

Fig. 10
Fig. 10

Left plot: seeing difference (T2–T3) versus wind direction for data from 15 September to 5 October. Right plot: seeing difference versus wind speed for same period. The error bars show the standard deviation of seeing difference in respective bins. The two plots show that the seeing difference between T2 and T3 changes with wind direction and speed and is thus at least partially caused by local turbulence effects.

Tables (3)

Tables Icon

Table 1 Detailed Parameters of the TMT DIMM Systems During the Campaign and Actual Site Testing

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Table 2 Dependence of Median Seeing and Remaining Number of Data Points on S 0 and the Corresponding Best-Fitting Factor f a

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Table 3 Dependence of Median Seeing Difference Between T2 and T3 (T2–T3) on Different Seeing-Dependent Strehl Ratio Limits a

Equations (5)

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I 2 D ( x , y ) = I 0 , 2 D e ( x 2 + y 2 / 2 σ 2 ) ,
I 1 D ( x ) = 2 π σ I 2 D ( x , 0 ) .
S 2 D = I 0 , 2 D I total 4 λ 2 π D 2 δ x 2 ,
S 2 D = I 0 ,1D I total 4 λ 2 2 π σ π D 2 δ x 2 .
S limit ( ϵ ) = S 0   exp [ 0 .134 f ( ϵ d 0.98 λ ) 5 / 3 ] .

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