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Twilight observations with MASS/DIMM
B. Safonov
Sternberg Astronomical Institute Moscow State University

Comprehensive Characterization of Astronomical Sites, 2010

B. Safonov (Moscow State University)

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Why observe in twilight with MASS/DIMM?

It's interesting how turbulence changes during transition from day to night. Evening twilight -- to predict conditions of OT in for thcoming night. Morning twilight -- to predict conditions of OT in for thcoming morning(for solar observations). Our ability to obser ve in twilight is limited by the level of sky background. ASM includes 3 devices sensitive to background: guider, DIMM and MASS.

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Background in guider.
CCD with angular scale 10"/px relatively sensitive to background. Serves for rough pointing to the star (proper pointing accuracy of telescope is quite poor).

Pointing is possible if h < -1 .

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Background in DIMM.
CCD with scale 0.623"/px DIMM is less sensitive to background. Relatively large background doesn't affect measurements.

We can observe up to h = +13

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Background in MASS.
Photometer with diaphragm to background. = 220" MASS is very sensitive

Approximation works for h < -6

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Statistics of MASS/DIMM twilight observations.
Regular automatic observations in twilight star ted in November'09. At the moment DIMM works at h < 0 , MASS works at h < -6 .

Table: Total duration of twilight measurement.
twilight total[days] civil[hours] nautical[hours] astronomical[hours]
B. Safonov (Moscow State University)

h limits (-6 , 0 ) (-12 , -6 ) (-18 , -12 )

DIMM evening morning 60 58 24.9 38.0 24.8 32.1 39.1 48.3

MASS evening morning

24.4 39.5

32.5 48.9
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Twilight observations with MASS/DIMM


Variation of MASS & DIMM seeings: running quar tiles.
Quar tiles of MASS & DIMM seeings depending on sun altitude.

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Variation of MASS & DIMM seeings: distributions.
A closer look to distributions of seeings.

There is no significant dependence.
B. Safonov (Moscow State University) Twilight observations with MASS/DIMM SITE 2010 8 / 17


Variation of isoplanatic patch and time constant.
Cumulative distribution functions.

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Variation of turbulent layers intensities.

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Classification of nights.
Take a parameter that we want to predict. Let's suppose this is DIMM seeing. Compute its mean T using twilight measurements 0 > h > -12 Classify night using T .
Table: Classes.

class A B C D

T (0, (q1 (q2 (q3

q1 ] , q2 ] , q3 ] , ]

where qi -- quar tiles of distribution of seeing.
B. Safonov (Moscow State University) Twilight observations with MASS/DIMM SITE 2010 11 / 17


Prediction of DIMM seeing: distributions.
Distribution of DIMM seeing depending on night class.

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Prediction of MASS seeing: distributions.
Distribution of MASS seeing depending on night class.

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Prediction of MASS & DIMM seeings: percentiles.

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Prediction of seeing.
Cases of long and shor t preparation time.

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Conclusions

Limiting sun altitudes for current configuration:
Guider h < 0 DIMM h < +10 MASS h < -6

It appears that there were no significant changes in OT characteristics for investigated interval of sun altitudes (h < 0). Practical use -- "prediction of seeing".
If obser vational program switching takes more than 20-30 minutes. If there is an obser vational program for twilight.

This is all valid for our par ticular summit -- Mt. Shatdjatmaz!

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Fur ther upgrade

What we can do to reduce effect of background on: MASS
Reduce jitter of mount. Shrink MASS diaphragm to 20".

Guider
Improve mount pointing accuracy. Remove guider.

B. Safonov (Moscow State University)

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