Astronomical Seeing at Maidanak Observatory During the Year 2018

galaxies

Article Astronomical Seeing at Maidanak Observatory during the Year 2018

Yusufjon Tillayev 1,2,*, Azimjon Azimov 1 and Aktam Haﬁzov 1

1 Ulugh Beg Astronomical Institute of the Uzbekistan Academy of Sciences, Astronomy str. 33, Tashkent 100052, Uzbekistan; [email protected] (A.A.); [email protected] (A.H.) 2 Department of Astronomy and Atmospheric Physics, Physics Faculty, National University of Uzbekistan, 4 University str., Tashkent 100174, Uzbekistan * Correspondence: [email protected]

Abstract: Results of a four-month campaign of astronomical seeing measurements at Maidanak astronomical observatory (MAO) are presented. A differential image motion monitor (DIMM) was used for seeing estimations during the period from August to November 2018. The observation was organized within the framework of a site testing for a new telescope which is going to be installed at the observatory. The median value of seeing for the entire period was determined as 0.70 arcseconds, which agrees well with the results of the period 1996–2003. The comparison of monthly values showed that some monthly median values differ from the seasonal trend of the previous period.

Keywords: astronomical seeing; terrestrial atmosphere; optical turbulence; differential image motion monitor

1. Introduction Citation: Tillayev, Y.; Azimov, A.; Hafizov, A. Astronomical Seeing at The performance of ground-based astronomical observatories is limited by the terres- Maidanak Observatory during the trial atmosphere. Astronomical seeing is one of the key parameters characterizing overall Year 2018. Galaxies 2021, 9, 38. turbulence of the atmosphere above the observatory. The optical efficiency of telescopes https://doi.org/10.3390/galaxies depends largely on the astronomical seeing. ◦ ◦ 9020038 Maidanak astronomical observatory (MAO, longitude 66 56 E, latitude 38 41 N, 2600 m above sea level) is located at the south-east of the Republic of Uzbekistan. It is the Academic Editor: Francois Hammer main observational facility of the Ulugh Beg Astronomical Institute (UBAI), Uzbekistan Academy of Sciences. There are seven telescopes with 50-cm to 1.5-m apertures in the Received: 10 March 2021 observatory [1]. Observations of different astronomical objects are carried out at MAO: Accepted: 18 May 2021 gravitational-wave optical counterparts within the framework of the GRANDMA collabo- Published: 21 May 2021 ration; gamma-ray bursts (GRB), gravitationally lensed systems (GLS), blazars, supernova, variable stars, exoplanets, and minor planets [2–8]. Publisher’s Note: MDPI stays neutral Optical properties of the atmosphere have been studied at MAO since it was founded with regard to jurisdictional claims in in the late 1960s. The previous site testing campaign began in 1996 within the framework published maps and institutional affil- of the international collaboration led by the European Southern Observatory (ESO) [9]. iations. Results of site testing studies at Maidanak Observatory in Uzbekistan in this period and a comparison to leading observatories showed that atmospheric parameters place Maidanak Observatory among the best international astronomical sites for high angular resolution observations [10]. Copyright: © 2021 by the authors. The amount of clear night-time hours for Mt. Maidanak is about 60% of available Licensee MDPI, Basel, Switzerland. yearly dark time—in absolute units this corresponds to 2000 h. The summer observing This article is an open access article capacity of Mt. Maidanak is two times (more than 90% in July) higher than in winter (up to distributed under the terms and 50% in February). conditions of the Creative Commons The monitoring of astronomical seeing was carried out at Maidanak observatory Attribution (CC BY) license (https:// during 1996–2003 using ESO DIMM [11] which was previously used for site testing of creativecommons.org/licenses/by/ ESO observatories of La Silla and Paranal in Chile. The statistics of the whole timeseries 4.0/).

Galaxies 2021, 9, 38. https://doi.org/10.3390/galaxies9020038 https://www.mdpi.com/journal/galaxies Galaxies 2021, 9, 38 2 of 7

revealed that the mean value of seeing is 0.76 arcseconds, the median is 0.71 arcseconds. The best monthly median seeing, 0.62 arcseconds, was observed in November [12]. In 2017, a project with a new 4-m telescope was initiated in Uzbekistan. Obviously, the ﬁrst candidate for the place of installation was Maidanak Observatory. This acted as a new motivation to start the monitoring of astronomical seeing and other atmospheric parameters at MAO. During the monitoring, it was planned to identify the following: - The current value of astronomical seeing; - Whether the astronomical seeing has changed or not; - How much the astronomical seeing has changed; - Causes of astronomical seeing changes; - The monitoring of astronomical seeing on several hills at the Maidanak Observatory; - The selection of a suitable location for the telescope; - The increase of the efﬁciency of the telescope.

2. Materials and Methods The light from the stars passes through the Earth’s atmosphere, and the wave front coming in the same phase reaches the Earth’s surface in different phases. As a result, the diffraction pattern of the stars formed in the telescope changes. The resolution θ of a telescope limited by atmospheric turbulence, as deﬁned by the Strehl criterion, is θ [11]:

4 λ θ = . (1) π r0

Here, r0 is Fried’s parameter, and λ—is the wavelength. Fried’s parameter can be measured from the image motion using a small telescope [11]. Astronomical seeing is measured by the FWHM of the intensity of the star proﬁle, which is the best angular resolution (in arcseconds) that can be achieved by an optical telescope in a long exposure time. Taking into account the Earth’s atmosphere, the value of astronomical seeing is deﬁned as εFWHM and is determined according to the following formula:

λ εFWHM = 0.98 (2) r0 One of the most effective methods of measuring astronomical seeing is the differential image method realized in a DIMMs [13]. In our case, it is based on a Celestron-11 telescope with a 279 mm primary mirror and a focal length of 2800 mm (Figure1). Two circular masks are installed on the Schmidt corrector of the telescope. The masks are of 5 cm in diameter and the distance between the centers is of 19 cm. A 195-arcseconds wedge prism is installed on one of the holes, which creates the second image of the stars. A Santa Barbara instrument group’s (SBIG) ST-5C camera is used as a detector on the focal plane of the telescope. Due to atmospheric turbulence, image motion occurs—the distance between two images changes in two directions. The principle of the DIMM is to determine the relative displacements of centroids of two CCD-images along the x (longitudinal) and y (trans- verse) directions. The differential image motion variations of the set of the 200 dx and dy measurements are used to calculate εFWHM according to [13]. This determines the value of astronomical seeing in units of arcseconds. The smaller the motion of the star (the smaller the astronomical seeing), the more suitable the atmosphere of the observatory. In the world’s leading astronomical observations, the value of the seeing is in the order of 0.6–0.8 arcseconds. Galaxies 2021, 9, x FOR PEER REVIEW 3 of 7 Galaxies 2021, 9, 38 3 of 7

Figure 1. Optical scheme of the DIMM. M1-main optical mirror, M2-secondary hyperbolic mirror, Figure 1. Optical scheme of the DIMM. M1- main optical mirror, M2- secondary hyperbolic mirror, W1-first slit, W2-second prismatic slit. W1- first slit, W2- second prismatic slit. The DIMM we used at Maidanak Observatory in 2018 is the modified version of the instrumentThe DIMM used we used in 1996–2003. at Maidanak During Observatory 1996–2003, in we2018 used is the ESO-DIMM modified version [11] with of the the camerainstrument SBIG used ST-5, in a 1996–2003. serial port During camera. 1996– Since2003, the speed we used of dataESO-DIMM transmission [11] with in the the serial cam- portera SBIG was limited,ST-5, a serial the time port requiredcamera. Since for one the exposure speed of wasdata abouttransmission 4 min for in the 40 imagesserial port in 2was different limited, exposures. the time required In 2007, for we one replaced exposu there was camera about with 4 min a more for 40 rapid images one—ST-5C in 2 differ- withent exposures. a parallel interface In 2007, we but replaced having the the same came pixelra with size a andmore parameters. rapid one—ST-5C We used with software a par- basedallel interface on Cerro-Tololo but having Inter-American the same pixel observatories’ size and parameters. (CTIO) Robodimm We used software [14]. Despite based the on softwareCerro-Tololo being Inter-American different, we used observatories’ the same method (CTIO) described Robodimm in [14]. [11] andDespite updated the software in [13]. beingIn different, order to havewe used a good the same signal-to-noise method described ratio, bright in [11] stars and are updated observed in [13]. close to the zenithIn to order have to the have air mass a good be atsignal-to-noise its smallest. A rati particularo, bright star stars is observedare observed for a close maximum to the ofzenith 4 h—two to have hours the air before mass and be at after its smallest. culmination. A particular A part star of the is observed list of stars for selecteda maximum for observationof 4 h—two is hours given before in Table and1. Itafter lists culmination. the names, magnitudes,A part of the and list equatorialof stars selected coordinates for ob- ofservation some of is the given bright in starsTable that 1. It werelists the observed. names, magnitudes, and equatorial coordinates of some of the bright stars that were observed. Table 1. A partial list of the observed stars. Table 1. A partial list of the observed stars. Star Apparent Magnitude (m) Right Ascension (α, 2000) Declination (δ, 2000) ◦ α And 2.05 Right00h08 Ascensionm23s.25 +29 0502500.55 Star Apparent Magnitude (m) Declination◦ (δ, 2000) β Peg 2.48 23h(03α,m 2000)46s.45 +28 0405800.02 ◦ α CygAnd 1.33 2.05 20 00hh4108mm2523s.91s.25 +45+2916°05049′25”.5500.21 ◦ 0 00 αβ LyrPeg 0.09 2.48 18 23hh3603mm5646s.33s.45 +38+2847°0401′58”.02.29 α h m s ◦ 0 00 α AriCyg 2.00 1.33 02 20h0741m1025.29s.91 +23+4527°1645′49”.21.94 α Lyr 0.09 18h36m56s.33 +38°47′01”.29 αOne Ari of the most important 2.00 aspects of measuring 02h07m astronomical10s.29 seeing+23 is°27 choosing′45”.94 the optimal exposure time (in milliseconds). It must be big enough to have two bright images of theOne same of starthe most with important the highest aspects possible of measur signal-to-noiseing astronomical ratio in orderseeing to ismeasure choosing the the centroidsoptimal exposure of images time accurately. (in milliseconds). At the same It must time, be the big exposure enough time to have should two bebright as short images as possibleof the same to “freeze” star with the the atmosphere. highest possible To overcome signal-to-noise this situation, ratio an in “interlaced order to measure exposures” the methodcentroids is of used—first images accurately. DIMM takes At the exposure same time,τ1 (say, the 5exposure ms) and timeτ2 (double should of beτ 1as—10 short ms). as Then,possible using to “freeze” two different the atmosphere. exposure times, To overcome it “extrapolates” this situation, seeing an to “interlaced 0 exposure timeexposures” using Equationmethod is (3), used—first also described DIMM in [takes13]. exposure τ1 (say, 5 ms) and τ2 (double of τ1—10 ms). Then,For using each two exposure different time, exposure the differential times, it displacements “extrapolates” of seeing the centroids to 0 exposure of the time images us- ining x andEquation y coordinates (3), also described are measured. in [13]. The variance of the set of the data is used for the estimationsFor each of theexposure values time, of ε1 andthe differentialε2 in two exposures displacementsτ1 and ofτ2 .the Then, centroids using theof the following images expression,in x and y wecoordinates can find theare valuemeasured. of seeing Theε variance0 [15]: of the set of the data is used for the

estimations of the values of ε1 and ε2 in two exposures7/3 𝜏1 and 𝜏2. Then, using the following ε0 = 0.5(c1ε1 + c ε2), (3) expression, we can find the value of seeing ε0 [15]:1

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Galaxies 2021, 9, 38 / 4 of 7 (3) 𝜀 =0.5(𝑐𝜀 + 𝑐 𝜀), 3/4 where c1 = (ε1/ε2) . To reduce the statistical noise, factor c1 is smoothed over time using a moving average method3/4 and then used in (3) to correct individual measurements [15]. where c1 = (ε1/ε2) . To reduce the statistical noise, factor c1 is smoothed over time using a Inmoving order averageto calculate method one seeing and then value used for in a (3) set to of correct 200 measurements, individual measurements the differential [15 ]. motions (dxIn orderand dy) to are calculate recorded one in seeing every value exposure for a time. set of 200 measurements, the differential motions (dx and dy) are recorded in every exposure time. 3. Results In3. Resultsearly August, 2018, the DIMM device was installed on the same six-meter-high platform whereIn early it August,was used 2018, in the the previous DIMM device period. was Monitoring installed onof astronomical the same six-meter-high seeing usingplatform the DIMM where telescope it was began used on in theAugust previous 4 and period.continued Monitoring until November of astronomical 20. seeing Onusing the the basis DIMM of the telescope observations began at on the August Maidanak 4 and Observatory, continued until the November values of 20.astronomical seeingOn the obtained basis of thefrom observations 4 August to at 20 the November Maidanak Observatory,were processed the and values analyzed. of astronom- Here,ical the seeingastronomical obtained seeing from are 4 August calculated to 20 for November the wavelength were processedof as λ = 500 and nm analyzed. and they Here, are reducedthe astronomical to zenith. seeing are calculated for the wavelength of as λ = 500 nm and they are Figurereduced 2 toshows zenith. the whole timeseries of the astronomical seeing results obtained at the MaidanakFigure Observatory.2 shows the whole timeseries of the astronomical seeing results obtained at the Maidanak Observatory.

Figure 2. Astronomical seeing measured in August–November, 2018. Figure 2. Astronomical seeing measured in August–November, 2018. The cumulative distribution of astronomical seeing is shown in Figure3. The blue Thecolor cumulative shows the distribution cumulative of distribution astronomical of astronomicalseeing is shown seeing in Figure obtained 3. The in 2018. blue The color dashedshows linethe cumulative shows the cumulative distribution distribution of astronomical of astronomical seeing obtained seeing in obtained 2018. The in 1996– dashed2003. line As shows can be the seen cumulative from this distribution graph, the median of astronomical value ofastronomical seeing obtained seeing in 1996– for 2018 is 2003.0.70 As can arcseconds be seen from and for this 1996–2003 graph, the it me wasdian 0.71 value arcseconds of astronomical [12]. seeing for 2018 is 0.70 arcseconds and for 1996–2003 it was 0.71 arcseconds [12].

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Figure 3. Cumulative distribution of astronomical seeing for the period August–November, 2018 Figure 3. Cumulative distribution of astronomical seeing for the period August–November, 2018 and from 1996 to 2003. and from 1996 to 2003. The Table2 shows the average, median values of astronomical seeing for each month, asThe well Table as values 2 shows at the average, 25% and median 75% percentile. values of The astronomical median value seeing of astronomicalfor each month, seeing as wellis 0.77 as values for August, at the 0.6625% forand September, 75% percentile. 0.65 for The October, median and value 0.75 of for astronomical November. seeing The value is 0.77at 25%for August, of all points 0.66 for obtained September, is 0.60 0.65 arcseconds. for October, The valueand 0.75 at 75% for November. is 0.83 arcseconds. The value To ﬁnd at 25%out of how all goodpoints or obtained bad these is astronomical0.60 arcseconds. seeing The indicatorsvalue at 75% are andis 0.83 how arcseconds. much they To have findchanged, out how wegood compared or bad these the astronomical astronomical seeingseeing resultsindicators obtained are and using how the much same they DIMM haveinstrument changed, inwe 1996–2003. compared the astronomical seeing results obtained using the same DIMM instrument in 1996–2003. Table 2. Astronomical seeing statistics for each month for Maidanak Observatory. Table 2. Astronomical seeing statistics for each month for Maidanak Observatory. Months 25% Median 75% Average Months 25% Median 75% Average August 0.67 0.77 0.87 0.78 AugustSeptember 0.67 0.580.77 0.660.87 0.77 0.78 0.69 SeptemberOctober 0.58 0.560.66 0.650.77 0.78 0.69 0.68 OctoberNovember 0.56 0.61 0.65 0.750.78 0.91 0.68 0.78 Total 0.60 0.70 0.83 0.73 November 0.61 0.75 0.91 0.78 Total 0.60 0.70 0.83 0.73 A comparison of results obtained in two different periods at the Maidanak Observatory isA presented comparison in Tableof results3. This obtained table shows in two the different monthly periods median at values the Maidanak of astronomical Observa- seeing toryobtained is presented during in Table the periods 3. This 1996-2003 table shows and the 2018. monthly According median to the values ﬁrst periodof astronomical data, the best seeingvalue obtained for astronomical during the periods seeing was1996-2003 in November and 2018. (0.65 According arcseconds), to the howeverfirst period our data, dataset the bestshows value that for October astronomical has better seeing seeing was than in November November (0.65 in this arcseconds), particular year.however Overall, our the datasetmedian shows value that ofOctober astronomical has better seeing seeing at thethan Maidanak November Observatory in this particular was found year. Over- to be 0.70 all, thearcseconds median value against of 0.71astronomical arcseconds seeing in 1996–2003. at the Maidanak The comparison Observatory shows was found that median to be 0.70values arcseconds for both against of the periods 0.71 arcseconds agree, the in difference 1996–2003. is negligible The comparison (0.01 arcseconds). shows that However, me- dianin values the particular for both year of the 2018, periods the median agree, monthly the difference seeing isis highernegligible in August (0.01 (0.05arcseconds). arcseconds) However,and in in November the particular (0.10 year arcseconds), 2018, the butmedi slightlyan monthly better seeing in September is higher and in August October. (0.05 arcseconds) and in November (0.10 arcseconds), but slightly better in September and Oc- tober.

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Table 3. Median values of astronomical seeing obtained at the Maidanak Observatory in 1996–2003 and August–November 2018.

1996–2003 2018 Months Difference (εFWHM, Arcseconds) (εFWHM, Arcseconds) August 0.72 0.77 +0.05 September 0.70 0.66 −0.04 October 0.69 0.65 −0.03 November 0.65 0.75 +0.10 Total 0.71 0.70 −0.01

4. Conclusions Overall statistics of astronomical seeing measured in this campaign are in good agreement with those of the period 1996–2003. Since the measurements carried out include the period where seeing is good (September–November) it was expected that the median value would be lower than the 0.70 arcseconds, but this was not the case in 2018. Due to the short duration of the measurement campaign in 2018, it is not possible to draw a complete conclusion. In order to ﬁnd the reasons of the seasonal changes, a continuous monitoring of this parameter is required. This requires regular observations of at least one full year or more.

Author Contributions: Y.T. participated in the writing of the article, data processing, and analy- sis. A.A. participated in observations, data processing, and article writing. A.H. took part in the observations. All authors have read and agreed to the published version of the manuscript. Funding: This research work was carried out and funded under the FA-Atex-2018-392 “Study of the astroclimate of the Maidanak high mountain observatory to optimize the monitoring programs of the 4-m telescope”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: We will post the information on our website. Acknowledgments: We would like to thank S. A. Ehgamberdiev, S. P. Ilyasov, S. B. Tursunkulov and R. R. Hazratov for their help in conducting this research work and writing the article. The authors are grateful to A. Tokovinin for kindly providing the RoboDIMM code which helped much in coding local observational software. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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