The Robotic MAAO 0.7 m Telescope System: Performance and Standard Photometric System

The optical system of the 0.7 m telescope is designed as a Corrected Dall-Kirkham system (CDK700) with a focal ratio of 6.5, which is produced by PlaneWave Instruments. It consists of three main components: an ellipsoidal primary mirror (700 mm in diameter), a spherical secondary mirror (312.4 mm in diameter), and an additional lens group to prevent image distortions such as coma, off-axis astigmatism, and field curvature. Both mirrors are made from fused silica, a material known for its excellent optical properties. Figure 1 shows the telescope mounted on an alt-azimuth mount system and equipped with dual Nasmyth foci, allowing for visual and CCD observations.

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For the mount performance, slewing speed can reach up to 50 deg s⁻¹ at maximum. The pointing accuracy is 36 (rms) from the pointing model recently built with 102 stars at altitudes ranging from 20 to 75 deg. The tracking accuracy is also measured using a similar method in previous studies (e.g., Hardy et al. 2015; Im et al. 2015; Zhang et al. 2020) by obtaining 60 images of each 1 minute single exposure at an altitude of ∼50 deg. The angular distance between the center coordinates of a star in each image is accumulated, resulting in 006 min⁻¹, which is in agreement with the manufacturer's specifications.

On the 0.7 m telescope, an SBIG STX-16803 CCD camera is currently attached. The camera uses a KODAK's KAF-16803 chip with a 4096 × 4096 pixel array in each pixel size of 9 μm × 9 μm. We investigated the characteristics of this CCD in the dome with calibration images.

2.2.1. Pixel Scale, Gain, Readout Noise, and Flat Image

The pixel scale (P) is given by

$$\begin{eqnarray}&&P=\displaystyle \frac{206265\times \mu }{1000\times f}\,(^{\prime\prime} \,{\mathrm{pixel}}^{-1}),\end{eqnarray} \tag{ 1 }$$

where μ is the CCD pixel size in the unit of μm and f is the focal length in the unit of mm. Considering the focal length of 4540 mm of the 0.7 m telescope, P is calculated as 041 pixel⁻¹ from Equation (1). Then a FOV is $27\buildrel{\,\prime}\over{.} 9\times 27\buildrel{\,\prime}\over{.} 9$.

In addition, we obtain a series of bias and sky-flat images. Taking the average values of the 2D arrays of each image (${\bar{B}}_{1}$, ${\bar{B}}_{2}$, ${\bar{F}}_{1}$, and ${\bar{F}}_{2}$), the gain can be described as

$$\begin{eqnarray}&&\mathrm{Gain}=\displaystyle \frac{({\bar{F}}_{1}+{\bar{F}}_{2})-({\bar{B}}_{1}+{\bar{B}}_{2})}{{\sigma }_{{F}_{1}-{F}_{2}}^{2}-{\sigma }_{{B}_{1}-{B}_{2}}^{2}},\end{eqnarray} \tag{ 2 }$$

where ${\sigma }_{{F}_{1}-{F}_{2}}$ and ${\sigma }_{{B}_{1}-{B}_{2}}$ are the standard deviation of the difference images of two bias (B₁ − B₂) and flat-field images (F₁ − F₂). Flat images are flat-corrected using the master flat. The readout noise is calculated as

$$\begin{eqnarray}&&\mathrm{Readout}\,\mathrm{Noise}=\displaystyle \frac{\mathrm{Gain}\cdot {\sigma }_{{B}_{1}-{B}_{2}}}{\sqrt{2}}.\end{eqnarray} \tag{ 3 }$$

Using Equations (2) and (3), we obtain the gain and the readout noise as 1.30 ± 0.04 e⁻ ADU⁻¹ and 12.62 ± 0.25 e⁻, which are consistent with the values in the specification from the manufacturer within the error.

Furthermore, during the comparison between sky-flats and dome-flats by dividing them, the displacement of the donut pattern (∼20–30 pixels) is found, but this issue arises among sky-flats or dome-flats and sometimes does not occur. This offset issue is under investigation to resolve.

2.2.2. Residual Signal

We investigate the residual signal remaining at the position of a bright source after exposure. A bright star is exposed (300 s) and a series of 30 s dark frames are obtained before (9 frames; Group A) and after (60 frames; Group B) the star's image. Figure 2 shows the signal variation with the image sequence within a 10 pixel diameter circular aperture (assuming 41 seeing) at the star's position in the pixel coordinates. Each data point is normalized with the mean value of Group A's signal, and the star's signal (Image sequence = 10) is not shown due to its high signal value (45.88). We find the residual signals in Group B shown in the image thumbnails in Figure 2. The signal of the first dark frame in Group B (Image sequence = 11) is 1% higher than the mean value of Group A's signal, resulting in a magnitude difference of 0.011 mag. The effect of the residual signal disappears within 3 times the standard deviation of Group A's signal after 8 readouts. Therefore, observers have to consider the residual signal for the photometry of the faint objects.

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2.2.3. Linearity

It is important to check if the input charges can be converted into the output electric signal with a simple linear relation. A series of dome-flat images taken with V-band are obtained by progressively increasing exposure time while the dome-flat lamps are turned on and the telescope is directed toward the dome-flat panel. Figure 3 shows a variation of mean counts of the dome-flat images from 0.1 to 240 s exposure. The pixels started to be saturated from 215 s exposure. The mean counts are well fitted with the linear relation up to ∼61,000 ADU within 2.5% and ∼57,000 ADU within 1% (${\chi }_{\nu }^{2}=0.16$). Even in the exposure time shorter than 1 s, the linearity can be found ≳100 ADU within 5%. During this process, any feature of the shutter pattern for the short-exposure images is not found.

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2.2.4. Dark Current

We also investigate a variation of the dark current resulting from the thermal electron with the CCD cooling temperatures from −30°C to −1.1° C. At −20° C, the dark current is 0.01 e⁻ pixel⁻¹ s⁻¹ as presented in Figure 4. The mean values from the bias-subtracted and combined dark frames (180 s exposure) are taken. The dark frame obtained at the lowest temperature (−30°C) is used as a reference to remove any other signals not dependent on the CCD temperature by subtracting those taken at the high temperature.

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Currently, the filter wheel SBIG-FW7 attached to the CCD camera has a set of 5 broadband (Bessell UBVRI) and 3 narrow band filters (Hα, O iii, and S ii) manufactured by Chroma Technology, Co. with a size of 50 mm × 50 mm. The filter transmission curve altogether with the quantum efficiency (QE) of CCD and the telescope optics throughput is provided in Figure 5. For CCD, the QE peaks at 60% at 5500 Å. For optics throughput, the corrector lenses with the standard anti-reflection coatings and the first surface mirrors with the enhanced Aluminium reflective coatings are considered, showing the peak (82%) at 6000 Å.

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In summary, the overall characteristics of the CCD camera on the MAAO 0.7 m telescope are presented in Table 1.

Table 1. Specifications of the Current CCD Camera (SBIG STX-16803)

Properties	Values
Sensor chip	KAF-16803 (36.8 mm × 36.8 mm)
Pixel size	9 μm × 9 μm
Pixel arrays	4,096 × 4,096
Readout time a	9 s
QE peak	60%
Pixel scale	0 41 pixel−1
Field of view	279 × 279
Gain	1.30 ± 0.04 e− ADU−1
Readout noise	12.62 ± 0.25 e−
Linearity	350−57,000 ADU (within 1%)
Full well a	100,000 e−
Dark current	0.01 e− pixel−1 s−1 at −20° C

Note.

^a The manufacturer's data are adopted.

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Two personal computers (PCs) control the system in a 64-bit Windows operating system environment. One is the main control PC connected to the 0.7 m telescope system and the other is the automatic weather station (AWS) PC. Figure 6 summarizes the system configuration at MAAO.

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3.1.1. Telescope System

3.1.2. Weather Monitoring System

An one-day run of the robotic observation involves 7 sessions: StartUp, Observation, AutoFocus, Idle, CalFrame, ShutDown, and FileTransfer, and they are controlled by a scheduler. Figure 7 illustrates a sequence of these sessions.

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(i) StartUp session can start when the solar altitude is below −18 deg and when the public event is finished (after 9 PM/10 PM in winter/summer). This session includes software initialization, azimuth synchronization of the dome and scope, dome opening, tertiary mirror position switching for CCD imaging, and CCD chip cooling. When the weather condition is poor, the scope enters Idle session.

(ii) Observation session performs observations based on scripts. The observation script, an ASCII file, must contain the following columns: object name (NAME), R.A. in J2000 (RA_J2000), decl. in J2000 (DEC_J2000), rise and set times above a specific altitude in local time (RISE(LT), SET(LT)), transit time (TRANSIT(LT)), filter (FILTER), single exposure time (EXPTIME), CCD binning (BINNING), the number of exposures to repeat (COUNTS), and observation starting time in local time (OBSTIME).

(iii) AutoFocus session uses the default auto-focusing function supported in PWI2. In the AutoFocus session, an optimal focus value can be determined by using a relationship between PSF sizes of a point source and focus values in a series of short exposure images. Users have the option to perform AutoFocus manually by generating an Observation session with "AutoFocus" in the NAME column. An initial auto-focusing procedure is performed every StartUp session.

(iv) CalFrame session takes calibration frames (bias, dark, and dome-flats) before ShutDown session. For dome-flats, after the dome closed, the scope points to the dome-flat panel (Azimuth = 267 deg, altitude = 51.5 deg), and the dome moves to the opposite position (Azimuth = 88.3 deg).

(v) Idle session moves the scope to the parking position, disables tracking, and closes the dome. The observation resumes when the weather conditions have remained safe for 30 minutes.

(vi) ShutDown session is set to automatically initiate when the solar elevation exceeds −18 deg.

(vii) FileTransfer session automatically sends observed data obtained during the nighttime using the file transfer protocol (FTP) to the server. The observation is summarized into an observation log and sent to the observer via email.

During the execution, the scheduler checks the solar altitude, local time, and weather data every 60 s to finish or halt the observation. An operation log is also generated after each command.

To explore the optical performance and tracking accuracy, we obtain the images with extending exposure times from 5 to 600 s, including 10, 30, 60, 120, 180, and 300 s. The left panel in Figure 8 shows the total FOV divided into 16 image sections a 60 s exposure of an open cluster NGC 0884. The PSF model in each section is built using PSFEx (Bertin 2013) from the detected stars with a high S/N ratio at least 20 but not saturated by setting the PSFEx parameters of SAMPLE_MINSN = 20 and SATUR_LEVEL = 50000, respectively. The right panel shows the PSF models produced in each image section with the ellipticity values (ELLIPTICITY from SExtractor, Bertin & Arnouts 1996). Though the PSF shape on the left-bottom side is slightly elongated (0.1 at maximum), we find uniform PSF shapes over the FOV.

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Photometric calibration is required because the photometric system depends on the characteristics of optics, imaging devices, and observing conditions. To measure the photometric parameters using data obtained at MAAO, we observed several standard star fields (SA 23 and SA 35) presented in Landolt (2013). The observations were performed in the clear sky, and the observation information is summarized in Table 2. After data reduction, we measure the magnitudes of the standard stars using the SExtractor with the 14'' diameter circular aperture to match with the same size aperture in Landolt (2013). We obtained the photometric parameters using the following equations:

$$\begin{eqnarray}&&b-B={z}_{B}+{k}_{B}^{\prime} X+{k}_{B}^{\prime\prime} (B-V)X+{C}_{B}(B-V),\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&v-V={z}_{V}+{k}_{V}^{\prime} X+{k}_{V}^{\prime\prime} (B-V)X+{C}_{V}(B-V),\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&r-R={z}_{R}+{k}_{R}^{\prime} X+{k}_{R}^{\prime\prime} (V-R)X+{C}_{R}(V-R),\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&i-I={z}_{I}+{k}_{I}^{\prime} X+{k}_{I}^{\prime\prime} (V-I)X+{C}_{I}(V-I).\end{eqnarray} \tag{ 7 }$$

In these equations, the observed magnitudes are denoted as b, v, r, and i and the standard magnitudes are B, V, R, and I provided from Landolt (2013). Additionally, z_B , z_V , z_R , and z_I correspond to the zero-points for the magnitudes producing a signal of 1 e^- s⁻¹ in each band. ${k}_{B}^{{\prime} }$, ${k}_{V}^{{\prime} }$, ${k}_{R}^{{\prime} }$, and ${k}_{I}^{{\prime} }$ are the first-order atmospheric extinction coefficients and ${k}_{B}^{{\prime\prime} }$, ${k}_{V}^{{\prime\prime} }$, ${k}_{R}^{{\prime\prime} }$, and ${k}_{I}^{{\prime\prime} }$ are the second-order atmospheric extinction coefficients. Since the second-order extinction coefficients are small, they can be negligible. The color term coefficients are C_B , C_V , C_R , and C_I , and X represents the airmass. Table 3 presents the resultant of the calibration.

Table 3. The Resultants of Standard Calibration

Filter	$k^{\prime}$	C	k''	z	Nstar	σrms
B	0.477 ± 0.032	0.011 ± 0.070	−0.070 ± 0.047	−21.632 ± 0.049	11	0.028
V	0.328 ± 0.013	0.102 ± 0.027	−0.016 ± 0.017	−21.912 ± 0.021	14	0.021
B	1.375 ± 0.086	−0.182 ± 0.237	−0.038 ± 0.138	−21.911 ± 0.148	7	0.039
V	1.068 ± 0.041	0.197 ± 0.099	−0.090 ± 0.057	−22.181 ± 0.071	7	0.016
R	0.752 ± 0.028	0.012 ± 0.121	0.084 ± 0.069	−21.861 ± 0.049	7	0.010
I	0.642 ± 0.088	0.192 ± 0.118	−0.100 ± 0.067	−21.081 ± 0.088	7	0.019
B	0.560 ± 0.036	−0.163 ± 0.086	0.031 ± 0.053	−21.777 ± 0.059	7	0.019
V	0.311 ± 0.020	−0.007 ± 0.045	0.072 ± 0.028	−21.960 ± 0.032	7	0.011
R	0.221 ± 0.027	0.024 ± 0.107	0.110 ± 0.066	−21.865 ± 0.044	7	0.012
I	0.120 ± 0.022	−0.014 ± 0.048	0.036 ± 0.029	−20.912 ± 0.037	7	0.009

Note. N_star is the Number of Stars used in the Calibration Process.

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Figure 9 shows the residuals of the measured magnitudes in each observing date. The extinction coefficients measured on 2023 February 20th and 2024 March 20th are comparable to those measured in other Korean observatories (See Table 4). Considering the analysis in Kim et al. (2008), the heavy extinction observed on 2023 February 28th might be related to a considerable amount of fine dust in spring season or the possibility of thin cirrus clouds.

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We measure limiting magnitudes of images in BVRI-band. A spiral galaxy NGC 3147 was observed in 10 minutes exposure under a clear dark night on 2023 February 20th as shown in the left panel in Figure 10. We perform the photometry using the SExtractor with a 2 × FWHM aperture diameter. The FWHM values exhibit variation ranging from 28 to 38 with the filter and the focus. The photometric catalog from the Data Release 1 of Pan-STARRS ¹⁸ (PS1; Tonry et al. 2012) is used as a reference catalog to calculate the zero-point magnitudes. We convert PS1 gri magnitudes to BVRI magnitudes (Refer Section 2.1 in Lim et al. 2023). As a result, the 5σ limiting magnitude in R-band for the 10 minutes exposure can be achieved as ∼20 mag, which is comparable to that of other telescopes measured in Korea (Refer Table 1 in Im et al. 2021). The other measured limiting magnitudes are summarized in Table 5.

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The U-band limiting magnitude is 16.8 AB mag. Due to the low sensitivity of the U-band, all the U-band images of SA 32 field obtained on 2023 March 29th with the total integrate time of 150 minutes are stacked. The zero-point magnitude is measured using the magnitudes of standard stars from Landolt (2013) and is converted into the AB system using Blanton & Roweis (2007).

After cleaning the mirror on 2023 May 30th, we additionally measured the limiting magnitudes of 5 minutes exposure images of M101 field on 2023 June 16th. We calibrated the observed magnitudes with the same manner using the stars in the data release 9 (DR9) of the AAVSO photometric all-sky survey (Henden et al. 2016) as comparison stars. The zero-points showed an improvement of ∼0.4 mag.