2.1.

Project Status and Approximate Timeline

As of the writing of this manuscript, MINERVA telescopes 1 and 2 are located on the Caltech campus in the first Aqawan (see Sec. 3.2), where we have been developing the MINERVA Robotic Software (MRS) and validating the performance of the various hardware components. The hardware on-site has been fully tested, and now the goal of this facility is to achieve coordinated, automated control of both telescopes and Aqawan 1. This is expected to be complete by early 2015, and once this goal has been reached, the entire facility will be moved to Mt. Hopkins.

The infrastructure at Mt. Hopkins necessary for the relocation of the Aqawans and telescopes has been completed. Aqawan 2 has been constructed in California and was transported to Mt. Hopkins on December 9, 2014. MINERVA telescopes 3 and 4 have had their performance validated and were relocated to Aqawan 2 at Mt. Hopkins on December 15, 2014.

The custom room designed for the spectrograph is currently under construction. The outermost layer in a three-stage environmental control scheme will be a 100 K clean room temperature stabilized to $\pm 1°\mathrm{C}$, peak-to-peak. A second, interior room will then be erected inside which the spectrograph will be mounted with its critical optical elements inside a vacuum chamber. The manufacturing of the spectrograph has been completed by Callaghan Innovation in New Zealand. Work is now underway on the input optics as well as the iodine cell mount. It is undergoing lab tests and awaits the completion of the spectrograph room at Mt. Hopkins. Delivery of the spectrograph is expected by the end of the first quarter of 2015 when on-sky commissioning will begin.

Allowing for minor unforeseen delays, fully automated, robotic control of the array and spectrograph is expected by mid-year 2015. The primary survey is projected to begin by the end of the year.

3.1.

CDK-700 by PlaneWave

The PlaneWave CDK-700 is a 0.7-m, altitude/azimuth mounted telescope system.^81,82 It has a compact design, standing just $<8\mathrm{ft}$ tall when pointing at the zenith, with a 5-ft radius of maximum extent when pointing horizontally. The telescopes use a corrected Dall-Kirkham (CDK) optical setup consisting of an elliptical primary mirror, a spherical secondary mirror, and a pair of correcting lenses to remove off-axis coma, astigmatism, and field curvature. This results in a flatter, more coma and astigmatism-free field than the Ritchey-Chretien design, with the added benefit that the spherical secondary mirror makes alignment forgiving compared to the hyperbolic secondary of the Ritchey-Chretien design. The CDK-700 has dual Nasmyth ports with output beams at $f/6.5$ accessed with a rotating tertiary mirror. The CDK-700 specifications are summarized in Table 1.

Table 1

CDK-700 specifications.

The telescope pointing is controlled by two direct-drive electromagnetic motors with an encoder resolution of 81 mas resulting in a pointing accuracy of 10 arcsec RMS, a pointing precision of 2 arcsec, and a tracking accuracy of 1 arcsec over a 3-min period. The slew rate is $15\mathrm{deg}{\mathrm{s}}^{-1}$, which keeps slew times between any two points in the sky to $<\sim 10\mathrm{s}$. The focus mechanism and image derotator are combined into a single, motor-controlled unit that can be remotely adjusted. Cooling fans and temperature sensors are used to keep the primary mirror in thermal equilibrium, and the control software is built to automatically correct for perturbations, such as wind gusts.

Section 6 presents the performance validation for our telescopes, including control, pointing, guiding, and throughput. The commissioning performed over the past two years at Caltech has proven the feasibility of using high-end, off-the-shelf hardware for professional astronomical research. Thereby, MINERVA has retired many of the risks involved in developing an observational facility from the ground up. Figure 3 shows an image of the MINERVA telescopes at the Caltech test site.

Fig. 3

Telescope 2, a PlaneWave CDK-700, is shown inside the MINERVA Aqawan telescope enclosure at the Caltech commissioning site.

3.2.

Aqawan Telescope Enclosures

An Aqawan (Chumash native American word for “to be dry”) is a telescope enclosure developed by Las Cumbres Observatory Global Telescope for their 0.4-m telescopes specifically designed for remote, robotic operations around the world.⁸³ The design offers full access to the sky, limiting the effects of dome seeing, and eliminates the need to coordinate dome slit positioning while maintaining a relatively small footprint. We have purchased two custom-built Aqawans with longer sides that can each accommodate two CDK-700s without any possibility of collision and can close safely with the telescopes pointed in any orientation. Stronger motors with a higher gear ratio were also installed to handle the heavier roof panels. Figure 4 shows the design and realization of our first Aqawan, which has been delivered to the Caltech campus for commissioning.

Fig. 4

(a) Design drawing of our custom Aqawan telescope enclosure with two PlaneWave CDK-700 telescopes inside. (b) MINERVA commissioning site on the Caltech campus showing the open Aqawan and telescopes 1 and 2 inside.

The Aqawan receives $208\mathrm{V}/30$ A three phase power that is converted to 24 V dc within its control panel. This power runs through an internal uninterruptible power supply (UPS) and powers a programmable automated controller (PAC) that controls all the functionality of the enclosure. In addition to basic opening and closing, the Aqawan has many auxiliary features, including a web camera that provides 360 deg coverage, a temperature and humidity sensor, fans to promote temperature equalization, fluorescent lighting, and a smoke alarm. Communication to the Aqawan PAC is established via TCP/IP, with commands consisting of ASCII strings. The Aqawan firmware is designed such that if a “heartbeat” command is not issued each minute, the roof automatically closes. This feature along with built-in backup power and the ability for the Aqawan to close with the telescopes in any configuration offers safety against power and connectivity failure modes.

3.3.

Fiber and Fiber Coupling

Astronomical light collected from MINERVA’s four 0.7-m telescopes will feed a stabilized spectrograph via fiber optic cables. Using a fiber feed offers many advantages and some challenges. Single-mode fibers offer superior control of the instrumental profile. However, they are diffraction-limited by nature and require a high-performance adaptive optics system for efficient coupling of starlight. Multimodal fibers couple to starlight much more easily, but they have a near- and far-field output that is variable. The near-field variations are due to the interference of modes at the output of the fiber, and when imaged on the detector, these variations can limit the signal-to-noise ratio of the observations. This effect can be mitigated by use of an octagonal fiber, which mixes the modes as they traverse the fiber.^84,85 Physical agitation of the fiber enhances this effect.⁸⁶

The far-field uniformity and stability are also important as the far field is incident on the echelle grating. Different grooves will be illuminated if the spatial intensity distribution changes, which can introduce spurious wavelength shifts as the grooves are not identical. One way to improve this performance is to introduce a double scrambler, which inverts the field and angle distribution, at a cost of reduced throughput.⁸⁷ This is an option that MINERVA will explore further if necessary.

We will use $50\text{-}\mu \mathrm{m}$ octagonal fibers with a $94\mu \mathrm{m}$ cladding diameter and a numerical aperture of 0.22 to feed light collected from each of the four 0.7-m telescopes to our stabilized spectrograph. Starlight will be coupled to our fibers at the native $f/6.5$ of the CDK-700 bypassing the need for small optics for the purpose of better throughput. KiwiSpec-MINERVA is designed to accommodate an $f/5.0$ beam or larger, permitting up to 20% focal ratio degradation before impacting overall throughput. Therefore, our fibers project onto the sky with a 2.27 arcsec diameter.

Our fiber coupling system consists of a fiber acquisition unit (FAU) and control software that can interface with the unit and provide closed-loop guiding with the telescope control. Besides KiwiSpec-MINERVA, this is one of the only truly custom hardware components of MINERVA. The details of this system are presented in Ref. 72, and here we present only a brief review.

The FAU has three accessible optical paths that are based upon the design presented in Ref. 88. In the primary path, the telescope beam is fed directly into the fiber. Separately, there is an optical path that relays a portion of the light to a guiding camera via a pellicle. Finally, there is a set of relay lenses and a corner retroreflector. If the fiber is illuminated from the exit end near the spectrograph, the input end of the fiber tip will be imaged on the guide camera. The corner retroreflector guarantees that misalignments of the optics do not affect the image position; this allows the determination of the pixel position on the guider that corresponds to the fiber tip and, hence, a setpoint for guiding. An annotated schematic of the FAU design is shown in Fig. 5, and a picture of one of the FAUs mounted on a CDK-700 can be seen in Fig. 6.

Fig. 5

Ray-trace illustration of the fiber acquisition unit (FAU), which measures 305 mm ($\sim 12\mathrm{in.}$) across. Light enters from the right, delivered by the CDK-700 with an $f/6.5$ beam. A pellicle reflects 2% of the beam to an SBIG ST-i guide camera with a V-band filter, matching the wavelengths used for radial velocity observations. The remaining 98% of the beam comes to a focus on the fiber tip. We include an achromatic lens and corner cube on the opposite side of the pellicle such that the fiber tip can be imaged onto the guide camera via backillumination. This allows for quickly determining the location of the fiber tip on the guide camera for precise guiding.

Fig. 6

The MINERVA FAU mounted on one of the MINERVA telescopes with all major components labeled. The focuser/derotator will be disengaged for standard spectroscopic observations.

3.4.

KiwiSpec-MINERVA

KiwiSpec-MINERVA is an adaptation of an existing spectrograph design^89,90 that leverages the successes of existing facilities^91–93 and techniques.^73,94 This critical component of our facility will be described in detail in a forthcoming publication. Here, we provide a brief description of KiwiSpec-MINERVA for the sake of completeness of this publication.

It is a bench mounted, fiber-fed spectrograph of asymmetric, white pupil design. Primary dispersion is achieved using an R4 echelle, and a volume phase holographic grism is used for cross-dispersion. Four distinct traces will be imaged on a $2\mathrm{k}\times 2\mathrm{k}$ detector covering a spectral range from 500 to 630 nm over 26 echelle orders. The resolving power of the spectrograph is $R\approx \mathrm{80,000}$. Two additional calibration fibers bracket the four science fibers and provide stable wavelength calibration with the use of a stabilized etalon wavelength source (as simulated in Fig. 7), or Thorium-Argon light. A subsection of a simulated echellogram created using ray-tracing techniques⁹⁵ is presented in Fig. 7. Each science trace corresponds to the input from an independent telescope and fiber system.

Fig. 7

A subsection of a simulated MINERVA echellogram showing four science spectra bracketed by a simulated stabilized etalon wavelength source. Each science fiber trace represents the input from one of the four MINERVA telescopes that will be observing the same astronomical target.

The estimated throughput of the prototype spectrograph from which the KiwiSpec-MINERVA has been designed is $\sim 30\%$. This has been validated with on-sky measurements taken at Mt. John Observatory in New Zealand.⁹⁰ For these tests, the mirrors for the prototype spectrograph were bare aluminum, the lenses had only single layer antireflection coatings, and several off-the-shelf optical components were used, all of which contribute to a suboptimal throughput. Improvements have now been implemented, including high-efficiency coatings and the installation of a new custom camera such that the throughput is expected to increase to $\sim 45\%$. The throughput of our fiber system is expected to be $\approx 70\%$, and the total throughput of our system up to the entrance slit of the spectrograph is expected to be $\approx 50\%$ (see Secs. 6.1 and 6.4). Therefore, the total throughput of our system including losses due to sky extinction at the Fred Lawrence Whipple Observatory (FLWO) is expected to be 10% or greater.

The line spread function will be sampled with 3 pixels and a minimum of 4 null pixels will lie between each trace. The spectral extraction and data reduction pipelines are currently being developed. The strategy is to model the two-dimensional spectrum directly, i.e., all orders will be modeled simultaneously such that cross-contamination between orders and scattered light will be accounted for in the model. This approach is likely only possible in the high signal-to-noise regime in which MINERVA will be working. The fact that all telescopes will be observing the same target simultaneously will also help to mitigate the effects of cross-contamination between orders.

The spectrograph will be placed inside a purpose-built, two-stage room and the critical components will reside inside a vacuum chamber stabilized to $\pm 0.01°\mathrm{C}$. An iodine cell will be mounted off the optical bench and can be inserted or removed from the optical path, allowing the option of simultaneous wavelength calibration of the science traces in the echellogram.

3.5.

Cameras and Filters

Each MINERVA telescope will also be equipped with a wide-field CCD camera on one of the two Nasmyth ports available on the CDK-700. The array will incorporate three different camera models for its four telescopes. Two of the MINERVA telescopes will be equipped with identical Andor iKON-L cameras.⁹⁶ These cameras have a back-illuminated sensor with wide band (BV) coating and $2048\times 2048$ square $13.5\mu \mathrm{m}$ pixels for a total chip size of 27.6 mm corresponding to a 20.9 arcmin field. Figure 8 shows an image of one of these cameras mounted on a MINERVA telescope. A third telescope will be equipped with an additional Andor iKON-L that is identical to the two described above except that it will contain a deep depletion sensor with fringe suppression (BR-DD). This camera is sensitive to light at wavelengths out to $1\mu \mathrm{m}$ and can be used for precision photometry in the near-infrared (e.g., $i^{\prime }$, $z^{\prime }$, and $Y$). The iKON-L cameras come with a four-stage thermoelectric cooling system that can achieve operating temperatures $\approx -80°\mathrm{C}$, nearly eliminating dark current. The third camera model will be an Apogee Aspen CG230⁹⁷ with $2048\times 2048$ $15\times 15\mu {\mathrm{m}}^2\text{pixels}$ for a chip size of 30.7 mm corresponding to a 23.2 arcmin field.

Fig. 8

The optics chain for MINERVA photometry with major components along the light path to the camera labeled.

Each MINERVA telescope has a filter wheel on the photometry port, and we employ two different Apogee filter wheel models, each with 50 mm square slots. Three of the MINERVA telescopes have the AFW50-7s filter wheel, while one telescope has a custom double filter wheel comprising two AFW50-10s wheels mounted back to back. One open slot in each 10-slot wheel allows access to the other 18 slots. Currently, the filters available to the MINERVA system are the Johnson $U$, $B$, $V$, $R$, and $I$; second-generation Sloan $g^{\prime }$, $r^{\prime }$, $i^{\prime }$, and $z^{\prime }$; narrow band $H$ $\alpha$, [Sii], [Oiii], and amateur filters, $L$, $R$, $G$, and $B$. The Andor cameras are mated to the filter wheels through a custom adaptor plate designed by Paul Gardner of Caltech Optical Observatories and implemented by Andor.

6.1.

Telescope Throughput

The equation for the photoelectron detection rate is

Camera gain was directly measured from flat field images, and the theoretical transmission and reflectance of our optical elements were supplied by the manufacturers and are shown in Fig. 11. Multiplying these curves together (including the Nasmyth lens curve four times to account for the four surfaces on two lenses) and then integrating over the the $V$ filter band gives a theoretical upper limit to the telescope throughput of ${\langle \tau \rangle }_V(X=0)=84\%$.

Fig. 11

Transmission and reflectance of our optical elements as a function of wavelength. The reflectance of the tertiary mirror was measured at a 45 deg incident angle, and the Nasmyth lens transmission is for a single lens surface. The quantum efficiency of the Andor camera (BV chip) is shown for reference, as is the Johnson $V$ bandpass, which roughly corresponds to the spectral range of KiwiSpec-MINERVA.

To calibrate the throughput of our telescopes, we observed bright ($V<12$) standard stars.^103,104 The standard stars were observed in sequences of between 3 and 10 images per pointing with integration times adjusted to give high counts within the linear regime of the CCD. The signal-to-noise achieved at each observation was typically a few hundred. We then cycled through a list of standard stars for a given night between three and five times producing measurements over a range of airmasses and at different azimuthal angles.

The total counts measured within the integration time of each observation were converted into a $V$-band averaged throughput for a calculated airmass by rearranging Eq. (1):

We were able to collect data over airmasses between 1 and 2 with limited horizons at both the Caltech and Rancho Dominguez sites. Observations were spread as widely in the azimuthal angle as possible on a given night, and we repeated observations several times to average over a large variance in the individual measurements that we attribute to a highly variable aerosol content in the Los Angeles basin atmosphere. Figure 12 shows the cumulative results of our throughput measurements using different telescopes and different cameras over the course of five separate nights. The overall best fit for the mean telescope throughput is 70% with a high formal error of 30% due to the covariance between the atmospheric extinction—which is poorly constrained by our data—and the throughput scale. However, the best fit value for the $V$ band atmospheric extinction, $0.27\mathrm{mags}/\text{airmass}$, is close to what we would expect and lends credence to the derived throughput of $\sim 70\%$. Fits from individual nights and sources using different telescopes and cameras also agree with this value within errors.

Fig. 12

Summary and fit of our throughput observations. The total throughput of the CDK-700 optics is estimated to be $\sim 70\%$ in agreement with expectations.

6.2.

Vignetting

To characterize the level of vignetting along our optical path, we perform astronomical observations and compare with the optical model of the telescope. The optical model of the telescope is summarized in Fig. 13, where the vignetting percentage with and without mirror baffles and RMS spot size is plotted as a function of the distance from the optical axis in the focal plane. For the $f/6.5$ optics of CDK-700, this corresponds to $45.3\text{arcsec}/\mathrm{mm}$.

Fig. 13

Optical model of the CDK-700 provided by PlaneWave Inc. showing the expected level of vignetting in percent as a function of off-axis distance (blue dots and line) and the RMS spot size in microns (gray dots and line). The on-sky measurement of Section 6.2 is shown in gold.

The vignetting experiments used a bright standard star at high elevation, SA 111 773 ($V=8.97$), observed alternately at the center of the CCD image and near to the four corners of the chip, 15.5 arcmin off the field center. We use standard aperture photometry to derive the flux of the star in each of the positions and then fit a polynomial function to the measured flux values of the standard star at the center of the chip to account for varying atmospheric conditions over the course of the observations. These variations were at the 1% level. The flux values normalized to the polynomial fit reveal the relative flux decrement observed with the star at the corners of the CCD. The results from our observations using the fourth MINERVA telescope performed on September 9, 2014, are shown in Fig. 14. The average vignetting measured at the off-axis positions is $4.9\%\pm 0.3\%$. This value is $\sim 0.8\%$ above the expected level, which is a statistically significant discrepancy given our measurement accuracy. However, this level of vignetting at the edge of our photometric field can be calibrated straightforwardly and is not expected to adversely affect our science goals.

Fig. 14

Results from our on-sky vignetting test. A bright standard star was observed on September 9, 2014 alternately between the field center and the four corners of the CCD chip with telescope baffles in place. After correcting for zeropoint drift due to atmospheric changes, we derive a $4.9\%\pm 0.3\%$ relative vignetting at an off-axis distance of 15.5 arcmin.

6.3.

Pointing, Guiding, and Source Acquisition

The pointing and guiding performance of CDK-700 is dependent on a pointing model that converts astronomical coordinates into altitude and azimuth positions. The MINERVA commissioning site has limited sky access due to buildings and foliage preventing a full sky pointing model. Despite this limitation, we have found that the pointing and guiding of our telescopes typically achieve or exceed the specifications of Table 1. This level of performance surpasses our requirements to place our RV target stars within the 2 arcmin field of view of our active guiding cameras.

The median seeing at the Mt. Hopkins Ridge site is about a factor of two better than our fiber diameters (1.2 arcsec versus 2.3 arcsec), ensuring that minimal flux will be lost at the wings of the seeing disk given adequate guiding. The CDK-700 telescope open-loop tracking is accurate to a couple of arc seconds over the typical integration times of our primary program. Therefore, we have implemented a modified positional proportional-integral-derivative-type controller to correct for drifts and other inaccuracies. The controller input is the star position on the camera, and the output is the altitude and azimuth offsets to the telescope mount. It typically sends corrections once every few seconds. The optical design of the telescope allows us to perform a one-time calibration of the camera field rotation if the telescope derotator is turned off, which is ideal for high-cadence observing.

Once a target star arrives within the field of view of our guide cameras, the controller actively guides the telescope such that the star is placed on the calibrated pixel location of the fiber tip. The fiber tip location can be calibrated as frequently as needed to minimize losses due to offsets between the guiding center and the true center of the fiber. We have tested the stability of the fiber position on the guide camera and have found no measurable drifts or systematic offsets on day timescales. We have not measured the temporal stability over timescales from days to weeks. However, we do not anticipate that frequent calibrations will be needed as the optics are rigid and the fiber is strain relieved. Also, for these bright stars, reflections off the fiber cladding during observations can be detected and used as a secondary check for optical alignment.

Currently, the controller operates near the optimal level, showing an RMS pointing precision of $\sim 0.2\text{arcsec}$, dominated by uncertainties in the measurement from seeing variations. Simulations of the required pointing accuracy indicate that coupling penalties $<5\%$ are incurred for a pointing accuracy of 0.2 arcsec RMS at any seeing from 0.5 arcsec to 2.5 arcsec. It is thus unlikely that the control system is contributing to any major loss of throughput in the system. The most convincing evidence that the controller is not adding significant noise to the pointing system is provided by the amplitude spectral density of the pointing errors; the error is comparable or lower at all sensed frequencies when the telescope is guiding (see Fig. 2 of Ref. 72). Typical guide camera exposure times are 0.1 s for stars from 4 to 6 mag, allowing sufficient sensitivity to successfully guide on the dimmest targets in our target list.

Results from a guiding test are shown in Fig. 15. For this test, first presented in Ref. 72, the location of a bright star was tracked on the FAU guide camera up until $\sim 49\mathrm{s}$, after which active guiding was initiated. The time to acquire the source on the chosen pixel (here $x,y=\mathrm{160,150}$) was $\sim 20\mathrm{s}$. Although this controller has not been optimized to minimize the acquisition time, we use this result as the basis for total source acquisition time including telescope slew time.

Fig. 15

Results from the FAU guiding test,⁷² here showing a zoom-in of the source acquistion. The active guiding system was initiated around 49 s, and the position of the star stabilized on the target pixel in $\sim 20\mathrm{s}$.

6.4.

Fiber Coupling and Throughput

The theoretical throughput curve for our $50\text{-}\mu \mathrm{m}$ octagonal fibers has been calculated as a function of astronomical seeing based on the vendor supplied transmission specifications for the pellicle, the fiber transmission, and the expected transmission calculated for the input and output reflectance (see Fig. 3 of Ref. 72). On-sky tests of the fiber coupling device performed from the MINERVA commissioning site on the Caltech campus in $\sim 2\text{arcsec}$ seeing conditions show very good agreement with these expected values in consideration of fiber losses, reflection losses, and coupling efficiencies.

On-sky throughput observations performed on the Caltech commissioning site have confirmed the expected performance of the FAU design achieving 50% measured efficiency (45% throughput). Further tests will be needed at FLWO to validate the performance at more optimal conditions. The final version of this instrument will be deployed later this year and will incorporate minor modifications, such as a customized pellicle for slightly higher transmission (98 versus 92%). These results suggest that the throughput of our fiber system will be roughly 70% at FLWO where the median seeing is 1.2 arcsec. With these results and those of Sec. 6.1, we expect to lose $\sim 50\%$ of the astronomical light from our telescope and fiber systems excluding losses from the atmosphere.

7.1.

High-Precision Photometry of 16 Cygnus

The secondary science goal of MINERVA is to search the transit windows of known and newly discovered super-Earths detected by the RV technique, including potential detections from the MINERVA target list. The transit of a 3 $R_{\oplus }$ planet around a Sun-like star ($0.8\lesssim M_{\star }/M_{\odot }\lesssim 1.2$) produces a decrement of light on the order of 1 mmag. This is an approximate upper bound of what would be considered a super-Earth and, therefore, represents a lower limit to the precision that must be achieved with MINERVA for our secondary science program to be viable. The timescale for this precision is also important. A super-Earth in its respective habitable zone of a Sun-like star will transit with a duration of $\sim 13\mathrm{h}$ and have an ingress/egress time of $\sim 20\min$. Planets closer to their host star are easier to detect, have higher transit probabilities, and shorter transit durations. Therefore, this level of precision should be attained on timescales $<20\min$.

On the evening of UT September 18, 2014, we observed one of the ${\eta }_{\oplus }$ targets that will be part of our RV survey, 16 Cyg AB ($V=5.95/6.20$, also HD $186,408/186,427$). The first MINERVA telescope was equipped with an Andor iKON-L camera, an SBIG ST-i guide camera, and a seven-slot filter wheel. The telescope was controlled through the PWI interface, while the camera and active guiding were controlled through Maxim DL.¹⁰⁵ A series of flats were taken during twilight. There were some clouds in the East, but the area of sky where we were observing remained clear throughout our observations.

Once on our target field, we aggressively defocused the camera while keeping our guide camera focused on a suitable guide star $\sim 20\text{arcmin}$ off-axis. The defocusing allows for the ample flux from our target to be spread over many more pixels mitigating the photometric error contributions from pixel-to-pixel variations and Poisson noise from the target itself. A second and important benefit from defocusing is to allow for long enough integration times to preclude shutter effects. To this second end, we also chose to observe in $z^{\prime }$ band where the quantum efficiency of our camera is a factor of 2 below peak. A series of bias and dark frames were taken with the same setup following our observations.

Using 7 s integrations and full frame readouts, we obtained 208 images of the 16 Cyg field spanning almost exactly an hour from UT 0352 to UT 0453. We actively guided throughout the course of these observations, but we did not achieve optimal guiding results. A drift of a few pixels was seen over the course of these observations, and there was one episode of a fairly large ($\sim 7\text{pixels}$ or 4 arcsec) guiding excursion that took place over $\sim 1.5\min$. The frames were bias and dark subtracted, and divided by our calibrated median twilight flats to correct for pixel-to-pixel variations.

To extract the photometry from the calibrated science images, we used the multiaperture mode of AstroImageJ,¹⁰⁶ which uses simple aperture photometry and sky-background subtraction. For all of the stars for which we measured a lightcurve, we used a constant aperture size of 30 pixels (18.4 arcsec) and a sky annulus with an inner radius of 90 pixels and an outer radius of 100 pixels. The rather large sky annulus was necessary so that the background annuli centered on each of the stars in 16 Cyg did not include the other member of the binary. On each science image, we recentered the apertures on the stellar centroids using the center-of-light method.¹⁰⁷

We used a set of five nearby comparison stars to remove systematics in the light curves of 16 Cyg A and B. The set of comparison stars included the corresponding other member of the 16 Cyg binary, which effectively provided all of the comparison information, as the next brightest comparison star was $\sim 3.5$ magnitudes fainter. To remove any lingering systematic trends in the data, we then performed a linear detrending against airmass.

The detrended photometric time series of both 16 Cyg A and B are shown in Fig. 16, where the individual points have been binned into 1-min intervals. The Allan Variance of 16 Cyg A is plotted in Fig. 17. The RMS of the unbinned photometry is 2.7 mmag, while we achieved sub-mmag precision on $\sim 3$- to 5-min timescales. The stability of the atmosphere on Mt. Hopkins is considerably better than in Pasadena. Therefore, these first results from our commissioning site support the prospect of routinely achieving sub-mmag photometric precision from FLWO.

Fig. 16

Detrended photometric time series of the 16 Cyg A and B observations performed on UT September 18, 2014 from the MINERVA commissioning site in Pasadena, California. The individual 7-s exposures have been binned into 1-min intervals.

Fig. 17

Allan Variance plot of the photometric time series of 16 Cygnus A performed on UT September 18, 2014 from the MINERVA commissioning site in Pasadena, California. On the 17-s duty cycle of our observations, a 2.7-mmag precision was achieved that bins down to $<1\mathrm{mmag}$ on 3- to 5-min timescales.

7.2.

WASP-52b: New Transit Observations and Modeling

WASP-52b is an inflated hot-Jupiter with $M=0.5{\mathrm{M}}_{\mathrm{J}}$ and $R=1.3{\mathrm{R}}_{\mathrm{J}}$ in a slightly misaligned, 1.75 d orbit.¹⁰⁸ The transits of WASP-52b were first observed by the SuperWASP survey⁷⁶ in 2008 and 2009, and the most recent observations in the literature are precision light curves obtained in September 2011.¹⁰⁸ The host star is reported to have a mass of 0.87 $M_{\odot }$ and a rotation period of 11.8 days, suggesting a gyrochronological age of 0.4 Gyr. The age, small orbital distance, and obliquity of this planet may have important implications for the mechanism by which it formed and for the formation of hot-Jupiter systems in general.¹⁰⁹

We observed WASP-52 on the evening of UT September 18, 2014, from the MINERVA commissioning site on the Caltech campus in Pasadena, California. It has a $V$ magnitude of $\sim 12$, $>200$ times fainter than 16 Cygnus. To maximize the signal-to-noise ratio of our observations of WASP-52, we again aggressively defocused. The observations were performed in an $r^{\prime }$ band, and we used an integration time of 120 s to build up high signal-to-noise on the defocused star image. Active offset guiding was used throughout the observations, but the guide star was faint and the target star drifted by $\sim 15\text{pixels}$ or 9 arcsec. This drift was little more than half the size of our defocused star images. The target was tracked for 4 h 19 min starting $\sim 13\min$. before the start of ingress.

We used the same AstroImageJ¹⁰⁶ reduction pipeline as in our 16 Cyg observations. We bias and dark subtracted our raw science images, before using a median twilight flat to remove image inhomogeneities. We then conducted simply aperture photometry with sky background subtraction on our calibrated images and extracted light curves for WASP-52 and 12 other nearby comparison stars. For all of the stars for which we extracted photometry, we used a fixed 20-pixel aperture radius (12.3 arcsec) and a sky annulus with an inner radius of 30 pixels and an outer radius of 50 pixels. We recentered the apertures on the individual stellar centroids in each calibrated image using the center-of-light method.¹⁰⁷

Figure 18 shows the calibrated and detrended photometry of WASP-52. We achieved 3 mmag precision on the 131-s duty cycle of these observations, which binned down to $\sim 1\mathrm{mmag}$ on 30-min timescales. The Allan Variance for this photometric time series can be seen in Fig. 19.

Fig. 18

Normalized and detrended light curve data for WASP-52 with best fit transit model overlaid. The transit center, $T_{\mathrm{C}}$, is observed to be 6918.79085 ${\mathrm{BJD}}_{\mathrm{TDB}}$.

Fig. 19

Allan Variance plot of the photometric time series of WASP-52 performed on UT September 18, 2014 from the MINERVA commissioning site in Pasadena, California. On the 131-s duty cycle of our observations, a 2.7-mmag precision was achieved that bins down to 1 mmag on 30-min timescales.

The midpoint Julian Date in Coordinated Universal Time (${\mathrm{JD}}_{\mathrm{UTC}}$) of each integration was recorded for each observation, which we convert to ${\mathrm{BJD}}_{\mathrm{TDB}}$.¹¹⁰ The light curve of WASP-52 was then fit using EXOFAST.¹¹¹ We employ priors on the planet period, $P$; the stellar age, Age; metallicity, [Fe/H]; and effective temperature, $T_{\mathrm{eff}}$; all taken from Ref. 108. The transit parameters are tied to the Yonsei-Yale stellar models¹¹² through the stellar density as determined by the scaled semimajor axis, $a/R_{\star }$. This, in turn, informs a prior for a quadratic stellar limb darkening model parameterized by $u_1$ and $u_2$.¹¹³ The remaining free parameters of the fit are the baseline flux, $F_0$; transit time, $T_{\mathrm{C}}$; $\cos i$; $R_P/R_{\star }$; $\log M_{\star }$; an additional noise term added in quadrature to the transit errors, ${\sigma }_r^2$; and an error scaling for the uncertainties in the photometric time series, TranScale. While the differential photometry technique accounts for the majority of the airmass effects on our photometry measurements, additional drift is noticed that we suspect is due to scattered light effects. We, therefore, include an additional nuisance parameter in the fit that accounts for a linear drift with airmass.

The differential evolution Markov Chain Monte Carlo (MCMC) sampler required 2080 burn-in steps and 13,074 steps after the burn-in for adequate mixing.¹¹¹ Figure 18 shows the best fit transit model overlaid on the detrended light curve data. The median parameter values and $1\sigma$ errors are reported in Table 2.

Table 2

Median values and 68% confidence interval for WASP-52b.

Our parameter distributions are in broad agreement with those of Ref. 108. However, 1 to $1.5\sigma$ discrepancies are seen between the scaled semimajor axis of the orbit, $a/R_{\star }$, and associated parameters. This discrepancy may arise from the difference in parametrization of the stellar limb darkening (Hebrard et al. use a four-parameter limb darkening model, we use two) or from the MCMC chains stepping in different parameters, which introduces different priors.

The mid-transit time we derive for WASP-52b is $T_{\mathrm{C}}=23\pm 31\mathrm{s}$ advanced from the predicted mid-transit time, 643 transits from the literature ephemeris. This is consistent with the published ephemeris and extends the time baseline of WASP-52b transit photometry to approximately six years. The possibility of a third companion was mentioned in Ref. 108 based on a potential acceleration of $30{\mathrm{m}\mathrm{s}}^{-1}{\text{year}}^{-1}$ seen in the RV data. If real, this could be due to a massive giant planet at a few to several astronomical unit (AU) or a stellar companion with mass $<0.8$ $M_{\odot }$ further out.¹¹⁴ At those large orbital distances, the light travel time effect would dominate a transit timing variation (TTV) signal.¹¹⁵ Using the lack of TTVs in our data as a constraint, we can rule out the existence of brown dwarfs, $M\gtrsim 10$ to $15{\mathrm{M}}_{\mathrm{Jup}}$, out to $\sim 6\mathrm{AU}$.

We also examine the photometry data from Ref. 108 and find no significant variation of the transit depths in the $r$ band for three epochs with full transit coverage. Independent fits to two epochs of data taken with the EulerCam on the 1.2-m Euler-Swiss telescope in La Silla, Chile, and one epoch of data from the 0.94-m James Gregory Telescope in St. Andrews, Scotland, agree with our transit depth to within $1\sigma$. This suggests that WASP-52b may be a suitable target for follow-up observations of occultation events for the purpose of atmospheric studies.¹¹⁶

8.1.

Future Prospects: MINERVA-South and MINERVA-Red

In addition to the primary survey presented in this article, the modularity of the MINERVA design offers several opportunities for expansion that are already being pursued. The abundant yield of transiting exoplanets around bright stars in both the Northern and Southern hemispheres expected from the Kepler K2 Ecliptic Mission⁷⁹ and the upcoming TESS⁸⁰ will require intensive RV follow-up. Planets with periods up to $\sim 100\mathrm{d}$ will be detected within the continuous viewing zone of TESS near the ecliptic poles that will require long-term RV monitoring. As has been made clear from the Kepler prime mission, the most interesting planets will require a substantial amount of dedicated telescope time.^117–119 Hence, there are significant opportunities for a MINERVA-like facility in the Southern hemisphere. MINERVA-South will take advantage of the same economies as described in Sec. 2, with the added advantage that our team’s investment in software/hardware development and operational expertise will easily translate to the Southern facility. We expect MINERVA-South to be of essentially the same design as MINERVA, sited in Australia or Chile, and operational by 2018 to capitalize on the coming flood of planet candidates from K2 and TESS.

In the near term, we are expanding the reach of the MINERVA project to include nearby M stars with a second instrument specifically designed for gathering precise RVs of the closest low-mass stars to the Sun. Recent results from Kepler and ground based surveys indicate that compact systems of planets in orbit around low-mass stars may be extremely common.^35,49,50 Statistically speaking, we expect some of the closest stars to the Sun to host systems of planets. However, these small, cool stars are often too faint to observe at the optical wavelengths where most precision RV instruments operate, including KiwiSpec-MINERVA. MINERVA-Red, a parallel effort to the main MINERVA survey, will specifically target a small sample of nearby mid- to late-M stars.

The MINERVA-Red instrument is a fiber-fed echelle spectrograph housed in a vacuum chamber and optimized to cover the 800 to 900 nm spectral region at a resolution of $R\approx \mathrm{55,000}$. The instrument is designed around a single-mode fiber input, which allows the instrument to be very compact and stable, and also eliminates modal noise as a source of RV error. While the single-mode fiber reduces the possible coupling efficiency of starlight into the fiber, special attention has been paid to maximizing the optical throughput of the rest of the instrument. To that end, the spectrograph will operate with a pair of PlaneWave CDK-700 telescopes that have gold mirror coatings, significantly enhancing the telescope efficiency in the wavelength range of interest. These will be in addition to the four CDK-700s that will be used for the primary survey. The instrument optics are all optimized for this relatively narrow spectral range, and a deep depletion detector will ensure high quantum efficiency and low fringing. The MINERVA-Red instrument is currently under construction and is slated to be deployed with the first of its two telescopes at the Mt. Hopkins site by mid-year 2015.