Principales barreras que limitan la implementación del acompañamiento espiritual en la

3.3.6.1 Camera requirements

The secondary science goal (and initial science results) for the MINERVA array is photometric followup to newly discovered or previously known exoplanets. Specifically for transit photometry, a super-Earth type planet (for example, 3 Earth radii) around a K-dwarf will produce a transit decrement of ∆m= 2.5 log " 1− _3R e 0.8R 2# = 1.3 mmag (3.15)

A “typical” transit time across a stellar surface can be estimated as

ttrans,max= 2R vp = R 2πa⊕/T⊕ ≈12 hours (3.16)

and similarly, the ingress and egress time for the planet to cross the limb of the host star can be estimated as

tin/egress=ttrans,max

Rp

R

≈20 minutes (3.17)

The transit time is more of an upper bound as the planet is unlikely to transit across the full diameter of the host star, and most planets that are found in closer-in orbits. The ingress and egress time are more insensitive; ingress and egress times of 10 to 40 minutes capture a very large range of planetary and stellar systems. Therefore, a sensitivity requirement can be stated as needing a photometric precision of about 1 mmag over a timescale less than 20 minutes, as multiple data points are needed to accurately characterize a transit.

For this level of transit precision, experience has shown that the atmosphere sets the most severe constraints on transit precision. For comparison, Kepler, a space mission optimized for transiting planets, had a photometric precision of 30 ppm, or equivalently about 30µmag. From the ground, in good atmospheric seeing conditions, many groups have reported a limit to ground-based photometry of a bit less than 1 mmag; see for example L´opez-Morales (2006), Mann et al. (2011). To reach this limit, differential measurements are absolutely required for ground-based work—that is, reference stars of a similar magnitude need to be measured as close to simultaneously with the target star as possible. This requirement leads to the need for a wide field of view, so that more stars can be

used as simultaneous references. Of course, field of view depends on both the optical performance of the telescope and the detector size. Independently of the size of the usable field, the detector needs to have acceptable dark current and readout noise. Additionally, detector nonuniformities can adversely affect long-term precision, so the need for active guiding to keep stars on the same pixels in measurements spanning many nights is necessary. This requires a high level of pointing control from the telescope, though not as high as in fiber guiding.

A tertiary soft design requirement for MINERVA is a camera suitable for outreach, particularly in the form of undergraduate-level lab courses which typically do not have access to telescopes. This requirement is derived from the demands of certain funding agencies that significant time be used for outreach, often at the 10% level. From this perspective, apart from the precision necessary to do basic eclipsing binary work, transit photometry, multi-filter spectral characterization, etc, it is useful to have a camera with a field of view large enough to image solar system objects, Messier objects, and other diverse celestial objects. For these kinds of observations, narrow-band filters can be very useful as well, as they can be used to both limit the flux from very bright objects and to reveal phenomena like H II regions in galaxies, which are frequently found in undergraduate astronomy curricula.

One advantage of MINERVA is that the telescopes are situated further apart than the typical coherence length of the atmosphere. This means that transit photometry datasets are expected to be weakly correlated, if at all, when measured simultaneously with the same filter from multiple telescopes. On a similar note, when measured simultaneously with different filters on each telescope, interesting constraints on the planetary atmosphere can be made.

3.3.6.2 Photometry optics and detector

The first camera we purchased (Apogee U230) had completely unacceptable transient noise proper- ties which resulted in low photometric precision and an inability to properly flat field. Sending it back to the manufacturer, resulted in months long delays and radio silence, and eventually a new camera with a host of other problems. We gave up and moved away from Apogee in the end.

The model we eventually chose for the standard photometric camera was the Andor iKon-L with the BV chip for three out of four telescopes and the BEX2-DD chip for the other. The advantages of this camera are high efficiency, high dynamic range, and an efficient thermoelectric cooler without the need for liquid coolants, which can be less reliable. The BV chips have a quantum efficiency of greater than 95% from 500 to 650 nanometers; the BEX2-DD has a quantum efficiency that is lower but covers a much greater wavelength range, higher than 80% from 380 to 900 nanometers. Camera parameters are presented in Figure 3.3.

Our camera setup is identical for each telescope except for the choices of filters. Each camera is mounted via custom adapter to an Apogee AFW50-7S filter wheel, which have seven accessible slots.

(We also tested a dual-ten slot filter wheel on one of telescopes, but this proved to be unreliable with frequent wheel slip due to a fundamentally poor mechanical design.) The filter wheels are connected to an off-axis guider optic which sends light to an ST-i guide camera with a field of view of 4 x 6 arcminutes. (The guide camera is the same model as used in the fiber acquisition unit.) The entire unit is connected to the telescope focuser and derotator. An image of the setup can be seen in Figure 3.15. The current filters are standard Sloan g0, r0, i0, z0, which are present on every filter wheel, as well as combinations ofU, B, V, R, I,, Hα, [S II], [OIII]. Typically, a slot is left open for luminance measurements. The filters where provided by Astrodon Photometrics.

Active Pixels 2048 x 2048

Sensor Size 27.6 x 27.6 mm

Pixel Size 13.5µm x 13.5µm

Well depth (typical) 100,000 e-

Max. Readout Rate 5 MHz

Read Noise 2.9 e- (min)

Maximum Cooling -100 C

Frame Rate 0.95 fps(max)

Coating BV (3x), BEX2-DD(1x)

Dark current (-70 C) 0.00013 (BV), 0.006 (BEX2-DD)

Usable field 21’ x 21’

Johnson filters U, B, V, R, I

Sloan filters g0, r0, i0, z0

Narrow filters Hα, [S II], [OIII]

Table 3.3: Andor iKon-L parameters and filters

3.3.6.3 Performance Validation

We measured the single-telescope photometric precision at the MINERVA test site in Pasadena. The typical test procedure involved heavily defocusing the field to reduce the effects of interpixel variations, Poisson noise, and shutter effects which are annoying to accurately cancel with very short exposures. This slightly complicates the pointing calibration, as the guide camera must be operated in focus while the science camera is out of focus, and without an automatic adjustment a one-time manual adjustment is required. Exposures are taken every∼10 seconds.

One of our target stars was 16 Cyg, a visual binary with a separation of about 30”. We measured this star for about one hour (about 200 images) while guiding on an off-axis star 20 arcminutes away. The guider was not functioning perfectly as there were some unwanted drifts over the course of the observation of a few pixels. Following the standard CCD reductions of bias, dark, and flatfield correction, we extracted aperture photometry using a simple annular method found in the program AstroImageJ, using a 30 pixel aperture for the defocused spot and 90 to 100 pixels for the sky annulus. There were five appropriate reference stars in the field for lightcurve detrending and systematics control.

The results of the above analysis showed a single-shot photometric precision of 2.7 mmags, with a binned precision of less than 1 mmag on 3-5 minute timescales, see Figures 3.16 and 3.17. This is well within the required precision characterized in the previous section. Given the unpleasant light conditions at the MINERVA test site in Pasadena (e.g. passing cars, nearby streetlights, etc), the underperformance of the guider in this particular test, and the simple analysis involved, it is encouraging to see that we achieve the required level of precision already. This result strongly suggests we will be able to achieve this level of photometric performance routinely at the observatory site.