Proposal #256

Dear AAVSONet Telescope Allocation Committee,

As part of the ExoClock project and also member of AAVSO, I would like to propose the observation of transits for the exoplanet NGTS-1b using the Optical Craftsman 0.61m Telescope (OC61), located at the Mount John University Observatory (MJUO), via the AAVSONet.
The ExoClock project, conducted by the ARIEL Ephemerides Working Group, is part of the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) Space Mission, and has as its main objective refine the transit timing for future exoplanet observations using the ARIEL space telescope.

The confirmed hot Jupiter-sized exoplanet NGTS-1b is a particularly interesting target for follow-up observations due to the following factors:
- There are no observations registered, up until this date, in both AAVSO Exoplanet Database or in the AAVSO WebObs Database for this target;
- The ephemeris uncertainty for the transits is currently still high. Also, a drift of 15.9 minutes has been applied to the prediction of its timing in accordance to only two previous observations already submitted by others to the ExoClock database;
- It was the third and most massive known exoplanet transiting an M-dwarf star at the time of its discovery, having the habitable zone very close to the host star, just like the other two most important planetary systems: Proxima Centauri and Trappist-1. This makes NGTS-1b a strong candidate for future characterization via the ARIEL space telescope and also the James Webb Space Telescope (JWST), benefiting significantly from ground-based observations.

In addition, the 24" OC61 telescope is particularly required for these observations due to the following factors:
- The faint target having V-mag of 15.57, and the large transit depth of 22.8 mmag, is particularly well suited for the capabilities of the 24" OC61 telescope;
- The observability of the southern celestial hemisphere at the Mount John University Observatory is extremely favorable for the target at -36.63° in declination, and its elevation at 1029 meters and Class 2 Bortle scale sky should be also helpful;
- The short transit duration of 1.2 hour, plus the required baseline observation of 1 hour pre-ingress and another 1 hour post-egress, leads to a duration of only 3.1 hours for the observing session. According to the Dr Karen Pollard (Director UC Mt John Observatory), this should provide a good match for the climate variations at the New Zealand site, and should also help minimize the conflict with other observation activities currently in place.

Personally, I have already contributed to the NASA Exoplanet Watch/AAVSO Exoplanet Database with over 100 exoplanet transits (under observer code KADB) and also over 45 of the transits already published in the ExoClock database, using both the 24" robotic telescope at the Madrona Peak Observatory (MPO) and the 6" Cecillia telescope as part of the Harvard & Smithsonian MicroObservatory network, besides others from my personal observatory.

Finally, the observations of NGTS-1 would require a high cadence, having the integration time adjusted accordingly in order not to exceed an interval of 2 minutes between exposures and also having the minimum dead time between them as low as possible, in order not to impact the precision of the transit model fit to the data. Also, the specific observations should be scheduled according to the predicted transit times provided by the ExoClock schedule calculator service, and preferably done using a Johnson-Cousins Rc filter, in order to reduce systematics and minimize the strength of the limb-darkening effect, producing a transit light-curve with better defined ingress and egress transitions.
The following list of predicted transit observations are suggested for the next 90 days (already corrected for the drift of 15.9 minutes):
- 2022-03-01 09:27 (Session start time in UTC): transit with 100% of the baseline observable, with 2% Moon at 98° distance;
- 2022-03-09 08:04 (Session start time in UTC): transit with 70% of the baseline observable due to partial pre-ingress, and 40% Moon at 62° distance;
- 2022-04-23 08:13 (Session start time in UTC): transit with 64% of the baseline observable due to partial post-egress, and 51% Moon at 107° distance;
- 2022-05-01 06:50 (Session start time in UTC): transit with 76% of the baseline observable due to a partial pre-ingress, and 0% Moon at 65° distance.

Cordially,
Andre Kovacs