3I/ATLAS
Fraser Gillan
Observing 3I/ATLAS (C/2025 N1 (ATLAS))
To observe comets or asteroids, it is best to take multiple shorter exposures using sidereal tracking—this makes future magnitude calculations easier. A shift-and-stack method can then be applied in post-processing to align the comet across frames and increase signal-to-noise without trailing.
Use the JPL Horizons Ephemeris (https://ssd.jpl.nasa.gov/horizons/app.html#/) to obtain the predicted right ascension, declination, and magnitude (T-mag in Horizons). Point the telescope to centre the comet in the field of view, and allow it to track at the sidereal rate throughout the exposure sequence—do not manually adjust or re-centre during the run.
For exposure times, I would recommend checking the current sky motion at either JPL Horizons (Sky_motion) or https://minorplanetcenter.net/iau/MPEph/MPEph.html and adjusting the exposure times so the sky motion doesn't exceed the seeing at the given site (currently moving 1.25’’/min so in the case of 2’’ seeing, the max exposure time should be less than 90 seconds).
Redder filters are preferred for cometary dust as it is most sensitive in this region, and bluer filters are more sensitive to the gas in the coma. However, please observe with whatever equipment and filters you have available - every contribution helps!
Obtain bias images, sky flats, and other calibration frames as usual for image reduction.
Note: To confirm the comet is captured in your images, try blinking the first and last frames and look for an object that moves unlike the background stars.
Once the observations are made, please do not send them to BHTOM as we are currently unable to process them. Instead, please send them to: https://drive.google.com/drive/folders/1BB6JCcXAserPzXur2ADqTaEUZgajgcgq?usp=sharing and use the sheet file to log your data. Please create a separate folder for your observatory.
Clear skies!
