The Comet Observation Database (COBS) - Recent feature upgrades

Introduction

Since its launch in 2010, COBS has become an essential resource for both amateur and professional astronomers. Currently, it contains over 288,700 observations of more than 1,640 different comets, making it the most extensive comet observation database available [1].

Recent feature upgrades have significantly expanded COBS's functionalities. A major addition is the ability to fit comet light curves using the Henyey-Greenstein (HG) model, which explicitly accounts for the forward scattering of sunlight by dust in the comet’s coma, which can cause comets to appear substantially brighter at large phase angles [2, 3]. This option allows more accurate predictions of visual comet magnitudes.

In addition to analysis upgrades, the COBS Observation planner has also been enhanced. It now offers users expanded filtering options for selecting comet targets based on observational constraints like altitude, limiting magnitude, and proximity to the Sun or Moon. These improvements facilitate better session planning and help observers optimize their observing opportunities.

Scatter Fit of Comet’s Light Curve

The COBS analysis page was recently upgraded to support advanced light-curve fitting. It now allows users to apply a compound Henyey-Greenstein (HG) function to model the scattering of light by dust particles in comets coma. The fitting process is performed using Python’s SciPy library. The parameters fitted, along with their corresponding ranges, are summarized in Table 1. These parameters describe the comet's intrinsic brightness, its photometric behavior with respect to heliocentric distance, and the scattering properties of dust within the coma.

Table 1: Parameter Ranges for Henyey-Greenstein Function Fitting [3].

Parameter	Symbol	Range	Description
Absolute magnitude at 1 AU from Sun and Earth	H0	-5 to 20	Intrinsic brightness of the comet
Photometric slope parameter	n	2 to 6	Describes the rate of brightness change with heliocentric distance
Forward scattering asymmetry factor	gf	0.8 to 1	Degree of forward scattering
Backward scattering asymmetry factor	gb	-1 to -0.5	Degree of backward scattering
Partitioning coefficient	k	0.8 to 1	Ratio between forward and backward scattering contributions
Dust-to-gas brightness ratio	δ90	1 to 10	Relative contribution of dust scattering to gas emission in the coma

This new functionality was tested on Comet C/2023 A3 (Tsuchinshan–ATLAS) [4]. Early predictions for that comet indicated that the comet would exhibit strong forward scattering effects following its perihelion passage, significantly enhancing its observed brightness [5].

As illustrated in Figure 1 the standard light-curve fit (solid line) systematically underestimated the comet's brightness during early October 2024, shortly after perihelion on September 22, 2024, when the phase angle approached 180°. This discrepancy is most notable near the peak of the light curve, where forward scattering dominates the apparent visual magnitude. By applying the scatter-fit model (dashed line), the fitted curve aligns much more closely with the observed data points.

This comparison demonstrates the critical importance of including forward scattering effects in light-curve analyses for comets observed at large phase angles.

Figure 1: Comparison between standard comet light curve fit and fit corrected for light scatter due to forward scattering.

Upgrade of the COBS Observation Planner

The COBS Observation Planner was recently enhanced to assist observers in efficiently selecting and prioritizing comet targets. Users can specify an session date, observer location (either manually or by selecting from MPC location codes), and the session start time, which is based on the Sun's altitude, ranging from sunset to the end of astronomical twilight.

Additional filters, such as limiting magnitude, minimum target altitude, and minimum elongation from the Sun and Moon, allow further refinement of the candidate target list (Figure 2).

Figure 2: User interface of the upgraded COBS Observation Planner, showing options for setting observational constraints.

After submitting the input parameters, COBS calculates significant astronomical events and generates a list of potential comet targets. For each target, the optimal viewing time, coordinates, altitude, and apparent motion are provided (Figure 3).

Figure 3: Example of a generated list of potential comet targets produced by the COBS Observation planner. Three targets have been selected for further ephemeris generation.

Observers can then select multiple targets and quickly generate detailed ephemerides for the selected date, aiding real-time observation planning (Figure 4).

Figure 4: Ephemeris generated for the selected targets, showing detailed positional data for the observation session. This data can be used directly at the telescope or be exported for further planning.

In addition, the target list now includes a direct link to a comet finding chart provided by in-the-sky.org [6]. These charts show the comet’s position in the sky and display comparison stars labeled with their V-band magnitudes, making them particularly valuable for visual observers.

Figure 5: Example of a finding chart from in-the-sky.org for comet P/2010 H2 (Vales), showing the comet's path and nearby comparison stars with labeled V-band magnitudes.

Summary and Conclusions

The Comet Observation Database (COBS) remains a vital and evolving tool for the comet observing community. Recent upgrades, particularly the introduction of scatter fitting using the Henyey-Greenstein model, have greatly improved the ability to analyse and interpret the brightness evolution of comets, especially in cases where forward scattering significantly impacts observed magnitudes. While the scatter fitting feature provides a valuable analytical improvement, users should apply it with caution to avoid overfitting, which could misrepresent a comet's true behavior.

The enhanced COBS Observation Planner provides an intuitive and flexible means of planning observing sessions, considering key observational factors such as object altitude, solar and moon elongation. Users can efficiently generate, refine, and export target lists, enhancing their productivity at the telescope.

Through continuous upgrades, COBS reinforces its role as a vital platform for the cometary science community, helping observers to maximize the scientific value of their observations.

Acknowledgements

We gratefully acknowledge Dominic Ford, author of in-the-sky.org, for adapting the comet finding chart output to meet the specific needs of visual comet observers.