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MNRAS has recently published an article by Aaron Watkins and collaborators (including Sugata Kaviraj and Chris Collins) discussing the merits and pitfalls of different kinds of sky subtraction techniques.  The paper, Strategies for optimal sky subtraction in the low surface brightness regime, describes experiments using fully synthetic images to investigate three different techniques, two commonly used in low surface brightness surveys, another experimental.  

The study's results quantify the impact of undetected flux on sky models, which tends to bias estimated sky brightnesses high, risking over-subtraction of flux. If the sky is modelled with a complex function (for example, a high-order polynomial, or a spline interpolation), that over-subtraction can occur locally, leading to artificial divots surrounding extended objects like galaxies, or even objects which are simply located close together on the sky.  However, the results demonstrate that when a simple model is used, and when proper care is taken to mask detected astronomical objects to low surface brightness levels, this bias can be reduced to negligible amounts.  

Even for a survey as deep as LSST, any sky subtraction technique traditionally used in low surface brightness surveys can still be applied safely, so long as empirical corrections are made for scattered light and undetected faint sources.

The paper justifies the recommendations Watkins and collaborators have proposed to LSST's data management (DM) team regarding the survey's pipeline sky subtraction.  Working alongside DM, Watkins et al. found that the existing algorithm suffers from two problems: insufficient masking of low surface brightness flux, and too complex a sky model.  Following the paper's results, the team found that adjusting the algorithm to use a much simpler model, even without an improvement to the masking, proves very successful at minimizing the impact of the sky subtraction on the flux of extended or clustered objects.  In fact, a preliminary investigation suggests that the proposed revised algorithm might benefit more than just low surface brightness science: a number of DM's photometric quality metrics appear to improve slightly when the revised algorithm is used, compared to the default pipeline.  However, the full impact of the proposed change is still being investigated.

Examples of the synthetic images used to conduct the tests on sky subtraction routines, showcasing the kinds of model sources injected for four different experiment types.  

• The top-left panel shows a sparsely populated control field.  

• The top-right panel shows that same image, but convolved with an extended scattered light model derived from the Subaru Telescope's Hyper Suprime-Cam imager, to investigate the impact of that scattered light on sky models.

• The bottom-left panel shows an image simulating data processed through a typical low surface brightness--optimised data reduction routine, in which that scattered light is subtracted from the stars (but not the galaxies).

• Finally, the bottom-right panel shows an image which includes models of many faint, high-redshift sources drawn from the New Horizon simulation, to investigate the impact of undetected faint objects on sky models (also known as extragalactic background light).

The latest science release from Rubin outlines how a discovery could trigger a space mission to a fast moving target.

The first space missions that launched before a primary target has been identified is in development: the ESA/JAXA Comet Interceptor mission is due to launch with the ESA ARIEL spacecraft in 2029. A Rubin discovery could see the multi-element spacecraft directed to a long-period solar system comet or interstellar object passing by the sun for the first time.

Rubin could also identify flyby opportunities for missions that are already in progress, including NASA’s Lucy, whose shape and orbit are represented in the image, left.

Colin Snodgrass, Mission Deputy Principal Investigator for the ESA/JAXA Comet Interceptor comments: “LSST is expected to discover vast numbers of Solar System objects, but what is really exciting for me is that we expect to discover new comets approaching the Sun for the first time at much larger distances than we currently do. This will help us answer questions about how cometary activity works far from the Sun, where it is too cold for water ice to sublimate, and also is a key enabling technology that allowed us to propose Comet Interceptor.

“We expect to have years to react to a discovery instead of months, and while this is still too short a time to plan and build a space mission from scratch, it is enough to get the spacecraft into the right place at the right time from a parking orbit in space. LSST:UK will make an important contribution to this through the Adler system that we are developing in Edinburgh and Belfast, that will (along with other goals) flag comets that could be potential mission targets amongst the LSST discoveries.”

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