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A Very Marginal Image of a Comet
So this is likely one of the worst images of a comet you are going to see. But that was sort of the point. This is a Track & Stack in Astrometrica of three 4-min exposures from last night. The remote imaging system used was T21, located just a few miles down the road in Mayhill, NM, via It is a Planewave 17" CDK with an FLI-PL6303E camera. 

As with most of my imaging these days, this was a SkyTools 4 test. A little backstory first: stellar and diffuse objects are modeled differently. A diffuse object, such as a nebula or galaxy, is modeled as one typical image pixel sampling a continuous surface brightness. A stellar object is modeled as a Gaussian Point Spread function. The details aren't really important-what matters is that comets are both of these things, and to varying degrees depending on the comet. Some comets look like stars with no visible coma, while others are completely diffuse, with no central bright spot. Most lie somewhere in between. This makes comets really difficult to model.

SkyTools 4 told me that comet 250P/Larsen would be high in the sky at around 3 AM, and with an integrated magnitude of 19.1, SkyTools predicted that it would be a difficult target. In fact, SkyTools predicted that even once I stacked three 4-min exposures the Signal to Noise Ratio (SNR) would only be 8. An SNR of at least 5 is typically required to do fairly accurate astrometry. So SkyTools was predicting that this would be a marginal observation.

And indeed it was! The SNR measured by Astrometrica was 6, which is quite close. This relatively small difference is actually quite remarkable, given how difficult comets are to model. Being able to fairly accurately predict how long of an exposure is required to obtain good astrometry is a rather useful thing!

Incidentally, SkyTools 4 also told me that the comet would only move by 3 pixels over the course of the three exposures, and that it would move by 1 pixel during each of them.

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Clear skies,
Head Dude at Skyhound

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