OASI: C/2020 F8

Orwell Astronomical Society (Ipswich)

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Comet C/2020 F8 (SWAN), 20 May 2020

Comet C/2020 F8 (SWAN) was discovered on 13 April 2020 by M. Mattiazzo in the public website of low-resolution hydrogen Lyman-alpha images obtained with the Solar Wind Anisotropies (SWAN) camera on the Solar and Heliospheric Observer (SOHO) spacecraft.

On the morning of 20 May I ventured into the countryside north of Ipswich to gain a clear NW horizon, primarily to photograph comet SWAN, and with a secondary objective to attempt to capture both comets SWAN and ATLAS in the same field of view. (There had been hopes earlier in the the year that C/2019 Y4 (ATLAS) would appear spectacular in May. Unfortunately, the comet disintegrated, so did not become prominent; however the remnants are still in orbit around the Sun.)

I could not see the comet either with the naked eye nor binoculars. Fortunately, it was located near the bright star Algol, so was relatively easy to locate in the camera screen and, from just after 01:00 UT, for the best part of an hour, I recorded it as a faint smudge.

There were three obstacles to stacking the images:

The background brightness of the sky changed considerably during imaging. At the start, the comet was at altitude 5.5° and the Sun at -16°; at the end, the comet had risen to 9° in a brightening sky with the Sun only 13° below the horizon.
The effect of refraction was marked close to the horizon, varying from top to bottom of the field of view. As the comet climbed in the sky, the variation of refraction across the field of view itself varied. The comet was too faint to be used to align images, so instead I used a star at a similar altitude. The resultant image stack (not shown) has point stars in the middle of the image and trailed stars both above and below due to the variation in refraction. It looks a mess!
There was cirrus cloud or haze parallel to the horizon in the distance.

I used predictions from JPL Horizons for the rate of movement of the comet across the sky.

In the following image, Comet SWAN is easily visible to the right of the image and, to the left is a faint smudge that is Comet ATLAS. Realigning the images on ATLAS does not really improve things.

Nigel Evans