Perhaps no other object has such a wide gulf between its
naked eye impression compared to its appearance in a telescope than Venus. In
our twilight sky Venus demands attention, shining like a brilliant diamond.
Whenever the crescent Moon stops by for a visit, I am always up for grabbing a
few shots (even though I already have many prior encounters documented). And
when our sister planet pairs with another planet (as it did earlier this year
with Jupiter) or the Pleiades, amateur astronomers turn into paparazzi and
flood the online galleries with their glamor shots.
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| Venus & Jupiter Feb 28, 2023 |
But few of those amateur astronomers will tarry very long with Venus in the eyepiece. The view is that of a dazzling, featureless cue ball emulating one of the Moon’s phases. Nothing stands out so it’s a quick check-in and on to something of greater interest.
With patience and some filtering there is more to see. A
deep violet Wratten #47 filter not only knocks down the glare but provides
subtle boost to the very low contrast features in the Venusian clouds. While at
times one might make out a dusky region on the planet, the most common feature
that appears is a brighter region at one (or both) of the poles. Know as a “cusp
cap” it is an actual feature and not just an artifact manufactured by our brain.
Large Hadley cells rise high into the atmosphere from the hot equatorial
regions and then sink back down in the high temperate latitudes, forming a slightly
darker “cusp collar” bordering the cooler cusp cap.
Planetary imagers also find themselves stymied by Venus’
reluctance to share features. Like most others, my prior attempts to record details on the 2nd rock from the Sun using “lucky imaging”
techniques produced crisp but bland captures of the globe.
Our professional brethren have also struggled with Venus
until about a century ago when astronomer Frank Elmore Ross targeted it
using Mount Wilson's 60- & 100-inch reflectors. His gig before that was a
decade at Eastman Kodak studying photographic emulsions and filters, which led
him to make photographs in IR and the newly released UV filter. While the IR
failed to penetrate the cloud layer to show details as he had hoped, the UV
unexpectedly did. Curiously, there was not much follow up to his
discovery until images acquired by French amateur Charles Boyer nailed down a
rotation period of about 4 days. His results were published in 1960 in Icarus
(and rejected by none other than Carl Sagan). While radar data eventually established
a retrograde rotation of 243 days for the planet, Boyer’s observations of a 4-day “super-rotation” of the Venusian atmosphere were eventually confirmed by Mariner 10. Thus we end up
having two longitudinal “systems” for tracking central meridian on Venus: CM I
for the surface, and CM II for the upper atmosphere clouds.
Over the last ten years amateur planetary imagers have
gradually been targeting Venus using a UV filter and getting some nice results.
UV filters are pricey (north of $200) and sometimes backordered for months. I
finally budgeted for the Atrodon UVenus filter which was heralded as having some
of the best UV transmission. Renowned imager Damien Peach put together a video
on his patreon channel
that provided further tips on obtaining a successful UV image such as combining
it with an infrared blocking filter since many UV filters leak IR which can
smear the image.
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| Sample transmission for UV filters |
With Venus approaching its greatest eastern elongation
(distance in our sky) from the Sun, it was time to try again to capture cloud
details on our neighboring planet. Starting at sundown I worked on centering
the planet with good focus to begin acquiring my UV videos. Reasonably calm
seeing is really important for achieving that focus and often not present when
dealing with an object like Venus that is never more than about 40°
high, but I finally got that on my May 15th session. I confess that I
was pretty ecstatic being able to make out what seemed to be a southern cusp
cap on the image shown on the laptop during acquisition, implying I would have
something worthwhile to work with. And the next day when putting the video through
my processing workflow I ended up with a nice result, clearly showing north and
south cusp caps with some of the darker collar. The center of the planet shows segments
of lighter and darker clouds.
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| Venus May 15, 2023 in UV |
In my researching about capturing Venus in ultraviolet I came across
a few sources that argued that introducing any sort of lens element in the
imaging train (e.g., a Barlow lens, an SCT corrector plate) was to be avoided if possible since these can cut down
on the amount of UV light considerably. The ideal situation is a mirror
telescope such as my 10” Newtonian. For the heck of it I did a capture at the
end of the session using the Barlow. While the seeing may have deteriorated by
that point, my results certainly seemed to confirm that the benefit of a larger
image was far offset by the decline in sharpness of the low contrast cloud
features.
So, what do the darker regions represent? In classic Venus fashion the planet will not give up that information yet. Some scientists believe that it is a photochemical reaction in the atmosphere that results in UV absorbing material. However, there are even some who argue it could be cloud based microbial life generating these regions and liken it to algae blooms that we see here on Earth. With a bevy of spacecraft heading Venus' way later this decade we may eventually get an answer to this question.
As usual I submitted the final results to ALPO. A few hours later I heard back from Julius Benton, the Venus section coordinator. “Thanks. Your UV image looks very good!” was great feedback to receive, and confirmation that the multi-year journey to acquire the equipment and knowledge on how to make Venus finally give up some details was satisfying indeed.
