{"id":42752,"date":"2023-03-23T09:52:45","date_gmt":"2023-03-23T09:52:45","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=42752"},"modified":"2023-03-23T10:15:48","modified_gmt":"2023-03-23T10:15:48","slug":"capture-experiments-with-venus","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2023\/03\/23\/capture-experiments-with-venus\/","title":{"rendered":"Capture: Experiments with Venus"},"content":{"rendered":"\n
CAPTURE<\/h5>\n\n

Experiments with Venus<\/h2>\n\n

Try out these activities while the Evening Star is shining bright in our skies <\/p>\n\n

\"\"
You can produce diffraction spikes by stopping down a camera lens, using Venus as the bright light source. As shown here with both the Moon and Venus, the spike quality can vary between lenses <\/figcaption><\/figure>\n\n

Venus is well-placed in the evening twilight this month, an intense blaze of light, difficult to miss in clear skies. It\u2019s an interesting object astronomically, showing subtle atmospheric markings through the eyepiece, as well as displaying striking changes in phase and apparent diameter. <\/p>\n\n

It\u2019s interesting for another reason too. Being so intensely bright \u2013 the brightest of all the planets \u2013 it\u2019s essentially a point source at low magnification, and ideal for demonstrating experiments with light. When placed in a dark sky, it\u2019s possible to experience its light casting a shadow \u2013 okay, it\u2019s a dim shadow, but it\u2019s one that can be photographed. Being very close to a point light source, the shadow cast is really sharp and well-defined too. <\/p>\n\n

To record this elusive effect, you\u2019ll need to be somewhere where there are no other light sources. Even a bright sky can interfere. A west or northwestfacing window is ideal, as long as you can turn all of the lights off. If you can\u2019t do this, use a large cardboard box with the opening pointed at Venus. Either way, you\u2019ll need a screen on which the shadow will be cast. This can be as simple as a light-coloured wall or several large white sheets of paper. <\/p>\n\n

You\u2019ll need a shadow-casting subject as well. This can be a shape cut from card pasted on a window or suspended in front of the box. Then it\u2019s simply a case of pointing a camera at the screen, focusing and taking your shot. A high ISO is recommended to keep the exposure times short; take too long and the razorsharp edge of the shadow will blur as Venus moves relative to the horizon. <\/p>\n\n

Make a rainbow <\/strong><\/h4>\n\n

Another interesting effect to see with Venus is something which is a nuisance for planetary imaging: atmospheric dispersion. This is caused by the light-dispersion quality of our atmosphere. Essentially, the atmosphere acts like a giant prism, refracting light that passes through it. The thicker the atmospheric layer, the more bending that takes place. In other words, the bending is greater at lower altitude. As white light passes through the atmosphere, different wavelengths of light get refracted by different amounts, resulting in an object\u2019s light being spread into a small spectrum. The effect is noticeable around the edge of the Moon, but the brilliance of Venus means it\u2019s easier to stay with the effect at lower altitude. Using a long-focal-length lens or telescope, it\u2019s possible to see Venus completely smear into a small rainbow of colour. <\/p>\n\n

Finally, the planet\u2019s brilliance and point-source nature make it ideal for experimenting with diffraction. A low-power reflector will create diffraction spikes easily and some general photographic lenses can produce them when the lens aperture is stopped down. Alternatively, if using a refractor, cotton thread placed over the front aperture is a great way to show these physical effects with the Venusian light. <\/p>\n\n

Equipment:<\/strong> DSLR camera with lens, a tripod or, preferably, a tracking mount <\/p>\n\n

Send your images to: gallery@skyatnightmagazine.com <\/a><\/strong><\/strong><\/p>\n\n

<\/div>
\n

Step by step <\/strong><\/h4>\n\n\n\n
\"\"<\/figure>\n\n\n\n
STEP 1 <\/strong><\/h5>\n\n\n\n

A west or northwest-facing window is ideal for capturing the shadow of Venus. In a darkened room, attach a shape to a window pane to act as a shadow caster \u2013 in this case a cardboard cutout of the symbol of Venus. The line from Venus through the target should ideally end up on a flat \u2018screen\u2019 wall. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
STEP 2 <\/strong><\/h5>\n\n\n\n

Despite Venus being such a brilliant planet, its shadow is still dim and will require an extended exposure. A mid to high ISO and low f\/ number are essential. In this example, there was a tree between Venus and our shadow target. An exposure of 20 seconds was required. If exposures are too long, the shadow edges will blur. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
STEP 3 <\/strong><\/h5>\n\n\n\n

This sequence, imaged through a low-power telescope as Venus was setting, shows the effects of atmospheric dispersion. Venus\u2019s light is spread into a spectrum, varying in intensity according to atmospheric stability (seeing). Just before setting, the green edge of Venus detached, creating a Venusian \u2018green flash\u2019. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
STEP 4 <\/strong><\/h5>\n\n\n\n

Although dispersion is easier to record with bright Venus, it\u2019s not the only object to show it. Mercury is well-placed this month too. It appears brightest at the start of the month, but its proximity to the Sun means you\u2019ll typically need to record its colour spread against twilit skies, as shown here. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
STEP 5 <\/strong><\/h5>\n\n\n\n

A refractor can be made into a diffraction generator by creating a cardboard collar around the front of the scope and attaching cotton \u2018vanes\u2019 in various orientations. Attach a camera to the scope and, using a mid-value ISO, focus (the spikes are useful for this) and take a shot of Venus, using an exposure of a few seconds. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n
STEP 6 <\/strong><\/h5>\n\n\n\n

Start off with a simple cotton line across the telescope\u2019s aperture diameter and go from there. A four-vane pattern generates eight spikes, but half are coincident with the other half, so you only see four. Three vanes arranged at 120\u00b0 intervals don\u2019t have this issue and here you\u2019ll see six vanes with 60\u00b0 between them. <\/p>\n<\/div><\/section>\n\n

\"\"<\/figure><\/div>\n\n

Pete Lawrence is an expert astro-imager and a presenter on The Sky at Night<\/em>.

<\/p>\n\n

ALL PICTURES: PETE LAWRENCE<\/p>\n","protected":false},"excerpt":{"rendered":"

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