How to safely capture the planet’s slender crescent around inferior conjunction
CAUTION
Never observe or image the Sun with the naked eye or any unfiltered optical instrument
Venus reaches inferior conjunction on 9 January, a time when it lines up with the Sun in the sky on the part of its orbit nearest to Earth. This is an exciting time for Venus watchers as, during the evening approach to inferior conjunction and morning exit from it, Venus appears as a slender crescent. There are safety considerations here: viewing Venus as a crescent means it’s going to appear close to the Sun. So follow all the solar safety measures and never view the Sun with the naked eye or unfiltered optical equipment.
On 1 January, Venus appears separated from the Sun by 12˚, setting 75 minutes after the Sun below the southwest horizon. The apparent separation in degrees and the setting time decreases in the days that follow; the planet will probably be lost from general view during the middle of the first week of January.
If you have experience of viewing the planet during the day when the Sun is up, it will still be possible to see it at this time and probably through inferior conjunction, but as Venus passes 4.8˚ north of the Sun when it reaches this position, viewing it at this time should only be attempted if you know what you are doing. At this apparent separation from the Sun, there is a risk of injuring your eyes or damaging equipment.
Re-emergence from inferior conjunction into the morning sky is rapid and for those with a flat southeast horizon, the planet may be seen before sunrise from 14 January, rising an hour before the Sun.
An object of beauty
A telescopic view of Venus in January will show its beautiful crescent phase and the planet’s brilliance makes it ideal for imaging. The low altitude, however, will play havoc with the finesse of the crescent, wobbling the sharp cusps and rendering them blurry.
This can be offset by imaging during the day. Although the altitude of the planet never gets high, a little higher does make a difference. Also, the effects of temperature gradients are less severe in the day.
A high frame rate camera fitted to a telescope will give the best results, and as you’re going to be imaging through blue sky during the day, it’s recommended to fit an infrared pass filter to the camera’s front. This has two benefits; firstly, it will darken the surrounding blue sky to black and second, it will reduce the effects of atmospheric turbulence.
Locating Venus in daylight carries a danger with it due to the proximity of the Sun. If your mount has setting circles, these are a great help. If you’re not sure what you’re doing, attempt to image when Venus is either in the morning sky before sunrise, or in the evening after sunset. Whichever method you use, it’s well worth capturing the Venusian crescent.
Recommended equipment: Telescope fitted with a high frame-rate camera; polar-aligned tracking mount with setting circles
Step by step
STEP 1
Use a camera setup that gives a 0.5-1.0°-width field of view. With the scope mounted and pointing away from the Sun, fit a full aperture, certified and pre-checked white light filter. Cap, filter or remove any finders and use the scope’s shadow to point at the Sun. Tweak the position until the Sun is seen by the camera.
STEP 2
Twist your camera so slewing in RA (Right Ascension) moves the Sun parallel to the screen’s bottom edge. Focus on the Sun as accurately as possible. If there are no sunspots, use its limb. Next, centre the Sun’s disc on screen. Obtain its current RA and dec. (declination) with a planetarium program such as Stellarium.
STEP 3
Using your mount’s RA setting circle, use the scale which increases in value when you slew east. Unlock the setting circle and rotate it to match the Sun’s RA value obtained in Step 2, then lock again. Do the same with the dec. scale, making sure you use the scale which increases in value when you slew north.
STEP 4
Look up the RA and dec. of Venus. Slew towards Venus in RA until the RA setting circle indicates the correct value. Slew in dec. until that setting circle matches. Early in January, Venus is north and east of the Sun; later in the month, it’s north and west. Ensure you’re pointing away from the Sun then remove the solar filter.
STEP 5
Venus may appear in frame, but typically you’ll need to do a small spiral search of the area, making sure you never point at the Sun. If the search area widens too much, give up and restart from Step 1. Once Venus is centred in the frame, try increasing the image scale with a Barlow lens; this will need re-focusing.
STEP 6
Adjust camera settings to balance a low gain and high frame rate. The use of an IR-pass filter helps to improve contrast. Monitor levels to avoid over-saturation and capture several thousand frames, processing results using a stacking program such as AutoStakkert! Point the telescope away from the Sun.
Send your images to: gallery@skyatnightmagazine.com
Pete Lawrence is an expert astro imager and a presenter on The Sky at Night