Track the progress of the dwarf planet as it passes across the open cluster in Taurus
Dwarf planet Ceres reaches opposition this month and by chance it does so while tracking across the Hyades open cluster in Taurus, the Bull. Shining at seventh magnitude, Ceres isn’t difficult to see, but normally you do need a few star-hopping skills to locate it. Within the Hyades, this isn’t the case – assuming you can find the Hyades of course! To find Ceres, follow the line of Orion’s Belt northwest to arrive at orange Aldebaran (Alpha (α Tauri). The Hyades is the sideways V-shaped pattern next to Aldebaran, which is visible to the naked eye and easy to find from the bright star.
On the trail of Ceres
Aldebaran marks the start point for Ceres’s track across the cluster, the dwarf planet being located just to the east-southeast of the star at the start of the month. It then moves to pass 10 arcminutes south of Aldebaran on the morning of 3 November. Its subsequent passage takes Ceres through the main V-shape of the Hyades, before eventually leaving the pattern on 20 November. On 30 November, Ceres sits 2.3˚ northwest of Gamma (γ) Tauri, the star that marks the bottom point of the ‘V’.
If you have equipment that can photograph at least down to mag. +8.0, and as long as you can image the main V-shape of the cluster in a single go, you should be able to record Ceres. Your setup doesn’t need to be complex and recording stars down to mag. +8.0 with modern cameras isn’t difficult.
A tracking mount will make the job easier, but it’s not essential. If you’re using a static mounted camera (on a tripod, for example) use the 500 Rule to work out the longest exposure you can make: it’s 500 divided by the focal length in millimetres of the lens you’re using. This assumes you’re using a full-format 35mm sensor. If you’re using a camera with a slightly smaller APS-C-type sensor, you’ll need to multiply your lens’s focal length by the correction factor, which is typically 1.6x.
If you use a lens with a focal length of 500mm, the longest exposure you can take without stars starting to trail will be: 500 ÷ 500 = 1 second. If you’re using a 50mm lens, you’ve got longer: 500 ÷ 50 = 10 seconds. If you’re using a 110mm lens attached to an APS-C camera with a 1.6x frame correction value, your longest exposure time is: 500 ÷ (1.6 x 110) = 2.8 seconds.
Using a static, non-tracking, mount all you need to do is adjust your camera and lens settings to record stars down to at least mag. +8.0, in exposures shorter than that given by the 500 Rule. Achieve this, and dwarf planet Ceres will be perfectly placed for you to photograph throughout November.
Recommended equipment: DSLR or equivalent, a lens, polar-aligned tracking mount (optional)
Step by step
STEP 1
A Hyades photo looks best if the cluster can ‘breathe’, so frame it with plenty of sky. A field of view measuring 12 x 8° allows the full month’s track with a decent sky margin to be recorded. A 170mm or shorter focal-length lens on a 35mm sensor achieves this. For APS-C sensors, use 110mm or shorter for a similar result.
STEP 2
Set the lens to manual. Select the lowest f/number and close by a stop or two to avoid frame edge distortions. Set the camera to ‘Manual’ mode. If you are using a tracking mount use a mid to low ISO setting, or select a mid ISO value for a fixed mount. A remote shutter release will help to stop any camera shake.
STEP 3
Focus as accurately as possible. The bright star Aldebaran will be good for this and it should show up clearly if your camera has a ‘Live View’ option. If not, you’ll need to focus as accurately as possible through the viewfinder. If you’re struggling with Aldebaran, try focusing on bright Jupiter earlier in the evening.
STEP 4
Once you’ve set the camera and lens, take a test exposure. If you have a static platform, use the 500 Rule to determine the longest non-trailed exposure (as described opposite). Examine the result and look for the mag. +8.0 stars we’ve circled here. If you can see them, then you are good to go for imaging Ceres.
STEP 5
If you’re using a tracking mount, your exposure time is limited by the accuracy of the mount’s drive, polar alignment and the sky’s brightness. A lower ISO will help bring out the redder hues of the ancient Hyades stars. Typically, bracketing your exposures to 30”, 60” and 90” will give you a good range of results to pick from.
STEP 6
Try to image the Hyades on as many clear nights as possible using the same setup. Keeping a similar orientation makes life a lot easier. Load your results into a layer-based editor, aligning the stars between layers. Flicking between layers will reveal Ceres’s movement as it tracks in front of the Hyades stars.
Pete Lawrence is an expert astro imager and a presenter on The Sky at Night
Send your images to: gallery@skyatnightmagazine.com