#dobsonian

Eaurp Guz at the eyepiece of a Skywatcher FlexTube 200P Dobsonian telescope

below the cut: unedited purely traditional art version.

(Above: This Dobsonian style reflector makes an excellent first telescope because it is simple, but powerful - showing you the moon and planets as well as dimmer objects like nebulas and bright galaxies. Many brands come in this format.)

Astronomy Skylights for this week (from December 17th, 2017) by Chris Vaughan. (Feel free to pass this along to friends and send me your comments, questions, and suggested topics.) I post these with photos at http://astrogeoguy.tumblr.com/ where the old editions are archived. You can also follow me on Twitter as @astrogeoguy! Unless otherwise noted, all times are Eastern Time. Please click this MailChimp link to subscribe to these emails. If you are a teacher or group leader interested joining me on a guided field trip to York University’s Allan I. Carswell Observatory, or another in your area, visit www.astrogeo.ca.

(Above: As the Earth orbits the sun, it’s polar axis points continuously at the same point in the sky. On the June and December Solstices, the northern hemisphere’s pole tilts directly towards and away from the sun respectively, causing the noon-day sun to reach its highest and lowest points in the sky for the year. A lower sun delivers weaker solar radiation for a shorter number of hours, giving us cold winter temperatures.)

Happy Holidays, everyone! Or, as we astronomers say, “Have a Happy Solstice and a Merry Perihelion!” 

For the Northern Hemisphere, the first day of winter, also called the Winter Solstice, occurs on Thursday, December 21st at 11:28 am Eastern Time. At that precise moment, the north pole of the Earth’s axis of rotation will be tilting directly away from the Sun. Every day, at local noon, the Sun reaches its highest position in the sky for that day. But at the Winter Solstice, that position is the lowest (i.e., farthest south, celestially) for the entire year, and we receive the shortest amount of daylight. The sunlight that we do get this time of year is diluted because it’s spread over a larger area, the same way a flashlight beam looks dimmer when you shine it obliquely at a wall (try it!). 

Fewer hours and weaker sunlight both translate into less received solar energy (insolation) and therefore colder temperatures! Good news for us, though - after Thursday, our days start growing longer again! For our friends in the Southern Hemisphere, the Sun will attain its highest noon-time height for the year on the solstice, and it marks the start of their summer season. 

It is NOT the case, as some people think, that we are colder in winter because we are farther to the Sun (a position called aphelion) – in actuality that happens every year in early July! On the contrary – we’re approaching Earth’s nearest position from the Sun (perihelion), which occurs every January 4, or thereabouts. 

Telescope Buying Tips

A few people have asked me about buying a first telescope. Two of the most important characteristics in telescope performance are its aperture (the diameter of the main “tube”) and the sturdyness of the mount/tripod. The aperture is the diameter of the main mirror or lens in the telescope, and the larger it is, the more light it can gather, allowing fainter objects to be seen more brightly and easily. A larger aperture also increases the telescope’s angular resolution, or sharpness, which is especially helpful for planets. 

In a refractor telescope (the type with the lens at the front end of the main tube), look for a minimum diameter of 60 mm. This type of telescope will offer nice views of the brighter planets, the Moon, double stars, and brighter nebulae and clusters. The performance of a telescope improves with the square of the aperture, so an 80 mm telescopes is (80*80)/(60*60) = 1.8 times more powerful than a 60 mm telescope! Nearly double! 

In reflector telescopes (the type with the large mirror inside the bottom of the main tube), diameters of 6 or 8 inches are common and affordable. Because of that aperture scaling rule, they outperform the refractor telescope by showing dimmer galaxies and clusters, especially from a dark sky, while also showing the same brighter objects. 

The second important factor when shopping is the mount, including the tripod. A high quality telescope on a rickety mount is practically useless. Every time you touch it, for focusing or centering an object, you set off a wild shaking of the image. This is one of the main reasons why cheap department store telescopes are poor investments – people get frustrated when they can’t put objects in the field of view. 

(Above: The Skywatcher BK 705AZ3 refractor features a resonable aperture and a sturdy alt-azmount and tripod with slow motion controls.)

Skywatcher makes a good affordable series of starter telescopes. Here’s a 70mm diameter model that ticks all the boxes. They have larger aperture versions, too. Look for the “AZ3” part of the model number. It’s a good, lightweight, and sturdy tripod that includes slow motion controls for following the object as the Earth turns. Any other brand with the same specifications should be okay, too. 

The type of telescope I recommend most for beginners is the Dobsonian style. While larger and heavier, they are simple and easy to set up and use. They are less portable, but are quite rugged and can be transported in the trunk of your car if padded decently. The beauty of the Dobsonian design is the extremely sturdy mounting system – not a tripod., but a swivelling and tilting box that holds the big tube. Once aimed at a target, a simple nudge with your finger will re-centre the object without any undue shaking. These types of telescopes typically show a generous amount of sky in their eyepiece, making finding targets much easier. I’ll post a picture of this type on my Tumblr blog here. 

The best brands of consumer telescopes include Meade Instruments, Celestron, Skywatcher, Orion, and iOptron. Some of the GTA telescope vendors include New Eyes Old Skies, Ontario Telescope, KW Telescope, and Khan Scope. Many of these vendors offer both new and used equipment, which can be a very cost effective solution. Dobsonians in particular can be found used on sites like Kijiji and the Canadian astronomy selling site Astrobuysell. Good used telescope prices range from $250 to $400. If you are buying used, make sure that you try the telescope on a star – to ensure that you can achieve a sharp pinpoint focus. One last tip – the eyepiece does the job of magnifying. So look for telescopes that accept eyepieces with 1.25” diameter barrels – the standard for quality equipment. That way you can add extra eyepieces, or upgrade to better quality ones later. 

Here’s an online beginner telescope buying guide from the Backyard Astronomer's Guide folks.

Public Events

I now have access to an inflatable planetarium and digital projection system for school and group presentation! Send me a note if you want to know more about that. 

Taking advantage of dark moonless evening skies this week, the members of the RASC Toronto Centre will gather for dark sky stargazing at Long Sault Conservation area, northeast of Oshawa on (only) the first clear evening from Monday to Thursday this week. You don’t need to be an RASC member, or own any equipment, to join them. Check here for details and watch the banner on their homepage or their Facebook page for the GO or NO-GO decision around 5 pm each day. 

On Monday evenings, York University’s Allan I. Carswell Observatory runs an online star party - broadcasting views from four telescopes/cameras, answering viewer questions, and taking requests! Details are here. And on Wednesday evenings after dark, they offer free public viewing through their telescopes. If it’s cloudy, the astronomers give tours and presentations. Details are here.

(Above: The annual Ursid Meteor Shower peaks on December 22 before dawn. Meteors can appear anywhere in the sky, but all Ursids will appear to travel away from the radiant above the Little Dipper.) 

Ursid Meteor Shower Peak

The annual Ursid meteor shower runs from December 17 to 23 annually. This year, it peaks during the wee hours of Friday, December 22, when seeing 10- 20 meteors per hour is possible under dark skies. The best time to watch will be from midnight until dawn that morning, but you can also watch after dinner. The moon will be a young waxing crescent that sets in early evening on the peak night, setting up good meteor watching conditions. The shower’s radiant is above the Little Dipper in Ursa Minor (Little Bear), near Polaris, but the meteors can appear anywhere. I wrote about meteor-watching tips last week.

The Moon and Planets

The New Moon phase, when the moon is crossing the imaginary line between Earth and the Sun, occurs at 1:30 am on Monday morning. So the moon will be hidden from view beside the Sun in the daytime sky. The first chance to see the moon again comes Monday at sunset, when its extremely slim silver crescent will be low in the western sky. Over the following few evenings, the moon will wax (grow thicker) and draw away from the Sun, but it will still set early in the evening – giving us dark moonless nights for most of this week. 

If you’re an early riser, you’ve probably been seeing a very bright object shining in the eastern dawn sky. That’s Jupiter, which rises about 4 am local time this week and is well above the southeastern horizon by dawn. Much dimmer and red-tinted, Mars, which rises before Jupiter, at about 3:30 am local time, is sitting to Jupiter’s upper right. On Monday morning, Mars will be about a fist diameter from Jupiter. But over the course of this week, Mars’ eastward orbital motion will carry it towards Jupiter, so the separation between them will shrink to a palm’s width next weekend.

(Above: The pre-dawn sky, shown here for 6:45 am local time on Decmeber 24, 2017, has all the naked eye planets this month - with Mars rising first, followed by Jupiter, and then Mercury. Over the course of this week, Mercury’s orbit swings it away from the sun, making it easy to spot about 6:45 am local time,)

Mercury re-appears in the pre-dawn sky this week. It’s going to be quite easy to see - especially later in the week when it has climbed farther from the sun. Between about 6:45 and 7 am local time, it will be the medium bright “star” sitting low in the southeastern sky. 

With Venus and Saturn hidden near the sun, Uranus and Neptune are the only planets left in the evening sky, setting about 2:30 am and 11 pm local time respectively. Blue-green Uranus is midway between the two chains of stars that form the dim constellation of Pisces (the Fishes). Tiny blue Neptune, only observable in a backyard telescope, is about half a finger’s width below the medium-bright star Hydor in Aquarius (the Water-Bearer).