This week in What’s Up is a holiday object, the Christmas Tree Cluster, and also a bit of discussion on telescope accessories. The Christmas Tree Cluster is so named because it vaguely resembles the shape of a Christmas tree. Like many named objects in the sky, it only sort of resembles the thing it’s named after. Humans like naming things after other things they are familiar with.

The Christmas Tree Cluster is in an active star-forming region with a wide mix of stars of different ages. You can visually see stars of different colors, red and blue, when looking at this cluster.

If you could see in the infrared, many more young blue stars would be visible hiding in the gas and dust of the nebula.

The Christmas Tree Cluster, also called NGC 2264, has an apparent magnitude of +3.9, so it may be faintly visible with the unaided eye from darker skies. To find it, draw a line between bright stars Procyon and everyone’s favorite star that hasn’t gone nova yet, Betelgeuse, then go to the midpoint of the line and look slightly above it.

Alternatively, it is slightly lower in declination than the star Castor in the constellation Gemini, in the direction of another bright star, Sirius. It is very much closer to Castor than to Sirius.

But an even easier way is to use the finder chart available from the show notes for today’s episode.

So the Christmas Tree Cluster is fairly easy to find and is visible in binoculars or a small telescope. It’s also quite large at about 20 arcminutes in size. That’s a little smaller than the angular size of the Moon at its furthest distance from Earth, but you won’t be able to make out the Christmas tree shape of the gas around the cluster without a telescope at moderate magnification.

The winter months are great for astrophotographers because the cold temperatures lead to less turbulence in the atmosphere. This makes it easier to take sharper images. The Christmas Tree Cluster is also an easy cluster to image, mainly because of its brightness. It fills a full-frame sensor at 1.25 meters focal length, such as that provided by a 125mm f/10 SCT, like the one Erik has, a Celestron Nexstar 5SE. If you go wider, say 400mm like in a Short Tube 80 refractor, you can also fit a few more nearby emission nebulae into the frame, NGC 2247 and IC 2169.

A really fun way to try out different sensor/optic combinations on different targets you might image before you buy is to use the free astronomy software Stellarium. If you add the specifications of your camera into the Oculars menu in the top right corner of the application, it will project a rectangle on the sky depicting the area and extent that particular combination of optics and sensor produces. The imaging mode looks like a rectangle and is the second option from the left in the menu on the top right. If you’re a visual astronomer, you can also use this feature for eyepieces. The eyepiece mode looks like a hand controller and is the first option on the left in the top-right menu.

Erik’s advice is to not buy a whole bunch of different eyepieces covering every possible power your telescope can produce. If you have a lot of eyepieces, you will end up spending a lot of money but less on each individual eyepiece, meaning you will probably end up with a large number of low-quality eyepieces that will gather dust. 

All you really need are four or five good eyepieces that are separated by about 50 times magnification. For example, Erik has 25mm, 12mm, 8mm, and 5mm eyepieces for his Nexstar 5SE, which provide approximately 50x, 100x, 150x, and 250x magnification, respectively. You can calculate the magnification an eyepiece will produce by dividing the focal length of the telescope in millimeters by the focal length of the eyepiece, also in millimeters.

We recommend the Astro Tech Paradigm or Agena Starguider Dual ED eyepieces as they are relatively inexpensive and have a wider apparent view than the standard eyepieces which come with a beginner telescope.

Be sure to check out the show notes for today’s show on DailySpace.org. We’ll include links to Stellarium and also some of the other gear we mentioned, including Amazon affiliate links so you can help support us.

More Information

NGC 2264 (SEDS)

NGC 2264 (Simbad)