Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into four sections; what you can see with the naked eye, with binoculars, with a small refracting telescope and with a Dobsonian reflector. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on July 4th. For July, your best viewing nights will be from July 1st to the 9th and July 23rd to the 31st. The Star chart below is set for Florence, SC on July 15th at 9 pm. Note: A table of contents for earlier columns is located in the February 2016 column.
Achromatic versus Apochromatic Lenses:
In last month’s column I mentioned the use of refractors for planetary viewing. In doing so, I mentioned the achromatic versus expensive apochromatic refractors. I did not have the space to discuss the differences between these two types of telescopes; so I will do so now.
When you look at objects in the night sky, they are usually white in color. As such, the white objects contain all the colors of the light spectrum; ROYGBIV. If you were to pass white light through a prism, each color would refract at a different angle, yielding a rainbow/spectrum. When you pass white light through a standard convex lens as found on simple refracting telescopes, it will come together at the focal point. However, in reality, due to color versus angle of refraction, not all the colors meet exactly at the focal point. This is called chromatic aberration. In general, your eyes may not notice this problem, but when viewing a tiny object such as a star or planet through your telescope, the result is lack of good sharpness; the object looks good, but not great. This effect is seen with achromatic telescopes. Apochromatic lenses utilized multiple lenses to correct this problem. The more costly the apochromatic lenses, the tighter the focal point of ROYGBIV. See diagram below.
The easiest way to check out chromatic aberration on your simple refracting telescope is to aim it at the Moon, and focus on the edge of the Moon. You will see that the Moon will have a blue/violet ring around its edge; check it out. You easily spend over $2000 on a large pair of binoculars and still see a blue/violet halo on the edge of the Moon.
Another perimeter affecting clarity is called spherical aberration. This is distortion is caused by how well the lenses are perfectly curved. Spherical aberration is most seen on inexpensive discount store refracting telescopes. If you buy a good achromatic refractor, spherical aberration is usually not a problem. With apochromatic lenses spherical aberration will not be a problem.
You can enjoy your astronomy hobby without ever finding the need to buy apochromatic telescopes. If one ever decides to invest in an apochromatic refractor, they usually invest in a 5 inch (130 mm) scope. The reason is that a 5 inch apo refractor has enough aperture to also see dim deep sky objects, thus not limiting yourself to only the planets and the Moon. Back to money. The 5 inch apochromatic refractor shown below costs $3000, which does not include a diagonal, lenses, tripod, finder, and tripod adaptor.
My suggestion is to spend your money on nice Dobsonians, and big binoculars, and when you go to star parties look through someone else’s apochromatic refractor.
The Summer Sky:
I find the winter and summer constellation to be equally enjoyable. Winter of course features Orion, and summer features Scorpius and Sagittarius. However, unlike winter, in the summer looking in the south toward Scorpius and Sagittarius brings you several other enjoyable sights. You are looking into the center of the Milky Way, therefore you will see a high density of stars, clusters and nebulae. Your biggest challenge will be to locate an area near Florence where you can see the Milky Way. If you can see the Milky Way at all, then with simple binoculars you should be able to locate many nebulae and clusters.Below is an image of many deep sky objects found in the region of Scorpius and Sagittarius.
In the image are objects listed by three different deep sky catalogs. The M numbers are Messier Objects, the NGC numbers are New General Catalog Objects and the IC numbers are from a supplement to the NGC published about 1900 known as the Index Catalogues.
Below is an offset chart adding a few more objects.
So take these two images print them out, and bring them outside to a dark sky site. Use a red light to see the printouts, and then use your binoculars to see how many objects you can find. You will find that you can easily find some of the brighter objects, and others may not be so obvious. Some like M20, the beautiful Trifid Nebula will be almost impossible to see without super dark skies.
If you find a nice dark location, using your naked eye only, see if you can locate the large wispy dark nebula known as the Dark Horse. This dark nebula is seen in both image above, and below, but the image below may help you to better define it. Its hind quarter is on the bottom, and the horse’s legs are to the right.
Note: This month onward, I added Dobsonian sights to the lists below.
Naked Eye Sights: The summer Milky Way and the Dark Horse nebula.
Binocular Sights (7 to 10 power): Follow the charts above and see how many targets you can find. I will also see how much I can find with my handheld binoculars.
Telescope Sights (60-100mm): Saturn continues to be the target for this month.
Dobsonian telescope (6 -8 inch): Saturn, and aim your scope at some of the southern sky objects shown in the images above.
See you next month!