Friday, August 19, 2011

Why aperture matters

When choosing a telescope, one of the most important questions that has to be addressed is the aperture, or diameter of the objective lens or primary mirror. That determines among other things, the size, weight, portability,  resolution and light gathering power a telescope has to offer. The larger the aperture, the more expensive the telescope of any given telescope will be. Furthermore, some types of telescopes are much more expensive than others of the same aperture. If observing galaxies and nebulae is what your telescope will be used for, then a telescope that offers the most aperture, and hence light gathering power is the best choice when observing capabilities alone are considered. There is no nirvana when it comes to aperture, a bigger telescope can always show dimmer objects than a smaller one from the same site. However, your muscles are only so strong, your vehicle only so big, and your bank account only so fat. Henceforth the goal when choosing a telescope is finding one large enough to show you want you want to see, yet is still affordable and isn't too heavy and bulky to transport and use easily.

With all of these factors are taken into consideration, Dobsonians are the best option for folks who want a telescope that can take them to the realm of the galaxies, and yet be affordable, portable and still have excellent optical and mechanical performance. Below are two drawings I made of the same globular star cluster from a heavily light polluted area. One was made through a 6-inch, the other through my 15-inch.

The top drawing I made with my small 6-inch Dob from a light polluted area at 60X. As you can see, it is unresolved and does not look at all like a globular cluster. From my light polluted front yard, it looks more like a comet, and even at high magnification only a few stars around the edges appear.

This is the same object from my light polluted front yard through the 15-inch telescope at 250X, which fully resolved it into stars and revealed clearly the star like inner core of thousands of stars packed into a region a few light light years across. In both cases I had the glare of thousands of streetlights lighting up the sky all around me. As you can see, a larger telescope does more even from a light polluted area, but it will be of much greater benefit from a dark site than a light polluted one.

Saturday, August 13, 2011

More drawings of deep sky objects

After my computer failed for good and I had no way to scan drawings for two months, I purchased a new computer and scanner and hence scanned the latest drawings I have made with the 15-inch telescope. While most were made from the airstrip, several were done from my driveway in the middle of heavy light pollution.

 M-15 is a compact but very bright globular star cluster in Pegasus, the only object of it's kind in fact that exists in the constellation. At 142X it was a small but very bright object that resembled a tiny pinch of salt or sand on dark fabric with the central core being a very tiny but very bright and star like. It is from this dense central region that X-rays are pouring out, possibly due to the presence of a black hole there feeding on interstellar gas and dust. At 250 and 400X the cluster becomes a swarm of glittering stars and at 572X the stars nearly fill the field of view. This 6th magnitude object is some 35,000 light years away and also possesses one of two planetary nebulae known to exist in globular star clusters. Known as Pease 1, I did not see it because it's barely non-stellar and the seeing was fair, effectively hiding it. Even so, to see it will require at least 500X to distinguish it from surrounding stars.

 M-2 is the brightest deep sky object in Aquarius and is the brightest of three globular clusters found there, the others being M-72 and the extremely faint NGC-7492. This star cluster only looks somewhat bigger than M-15, even though it is larger in reality due to the distance of more than 50,000 light years from Earth. Through the 15-inch no dense central core resembling the one in M-15 was evident, but the star cluster resolved nicely into stars at 250X.

M-51 is one of my favorite galaxies, and I look at it and it's SO companion NGC-5195 every chance I get. Through the 15-inch spiral arms are plainly apparent, along with brighter condensations that mark where star formation is underway and massive associations of O and B-type blue giants dominate the spiral arms. Recently however, M51 hosted yet another supernova, a type IIb and the third to appear in the past 15 years. The supernova occurred in the outer spiral arm and was plainly visible as a 14th magnitude start like dot the first time I looked for it. By the time this drawing was made, it brightened to a magnitude of 12.1. It is now fading and is back below 14th magnitude. In addition to the supernova and the spiral arms, hints of the "bridge" between M-51 and NGC-5195 was visible, along with NGC-5195's odd shape and structure. The nucleus of M-51 is bright and star like, with both galaxies and the supernova a splendid sight at 250X.

The pair of Messier galaxies M-59 and M-60 has always been a favorite sight for me when I had only a 6-inch telescope, and I wanted to revisit this field with the 15-inch. Two NGC-galaxies also appeared in the view and both can also be spotted with a 6-inch if the skies are reasonably clear and dark. All four of them are ellipticals with bright inner cores, resembling comets or unresolved globular star clusters instead of huge galaxies that are more massive than our own several times over. At 83X, this is a nice example of a quartet of galaxies.

M-87 is the most massive member of the Virgo cluster of galaxies and it is also the brightest radio source in Virgo as well, giving it the designation of Virgo A. This is a massive elliptical galaxy that swallowed up dozens of other galaxies over it's lifetime and also possesses over 1,000 globular clusters. In the center there is a super massive black hole that is spewing out jets of plasma at nearly the speed of light and this jet has been seen in telescopes as small as 16-inches in aperture. Through the 15-inch at the airstrip, I saw a large round object with a very bright inner core. Flanking it are two smaller elliptical galaxies, NGC-4476 and NGC-4478. This trio of galaxies makes an impressive sight at 217X through a large telescope.

The pair of bright elliptical galaxies M-84 and M-86, together with numerous other galaxies form the very heart of the Virgo Cluster of galaxies. These giant ellipticals, along with NGC-4435, NGC-4438, NGC-4387, NGC-4458, NGC-4461, NGC-4473 and a few others form "Markarian's Chain" named after an Armenian astrophysicist who found these galaxies are bound together as a small group of galaxies in the very center of the Virgo Cluster. Forming an arc two degrees long that extends into Coma Berenice's, this chain of galaxies is a marvelous assortment of elliptical and spiral galaxies for even a small telescope.

NGC-4435 and 4438 are an interesting pair of galaxies that have been interacting with M-86. Dubbed the eyes, NGC-4435 clearly shows signs of being disheveled by it's run in with the more massive elliptical, and traces of the structure seen in photos was apparent through the eyepiece. In contrast to NGC-4435, NGC-4438 has a more normal spiral form, since it has not been tugged on by M-86. The pair nicely fit in the field of an 8mm wide field eyepiece at 217X

M-80 has always been a challenging globular cluster for me even though it is very bright and easy to find. The same cannot be said for resolving it's stars, given that it's highly condensed. Through a 6-inch it looks like a comet. stars start to show up around the edges through a 10-inch on good nights, but through the 15-inch I saw it clearly as a globular cluster for the first time. It's even more concentrated than M-15, and looks very similar too, right down to the tiny, bright, star like inner core at 250X. This globular also has the distinction of being one that hosted a supernova in the center back in 1860. Probably a type 1A, the blast outshone all of the other stars put together at maximum. This object can be seen in just about any telescope, and is one of the best deep sky objects in Scorpius for owners of small telescopes who live where Scorpius does not scrape the horizon.

NGC-4458 and NGC-4461 are two more members of Markarian's Chain. Both are spiral galaxies of normal appearance and form. Fainter than NGC-4435 and 4438, they nevertheless are a nice pair of  galaxies for a 15-inch telescope at 217X.

NGC-4473 is another bright spiral galaxy that also belongs to Markarin's Chain. A single isolated galaxy, this spiral is larger and brighter than NGC-4458 and NGC-4461 with a bright core. A fine object for a 10-inch telescope, it is even better through a 15-inch at 217X.

NGC-5728 is one of a number of moderately bright and fainter galaxies in the constellation Libra, which is marked by the bright stars Zubenelgenubi and Zubeneschamali between Scorpius and Virgo. This barred spiral galaxy was small, oval shaped with a condensed, bright core. No sign of the spiral arms was apparent but the central bar gave it the appearance of an edge on galaxy. In spite of what was average sky conditions. NGC-5728 was a fairly easy and nice object through the 15-inch at 250X.

Another Libra galaxy is NGC-5812, an elliptical galaxy that is nearly round with a bright core. Easy to spot due to high surface brightness, this is probably the easiest galaxy for smaller telescopes in Libra. Resembling a comet more than anything else, it's a nice catch at 250X and can tolerate more magnification than that under good sky conditions.

NGC-5879 is one of many galaxies accessible to medium and large aperture amateur telescopes in the constellation Draco. This spiral galaxy is small in apparent size but has a reasonably high surface brightness, making it easy to find with the 15-inch. The bright inner core is distinct and the disk itself is bright enough to show up easily.

NGC-5981, 5982 and 5985 are a trio of galaxies in Draco that consists of a face-on spiral, elliptical and a edge on spiral galaxy. At 153X all three fit into the field of view nicely, with the elliptical galaxy NGC-5982 being the brightest of the three. I have always been able to see NGC-5982 and 5985,  which is the face-on spiral with a 10-inch, but the third galaxy has been elusive. Apparently the edge on spiral NGC-5981 is a lot fainter than it's apparent magnitude suggests, it was faintly visible through the 15-inch. This is a very nice trio for a medium to large aperture telescope.

One of my favorite deep sky objects has always been the Saturn Nebula or NGC-7009. Even through a 6-inch I can see the strong blue color and the distinctive outline of this planetary nebula. What I never seen clearly until now was the Saturn like ring, the ansae or the central star. The ansae did not appear because I was observing the nebula from the driveway, but the central star and ring did appear at 400X. The electric blue color was intense and oddly enough the view was not that much better through an O-II nebula filter either from home.

These are just a few of the many nebulae, star clusters and galaxies I have observed recently with the 15-inch, which has exceeded my expectations in every respect. I'm very pleased with how my handiwork has turned out, and will continue to observe and draw more deep sky objects with it in the future along with the 10-inch telescope. That telescope is much more convenient to use, but it doesn't show galaxies nearly as clearly as the 15-inch does. Even so, both telescopes as well as my original 6-inch are going to remain in active use for many years to come.

Friday, August 12, 2011

A marvel in the darkness

While a potentially very bright comet has just been found by the PANSTARRS telescope, there is another good comet for small telescopes and binoculars visible right now in the northern sky, C/2009P-Garrad. Two weeks ago it passed the bright globular cluster M-15 in Pegasus as it moves westwards towards the summer Milky Way. This comet will be unusual in that it is expected to stay bright enough for amateur astronomers to follow it visually with modest telescopes for months to come. It will stay high in the sky for months and away from the glare of the Sun. However it will pass close to the north celestial pole which for some will keep it near the horizon at that time. I made this drawing with my 15-inch truss-tube from the usual site at a semi-rural airstrip to the west of my city. At 142X the beginnings of a tail were plainly visible, and the comet itself was easy to see despite the rather poor skies that night. Currently the moon is too bright and the skies are too hazy for it to be worth the effort to look for the comet here, but in a week's time the moon will no longer interfere and observation will be possible, weather permitting. The Perseid meteor shower is also now peaking, but unfortunately the full moon will drown out all but a few of them. So I'm going to wait until next year to watch for them since the moon will be a thin waning crescent next year on August 12th. The comet however will be visible through much smaller telescopes than the 15-inch, which I was using that night mainly to look at galaxies and nebulae. For more information about the comet that had recently been discovered, refer to the links below. There is the possibility this one might become the second great comet of the twenty first century because it may become visible during the daytime! The first great comet of this century was McNaught, which blazed across the southern sky back in 2007 and was seen during the daytime from the northern hemisphere too.

I will be posting other drawings soon I made of galaxies, nebulae and star clusters through the 15-inch, and I will also include drawings of the same objects through small telescopes to illustrate why aperture matters.