I’m in dark skies, eastern Canada with my telescope. The Milky Way is quite apparent. Based on past advice here on the boards, I’m working my way through the Messier objects. The faintest thing I’ve found over the past few days is The Cigar Galaxy (apparent magnitude 8.4). How much fainter objects can I expect to find than that? For a noob like myself, it takes a fair amount of time to star hop my way to these things, so if it ain’t gonna happen, I don’t want to bother banging my head against the proverbial wall.
And just for clarity, I’m having a blast finding what I’ve found so far. I don’t want anyone thinking I’m getting frustrated or discouraged. Far from it!
What kind of telescope are you using and what’s the aperture?
And where precisely in Easter Canada? With truly dark skies (extremely rare in North America, but there are a handful of spots left), and with a 20" scope (which is within the range of a few backyard astronomers, but only just), you can see damn near everything.
Apparent magnitude measures the total brightness of an object. But what really matters is the surface brightness of the object.
For example, Barnard’s Loop is magnitude 5, but that light is spread over a huge area (about half the length of the constellation Orion) that it’s impossible to see with any telescope or binoculars. (Telescopes don’t amplify surface brightness, they just make things appear bigger.) On the other hand, the Eskimo Nebula is magnitude 10.1, but that light is concentrated in a tiny circle, smaller than the angular size of Jupiter. So it’s actually very easy to see. I routinely found it from downtown Boston with a 10" telescope.
I’m not sure if there’s a deep-sky catalog that lists surface brightness though.
Thank you for your answers!
Willcross Basic Nexstar 4"
Chronos The Maritimes. It’s dark but not crazy dark. There is still some light pollution from the small towns in the area and a city about 40 km away.
scr4 Thank you for the perspective. I hadn’t heard of the Eskimo Nebula. If I’m reading correctly Gemini isn’t visible in the North until later in the month. I’ll certainly look for it when I’m home.
Just saying I’m going to follow this thread.
I live pretty remote at 11,200 feet in elevation. The nearest town of 400 people is 4 miles away. Then a bigger town of 3000 is 15 miles away. A city, Denver is 100 miles away.
One of my biggest issues is where I can look TO. I am pretty much surrounded by mountains and trees. Can’t look north at all. Very small window. Google sky helps a lot.
D18: It sounds as if you’re in to ‘deep sky’ observing more than anything else. Believe me, I can relate! I spent hundreds, probably thousands, of hours searching for non-Messier galaxies, globulars, and the like. And, just as I think you were getting at, I’d say the search itself was great fun. It is tremendously satisfying to locate a faint object you’ve been searching for.
That said, it was my mistake to neglect ‘near sky’ projects/targets, i.e. things like double (and triple) stars, variable stars, etc. You have a Nexstar and, as a Maksutov-Cassegrain, it should be a great instrument for just that type of observing.
Have fun!
KarlYes, it is a blast! Sounds like you’ve got a pretty comprehensive sense of the night sky. You’ve got a good idea, and I’ll be sure to start doing that the next clear night. I’ve had the telescope for two years, but this is the first time I’ve gotten it to a dark, non-suburban sky, so I glommed onto the sexier stuff.
I believe you’re mistaken about that, despite your obviously deeper understanding of astronomy than mine.
The bigger the objective, the more light gathered, and thus the brighter the objects appear (at any given magnification).
To see dimmer objects, get a bigger telescope. I grew up using an Edmund Scientific 4 1/4" reflector, and had a lot of fun with that as a kid. Telescopes like that are best for seeing relatively small targets, rather than the big faint stuff.
That’s a nifty telescope. What I wouldn’t have given for something like that as a kid! Like my childhood telescope, it’s best for seeing fairly small targets.
Consider taking another approach: a big, inexpensive azimuth-mounted Newtonian. (Keep the Nexstar! But add another tool to the kit.) This is for seeing the rather large but faint (and remarkably colorful) targets. I confess I don’t know a lot about these, but someone I worked with described it in detail and it seemed very interesting. I didn’t take it up for practical reasons, but I’ve always wanted to. Unfortunately, I live in the city and have a wooded lot, no place to use a telescope.
The idea is that you can use a relatively simple mirror. For high powers, you need a parabolic mirror. You can get by with elliptical, which I believe my Edmund had, at medium powers. But for low power (up to 10x, IIRC), even a spherical mirror will do, and these are inexpensive. They sell kits where you grind it yourself and send it back for polishing. The polishing is the expensive part, but they know only a small percentage will ever be completed and sent back for polishing, so it ends up being very affordable if you have the patience. [PS: this is old info; hopefully someone here will pipe up with current situation.]
Since you’re not using high power, the target doesn’t move out of the field quickly, so an inexpensive azimuth mount is sufficient. (That’s as opposed to an equatorial mount, where you can keep the telescope pointed at the object by rotating on one axis parallel to the poles as the Earth rotates. Your Nexstar has an azimuth mount, because the computer can move on two axes at once. Without a computer, you’d want an equatorial mount for that scope. Yay computer!)
Oops. I omitted an important point: you can use a relatively simple but LARGE mirror (say, 12"). This allows you to gather a LOT of light and see faint but large objects, on a budget that’s less than your Nexstar.
I like the look of that Nexstar. I’m tempted to put one on my wish list! The problem is, where I would use it, I’m only there one week a year.
Don’t feel bad; this is an incredibly common misconception, so common that I myself suffered under it until CalMeacham corrected me. A larger aperture does collect more light, and this does allow for greater total brightness of an image… but it does this by allowing for greater magnification at (at best) the same surface brightness. This isn’t usually noticeable in astronomy, since so many objects of astronomical interest are effectively point sources, and so magnification doesn’t appear to increase the apparent size of an object. But it still applies: A telescope which did increase the surface brightness of its target would violate the Second Law, and in this house, we obey the laws of thermodynamics.
I think I’m not understanding total brightness versus surface brightness, then. Given that the apparent diameter of the Andromeda Galaxy is already much greater than the full Moon’s, how is it that a telescope makes it visible?
That’s not a matter of the telescope itself, but of long exposures on the camera attached to it. You could do something similar with an ordinary point-and-shoot, if it were mounted on a clock drive (though you’d have a harder time keeping stray light out).
And because many extended objects (galaxies & nebulae) have fairly high surface brightness, they’re just too small for naked human eyes to see. So just magnifying them is enough to allow us to see them.
Also, a large faint object is easier to see than a small and equally faint object.
The core of the Andromeda Galaxy is much smaller. And that’s about the only part of Andromeda Galaxy you can see under light-polluted skies, even with a telescope. Under a dark sky, the Andromeda Galaxy is visible to the naked eye.
One other thing, the telescope cannot amplify surface brightness, but it can definitely reduce the surface brightness if you use too high magnification. The optimal point (maximum magnification where surface brightness is not reduced) is where the exit pupil of the telescope matches your eye’s pupil diameter. This is usually given as “up to 7mm”, but of course it depends on the individual, and on the conditions.
Even with perfect skies and perfect human vision, you’re not going to see much more than the core of the Andromeda Galaxy. What you see in photos, that’s larger than the full moon, requires a long exposure.
What exactly do you mean by “surface brightness”? Clearly the larger the aperture for a given focal length, the brighter the image – the classic photographer’s f-stop. Which is precisely why large mirrors or lenses are necessary for photographing faint objects which otherwise simply wouldn’t appear within the available exposure time. Obviously a telescope can never increase the intrinsic per-square-foot brightness of an object that you would see if you were standing right in front of it. But a statement like “Telescopes don’t amplify surface brightness, they just make things appear bigger” seems confusing; for the ordinary meaning of “brightness” that would be more a statement about the eyepiece than the aperture – larger magnification eyepieces make the object larger but dimmer and less resolved, but larger apertures always make it brighter than smaller apertures (for a given focal length).
I mean apparent surface brightness, which is the amount of light entering your eye per unit solid angle. Watt per square degree, or photons per second per square degree, or watt per steradian.
Yes, but no amount of aperture will make an object brighter (in apparent surface brightness) than what you see without the telescope. So if you were looking at a huge white wall through a telescope, at very low power, the wall will look like it does without the telescope (not brighter or dimmer). If you increase the power, it will become dimmer. No telescope in the world can make the wall look brighter than it does to your naked eyes.
(Obviously I’m only talking about passive, optical telescopes here. Electronic night-vision systems are another matter.)
p.s. Note that by “telescope”, I mean a visual telescope including the eyepiece. And we’re talking about the brightness of the virtual image a human eye sees in the eyepiece. There is no “F stop” for a virtual image.