Do Dark Skies Really Matter?
Hong Kong is bad. Do not live in Hong Kong.
As somebody who gets asked a ton of questions in this hobby, the number one question I get is, "What can I see through my telescope?"
I find this as an interesting question, mostly because I believe this is something you should know BEFORE you buy your telescope. But because I am a pragmatist and I don't feel that there's any benefit to telling people how much they "screwed up," I will always try to help them get the most from the scope they have purchased.
I believe this question is derived due to a certain amount of expectations on behalf of the purchaser. Perhaps they took their new telescopes outside, probably in the middle of the city during a full moon, and then they couldn't understand why they couldn't see anything (other than the moon)? Perhaps they've seen a bunch of Hubble images and thought it should look like that? Or maybe they went to a local star party and bought their scope based upon some views through other scopes, only to find out that they cannot repeat what they saw?
Actually, I believe, simply, it is because these people (any observer really) probably lacks the proper perspective of what a telescope can really do under dark skies.
SIGNAL-to-NOISE RATIO FOR OBSERVERS
As an astroimager, I have come accustomed to computing the signal-noise ratio (S/N) for my images...or in the least, using this principal as a guiding thought for achieving the results I desire. However, S/N isn't just for imagers. An understanding of it yields a nice benefit for observers as well.
"Signal" is the easy part. It is the collection of wanted photons onto the visual receptors (or CCD pixel). But there is an ideal position (or pixel) where we expect to witness any given photon or photons. For example, collectively, if what we are seeing is a star, then we would expect to see a bright point of light, with diffraction rings and/or spikes radiating uniformly from the center. When our eyes do not witness this, then a savvy observer will understand that there is something hindering the "ideal" visual image. Could it be poor focus? Bad collimation? Horrible seeing? Tube currents?
When the photon misses its ideal location, this adds a degree of uncertainty in the image, which is technically what we call "noise" in our visual image. The take-away point here, for those hoping to see more through their scopes, is that optical quality, scope setup/alignment, hardware quality, and scope acclimatization are HUGE players in what you see and what you do NOT see.
You perceive this less on high S/N objects, like stars. Rather, it frustrates more on the low S/N objects, like the extended galaxy arms that you hope to see.
The nice thing about being a visual observer is that S/N is actually a moving target...a fluctuating thing. If you have given yourself the opportunity to SEE quality signal by taking care in setup and execution of the observation, then patience can reward you with glimpses of excellence, when the signal passes through the atmosphere quite untouched!
This is why experience as a telescope user is so important...veterans can greatly improve their potential to see quality signal by lessening the waste of photons spread away from the "ideal" location as noise.
So, keep your head still and wait until the object "snaps" into view.
LIGHT POLLUTION
So how does light pollution play into this? Certainly, that is noise, right?
No...not at all. It too is "signal"...it's just signal that's unwanted! It is a competitor to the object signal you are hoping to see. And in most cases this unwanted signal is competing strongly at the limiting threshold of our instruments, where "wanted" object signal is extremely faint. Limiting the "noise" by being a good observer gives us a shot of object detection when the atmosphere relents (giving us temporary moments of higher S/N), but wouldn't it be nice if that glowing moon or local stray light wasn't trying to crash the party!
And this is where DARK SKIES REALLY MATTER. If there is no light pollution to compete against the object signal, then there will be greater opportunity to detect the lower S/N parts of an object...the things we are really wanting to see.
ANECDOTAL EVIDENCE
So, for those who struggle with seeing objects from their present locations, let me state STRONGLY the importance of finding a dark sky site before you throw in the towel with your telescope. Using your scope in dark skies, if only ONCE, will provide you with the perspective of what is possible, even with the smallest of scopes in our hobby. You come away fully understanding the capabilities of the telescope you purchased, which are FAR GREATER than your city-observing-views have indicated.
Allow me to share some anecdotal evidence, something I wrote many years ago after spending time in some of America's darkest skies:
I find this as an interesting question, mostly because I believe this is something you should know BEFORE you buy your telescope. But because I am a pragmatist and I don't feel that there's any benefit to telling people how much they "screwed up," I will always try to help them get the most from the scope they have purchased.
I believe this question is derived due to a certain amount of expectations on behalf of the purchaser. Perhaps they took their new telescopes outside, probably in the middle of the city during a full moon, and then they couldn't understand why they couldn't see anything (other than the moon)? Perhaps they've seen a bunch of Hubble images and thought it should look like that? Or maybe they went to a local star party and bought their scope based upon some views through other scopes, only to find out that they cannot repeat what they saw?
Actually, I believe, simply, it is because these people (any observer really) probably lacks the proper perspective of what a telescope can really do under dark skies.
SIGNAL-to-NOISE RATIO FOR OBSERVERS
As an astroimager, I have come accustomed to computing the signal-noise ratio (S/N) for my images...or in the least, using this principal as a guiding thought for achieving the results I desire. However, S/N isn't just for imagers. An understanding of it yields a nice benefit for observers as well.
"Signal" is the easy part. It is the collection of wanted photons onto the visual receptors (or CCD pixel). But there is an ideal position (or pixel) where we expect to witness any given photon or photons. For example, collectively, if what we are seeing is a star, then we would expect to see a bright point of light, with diffraction rings and/or spikes radiating uniformly from the center. When our eyes do not witness this, then a savvy observer will understand that there is something hindering the "ideal" visual image. Could it be poor focus? Bad collimation? Horrible seeing? Tube currents?
When the photon misses its ideal location, this adds a degree of uncertainty in the image, which is technically what we call "noise" in our visual image. The take-away point here, for those hoping to see more through their scopes, is that optical quality, scope setup/alignment, hardware quality, and scope acclimatization are HUGE players in what you see and what you do NOT see.
You perceive this less on high S/N objects, like stars. Rather, it frustrates more on the low S/N objects, like the extended galaxy arms that you hope to see.
The nice thing about being a visual observer is that S/N is actually a moving target...a fluctuating thing. If you have given yourself the opportunity to SEE quality signal by taking care in setup and execution of the observation, then patience can reward you with glimpses of excellence, when the signal passes through the atmosphere quite untouched!
This is why experience as a telescope user is so important...veterans can greatly improve their potential to see quality signal by lessening the waste of photons spread away from the "ideal" location as noise.
So, keep your head still and wait until the object "snaps" into view.
LIGHT POLLUTION
So how does light pollution play into this? Certainly, that is noise, right?
No...not at all. It too is "signal"...it's just signal that's unwanted! It is a competitor to the object signal you are hoping to see. And in most cases this unwanted signal is competing strongly at the limiting threshold of our instruments, where "wanted" object signal is extremely faint. Limiting the "noise" by being a good observer gives us a shot of object detection when the atmosphere relents (giving us temporary moments of higher S/N), but wouldn't it be nice if that glowing moon or local stray light wasn't trying to crash the party!
And this is where DARK SKIES REALLY MATTER. If there is no light pollution to compete against the object signal, then there will be greater opportunity to detect the lower S/N parts of an object...the things we are really wanting to see.
ANECDOTAL EVIDENCE
So, for those who struggle with seeing objects from their present locations, let me state STRONGLY the importance of finding a dark sky site before you throw in the towel with your telescope. Using your scope in dark skies, if only ONCE, will provide you with the perspective of what is possible, even with the smallest of scopes in our hobby. You come away fully understanding the capabilities of the telescope you purchased, which are FAR GREATER than your city-observing-views have indicated.
Allow me to share some anecdotal evidence, something I wrote many years ago after spending time in some of America's darkest skies:
"Objects that can be marginally viewed at my home (mag 4.5 skies) in even the best conditions, and with the biggest of my scopes (10" LX200), are clearly noticeable with the smallest of my scopes’ FINDERS in these magnitude 8 skies. As an example, while my 10” LX200 was guiding on photographic objects, I was busy starhopping to some of the skies more prominent features with my 3” Tak. As I attempted to “hop” to M81 and M82, I discovered that hopping was hardly necessary. Using my 6x30mm Tak finderscope (!), I moved the scope to where I expected the objects to be from an angular measure to the Big Dipper. Much to my surprise, M81 and M82 showed up very clearly in that very small, albeit sharp finderscope! Looking at the objects through the 3” scope made my jaws drop. Both showed their entire surface shape at low and medium magnifications, though spiral structure in M81 did manage to allude me.
"I then decided to couple my Nikon F2 (film) camera to the Tak to catch a shot of both galaxies in the same field. Since my Nikon has the 6x magnification prism attached to it I knew I could frame the objects easily on the camera’s viewfinder. Much to my pleasure, BOTH galaxies were visible through the camera! I easily framed the shot, focused, and took a couple of 10 to 15 minute unguided exposures (I didn’t have a way to use my STV with the Tak), hoping that my GM-8 mount was accurately polar aligned. The results of that experiment will be known after I get the E200 slides developed [update : it didn't work]. But I ask you, has anybody here ever seen a Messier galaxy (aside from M31) through the viewfinder of your camera? That’s how dark these skies are."
- written after the 2002 Texas Star Party near Fort Davis, Texas
"I then decided to couple my Nikon F2 (film) camera to the Tak to catch a shot of both galaxies in the same field. Since my Nikon has the 6x magnification prism attached to it I knew I could frame the objects easily on the camera’s viewfinder. Much to my pleasure, BOTH galaxies were visible through the camera! I easily framed the shot, focused, and took a couple of 10 to 15 minute unguided exposures (I didn’t have a way to use my STV with the Tak), hoping that my GM-8 mount was accurately polar aligned. The results of that experiment will be known after I get the E200 slides developed [update : it didn't work]. But I ask you, has anybody here ever seen a Messier galaxy (aside from M31) through the viewfinder of your camera? That’s how dark these skies are."
- written after the 2002 Texas Star Party near Fort Davis, Texas
When a variety of mirror sizes are stacked like this, it's easier to see how much more area there is to collect light with bigger "apertures." Doubling the diameter yields 4x more light grasp.
There are a couple of points you should understand based on reading my statement here. First, notice that my evaluation of the quality of my skies used objective, data-driven language. At the point of making this observation, I was into my 6th year as an observer. By this time, I had been in the habit of documenting my observations based on my ability to see particular stars with the "naked eye." I had a good perspective with regard to what "poor," "average," and "great" means because I was using an actual, measurable standard (stellar magnitudes).
Second, I knew how to find objects (star-hopping) and I had expectations that came from familiarity with the sky and the objects themselves. I was very capable of relating what my eyes saw with what they had seen before AND the ability to gauge how "find-able" they were in context.
Finally, I had a variety of instruments with which to conduct my observations. I had a 10" SCT, a 3" refractor, two "finder scopes," a camera viewfinder, not to mention a pair of binoculars on the side. Thus, my understanding of what I was seeing was based on the size of the APERTURE being used...and when used together on a given night I had the perspective of what each instrument delivers.
The take away point for you, the reader, is that it takes time to build this perspective of what you can see through a scope and what dark skies can do for you. It also means that you need a framework for objectively evaluating your observations if you hope to make everything practical.
OBJECTIVE EVIDENCE
So, if you have raised this question based upon an initial impression of your first experience looking through the eyepiece, then you need to understand that it gets MUCH better with time and experience.
But how much better? And is there a way that we can understand and measure this, rather than just taking my own anecdotal evidence as the gospel truth?
Certainly there is, but that requires you gaining an understanding of what the magnitude scale is and what it means when you look through the eyepiece.
The Magnitude Scale
The limit of what you can see in any scope is a function of how dark the sky is and how big of aperture you are looking through.
There is a way to measure how much light "aperture" collects. The amount of light collected by a single instrument will be the area of the optic itself. The difference of light "grasp" between any two telescopes can be calculated simply by dividing the apertures of the two scopes and squaring them. So when comparing, for example, a 100mm (4 inch) scope with a 200mm (8 inch) scope, you get 4 times as much light through the bigger instrument.
But to understand how this relates to the stars and other celestial objects we need to understand how the magnitude scale works.
Think of your own eye ALSO as a telescope. The aperture of that scope is 8 mm - if you are young and less if you are older - which is the size of your pupil (aperture) when fully dark adapted. With this eye, you will see stars at a particular magnitude depending upon the light pollution conditions of your site. Perhaps at home you can see magnitude 2 stars with the naked eye, where I would assume you are living right in the middle of a big city. But if you take that same eye out to the dark country-side you will see stars somewhere in the area of 7 or 8 magnitude.
For every decrease in magnitude (i.e. from mag 1 to mag 2), it represents a 2.512 times decrease in detectible light. Or better said, a 1st magnitude star will be 2.512 times brighter than a 2nd magnitude star. This number is derived from a logarithm that says there is a 100 times increase of light for every 5 magnitudes (which is 2.512 to the 5th power). Therefore, a 6th magnitude star will be 100 times dimmer than a 1st magnitude star.
So, how much more light can you collect just by driving a few hours into the country-side?
It can be charted as follows using the star Vega, at Magnitude 0, as a point of reference:
Second, I knew how to find objects (star-hopping) and I had expectations that came from familiarity with the sky and the objects themselves. I was very capable of relating what my eyes saw with what they had seen before AND the ability to gauge how "find-able" they were in context.
Finally, I had a variety of instruments with which to conduct my observations. I had a 10" SCT, a 3" refractor, two "finder scopes," a camera viewfinder, not to mention a pair of binoculars on the side. Thus, my understanding of what I was seeing was based on the size of the APERTURE being used...and when used together on a given night I had the perspective of what each instrument delivers.
The take away point for you, the reader, is that it takes time to build this perspective of what you can see through a scope and what dark skies can do for you. It also means that you need a framework for objectively evaluating your observations if you hope to make everything practical.
OBJECTIVE EVIDENCE
So, if you have raised this question based upon an initial impression of your first experience looking through the eyepiece, then you need to understand that it gets MUCH better with time and experience.
But how much better? And is there a way that we can understand and measure this, rather than just taking my own anecdotal evidence as the gospel truth?
Certainly there is, but that requires you gaining an understanding of what the magnitude scale is and what it means when you look through the eyepiece.
The Magnitude Scale
The limit of what you can see in any scope is a function of how dark the sky is and how big of aperture you are looking through.
There is a way to measure how much light "aperture" collects. The amount of light collected by a single instrument will be the area of the optic itself. The difference of light "grasp" between any two telescopes can be calculated simply by dividing the apertures of the two scopes and squaring them. So when comparing, for example, a 100mm (4 inch) scope with a 200mm (8 inch) scope, you get 4 times as much light through the bigger instrument.
But to understand how this relates to the stars and other celestial objects we need to understand how the magnitude scale works.
Think of your own eye ALSO as a telescope. The aperture of that scope is 8 mm - if you are young and less if you are older - which is the size of your pupil (aperture) when fully dark adapted. With this eye, you will see stars at a particular magnitude depending upon the light pollution conditions of your site. Perhaps at home you can see magnitude 2 stars with the naked eye, where I would assume you are living right in the middle of a big city. But if you take that same eye out to the dark country-side you will see stars somewhere in the area of 7 or 8 magnitude.
For every decrease in magnitude (i.e. from mag 1 to mag 2), it represents a 2.512 times decrease in detectible light. Or better said, a 1st magnitude star will be 2.512 times brighter than a 2nd magnitude star. This number is derived from a logarithm that says there is a 100 times increase of light for every 5 magnitudes (which is 2.512 to the 5th power). Therefore, a 6th magnitude star will be 100 times dimmer than a 1st magnitude star.
So, how much more light can you collect just by driving a few hours into the country-side?
It can be charted as follows using the star Vega, at Magnitude 0, as a point of reference:
Magnitude |
Light Throughput |
Example |
-26 |
25.2 billion times brighter |
Sun |
-12 |
63,130 times brighter |
Full Moon |
-4 |
39.9 times brighter |
Venus |
-3 |
15.9 times brighter |
Jupiter |
-2 |
6.3 times brighter |
Sirius (-1.46 magnitude) |
-1 |
2.512 times brighter |
Canopus (-0.74) |
zero |
- |
Vega (+.02) |
1 |
2.512 times fainter |
Aldebaran (+.86) |
2 |
6.3 times fainter |
Polaris (North Star, +1.98) |
3 |
15.9 times fainter |
Alcyone (+2.87) |
4 |
39.9 times fainter |
M42 (Orion Nebula) |
5 |
100 times fainter |
M22 (Globular Cluster) |
6 |
251.2 times fainter |
Uranus |
7 or 8 |
Over 631 times fainter |
Naked eye limit in dark skies |
To compute your own table, the following formula should be used:
magnitude difference = log(change in brightness) multiplied by 2.5
Beyond the naked eye limit, we obviously need a little bit of help.
A 100mm (4 inch) telescope can see 156 times the amount of light that can be seen with the naked eye. According to the magnitude scale, this equates to approximately 5.2 magnitudes of improvement over the naked eye. Therefore, if you can see 7th magnitude objects with the naked eye, then you should expect to see at least 12th magnitude objects with a 100mm (4 inch) telescope. The aforementioned 200mm (8 inch) scope will show a 7 magnitude improvement over the naked eye. Therefore, if you can see 7th magnitude objects with the naked eye, then you should expect to see at least 14th magnitude objects with a 200mm (4 inch) telescope.
Thus, depending upon the sky conditions, you should expect the limiting magnitude of any telescope as follows:
A 100mm (4 inch) telescope can see 156 times the amount of light that can be seen with the naked eye. According to the magnitude scale, this equates to approximately 5.2 magnitudes of improvement over the naked eye. Therefore, if you can see 7th magnitude objects with the naked eye, then you should expect to see at least 12th magnitude objects with a 100mm (4 inch) telescope. The aforementioned 200mm (8 inch) scope will show a 7 magnitude improvement over the naked eye. Therefore, if you can see 7th magnitude objects with the naked eye, then you should expect to see at least 14th magnitude objects with a 200mm (4 inch) telescope.
Thus, depending upon the sky conditions, you should expect the limiting magnitude of any telescope as follows:
Scope size (times naked eye) |
City Skies (magnitude 2) |
Suburban (magnitude 4.5) |
Country (magnitude 7) |
50mm/2 inch (39x) |
6.0 |
8.5 |
11.0 |
75mm/3 inch (88x) |
6.9 |
9.4 |
11.9 |
100mm/4 inch (156x) |
7.2 |
9.7 |
12.2 |
125mm/5 inch (244x) |
8.0 |
10.5 |
13.0 |
150mm/6 inch (352x) |
8.4 |
10.9 |
13.4 |
175mm/7 inch (479x) |
8.7 |
11.2 |
13.7 |
200mm/8 inch (625x) |
9.0 |
11.5 |
14.0 |
225mm/9 inch (791x) |
9.3 |
11.8 |
14.3 |
250/10 inch (976x) |
9.5 |
12.0 |
14.5 |
275mm/11 inch (1182x) |
9.7 |
12.2 |
14.7 |
300mm/12 inch (1408x) |
9.9 |
12.4 |
14.9 |
350mm/14 inch (1916x) |
10.2 |
12.7 |
15.2 |
400mm/16 inch (2500x) |
10.5 |
13.0 |
15.5 |
500mm/20 inch (3904x) |
11.0 |
13.5 |
16.0 |
600mm/24 inch (5632x) |
11.4 |
13.9 |
16.4 |
750mm/30 inch (8789x) |
11.9 |
14.4 |
16.9 |
900mm/36 inch (12,656x) |
12.3 |
14.8 |
17.3 |
1 meter/40 inch (15,625x) |
12.5 |
15.0 |
17.5 |
My favorite dark sky site, CSAC near Crowell, Texas, shown on a light pollution map. This equates to limiting magnitude skies in the 7.0 to 7.5 range. It's only 3.5 hours from my DFW residence. More than likely, truly dark skies are closer than you think.
There are several VALUABLE things we can derive from this table...
HOW TO APPLY THIS INFORMATION
This chart is designed for stars. For example, if you have a 4 inch scope in dark skies, you should see ~12th magnitude stars. As for objects, that depends on the surface brightness of the object and the ability of your telescope to present that light in a way that is observable. In other words, it may not be enough for you to merely see an 11th magnitude galaxy. You might need more aperture or darker skies to see certain aspects of that galaxy that makes it interesting. It's the difference between seeing M51, the Whirlpool galaxy, as a "faint fuzzy" or as a galactic spiral.
Regarding deep sky objects (DSOs), the Messier catalog of 109 objects is the most observed list of deep sky splendors. The hardest to see of these objects might not be the ones designated with the faintest magnitude measurements. For example, M76, the Little Dumbbell Nebula, is normally considered the faintest object, at around 10.2 magnitude or so. But I find this object much easier to see than fainter galaxies such as M74 and M77 since their magnitudes are spread over a larger surface area. To see the faintest of the Messier objects, you will need to have a scope/sky combination that gives you no less than ~10.5 magnitudes. That might be a 2 inch scope in dark skies, a 6 inch scope in suburban skies, or a 12 inch scope in city skies.
But to truly observe these objects, you will need a combination of scope and sky that will reach into the 12th magnitude or greater range, since much of what might constitute "detail" in a galaxy will be more faint than its overall surface brightness. Only then will you be able to begin seeing certain features that are characteristic of the object itself.
It's no wonder that in dark skies with a 24" Dobsonian reflector, some of these objects begin to look like their photographs.
CONCLUSION
So, back to our original question...Do Dark Skies Really Matter? Not only is this answer quite obvious now, chances are you didn't realize exactly how much impact dark skies truly have to your telescope views!
Users of even small aperture scopes should not despair for this reason. Taken to a dark sky site, a 3 inch telescope makes not only the entire Messier list detectable, but you can begin to see detail in many of these objects. Conversely, users of 10 inch scopes used in the city might have difficulty seeing even the brightest of Messier objects with any amount of detail.
The difference between a 2 inch telescope and a 30 inch telescope is about 6 magnitudes. I practically gain 6 magnitudes by driving a few hours west. It's like owning a huge telescope with little cost to myself - imagine a 30" telescope in those skies!
Therefore, the easiest way to get these lost magnitudes back, without spending extra money, is to find a darker site. Below you will see the Bortle Scale, which can help you evaluate the sky conditions you find yourself in. And if you can't find Mag 7 skies, then a quick drive into the suburbs might be all you need to make your telescope perform MUCH BIGGER.
Don't sell that scope just yet. Give it a try and tell me I'm wrong!
- The darker the skies that you can observe in, the more you can see. In fact, by driving from Mag 2 city skies to Mag 7 country skies you will have a 100-fold increase in the amount of WANTED signal you are collecting. If doubling the aperture size of your scope only yields a 4-fold signal increase, then what do you think has the greater impact...dark skies or more aperture? And you were ready to throw away your scope! For shame!
- The more aperture you have, the more you can see (given equal optics) regardless of where you observe. Doubling the aperture yields 4 times the light increase...which is approximately a gain of 1.5 magnitudes. Thus, a 20-minute drive would probably gain you more than buying twice the telescope!
- Not everybody has the same eyes. Young people have an advantage in pupil size since this decreases as we age (5 to 6 mm for elderly folks). The 8mm pupil size that I used for the light comparison figures assumes a young, dark adapted eye. Typically, older people balance things out a bit by being more experienced as an observer, as such they have learned to use averted vision and other advanced skills.
- This assumes you are using equipment that does not limit the amount of light entering the eye, such as bad optics, unsteady mounts, and poorly performing eyepieces or barlows. It also assumes acclimatized optics, good focusing, and generally steady "seeing." In other words, you may not experience these numbers if you do not take care to limit the "noise" that comes with controllable factors like bad equipment, poor setup practices, and weak execution.
- Limiting magnitude determines if the light can be detected. Factors such as resolving power, contrast, atmospheric stability and clarity will influence how well you see this light. Ideally, these are the only limiting factors, those that are truly uncontrollable.
- Truly dark skies will be pushing 8th magnitude, so if your skies are darker than 7, just make the necessary adjustments to the magnitude scale above. As a reference, I use Mag 7 since this is a more realistic representation of what is available to us. Magnitude 8 is a world-class, rare, hard-to reach site for the vast majority of people. Mag 7 can be found by driving a few hours away from larger metropolitan areas, unless you are in the eastern part of the U.S. - my apologies to others around the world; just look at a dark sky map. For example, my favorite dark sky site, Comanche Springs Astronomy Campus (3RF) near Crowell, Texas is 3.5 hours from my central DFW home. This puts me in Bortle Class 2 skies (also see end of article) between 7.1 and 7.5 magnitude. See Dark Sky Map for my favorite way of finding dark skies for yourself!
- The worst skies according the Bortle scale are stated at less than 4th magnitude. But I find that 4th magnitude in a big metropolitan areas isn't very realistic. Perhaps that would be true IF all localized, direct sources of light are OFF and if I am looking straight at the zenith. In my central DFW home (a Bortle Class 8 or 9 site), I can barely make out Polaris (mag 2) on a typical, moon-free night. And because you have little control over street lights and other sky-robbing light pollution, I find that an all-sky Magnitude 4 would be impossibly generous. A casual look at the stars, like a newbie observer might experience for their first light views, are probably being limited to quite a bit worse than 4th magnitude, practically speaking. Hence, representing city skies as something close to 2nd magnitude in my own magnitude chart seems to make much more sense to me.
HOW TO APPLY THIS INFORMATION
This chart is designed for stars. For example, if you have a 4 inch scope in dark skies, you should see ~12th magnitude stars. As for objects, that depends on the surface brightness of the object and the ability of your telescope to present that light in a way that is observable. In other words, it may not be enough for you to merely see an 11th magnitude galaxy. You might need more aperture or darker skies to see certain aspects of that galaxy that makes it interesting. It's the difference between seeing M51, the Whirlpool galaxy, as a "faint fuzzy" or as a galactic spiral.
Regarding deep sky objects (DSOs), the Messier catalog of 109 objects is the most observed list of deep sky splendors. The hardest to see of these objects might not be the ones designated with the faintest magnitude measurements. For example, M76, the Little Dumbbell Nebula, is normally considered the faintest object, at around 10.2 magnitude or so. But I find this object much easier to see than fainter galaxies such as M74 and M77 since their magnitudes are spread over a larger surface area. To see the faintest of the Messier objects, you will need to have a scope/sky combination that gives you no less than ~10.5 magnitudes. That might be a 2 inch scope in dark skies, a 6 inch scope in suburban skies, or a 12 inch scope in city skies.
But to truly observe these objects, you will need a combination of scope and sky that will reach into the 12th magnitude or greater range, since much of what might constitute "detail" in a galaxy will be more faint than its overall surface brightness. Only then will you be able to begin seeing certain features that are characteristic of the object itself.
It's no wonder that in dark skies with a 24" Dobsonian reflector, some of these objects begin to look like their photographs.
CONCLUSION
So, back to our original question...Do Dark Skies Really Matter? Not only is this answer quite obvious now, chances are you didn't realize exactly how much impact dark skies truly have to your telescope views!
Users of even small aperture scopes should not despair for this reason. Taken to a dark sky site, a 3 inch telescope makes not only the entire Messier list detectable, but you can begin to see detail in many of these objects. Conversely, users of 10 inch scopes used in the city might have difficulty seeing even the brightest of Messier objects with any amount of detail.
The difference between a 2 inch telescope and a 30 inch telescope is about 6 magnitudes. I practically gain 6 magnitudes by driving a few hours west. It's like owning a huge telescope with little cost to myself - imagine a 30" telescope in those skies!
Therefore, the easiest way to get these lost magnitudes back, without spending extra money, is to find a darker site. Below you will see the Bortle Scale, which can help you evaluate the sky conditions you find yourself in. And if you can't find Mag 7 skies, then a quick drive into the suburbs might be all you need to make your telescope perform MUCH BIGGER.
Don't sell that scope just yet. Give it a try and tell me I'm wrong!
Bortle Sky Chart
