Results 1 to 2 of 2
Like Tree22Likes
  • 21 Post By KT4HX
  • 1 Post By BobDob

Thread: Why Can't I See That Galaxy?

  1. #1
    KT4HX's Avatar
    KT4HX is offline HYPER GIANT
    Points: 101,098, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 35.0%
    Achievements:
    365 Days+ Registered Achievement!First 1000 Experience Points750 Days+ Registered Achievement!1000 Days+ Registered Achievement!200+ Posts Achievement!
    Join Date
    Dec 2008
    Location
    Virginia
    Posts
    11,765
    Points
    101,098
    Level
    100
    Thanks
    1,432
    Thanked 8,084x 4,352 Posts
    Blog Entries
    200

    Default Why Can't I See That Galaxy?



    So, youíve taken your time to learn the bright stars, and you can point out many constellations, and know where the planets are in the sky. Youíve learned how to collimate to your satisfaction, if your scope requires it. Youíve also learned how to star hop pretty well, and locating a few of the brightest Messier objects is starting to become easy for you, and the Messier award is your first big goal. The importance of aperture is imprinted on your brain, and you are starting to get a grip on the impact of your local light pollution levels. Youíve also learned that things will not look like they do in the images, and laugh that you were ever that naive. You understand the need to allow your scope to reach thermal equilibrium in order to achieve stable views. All in all, you feel you are moving along the learning curve pretty well, and your confidence is increasing with each outing. You know you have a lot more to learn, but then who doesnít? You look through the list of Messier objects, and you notice that a few of the galaxies are pretty bright. Why, just look at M33 in Triangulum, with its magnitude of 5.7 - now thatís got to be easy. So you check Stellarium and find M33 will be up later tonight, and you consult the chart you will use to find it. You study the star hopping pattern until you feel pretty good about your chances. Again, at 5.7, you feel it is just going to jump right out at you as you slowly glide to its position. This is going to be sweet, pulling down some photons that took about three million years to get here. Suddenly you think about that, and speculate on what was happening on Earth when the light you will see tonight left M33, and it staggers the mind. You are so pumped and primed that you can barely wait until itís time to observe your first galaxy!

    Now fast forward a few hours to find yourself under the stars. Your scope is acclimated, your adjustments are complete, and your accessories are at the ready. You consult your charts with your trusty red light, and start the process. You are confident that you are at the right star field. You peer through your low magnification eyepiece to see - nothing! What, this canít be right! You start to second guess yourself. You double check your star hop and position, and feel they are right. Then you try every eyepiece you have, even trying a ridiculously high magnification that obliterates the stars in the field of view. You can almost hear your confidence hit the ground.

    While the scenario above may appear a little over dramatic, itís not far from reality for many observers, as they try to find their first galaxy. So what did you do wrong? Actually you didnít do anything wrong. You merely overlooked some basics that one needs to consider when targeting galaxies, as well as any extended, diffuse DSOs. You are familiar with magnitude, at least from the stand point that the lower the number, the brighter the star or object. And, perhaps youíve heard or read a passing reference to something called surface brightness as you were digesting information. Youíve heard of an objectís angular size in the sky, but maybe glossed over it as another annoying term that gets tossed around in this hobby. However, now that M33 has eluded you, it is time to start thinking about these things in a different light, and with more understanding. Maybe, just maybe, you will see that they play an important role in why you can see some galaxies, and not others, or the view is less impressive than what you had hoped. First, letís take a quick look at these terms in a non-complicated manner so they are easier to digest. By de-mystifying things a bit, it becomes easier to get a better overall picture of what is going on here.

    The first thing we notice about M33 is its magnitude, which is 5.7. But what does this tell us? Apparent magnitude, or just simply magnitude, is the brightness of an object as seen by an earthbound observer calculated in a way to remove the effects of our atmosphere. The magnitude of 5.7 for M33 is its apparent magnitude (or visual magnitude). Now stars are a point source of light, whereas, DSOs are not. So how do we get the apparent magnitude for a DSO. There are formulae and calculations associated with the process, but in simple terms here is what we need to grasp. If you take all the visible light coming from M33 and compact it down to a star-like point source, then it would be equivalent to a star of magnitude 5.7. While that should be a very simple concept to grasp, why isnít M33 an easy thing to see then? With that in mind, letís take a look at what else comes into play.

    Next we have an objectís angular size. This is its size in the sky from our perspective here on Earth. It is expressed in terms of degrees (į), arc minutes (Ď) and/or arc seconds (ď). So in the case of M33, its angular size is 1.1į x 41.6í. Comparatively, the full moon on average is about 32í in angular diameter, or just over 1/2į. What this means to you is that M33ís magnitude 5.7 light is being spread out over its angular dimension, which has a very profound effect.

    This now brings us to surface brightness, one of those many mysterious terms one encounters when they first get into the hobby. In a nutshell, this is a measure of an objectís magnitude per a specified unit area, most typically in square arc seconds, and is expressed in Magnitudes Per Square Arc Second (MPSAS). It can also be expressed in terms of Magnitudes Per Square Arc Minute (MPSAM) with a simple conversion.

    Ok, so what does this mumbo-jumbo really tell us about M33? We have already established that M33 has a magnitude of 5.7, which is based on scrunching all its visible light to a star-like point source, then calculating what stellar magnitude it is equivalent to. So now, as we take this light and start to spread it out over an area the size of M33, what do you suppose happens to its appearance? If you guessed it effectively dims the object, you would be correct. Because this light is smeared out over a larger area, it naturally dims the overall appearance. To be sure, there could be pockets of more intense light within the object (such as a galactic core), but overall, across the entire surface of the object as presented to us, a noticeable dimming effect occurs. Letís try a simple analogy that should drive this point home even better. Center a bright star in your telescope and bring it to focus. As one would expect you find it presents a bright, dazzling light. Now, slowly defocus it Ė what happens? You see that not only does the star become larger and larger, but it becomes dimmer and dimmer, to the point of effectively disappearing. You have just seen what happens to M33ís light as it spreads out over its angular dimension. The light coming from the star hasnít changed in magnitude, but you have increased its angular size as you see it through the scope, effectively reducing its surface brightness. Thatís kind of neat isnít it?

    So letís look at the actual surface brightness numbers for M33 so you can get a perspective. Surface brightness is also based on a formula that takes into account the objects magnitude and its area in square arc seconds. Since M33ís magnitude is 5.7 and its angular size is 1.1į x 41.6í, the surface brightness calculates out to 23.23 MPSAS. I find that some observers have a problem relating a number so large (and thus dim) to the listed magnitude of 5.7, that I usually convert the number to MPSAM by subtracting 8.89 (or you can round it to 8.9) from the MPSAS, which gives us an MPSAM of 14.34. Some feel more comfortable with that number rather than 23.23.

    So ultimately, what is this telling us about M33? If you remember at the beginning I mentioned something about you having learned about the impact of light pollution in your area, and that you understand that this artificial sky glow brightens the sky above you. The watchword here is contrast. Because the sky is now brighter (through artificial means), the contrast between the sky and M33 lessens, which means it is harder to see. When you apply what weíve learned, then it is easy to see why M33 becomes a tough challenge for many observers. Just like the star you defocused previously, it can become almost, if not entirely invisible. If your sky was truly pristine, with no artificial light pollution, then the only sky glow you have to worry about is of natural origin, predominantly star light. M33 will appear large and bold in appearance in a low magnification, wide field view. The reason is again, contrast. The light from M33 doesnít have to combat the artificially induced sky glow that is far brighter than natural sky glow, and it becomes easy to see. So you can see the more artificial sky glow that is introduced into your observing environment, the more objects like M33 will suffer, and for many, seemingly vanish from view.

    So what can one do in a case like this to help improve the contrast. First, one thing you canít do is change the overall effect of your general light pollution, other than to head to a dark site to escape its impact. There is absolutely no substitute for darker skies when it comes to improving your views. But, there are other things you can do locally to eke out as much contrast at the eyepiece as possible for your conditions. One is to make certain that you observe from a location in your yard that has no extraneous ground lighting intruding directly upon you. This is known as glare, rather than light pollution. Use a tree, fence, or building to block an offensive light if possible. It is worth giving up a small section of sky in order to shield yourself from these lights. If you cannot find a darker corner of the yard, I recommend building some sort of blocking screen to shield your position. Many have done this with PVC pipes and tarps, and there are numerous plans on the internet. Another thing you can do is to drape a dark cloth over your head and focuser as you observe to block any light that might be bothering you. Some will use an eye patch over their observing eye when not at the eyepiece in order to maintain best dark adaptation. It is imperative that you protect your eyes and the end of your OTA from direct light encroachment. Just a tiny bit of light coming into the corner of your eye while looking through the eyepiece can take away contrast. Enough light shining into the end of your OTA can cause glaring and loss of contrast as well. So it is very important to do everything you can to shield your position from these annoying lights.

    Aside from the above important factors, two other terms you need to become familiar with are seeing and transparency. Basically, seeing is the steadiness of the atmosphere, and transparency is the clarity of the atmosphere. We've all experienced nights of excellent seeing, where the moon and planets yield steady images, even at higher magnification, and the stars seem to barely twinkle at all. But, on the flip side we've all had nights where it was almost impossible to see the moon and planets clearly, and the stars twinkled madly. Usually we experience something in between those two extremes. Transparency can be directly impacted by such things as moisture (humidity), dust, pollen, smoke, etc., in the atmosphere. These cause light to be scattered and absorbed, thus increasing our ordinary level of sky glow. This characteristic is known at atmospheric extinction, and even the elevation above the horizon of a star or object can contribute. On nights of poor transparency, we can notice a drop off in the magnitude of stars we can see naked eye, and bright nebulae appear muted. These are not the best nights to try finding galaxies. Then on nights of excellent transparency, our unaided eye seems to pick up more stars, and the conditions for chasing extended and diffuse DSOs are at their best. However, again, those that lie nearer to a horizon can suffer from increased atmospheric extinction, and thus not appear as good as they would if they gained higher elevation. While both seeing and transparency are important to us, it is transparency that is more critical when trying to observe galaxies and other DSOs. Typically, though not always, nights of excellent seeing are poorer for transparency, and vice versa. So one should pay attention to both and be prepared to change observing plans as needed to take advantage of the prevailing conditions.

    Hopefully the above has clarified some important issues for you when it comes to observing extended and diffuse DSOs. The factors of magnitude, angular size and surface brightness are vastly important to understanding the observability of DSOs in general, and for the more extended and diffuse types in particular. Unfortunately, the vast majority of lists only contain the magnitude, which only tells part of the story. Often times they will also give the angular size, which helps as well. I simply donít understand however, why they fail to regularly list the surface brightness. True, one can calculate the surface brightness of any object if you have the other two, but why not just list it to begin with.

    I will leave you with one last thought then we will take a look at the Messier galaxies to give you a comparison. Using the above information gives you an understanding of why an object may or may not be easily seen. But, that should never be used to avoid trying to pursue an object. Of course there are limitations to how deep any specific aperture can go. But, I am a firm believer that the best way to become a better DSO observer comes from two basic actions. First and foremost, get out under the skies as often as you can. The more time you spend under the stars, the smarter you become about your observing practices. Observing dim objects is not a natural skill, it is an acquired one. Second is, never shy away from an observational challenge. That doesnít mean go hunting for a 15th magnitude galaxy with an 8 inch scope from the middle of New York City. What that means is be both realistic and bold in establishing goals. Donít get caught in an observational loop, whereby you are observing the same set of objects over and over again because it is easier than taking the next step forward. Move outside of your comfort zone. Expect to fail, but donít let those failures discourage you. Rather look at them as a learning tool as you progress. I have always been a firm believer that we become better at our craft through challenging both our equipmentís capability, and our own observing skills. That is how we grow, that is how we learn.

    Now letís take a look at the galaxies that are part of the Messier catalog. The below chart will provide the basic info for each object, and please note that the surface brightness is expressed in MPSAM. By reviewing the data below, one should get a sense of which galaxies should present the easier targets to start out with, especially when dealing with noticeable light pollution. As a side note, we often hear that many find M101 a huge challenge, and looking at the numbers below, it should be easy to understand why. However, with M31 we notice that there is quite a large differential between its magnitude and surface brightness. That is due to its large and intense core area. That is often the only portion that many observers see in M31. It has such intensity that it can push through light pollution, whereas the dimmer outer portions simply fade away into the sky glow. Because of this significant disparity in brightness levels within its structure, its numbers are a bit skewed. The brightness of the core drives the magnitude figure to the brighter end of the scale, while the larger and much fainter outer portions push the surface brightness dimmer. M31ís two satellites, M32 and M110, are of the same magnitude, but radically different appearances for most observers. From my backyard I have always seen M32, but on occasion M110 has faded into obscurity. If you look at the comparative surface brightness and angular size for both, that tells the story. M32 often looks like a very small, unresolved globular cluster, whereas M110 is more elongated and diffuse and always dimmer when dealing with significant sky glow. It should be easy to see that M33 and M101, despite their bright apparent magnitudes, might not be the best choices when trying to bag your first Messier galaxy. They are always best saved for nights of better transparency, or a trip to a darker location. One thing to remember here is that just a fraction of a magnitude can make a noticeable of difference when dealing with these types of objects. Particularly in the area of surface brightness.



    Object Constellation Magnitude Surface
    Brightness
    Angular
    Size
    Messier 31 Andromeda 3.4 13.56 3.2į x 1.0į
    Messier 32 Andromeda 8.1 12.46 8.5' x 6.5'
    Messier 33 Triangulum 5.7 14.34 1.1į x 41.6'
    Messier 49 Virgo 8.4 13.16 9.8' x 8.2'
    Messier 51 Canes Venatici 8.4 13.03 10.8' x 6.6'
    Messier 58 Virgo 9.7 13.35 6.0' x 4.8'
    Messier 59 Virgo 9.6 12.92 5.3' x 4.0'
    Messier 60 Virgo 8.8 12.98 7.6' x 6.2'
    Messier 61 Virgo 9.6 13.56 6.5' x 5.9'
    Messier 63 Canes Venatici 8.6 13.54 12.6' x 7.5'
    Messier 64 Coma Berenices 8.5 12.78 10.3' x 5.0'
    Messier 65 Leo 9.3 12.59 9.0' x 2.3'
    Messier 66 Leo 8.9 12.83 9.1' x 4.1'
    Messier 74 Pisces 9.4 14.33 10.0' x 9.4'
    Messier 77 Cetus 8.9 13.06 7.3' x 6.3'
    Messier 81 Ursa Major 6.9 13.04 14.9' x 11.5'
    Messier 82 Ursa Major 8.4 12.72 10.5' x 5.1'
    Messier 83 Hydra 7.5 13.01 13.1' x 12.2'
    Messier 84 Virgo 9.1 13.11 6.7' x 6.0'
    Messier 85 Coma Berenices 9.1 13.20 7.4' x 5.9'
    Messier 86 Virgo 8.9 13.38 9.8' x 6.3'
    Messier 87 Virgo 8.6 13.00 8.7' x 6.6'
    Messier 88 Coma Berenices 9.6 13.10 6.8' x 3.7'
    Messier 89 Virgo 9.8 13.31 5.3' x 4.8'
    Messier 90 Virgo 9.5 13.60 9.9' x 4.4'
    Messier 91 Coma Berenices 10.1 13.45 5.2' x 4.2'
    Messier 94 Canes Venatici 8.2 13.51 12.3' x 10.8'
    Messier 95 Leo 9.7 13.47 7.3' x 4.4'
    Messier 96 Leo 9.3 13.32 7.8' x 5.2'
    Messier 98 Coma Berenices 10.1 13.44 9.4' x x2.3
    Messier 99 Coma Berenices 9.9 13.37 5.3' x 4.6'
    Messier 100 Coma Berenices 9.4 13.55 7.5' x 6.1'
    Messier 101 Ursa Major 7.9 15.17 28.5' x 28.3'
    *Messier 102 Draco 9.9 13.16 6.5' x 3.1'
    Messier 104 Virgo 8.0 11.90 8.6' x 4.2'
    Messier 105 Leo 9.3 12.81 5.3' x 4.8'
    Messier 106 Canes Venatici 8.4 13.55 17.4' x 6.6'
    Messier 108 Ursa Major 10.0 13.29 8.6' x 2.4'
    Messier 109 Ursa Major 9.8 13.60 7.5' x 4.4'
    Messier 110 Andromeda 8.1 13.98 19.5' x 11.5'


    *
    The status of M102 is a subject of debate. In a letter dated May 6, 1783, from Pierre Mechain to Johann III Bernoulli in Berlin, Merchain indicated that his original observation of M102 was actually a duplicate observation of M101 that came about because of a chart error. However, Messier included M102 in his publication without personal verification of position or observation. Some believe there is sufficient evidence that both Mechain and Messier did observe NGC 5866 in Draco, and thus this is often accepted as M102, especially for award completion. However, note that both the Pocket Sky Atlas and Sky Atlas 2000 do not include a listing for M102. It is also excluded from other sources as well.

    Anyway, there you have it. I hope this all helps to make some sense of the larger picture when it comes to how well you may be able to see galaxies, and extended, diffuse DSOs in general. Having an understanding of the whys and wherefores should help with planning and execution when taking on these fascinating objects. Most importantly, get out there, observe and have fun. Happy viewing!
    Alan || My DSO Blog:
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.

    ---------------------------------------------------------------------------
    Astro Sky 17.5 f/4.5 Dob || Apertura AD12 f/5 Dob || Zhumell Z10 f/4.9 Dob
    ES AR127 f/6.5 and ES ED80 f/6 on Twilight-II Mount
    ES 82į 24mm, 18mm; Pentax XW 10mm, 7mm, 5mm
    ---------------------------------------------------------------------------
    Astronomers: We look into the past to see our future.

  2. The Following 13 Users Say Thank You to KT4HX For This Useful Post:

    + Show/Hide list of the thanked

    Alec in France (07-01-2015),AndersB (08-28-2015),andro-mike-eda (11-20-2014),Asterion (12-27-2015),Blu (07-07-2015),GreyDay (07-01-2015),Insomniac (06-27-2015),Jayhas (11-21-2015),John Baars (01-12-2019),Locutus (10-26-2015),Mhanis (09-04-2015),sketrip (11-09-2015),Skytopher (01-11-2016)

  3. #2
    BobDob's Avatar
    BobDob is offline Super Moderators
    Points: 54,091, Level: 100
    Level completed: 0%, Points required for next Level: 0
    Overall activity: 0%
    Achievements:
    Got three Friends20+ Friends Achievement!First 1000 Experience Points5+ Referrals Achievement!100+ Threads Achievement!
    Join Date
    Mar 2009
    Location
    Port Angeles, Washington
    Posts
    7,113
    Points
    54,091
    Level
    100
    Thanks
    5,234
    Thanked 2,895x 1,924 Posts

    Default Re: Why Can't I See That Galaxy?

    A discussion thread for this excellent paper appears at: Why Can't I See That Galaxy?

    This sticky is closed to comments.

    Clear skies, and thank you Alan!

    Bob
    KT4HX likes this.
    6 inch F/5 GEM mounted reflector

    The Forum Rules, or Terms of Service:
    To view links or images in signatures your post count must be 5 or greater. You currently have 0 signatures.


    >)))))*>

  4. The Following 2 Users Say Thank You to BobDob For This Useful Post:

    kingclinton (08-15-2014),KT4HX (04-01-2018)

 

 

Similar Threads

  1. Triangulum Galaxy and Andromeda Galaxy From Last Night
    By jschr338 in forum Astrophotography Forum
    Replies: 11
    Last Post: 11-01-2014, 11:05 AM
  2. Replies: 12
    Last Post: 10-29-2010, 06:06 PM
  3. Replies: 11
    Last Post: 04-14-2009, 01:46 AM
  4. Spiral galaxy or Elliptical galaxy Samenness
    By John Zinni in forum General Astronomy Forum
    Replies: 12
    Last Post: 12-06-2003, 06:07 PM
  5. Replies: 0
    Last Post: 10-16-2003, 06:00 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletinģ Version 4.2.0
Powered by vBulletinģ
All times are GMT. The time now is 12:21 PM.