#15: Full Moon: Dark Skies Just Ahead

     The time just before and after full moon is perfect for tweaking one's observing plans.  Spring constellations are available to us for such a short period of time compared to summer, autumn and winter ones that careful preparation is even more important than usual.  I have been trying to make a decent start on Leo, and beginning two nights after full moon I hope to resume my work.  Immediate observing projects are to observe and sketch M 66 and eg 3628.  As the moonrise is gradually set back far enough in the evening, I will leave my back deck and set up my scope farther afield in dark sky locations.  I don't know yet whether I'll be able to observe in the 'preceding' area of Leo, where I have a whole collection of faint objects remaining to see, or if I will have to immediately move my observing session more towards the middle and 'following' sections of Leo.  Leo begins at about 9h 22m r.a., and continues until 11h 58m r.a.  No doubt I will still get lots done in Leo if it's clear, but not necessarily from the first pages of my object list, which generally follows r.a. of the constellation I am observing.

     One thing is for certain; Leo will set relatively early during the night.  Though I have notes prepared for Lyra, I decided I needed something to bridge the gap between Leo and Lyra.  Bootes seems to fit the bill nicely, though I have already observed it in some detail with the 8" scope.  Bootes begins at 13h 16m r.a., continuing to 15h 12m r.a.  Bootes is a galaxy-filled constellation of difficult NGC objects, and it contains not a single Messier object or galaxy of real brightness.  Besides galaxies, there is a lone, very faint globular cluster.  Having already viewed and sketched the brightest galaxies and the cluster, there may be little remaining for me to see except the dozens of much fainter galaxies.  While I don't mind reviewing previous brighter galaxies in the newer 12", I also want to add new NGC objects to my life list.  The good news is that right about now where I live, the trees usually begin to leaf.  When this happens, a lot of ground light pollution is blocked and the sky grows a bit darker.  Any slight advantage might make the difference between seeing a faint galaxy or not seeing it.  This year in southern Canada, however, we seem considerably behind in seasonal changes, and I may have to wait one more full moon before the trees are of much help.

     Leo also offers two close-up charts in Uranometria.  Charts A10 and A11 are galaxy-filled playgrounds, and I hope to make a good start on Abell 1367 (Chart A11).  I haven't yet decided on an observing strategy for this remarkable area.  I will first determine which objects and how many are actually available to me by having a quick look there.  If many or most of them are visible, then I will likely enjoy a galaxy-filled observing orgy over several nights.  As I enjoy star hopping so much, I'm certain to love galaxy hopping, too!  If my skies do a poor job of allowing me decent access, then I will likely plan a road trip with my telescope for next spring to somewhere very dark in order to experience this feast properly.  In fact, an upcoming post will likely be about dark site road trips.  Where to go, when to go, why to go and which way to go.  So stay tuned.

     Full moon is also a perfect time to tune up the scope and accessories.  Cleaning eyepieces and the finder-scope is a great idea, and perhaps doing some collimating on the secondary and primary mirrors isn't such a bad idea, either.  And of course it is a perfect time to scour the web and catch up on some blog reading!  Until next time enjoy the full moon, and join me in anticipating the dark-sky nights that will come soon afterwards.

Mapman Mike

#13: More About Galaxies

     I said awhile back that 5 of every 6 non-stellar objects plotted in Uranometria 2000 are galaxies.  That only includes objects to 15th mag.  If we dip down to 17th or 18th mag, virtually every object past 15 is a galaxy.  They are difficult to avoid if observing faint objects.  Since this blog is currently concerned with the NGC catalogue (and much of the IC list), I will only discuss the main list at this time.  With over 7800 objects of all types in the NGC list, galaxies still reign numerically supreme.  It might be of interest to look at which constellations have the most NGC objects within their boundaries, and just what type those objects are.  Because I live and mostly observe at latitude 42 degrees north, I will only discuss those constellations which are visible from mid-northern latitudes.

     The constellation (north or south) with the most NGC objects is........Virgo!  Not a huge surprise to seasoned observers, I'm sure.  Virgo has 637 NGC objects.  And guess what?  636 of them are galaxies!  There is one lone globular cluster (gc 5634).

     Number two on the list with the most NGC objects is....Ursa Major.  There are 407 NGC objects.  Only two are non-galaxies!  There is a single very famous planetary nebula and one open cluster.

     Number three constellation is Cetus.  Some of it is a bit low in the sky for me, but it is all there.  Cetus has 383 objects.  There is one lonely and very low surface brightness planetary nebula (NGC 246).  The rest are galaxies.

     Number four on the list and close behind is Leo, with 378 NGC objects.   Every single one is a galaxy!  No relief here, though it is peppered with some lovely double stars and multiples.  I am currently beginning my extensive and hopefully exhaustive exploration of this early spring constellation.  It will take me many years.  To date, I have located 13 of its NGC galaxies, though I've officially logged only 9 so far (it`s been pretty cloudy).

     Number five is Eridanus, a constellation that winds from north to south, meaning that I cannot see all of it from my latitude.  With 315 NGC objects, it has one reflection nebula and one planetary nebula.  All the rest are galaxies.

     So, then, the top five constellations for northern latitude viewers contain a total of 2,120 NGC objects.  That`s about 2/7ths of the entire list.  And a whopping 6 of those objects are non-galaxies!  Let`s continue through the top ten NGC constellations.

Number six is Pegasus.  With 298 NGC objects, only 3 are non-galaxies.

Number seven is Coma Berenices.  293 objects.  It has 3 globulars.

Number eight is Draco.  287 objects.  2 planetary nebula.

Number nine is Pisces.  270 objects.  All galaxies.

Number ten is Bootes.  267 objects.  1 globular.

     We are now up to 3,535 objects, nearly half of the NGC list!  All are galaxies except for 15 objects!  I don't know about you folks, but I find this quite astonishing and somewhat amusing.  "Well," you say, "what about those rich milky way summer constellations?"  OK.  Let's look at Scorpius.  Lots of interesting objects there.  Scorpius has a grand total of 71 NGC objects.  There are some totally amazing ones, too.  However, there just isn't a whole lot of them compared to those galaxy constellations (Sagittarius has 77 NGC objects).

     After the top ten NGC constellations comes Hydra, Canes Venatici, Hercules, Andromeda, Aquarius, Cancer and Lynx.  All are very heavy with galaxies.  I am not trying to prove any point here, but merely stating some interesting (for me) facts.  And no, I did not count the objects myself from the atlas.  This website does it for me.  
http://www.nightskyatlas.com/constellations.jsp

I did, however, scan through each constellation's list to search for non-galaxies, so there is some original research involved.

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Here are a couple of older sketches each depicting a galaxy I saw with my previous scope, the amazing Edmund 8":

My first attempt to scan some early sketches.

eg 4565, Coma Berenices.  112x.

15'.8 x 2'.1  Vis. Mag. 9.5.  Sur. Br. 13.2.

One of the showpieces of the heavens.

eg 4448, Coma Berenices.  72x.

3'.9 x 1'.4  Vis. Mag. 11.2.  Sur. Br. 12.9

The star field is at least as interesting

as the galaxy.  The scale for this sketch

is obviously not the same as for the one

above--I cropped this one a lot.

Happy galaxy hunting!

 

Mapman Mike





#12: Visual Magnitude versus Surface Brightness

     Any discussion about observing galaxies must inevitably come round to the subject of the galaxy's brightness or lack of it in the eyepiece.  Galaxies that I could barely see in my previous 8" scope are now much easier to locate, identify and discuss using my 12" mirror.  However, an entire new spectrum of much fainter galaxies is now available, going well beyond what the 8" might have seen.  If I suddenly started using a 16" scope, then the faintest ones now seen with the 12" would appear brighter, and a whole slew of new and fainter galaxies would be available to challenge my view.  This process would literally go on and on with each larger mirror.  There is no end to the faintness of distant galaxies, as evidenced by Hubble's deep field explorations.

     I am still trying to determine the faintest galaxy that I can view in my nearby dark sky with the 12" scope.  So far it has to be NGC 2873 in Leo, with a visual magnitude of 15.4 and a surface brightness of mag. 13.6.  While there are several fine explanations on the web as to exactly what visual mag. is, and surface brightness, I will give my own version here without equations, based on my current understanding and experience.

     Visual magnitude refers to how much actual light is coming from the galaxy to the observer's eye.  How it reaches the eye is greatly affected by the object's size.  Some bright galaxies (Messier 66, for example) have a bright visual magnitude, making them seem relatively easy to see in a wide range of small telescopes.  The visual mag. for M 66 is given as 8.9.  If that object was a star, it would be considered somewhat faint but still easily observable in a small telescope.  NGC 2873, by comparison, has a visual mag. of 15.4, making it nearly impossible to see if it was a star, except with a moderately large scope.  If 2873 was as large an object in the eyepiece as M66, it would never be seen in amateur scopes.  And yet the surface brightness levels of the two galaxies are not that far apart.  For M66 it is given as mag. 12.7, and for 2873 it is mag. 13.6.  There is less than a magnitude difference between these two vastly differing objects!  And yet in fact there is more than six magnitudes of difference between them in actual visual light.  A difference of one magnitude is 2 1/2 times, with mag. 10 being 2 1/2 times fainter than mag. 9, for example.  This is most interesting.

     The size of a galaxy, then, is important in determining its brightness, and in making it easier for the eye to see it.  Without getting mathematical (not my strength), let us take 3 hypothetical galaxies as our examples.  Our first galaxy is going to be sized at 1'.1 x 1'.0 (arc minutes).  This is a somewhat small object, and virtually round.  If the visual mag, was, say, 13 (from all the light coming from it), then the surface brightness would be the same mag. as the visual one.  That exact size of object has the same visual mag. as its surface brightness. 

     However, if we now expand the galaxy to 2'.2 x 2'.0, then the surface brightness decreases (there is an easy formula for this).  The same amount of visual light is now spread over an area 4x as large.  A galaxy of mag 13 might now become one with a surface brightness of 14.7 (not an exact answer, just an example, but it will be 4x fainter). 

     If we now shrink the galaxy to something like 0'.55 x 0'.5, then the same amount of light now arrives in a more compact, star-like package.  The surface brightness now increases, so that our imaginary galaxy with a visual mag. of 13 now seems more like a 12 mag. galaxy (again, just a rough answer--the formula is available on-line if you want exact answers).

     The upshot of all this is that a large galaxy that looks bright on paper might actually be very hard to see at the eyepiece, because its light is spread out over a large area (NGC 45 in Cetus is a good example--that one also has a bright star attached that interferes with seeing the fainter haze of the galaxy)).  And a tiny galaxy like 2873  might seem at first to be impossible to see, but is actually a bit easier because of its very small size (its actual size is 0'.8 x 0'.2).  Which magnitude is the one that observers should pay attention to, visual or surface brightness?

     In my experience, neither one should be used apart from the other.  Even taken together, there are still so many other factors to consider when hunting galaxies.  Some of these factors include how high above the horizon it lies, if it is near a bright star, the transparency of the sky and overall seeing conditions, and even how tired you are.  And if the galaxy is the same brightness as the sky background, it will be invisible.  When galaxies get too far west I stop observing them, as that area of sky is very light polluted.  I take both figures with me outside on the clipboard, along with the size.  If the galaxy is very large and bright, then the surface brightness formula only works to a certain degree.  A combination of the two magnitudes is often a better way to go.  M66 lists a visual mag. of 8.9 and a surface brightness of mag. 12.7.  You can't see a 12.7 mag. galaxy in a 4" telescope, but you can see M66.  Halfway between 8.9 and 12.7 is magnitude 10.8, which seems closer to the truth in this case.  A 4" scope can now work very hard and show the galaxy pretty well on a decent night.  My 'split the difference' method is unscientific, but seems to work for me in the field.

M66--is it really only 0.9 magnitude brighter than

 NGC 2873?  M66's vast dimensions (9'.1 x 4'.1) make

its bright magnitude appear dimmer to the eye.

Both photos from

http://www.astrosurf.com/benoit/ngc.html

NGC 2872 on right.  NGC 2874 on left.  Both are easy

to locate in a 12" scope.  Much trickier is tiny NGC 2873,

lying just north of the two larger Leo galaxies,

 in the upper center of the photograph.  The photos

show true relative size of all galaxies depicted, including M 66, above. 

     The same goes for NGC 2873.  I could never hope to see a galaxy of 15.4 magnitude in my skies with a 12" scope, and the galaxy certainly doesn't reach its given surface brightness mag of 13.6 either.  Halfway between the two figures lies mag. 14.5, which gives a more reasonable ballpark estimate as to what my observing companion and I observed a few nights back.

     So for now, I am assuming that I can see a tiny mag. 14.5 galaxy in a good sky with my scope.  This will aid me in knowing which faint galaxies to attempt and which ones to pass up.
Of course the companion volume to Uranometria 2000 is also an awesome source of this information.

As ever, I am curious to hear what you think.
Mapman Mike