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Old 15-02-2019, 08:45 AM
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mental4astro (Alexander)
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Observing Galaxies - where are they??? A how and why guide.

This is the third installment of the article series I've started in observing the heavens. This third piece has proven to need a bit more consideration as galaxies, despite their true size, can be the most confounded to observe, so I've wanted to produce a piece that explains, encourages & fuels your quest to tackle these illusive denizens of the night sky.

~x.X.x~

Ok, you've settled in for a night of observing. You have your list of galaxies that you want to nail. The charts, books and apps all tell you these are all bright suckers that are well within your scope's light grasp. Sky is clear, no Moon about, and the scope is primed...

But hang on! Where are they?

How come I can't see them, or they are sooooo underwhelming?

Where are their arms?

Is my scope a lemon???

Have I been conned?...


The galaxies are there. So are there arms, and your scope is not a lemon. BUT yes, there is a little bit of a con going on. And it isn't a deliberate one, but one born out of misinformation and ignorance. I too fell for this, and it took me MANY years to figure out what was going on.

The quasi con explained.

Here's the root of the problem. It starts with the "visual magnitude" that charts, books and apps all display. A galaxy is described as say 'magnitude 3'. It should EASILY be visible in your scope, heck even to your naked eyes. But you cannot see it naked eye, and you are barely able to make out a mere soft brightening in the field of view of the eyepiece.

The problem lies in how & what the visual magnitude actually means! Stars are listed with their magnitude, and stars are a point source of light. And this is how the magnitude of a galaxy is described - as the entire light of the galaxy is put into a single point source! The kicker is that a galaxy is not a point source, but an extended object - it is spread out and has a size/dimensions. So the point source magnitude value is actually spread out across the entire dimensional space/size of the galaxy! That magnitude 3 galaxy you were trying to see has its light spread out over the same size occupied by four full moons! This is the case not just for galaxies, but for all deep sky object, nebulae, open clusters, globular clusters, etc.

Take the Large Magellanic Cloud. It's visual magnitude is something around magnitude 0, a 'bright' sucker, as bright as Alpha Centauri! It "should" be as easy to spot as Alpha Centauri even from light polluted skies. But it is difficult if not impossible to see the LMC from under light polluted skies, even though Alpha Centauri is easy to see. And under a dark sky it glows with a soft light. Remember, the LMC is huge in the sky, occupying a whopping 87 square° - the Moon's size by comparison is just 0.2 square°.

So, that brilliant magnitude 0 glow is spread out over a huge area, and unevenly too.

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This is the case with all galaxies.

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Situation explained.

~x.X.x~

In the next post, I'll give you some tips on how to best observe galaxies, what you can see in them, and why their arms are most difficult to see - alas this last part needs the use of BIG aperture... HOWEVER, even with small apertures if you know a trick or two, you will be able to spot a lot more galaxies than you may have until now...

Alex.
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Old 15-02-2019, 05:29 PM
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mental4astro (Alexander)
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Aperture is King - but not always...

Light pollution is the arch enemy of Faint Fuzzies. While there are a lot of nebulae that can still be seen under urban skies, galaxies are a different matter.

The best place to view galaxies is from a dark site.

Then, to be able to make out details such as the spiral arms, the bigger the aperture the better. Even those galaxies whose arms are easiest to identify, really a 12"scope is just about the smallest aperture. The one exception is M31, the Andromeda galaxy, whose dusty lane and arm structure is visible in binoculars from a dark site under transparent skies, and even NGC 253, the Silver Dollar galaxy shows this dusty lane structure.

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If you have access to a 12" or larger scope, again the larger the aperture the fainter and finer detail that can be seen within the arms.

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Apart from arms, the main details visible are the core and soft halo of the main body. There are a few galaxies that reveal dark, dusty lanes, and fortunately scopes as small as 4" can show these stark, intricate and lovely details. M104 the Sombrero galaxy, NGC 4565 the Needle galaxy & NGC 5128 the Hamburger galaxy, are all edge on galaxies that show a dark lane that can be seen through smaller apertures.

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There's another type of galaxy that shows another set of details. These are very active galaxies which as a result have their neat spiral structure disrupted. As a result they show a mottled, spotty structure.

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One other galaxy type is highly disrupted due to a recent close encounter with another galaxy, and the resulting gravitational tidal pull between the two bodies caused the galaxies to become very distorted. The Hamburger is also one such faint fuzzy. Some galaxies are also in the middle of this gravitational interplay, even merging process. The Antenna galaxy is a pair of such interacting, highly gravitationally distorted galaxies.

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Trick with small apertures

Smaller apertures, say 8" and smaller, have a trick up their sleeve - low magnification.

By dropping the magnification, the image become more concentrated, and hence intensify the image of those faint fuzzies in order to help spot them.

Details may be possible to make out, like the core and halo, but many galaxies, such as ellipticals, only glow as small round glows, nebula like.

Binoculars, especially 70mm and larger, are also very good in galaxy hunting. These are small aperture, low magnificaiton refractors, and the edge they have is the brain is so powerful that in combining the image from both eyes, a single image is produced that is brighter than that of a single refractor of the same aperture size.

Alex.
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Old 15-02-2019, 05:48 PM
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mental4astro (Alexander)
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Filters and galaxies

Nebula filters work by cutting out the majority of the optical spectrum, but transmitting those wavelengths at which nebulae glow.

Thing is with galaxies, they glow across the entire spectrum, and do not have the same spectral bias as nebulae do. As a result, nebula filters will not help with making galaxies easier to see.

If you have a scope 16" and larger, there is one thing that a nebula filter can help with in galaxies. There are a small handful of galaxies that have massive glowing hydrogen regions. These immense nebula regions are also massive stellar formation areas. A UHC type filter can help reveal these regions in these galaxies. To have a chance to spot these very difficult features, a dark sky is not only required, but also very good transparency. Unless transparency is as good as possible, your chances to spot these very faint features is next to none.

"Galaxy filters"

There are some filters that are marketed as being "galaxy filters". Some are produced by very reputable manufacturers such as Omega Optical (link). I have no experience with these, so I cannot comment on them. Maybe someone else here has, and can offer some comments on these filters.

Alex.
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Old 15-02-2019, 09:12 PM
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Thanks Alex!
Great read,and very educational!
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Old 16-02-2019, 04:03 PM
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The magic words : Surface brightness

Hi Alex & all,

Thanks Alex for such an informative set of posts but may I add just a little to them to tie it up with a neat little bow?

The key concept Alex has touched upon here without actually using the term is "surface brightness" and surface brightness -v- integrated magnitude. As Alex pointed out, the total brightness of a galaxy can sometimes be misleading when it comes to how much "punch" it has in the eyepiece.

Surface brightness magnitude is how bright, on average, each square arc-minute of the area a thing occupies is measured to be. Suburban skies (just the background sky with nothing in it) has a surface brightness of about 18 magnitudes per Sq arc-second. A rural sky 21 and a really top notch site about 22 magnitudes per arc-second. Outside Earth's atmosphere, the value is about 25 magnitudes per arc-second.

Obviously, the darker the native sky brightness is, the better the contrast there is going to be, between the galaxy and its background (all other relevant factors assumed to be equal). Many guides and planetarium software (not all) include such a surface brightness magnitude. The difference between the native surface brightness of the sky and the surface brightness of a galaxy is going to tell you how much contrast there is between the galaxy and the background.

A classic example of a relatively high surface brightness galaxy is M104 (The Sombrero Galaxy). It has (overall, averaged) a surface brightness magnitude of 11.6 -- ie every square arc-minute has the light from a magnitude +11.6 star smeared over that arc-minute. A classic low surface brightness galaxy is NGC 45 in Cetus. It has a relatively high integrated magnitude -- just below 10, but, it is a quite large (nearby) galaxy with a large surface area so the surface-brightness magnitude is +14.7

Conventional wisdom says a S.B magnitude of about 14.5 per arc-minute, given very good conditions is about the limit for a 30cm telescope, 25cm -14, 20cm about 13.5. The larger the difference between the native sky brightness and the surface brightness the better it will look. Telescopes always show more galaxies and more detail within them under a truly dark sky.

Surface brightnesses in observing literature are usually quoted in magnitudes per square arc minute. Because the magnitude scale is logarithmic, calculating surface brightness is not performed by simple division of magnitude by area. Instead, for a source with a total or integrated magnitude m extending over a visual area of A square arcseconds, the surface brightness S is given by

S = m + 2.5 ⋅ log (sub)10 ⁡ A .

Seeing detail:

So far I've been talking about averaged surface brightness, but some of the brighter galaxies show a great deal of variance between this bit and that bit with their haloes. Many of you will have seen the dark lane in M104 before. The reason it is visible in such small telescopes is there is a very high surface brightness contrast between the bright bit and the dark bit. Most of the galaxies where you can (in the right sized telescope) see some spiral structure is where there are large, or a better adjective is abrupt differences in surface brightness between "this bit" and "that bit". M51 (The Whirlpool Galaxy) is a classic example. There is a very big (and abrupt) (ie gradient) difference between the arm and the bit in between. Same goes for NGC 1365.

Larger telescopes allows you to detect (more easily) these differences in surface brightness between "this bit" and "that bit" -- you will more frequently and with greater ease and confidence be able to see spiral structure (in those objects with spiral structure) and other details in galaxies and other "non-point" (not stars) objects.

Another brief word on the subject of integrated magnitude. Not all supplied magnitudes you see in guide books, planetarium software etc are measured at visual wavelengths. If you see it prefixed with a (V) -- this is a visual magnitude. Our eyes have different sensitivities to photographic plates, CCDs and photo-electric photometers (both with and without coloured filters). If you see something like Magnitude (B)+12.5 this is magnitude measure from a blue sensitive plate, There are other prefixes like (p) photographic (ir) infra-red (z) another photographic system are sometimes useful as a loose guide but are a long way from being reliable so far as your eye is concerned.

Also, the fainter you go a great many of the supplied magnitudes (fluxes) in the literature become increasingly unreliable for the visual observer. Above (integrated) magnitude 12, nearly all of them are very accurate. Magnitude 13 many, probably most, magnitude 14 some might be a useful guide, magnitude 15 -- pirates code. You might grumble "why" -- there is a good reason that has to do with the professional study of galaxies and galaxy formation. It doesn't help much to have an integrated visual magnitude for most purposes in the professional world few bother.

Best,

L.

P.S I add this addendum to highlight to the reader that some of the measures I have used are arc-seconds and in other cases arc minutes -- allowances should be made accordingly on the figures.

P.P.S See further addendum below.

Last edited by ngcles; 18-02-2019 at 12:50 AM. Reason: To correct units
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Old 16-02-2019, 05:24 PM
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mental4astro (Alexander)
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Superb stuff, Les!

Thanks for your input. I was so caught up with the flow of my posts I forgot to complete where I was going with surface brightness and magnitude vs object size.

Thanks for picking up where I dropped the ball.

Alex.
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Old 16-02-2019, 06:13 PM
Hemi
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Brilliant!

Thank Alex and Les, great information and very helpful.

Best w

Hemi
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Old 17-02-2019, 12:16 AM
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Converting magnitudes per square arc minute to magnitudes per square arc second

Hi All,

This is the second addendum to my post above but I have placed it in a separate post for clarity. Read the first addendum at the bottom of my first comment first:

The conversion of surface brightness per arc minute to surface brightness magnitude per arc second so they are in the same units is more straightforward than you might think. There are 3600 square arc seconds per square arc minute. 2.5 * log (3600) = 8.8907.

So to convert (as a quite accurate approximation ) from one to the other, simply add 8.9 to any magnitude provided in arc minutes to get square arc seconds. This would provide an average S.B magnitude for M104 (The Sombrero Galaxy) of 20.5 magnitudes per arc second -- about 4.5 magnitudes higher than a perfect (in space) background of 25.0. Quite a contrast! For NGC 45 -- 23.6 quite a low contrast. How can the eye detect such faint light.

Well, you have to remember the brightness of one is superimposed over (on top of) the other. In theory even in a sunlit sky the very low surface brightness NGC 45 is still 1.4 magnitudes brighter than the daytime sky -- because its light is superimposed upon and added to the skies' luminance. But, the eye at such high brightness levels cannot detect the difference -- so it can't be detected. In modern parlance -- the sensor is flooded.

The same problem to a much lesser extent applies at night in the difference between a suburban sky and a rural one or better a premium site. The darker the native background + the larger the telescope (assuming good contrast elements in the 'scope), means it is easier for the eye to detect smaller differences between a very low surface brightness object and the native sky it is superimposed upon.

I hope that is some help for the reader.

Best,

L.
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Old 17-02-2019, 07:02 PM
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Thankyou Alex & Les,

Again, very informative posts that help us relatively inexperienced (as well as total Noobs) observers figure out how to go about seeing these things that seek to elude us.

I have this thread of info, as well as Alex's series on Nebula, Planets & Lunar compiled into three Word documents...

Happy to send them to someone if you would like to save yourself the effort or, maybe I can send them to an admin to have them posted in the Projects & Articles area..

Someone let me know via PM if they would like the documents I have compiled from these very useful threads....
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Old 22-02-2019, 02:55 PM
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Very useful, thanks Alex and Les.

Dear Mod - I suggest this is made "Sticky"?
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Old 23-02-2019, 08:31 AM
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Thanks Eric

With respect to making new stickies, new stickies are not being made. The Beginners Forum is too heavy with them alone as an example. I've corresponded with the mods about this, and also offered some ideas on how to restructure some of the fora so old stickies are placed into the one thread so they are not lost (but no new posts can be made) and allows for space for new and more up to date/relevant stickies to be started, but the administrators are not so enthusiastic about IIS to devote time to it anymore. I've even submitted articles and questions about articles to the admins, but nothing happened. No reply to my PMs either. I've been trying/agitating for change, as have the mods, but there's only so much that can happen if the admins are not enthusiastic anymore. Real shame.

Alex.
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Old 23-02-2019, 09:54 AM
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Thanks for the write up Alex. I remember looking for galaxies one night and thinking maybe everything on my telescope was out of alignment because I couldn't see a thing.

After a while, with good dark adaption and lots of effort I found the Sombrero galaxy as a luminous smudge, but that was sufficient for me and my imagination.

The magnitude of the galaxy really does make the beginner think it should be a matter and point and go but a bit of knowledge goes a long way and helps to understand what you are seeing.

Lots of great and practical information here, much appreciated
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Old 23-02-2019, 10:49 AM
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Quote:
Originally Posted by Placesinthedark View Post
The magnitude of the galaxy really does make the beginner think it should be a matter and point and go but a bit of knowledge goes a long way and helps to understand what you are seeing.
Or NOT seeing! This is probably the most frustrating part of it all.

"But why can't I see anything! It should be as easy as seeing a jumbo jet!"

Yup, it's happened to me too. And still does even after 35 years of using scopes.

In my other thread about Understanding Nebulae I discuss our human eyes, how to best exploit them and how to look through a scope. All of this also applies to observing galaxies.

Rush, and you will miss stuff. Be impatient, and you won't see it. Remember, astro is a quite pursuit. It demands that you slow down. Be patient with yourself, your eyes and your gear. Looking at faint things requires teaching yourself how to see again. It is a totally different skill-set from looking at things during bright daylight.

So, slow down, DAMN IT!

Alex.
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