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# What Can You See?

40 replies to this topic

### #26 Starman1

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Posted 16 August 2014 - 01:16 PM

Yes.

If the object is 1' x 1', then TIM and SB will be equal.

So if SB is HIGHER than TIM, the object must be quite small, and bright.

IF it's light were spread out to 1' x 1', then it would be dimmer, and that's what you see in the second example.

if you do not know the surface brightness, but your charts have TIM and size listed, then you can use this formula to calculate SB:

surface brightness in sq.arc-min=TIM +2.512log (.7854 x max' x min')

where max' and min' are the sizes in arc minutes (you may have to calculate the decimal equivalent if your chart says: 2' 6", which would be converted to 2.1').

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### #27 cpper

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Posted 16 August 2014 - 03:19 PM

Did you know it actually makes sense ?

Well....almost. Quoting you:

"There is the Total Integrated Magnitude (the one often quoted) which is the brightness of the galaxy if the galaxy were compressed to about one square arc minute.

Then there is Surface Brightness, which is the brightness of the extended galaxy per square arc-minute (or arc-second in different figures)."

Don't you missed an 'average' somewhere ? In my mind "compressed" and "extended" is the same thing, as long as both refer to 'fitting' an object in one square arc-minute. So both definitions of TIM and SB are in my mind the same thing. What would make sense to me : Let's say the size of an object is 5 square arc-minutes. We compress(since it's larger) the object to one arc-minute, and measure it's brightness, let's say 15 mag. Then the TIM is 15, but the SB is(/makes sense to me to be) 15/size, meaning 15/5=3, am I right ? Your post doesn't make clear(to me) that there is one more calculation(TIM/size) to do to get the SB. Or maybe it's because of my poor english...

Edited by cpper, 16 August 2014 - 03:42 PM.

### #28 Starman1

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Posted 16 August 2014 - 04:50 PM

Did you know it actually makes sense ?

Well....almost. Quoting you:

"There is the Total Integrated Magnitude (the one often quoted) which is the brightness of the galaxy if the galaxy were compressed to about one square arc minute.

Then there is Surface Brightness, which is the brightness of the extended galaxy per square arc-minute (or arc-second in different figures)."

Don't you missed an 'average' somewhere ? In my mind "compressed" and "extended" is the same thing, as long as both refer to 'fitting' an object in one square arc-minute. So both definitions of TIM and SB are in my mind the same thing. What would make sense to me : Let's say the size of an object is 5 square arc-minutes. We compress(since it's larger) the object to one arc-minute, and measure it's brightness, let's say 15 mag. Then the TIM is 15, but the SB is(/makes sense to me to be) 15/size, meaning 15/5=3, am I right ? Your post doesn't make clear(to me) that there is one more calculation(TIM/size) to do to get the SB. Or maybe it's because of my poor english...

Sorry for my omission, you're right.

Surface brightness is the AVERAGE brightness of each square arc minute of the object, or the object itself if smaller than 1'.

(most objects are actually measured in magnitudes per square arc-second and then converted to minutes).

TIM is the brightness of the object overall if made 1 square arc minute.

Hence, any object larger than one square arc minute will have an SB lower than the TIM, whereas any object smaller than 1 square arc minute will have an SB HIGHER than the TIM.

So, for example, a large object like M33 can be a TIM of 5.7, but a SB of 14.1 due to its huge size.

In contrast, NGC6905 (a bright planetary in Delphinus) has a TIM of 12.0, but a SB of 10.9 (it's smaller than 1').

Thank you for making me clarify that.

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### #29 cpper

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Posted 16 August 2014 - 05:38 PM

I thank you for answering all my questions and making everything clear !     ​

Stellarium states that NGC 6905 has TIM 12, SB 12.99,  and size 1'42", curious.

### #30 Starman1

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Posted 16 August 2014 - 06:45 PM

I thank you for answering all my questions and making everything clear !     ​

Stellarium states that NGC 6905 has TIM 12, SB 12.99,  and size 1'42", curious.

Well, this is due to the many discrepancies in size that can be applied to some objects.

Usually, the size is determined to the magnitude 25 (per square arc-second) isophote (line of equal brightness), and this is often a little deeper than we can see. As a result, many, if not most, deep sky objects are smaller in our scopes than the size listed in atlases.  We actually see most of the brightest portions, but less of the dimmest portions.

But many deep-sky objects continue to get bigger as the isophote gets fainter beyond magnitude 25.

You probably have seen M31 quoted as being 3 degrees, or 3.5 degrees.  Really deep photos show the galaxy is 5 to 5.5 degrees, and possibly much larger.

It has even been discussed that the halos of the Milky Way and M31 may already be in contact (!), and the halo of M31 extending out to 15 degrees.  We don't see that--it is inferrred from studies of similar galaxies.  Recent work suggests the Milky Way has more mass in its halo and M31 has less, while M31 has more stars.

So, just exactly how big is M31?

The same is true of planetary nebulae.  Before the "poof" that sent out the large chunk of material we see illuminated, these dying stars may have sent out many other "poofs" of gas from their atmospheres.

M57 is usually described as 1.5' x 1' in size, and, indeed, that is what most amateurs see.

But deep photos of M57 ( http://astrophoto.com/Ring.htm ) show it to be more like 4' x 4', and really deep photos show it larger still.

So it is with NGC6905.  It's normal size is 0.73' x 0.63', so its SB is brighter than the 12.0 TIM.

But, long exposures show it has large extended wings not normally seen or photographed ( http://de.wikipedia...._-_RHaBOIII.png ), doubling the size in that direction, and when that extra size is taken into account, the SB lowers significantly.

All I can tell you is that this one is fairly bright in a 4" refractor in dark skies, so it should be visible in brighter skies up to a point.

The part you'll see will be <1' across, so have a fairly high surface brightness (at least of the part you see).

In essence, the size of any deep sky object will be the size that you see.  And that will vary with aperture, darkness of sky, etc.

Just, alas, another reason why magnitudes are only approximate things.

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Posted 16 August 2014 - 08:12 PM

Living near Austin the sky glow or pollution is ever present.  I live in an orange zone and I may get up to 5 mag on a really good night.  I can not see the Milky Way from my back yard and that's a shame.  I usually observe from my back yard and occasionally the front drive way.  I'm currently looking for a decent observing location.  Growing up in west Texas I grew up with near perfect skies and the Milky Way was alive with jewels for the taking, g.  Now I long for those nights!  I get them when we sometimes travel to the Big Bend area.  I'm sure I can find something within a 20 to 30 mile drive that will certainly improve my seeing.  In the 60's and 70's when I lived in west Texas, I used to lay on the hood of the car with a pair of 7X50 binoculars and spend the whole night out.... man what wonderful memories....

### #32 rowdy388

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Posted 16 August 2014 - 10:27 PM

Mag 6 or better where I live in rural upstate New York.  I just roll the12" dob out onto the deck.  The stars at night ..are big and bright ...deep in the heart of NY State. (in or close to the Adirondacks  where I live anyway)

Except of course its always cloudy here so I never actually get to see any stars.

Dave Y

### #33 cpper

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Posted 17 August 2014 - 05:39 AM

Yep, the size...it explains everything.  I think the same goes for comets, with their long fade-out tail(s).

Tony, I also live in an orange zone, but can see 5.5 on a good night. Either my map isn't accurate or your transparency is bad.

Apollo

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Posted 17 August 2014 - 09:24 AM

Hey Andi - I'm not sure how accurate the color overlay is but I'm in an orange zone barely out of red.  I think transparency and age/condition of eyes is a factor as well...  clearer skies....

### #35 JonNPR

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Posted 17 August 2014 - 01:31 PM

I'm using the AL's Urban Observing Club program as an incentive to set up in our Red Zone backyard more often. Our club has a good dark sky site an hour or so from here, but the back yard is a bit closer! With street lights, lights from inside neighboring homes, and urban sky glow, the best I can usually get is about mag 4.3.

For me, the much worse condition is the small amount of sky visible due to trees and houses. Using SkyTools 3, I've sketched out a rough usable horizon and added it to the excellent program. Looking at ST's Overhead Sky chart, I've only got about a quarter of the sky visible. And a chunk of that to the north is wiped out by the city's light dome. On the positive side, much of what remains is the area close to zenith.

Some of the usual methods to improve urban observing have really helped a lot:

- Observe later at night, lights are reduced

- Use an observing hood for faint objects, to eliminate all ambient light at the eyepiece

- Talk to our next door neighbor about minimizing their worst indoor light, that shines directly down on me

- Use go-to mount for faintest objects, and for extended viewing at high magnifications.

I'm about half way through the AL Urban program now.

Jon

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Posted 17 August 2014 - 03:31 PM

Jon I've used similar actions but on my to get list is an observing hood.  I think that item would be very useful!

Edited by CelestronDaddy, 17 August 2014 - 03:33 PM.

### #37 penguinx64

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Posted 19 August 2014 - 06:36 PM

Magnitude 4 is the max for most nights in Holland.

Mag 6 on an OK night.

Mag 8 on a good night.

Mag 10-12 on a really really good night.

Heck, if I can see the Moon, it's better than most nights.

### #38 Gastrol

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Posted 19 August 2014 - 07:56 PM

I live under the downtown L.A. light dome and less than a mile away from Dodger Stadium.  I sometimes go to my observatory just to chill out, drink wine, and tinker with my scopes.  I might even roll the roof open when the planets are up......LOL...

### #39 havasman

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Posted 19 August 2014 - 08:41 PM

Tony, you might investigate Enchanted Rock State Park for observing opportunities as they either just received or have nearly completed the process of getting their dark sky rating. The Dark Skies Apparel hood is GREAT!

Edited by havasman, 19 August 2014 - 08:41 PM.

Apollo

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Posted 19 August 2014 - 09:13 PM

Tony, you might investigate Enchanted Rock State Park for observing opportunities as they either just received or have nearly completed the process of getting their dark sky rating. The Dark Skies Apparel hood is GREAT!

You are certainly correct on Enchanted Rock State Park.  I haven't observed there but I've heard good reports for observing around that area.  It's a bit of a drive for me... I live near Round Rock even though I have an Austin address.  I've been checking some areas out to my west that might be good and around 30 miles max.  The next item I'd like to get is the hood.  I think it would be very useful.....  Thanks ...

### #41 esd726

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Posted 19 August 2014 - 10:22 PM

Haven't ever really figured what my limiting magnitude is, I guess I need to.  I have been able to see M33 naked eye a couple times-enough that I can say I saw it-nothing I could show family though.  The Milky Way is very nice overhead and you can follow it easily South to Sagittarius and "breaking off" on to Ophiuchus  and even better going North.    I'm not sure what my backyard skies are, but I'm happy with them (when I can get out).  I would really like to see what DARK skies are like sometime though..... or maybe not, I might not be as content with MINE

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