How Do You Observe M32?

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turboscrew
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Re: How Do You Observe M32?

#21

Post by turboscrew »


- Juha

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Grab 'n go: Omegon AC 102/660 on AZ-3 mount
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Re: How Do You Observe M32?

#22

Post by Don Pensack »


turboscrew wrote: Sat Jun 12, 2021 12:29 pm Note, however, that (visual) magnitude is not linear scale, but (if I recall it right) about 2.7 exponential function.
That is: a star of magnitude 3 is about 2.7 times brighter (in terms of flux) than a magnitude 4 star.
The magnitude scale started with Ptolemy, who divided stars into levels or magnitudes, calling the brightest 1st Magnitude and so on down to the dimmest naked eye stars of 6th Magnitude.
Once we developed measuring devices, we saw that 1st Magnitude covered quite a spread but the difference from the dimmest 1st magnitude stars to the dimmest naked eye stars was about a 100:1 ratio.
Since the response of the eye is logarithmic, that meant each magnitude was the 5th root of 100 dimmer than the next, a ratio of 2.512.
And, it meant the 1st magnitude covered a spread from magnitude 1 all the way to -1.42 (Sirius).
The 5th root of 100 stuck, and has been the definition of a 1 magnitude difference ever since = 2.5118864

As for surface brightness, the calculators that compare it to Total Integrated Magnitude (the light compressed into what the eye sees as a point--about a 1' size) show magnitudes in magnitude per square arc minute or magnitude per square arc second.
The mpsas is what professionals prefer because so many objects are quite small. So you understand how it works, a 1' squareobject of magnitude 16 would have a surface brightness of magnitude 16. But a 2' object of magnitude 16 would have a surface brightness 1/4 as great,
or magnitude 17.2. The first object would appear brighter than the second even though both have the same magnitude. That explains how M33, with a total integrated magnitude of 5.7, and visible to the naked eye in a dark sky, can have a surface brightness of mag.14.1.

The size of any deep sky object is related to how long the exposure is, so professionals have defined the size of objects at the isophote (line of equal surface brightness) to be magnitude 25 per square arc-second, which is magnitude 16.1 per square arc-minute.
It corresponds to the average reach for a visual user in an amateur-size scope in dark skies. Of course, the objects are larger than that--the limit was set back when photographic plates were being used for imaging. Some observers see well beyond the magnitude 25 isophote.
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Re: How Do You Observe M32?

#23

Post by Don Pensack »


GCoyote wrote: Sat Jun 12, 2021 12:40 pm Understand, whereas the surface brightness is just the flux averaged over the apparent surface?
Yes, and that is a good point. Galaxies are not uniformly bright--their cores and nuclei are brighter than the fainter outer arms.
Two galaxies of identical size to the magnitude 25 isophote might have radically different core brightness peaks.
There is a "brightness gradient" from the edge to the center. This brightness gradient would be useful information to compare to surface brightness and total integrated magnitude to determine the visibility of at least a part of the galaxy.

Enter in the RC3 of Vaucouleurs (downloadable on-line). His listed of many thousands of brighter galaxies contains, for about the brightest couple thousand of them, a field called m'_e, which takes the brightness of the peak,
draws a brightness gradient down to the standard magnitude 25 isophote, finds the 50% point and lists the surface brightness of the brightest 50% of the galaxy.
I wrote an article explaining why this cannot be used as a "visibility index" that is on my website and elsewhere, but that figure forms a very interesting bit of information to compare to TIM and SB to give a more complete picture of the galaxy.
AND, it seems to correspond to the "apparent" brightness many amateurs see in a galaxy when the fainter parts are blocked by light pollution.
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Re: How Do You Observe M32?

#24

Post by turboscrew »


Don Pensack wrote: Sun Jun 13, 2021 3:52 pm
GCoyote wrote: Sat Jun 12, 2021 12:40 pm Understand, whereas the surface brightness is just the flux averaged over the apparent surface?
Yes, and that is a good point. Galaxies are not uniformly bright--their cores and nuclei are brighter than the fainter outer arms.
Two galaxies of identical size to the magnitude 25 isophote might have radically different core brightness peaks.
There is a "brightness gradient" from the edge to the center. This brightness gradient would be useful information to compare to surface brightness and total integrated magnitude to determine the visibility of at least a part of the galaxy.

Enter in the RC3 of Vaucouleurs (downloadable on-line). His listed of many thousands of brighter galaxies contains, for about the brightest couple thousand of them, a field called m'_e, which takes the brightness of the peak,
draws a brightness gradient down to the standard magnitude 25 isophote, finds the 50% point and lists the surface brightness of the brightest 50% of the galaxy.
I wrote an article explaining why this cannot be used as a "visibility index" that is on my website and elsewhere, but that figure forms a very interesting bit of information to compare to TIM and SB to give a more complete picture of the galaxy.
AND, it seems to correspond to the "apparent" brightness many amateurs see in a galaxy when the fainter parts are blocked by light pollution.
This? https://cdsarc.unistra.fr/viz-bin/cat/VII/155
- Juha

Senior Embedded SW Designer
Telescope: OrionOptics XV12, Mount: CEM120, Tri-pier 360 and alternative dobson mount.
Grab 'n go: Omegon AC 102/660 on AZ-3 mount
Eyepieces: 26 mm Omegon SWAN 70°, 15 mm TV Plössl, 12.5 mm Baader Morpheus, 10 mm TV Delos, 6 mm Baader Classic Ortho, 5 mm TV DeLite, 4 mm and 3 mm TV Radians
Cameras: ZWO ASI 294MM Pro, Omegon veLOX 178C
OAG: TS-Optics TSOAG09, ZWO EFW 7 x 36 mm, ZWO filter sets: LRGB and Ha/OIII/SII
Explore Scientific HR 2" coma corrector, Meade x3 1.25" Barlow, TV PowerMate 4x 2"
Some filters (#80A, ND-96, ND-09, Astronomik UHC)
Laptop: Acer Enduro Urban N3 semi-rugged, Windows 11
LAT 61° 28' 10.9" N, Bortle 5

I don't suffer from insanity. I'm enjoying every minute of it.

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Re: How Do You Observe M32?

#25

Post by Don Pensack »


Yes, that's it.
Here is another link that also has the indices and where you can look at it online without a download:
https://heasarc.gsfc.nasa.gov/W3Browse/all/rc3.html
Astronomer since 1963
Currently using a 12.5" dob and a 4" apo refractor
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