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SKY DARKNESS, MAGNITUDE, AND TELESCOPE SIZE

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SKY DARKNESS, MAGNITUDE, AND TELESCOPE SIZE

Every observing guide seems to list a figure, Magnitude, for the deep-sky objects. This magnitude figure is generally the ¡°total integrated magnitude¡± of the object. Imagine reducing the object to the size of a star, or blowing up the image of a star to the size of the galaxy, and you have a rough approximation of the Total Integrated Magnitude (henceforth called TIM) of that extended object.

Many, but not all, observing guides also list Surface Brightness Magnitude (henceforth called SB) as an additional magnitude of the deep-sky objects in the lists. This can be calculated more than one way, but is usually expressed as the average brightness of each square arc-minute (1/3600 of a square degree) of the extended object. This figure is usually lower than the TIM figure.

The issue for both of these magnitudes is where to cut off the ¡°edge¡± of the deep-sky object. Longer time-exposures of most deep-sky objects just keep showing ever-increasing sizes for those objects. So an arbitrary cut-off point, the faintness ¡°isophote¡± (contour line of equal brightness) is often set at magnitude 25.0 per square arc-second (very approximately mag.16.33 in sq. arc-min parlance). This is about the brightness ¡°edge cutoff¡± easily seen on most of the photographic plates taken for magnitude studies at the professional observatories.

The relevance of these two measurements for we amateur observers is that the chosen cutoff is usually fainter than we can see so we don¡¯t actually see all the surface area used for the calculation of the overall brightness of the galaxies in question. Accordingly, the TIM calculation OVERSTATES the brightness we see because it includes a lot of area we can¡¯t see, and the SB calculation UNDERSTATES the brightness of the object because it calculates the average brightness using a lot of faint area we can¡¯t see.

You can understand all that, and still be confused about which figure has the most relevance for the amateur observer.

Take M33, for example. At magnitude 5.7 (TIM), it¡¯s visible to the naked eye in a dark site. Yet, it¡¯s not easy in most scopes. A lot of beginners have trouble finding it. The SB figure tells why¡ªit¡¯s magnitude 14.1 per square arc-minute. That¡¯s fairly faint and though it should be within reach of most small scopes, it certainly won¡¯t stand out as spectacularly as, for example, M32, (the companion of the Andromeda Galaxy) which, although small, is outstandingly bright.

The point is you should look at both figures, if both are available, because Integrated Magnitude can only tell you how bright the object is overall not how bright most of the object seems. And Surface Brightness can only tell you how bright the object is on the average, not how bright the overall object is. Lowering the magnification makes the surface brightness higher but it has no effect on Integrated Magnitude.

Additionally, light pollution (the brightening of the sky caused by inappropriate use of outdoor lighting) makes fainter details and objects harder to see (or even invisible), so we have to compensate for the brightness of the sky when we are estimating how faint we can see. Our CALCULATOR does just that.

But how do you estimate the brightness of the night sky? How can you determine the NELM (naked eye limiting magnitude) to plug into the calculator? How can you rate your observing site? Here is a simple chart that shows the brightness of stars near Polaris. Everyone in the northern hemisphere can see Polaris in the Little Dipper, so you can use this chart to come up with a Limiting Magnitude for your site:


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DEEP SKY TELESCOPE FILTERS

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Lumicon Deep Sky Filter 48mm
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Lumicon Deep Sky Filter 48mm
Item No. 735-20112
Manuf. No. Lumicon LF3015

The best light pollution filter on the market today. Blocks all high & low pressure mercury and sodium vapor lamp light, neon lights and airglow, while transmitting the rest of the visible spectrum. Excellent for both visual and photographic applications.
MSRP: $280.00
Our Price: $199.95
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You Save $80.05

Lumicon Deep Sky 1.25 inch Filter
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Lumicon Deep Sky 1.25" Filter
Item No. 735-20102
Manuf. No. Lumicon LF3010

The best light pollution filter on the market today. Blocks all high & low pressure mercury and sodium vapor lamp light, neon lights and airglow, while transmitting the rest of the visible spectrum.
MSRP: $140.00
Our Price: $99.95
Buy Lumicon Deep Sky 1.25 inch Filter
You Save $40.05

LUMICON #29 Deep Red Filter - 2 inch
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LUMICON #29 Deep Red Filter - 2"
Item No. 735-22040
Manuf. No. Lumicon LF2045

Increase contrast and detail on the moon and planets! For use with standard 2'' eyepieces and accessories.
MSRP: $38.00
Our Price: $29.95
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You Save $8.05

Lumicon #12 Deep Yellow Filter - 2 inch
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Lumicon #12 Deep Yellow Filter - 2"
Item No. 735-22015
Manuf. No. Lumicon LF2020

GOOD FOR VARIOUS DETAILS OF MARS, SATURN, NEPTUNE.....
MSRP: $38.00
Our Price: $29.95
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LUMICON #29 Deep Red Filter - 1.25''
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LUMICON #29 Deep Red Filter - 1.25''
Item No. 735-21040
Manuf. No. Lumicon LF1045

Increase contrast and detail on the moon and planets! For use with standard 1.25'' eyepieces and accessories.
MSRP: $25.00
Our Price: $19.95
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Lumicon #12 Deep Yellow Filter - 1.25 inch
Lumicon #12 Deep Yellow Filter - 1.25"
Item No. 735-21015
Manuf. No. Lumicon LF1020

ULTIMATE CONTRAST ON LUNAR AND PLANETARY DETAIL
MSRP: $25.00
Our Price: $19.95
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