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Magnitude limit of your seeing in binoculars

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#1 Blair

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Posted 09 December 2003 - 09:33 AM

I'm just curious on what others see, on average, in their binocs in their usual observing spot on a moonless, clear night. I just bought Oberwerks 11X70mm and have just ordered their 8X56mm. As soon as the Moon moves on I'll post what I can see in my 11X70s. Please state sight conditions (very little, some or quite a bit of light pollution for example) and type of binocs. Thanks to all those whose reply. :question:

#2 EdZ

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Posted 09 December 2003 - 12:20 PM

Binoculars I used were:
Oberwerk Deluxe 20x80
Fujinon 16x70 FMT-SX
Oberwerk ’03 15x70
Oberwerk ’02 15x70
Pentax WP 16x60
Pentax PCF III 12x50
Orion Ultraview 10x50
Swift Ultralite 8x42

All binoculars were mounted on stable tripods. Viewing from my front yard. Light pollution not evident, but city lights 15 miles away on the southern horizon.

In mag 5.2 to 5.6 skies I recorded these scope observations to lend credence to all subsequent binocular observations. The increase of magnification needed in the SCT to see a mag 12.0 star gives a very good indication of the benefit magnification provides to Limiting Magnitude. Also, it gives a pretty good indication of what kind of binocular it would take to see a mag 12.0 star.

Mag 11.0 stars were first seen in a Stellervue AT1010 78mm scope at 40x. A mag 11.4 star was not seen in 78mm at 40x but was barely seen at 65x.

A TV85 did not see mag 11.3 at 40x but 11.3 could be seen at 100x. The TV85 did not see mag 11.9 or 12.0 even at 100x.

A Celestron C5 5” (125mm) SCT scope with a 30mm Ultima at 45x could just barely make out mag 11.2-11.3 stars, none higher. The 125mm scope did not see mag 11.9 or 12.0 stars until magnification was increased to about 100x. With the 5”, a 12.0 mag star was not seen with a 26mm Meade SP at 50x. It was momentarily glimpsed using 18mm UO ortho at 75x and was seen steady using 12.5 mm UO ortho at 110x. With this C5 scope I have recorded a maximum limit of mag 13.1, but only at magnifications up around 150x-175x.

Assuming a 20% gain for two-eyed vs. one-eyed viewing, a 78mm aperture is approximately equivalent to a 70mm binocular and the 85mm scope is approximate to a 77mm binocular. The 125mm SCT has the equivalent light gathering (due to c.o.) of 115mm aperture, approximately equal to a 105mm binocular.


MY BINOCULAR OBSERVATIONS
Fujinon FMT-SX 16x70, Oberwerk 15x70/03, Oberwerk 15x70/’02, Pentax PCF-WP 16x60 and PCF III 12x50, Orion Ultraview 10x50, Swift Ultralite 8x42
SKIES NELM 5.6 direct, 5.8 suspected averted.
Transparency 9-10 and Seeing very good. seeing improved with time

Swift 8x42 –several 9.5 seen, saw 9.6 and 9.75 averted. 10.0 was suspected averted.

Orion 10x50 - wide-field exhibited poor sharpness in outer 40% of fov. LM was never seen in outer 40% of fov, needed to be on-axis to see best LM. Best observations were 9.8 to 10.0 direct.

Pentax 12x50 – See 10.2, 10.3 averted, and 10.5 averted suspected.

Pentax 16x60 - see 10.5 direct, 10.7 averted and 10.8 suspected averted. Did not see several others at 10.7 to 10.8.

Oberwerk 15x70/'02 - 10.5 was the faintest star that could be seen. A mag10.6 star was seen averted. 10.8 could not be found with the ’02 Oberwerks

Oberwerk 15x70/’03 –see 10.4, 10.6 and 10.7. Did not see 10.5, 10.7, but the 15x70/’03 could see 10.8.

Fujinon 16x70 – Saw both 10.83 and 10.83 at the same time. Saw 10.76 and 10.8 at same time. Did not see 10.86, 10.93, 10.9, 10.96 or 11.0.


Summary Results, in NELM 5.6-5.8 skies:
Oberwerk 20x80 see 10.8 direct (lower NELM skies).
Fujinon 16x70 see 10.8 direct.
Oberwerk 15x70/03 see 10.7 direct, 10.8 averted.
Oberwerk 15x70/02 see 10.5 direct, 10.6 averted.
Pentax 16x60 see 10.5 direct, 10.7 averted, some brighter not seen.
Pentax 12x50 see 10.3 direct, 10.5 averted, some brighter not seen.
Orion 10x50 see 10.0 direct, none fainter averted.
Swift 8x42 see 9.5 direct, 9.8 averted.


The acuity of the observer will have an affect on LM. An observer with better acuity will record a deeper naked eye limit but also the more acute observer may in fact see fainter stars in the eyepiece.

The altitude of the stars observed will have an affect on LM. Skies closer to zenith are observed through less air and also have a better chance of being darker.

The experience, persistence, concentration level and patience of the observer will have an impact on LM.

The Pleiades is a good object to use for recording the NELM. Another area for recording NELM is the V of Taurus. Both of these NELM targets provide for a variety of stars ranging from bright to 6th magnitude.

If double stars are included in the survey, the integrated magnitude must be determined. An example of this is gamma Delphinus. With components of mag 4.5 and 5.5 separated by 9.6” (4.5-5.5/9.6”), it has an integrated magnitude of 4.15 that can easily be seen when compared to the other stars in Delphinus.

It was very difficult even with the best 70mm binoculars to see stars beyond mag 9.7. A passing glance in the eyepiece would not show stars beyond 10th mag. It required a concentrated period at the eyepiece, allowing gaze to move around. Once glimpsed, stars beyond mag10 could then be seen directly.

More than once I noted that bumping the binoculars and causing any minor shake eliminated most stars over mag 9.0 from view. As soon as they settled quite after 5 or 10 seconds, 9.5 and 9.7 mag stars were readily seen. Absolute steadiness and some persistence is required to see mag 10.2 and 10.3. Continued persistence and effort is required to see mag 10.4. Stars beyond that are not seen in any binocular without expending a considerable concentrated effort, sometimes over as much as several minutes.

Without a steady mount the average observer is probably not seeing beyond mag 9.5 with any binocular up to 70mm. Hand held observations with most of the above binoculars gave results 0.5 to 1.0 mag less than when mounted. Without considerable effort the average observer is not seeing much beyond mag 10.2 to 10.3. The practiced observer using 70mm mounted binoculars will see stars of 10.5 to 10.8 only with considerable effort and absolutely still binoculars. The faintest star observed was mag 10.83. There were two stars at this magnitude seen on several observations with the Fujinon 16x70s and the Oberwerk 15x70/’03 and the Oberwerk 20x80s. No fainter stars were found with 20x80s, although that can be attributed to some difference in quality and a lower NELM observation.

It is not likely anyone will see mag12 stars in anything but the largest 100mm+ binoculars under mag 6.5+ skies.

edz



#3 Blair

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Posted 09 December 2003 - 02:44 PM

Edz thanks for the great reply.

I think it will be really interesting to see what can be seen from different locations, light pollution, eyes and equipment.

Thanks again Edz.

Blair




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