Telescope Reviews: How to Determine Limiting Magnitude

Leaving the Variable Stars out is a good idea. Even reference to the recent observations recorded on the pages of the AAVSO shows that recorded observations and estimates of magnitude can vary by as much as 0.3-0.4 mags on any given night from one observor to another.

Doubles however can be used because there is a fixed formula to determine the integrated magnitude of any double given the magnitudes of the two components. I use doubles on my charts.

Although it eventually happens through repeated use, I don't try to memorize any of my charts. Rarely am I out without my logbooks. All my drawings are in my logbook. I simply refer to the drawing, or I make a sketch and refer to the logbook later.

I've copied to here some excerpts from my Lab Report "Limiting magnitude in Binoculars". Some useful information.

Assessing and Recording NELM

There is some difficulty associated with determining NELM. To me, it seems much more difficult observing a star unaided at the limits of vision than it does observing the limits of vision through optics. Add to my previous discussion that observing LM requires considerable persistence and you can see that determining NELM may require just as much effort as determining LM in an instrument. In NELM 5.8 skies, while I can observe stars of 4.5 to 5.0mag directly rather quickly, it sometimes requires 5 minutes of concentrated gazing to record a 5.7 or 5.8mag star.

There sometimes may not be a sufficient variety of magnitudes in close proximity to step up in progression without moving the gaze to an entirely different area of the sky. Although I have at least five different indexed charts that I use for determining limiting magnitude, there are some gaps on every one of those charts, usually on the order of 0.2 to 0.3 mag. Determining LM almost always requires observing and noting more than one location.

When stars are too close in proximity to one another it can be difficult to tell how many are seen. CR399, Brocchi’s Cluster has a good array of stars ranging from mag 5.2 to 7.2, however the 5.2, 5.6 and 5.8 stars are so close together (separated by 15 arcminutes) for naked eye observation, even when it can be determined more than one star is visible, seldom can it be determined if three or only two are being seen. So, with this target, often times I cannot differentiate between mag 5.6 and 5.8. Both very dark skies and excellent visual acuity would help with this effort.

If double stars are included in the survey, the integrated magnitude must be determined. A good 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.

If conditions appear to be changing throughout the evening, NELM should be recorded again. On more than one occasion I have had NELM improve or decline in less than an hour.

NELM can vary from one area of the sky to another and certainly will vary from horizon to zenith. Observations should be made in the vicinity of the planned targets.

A sufficient variety of stars, even of similar magnitude, should be attempted. Sometimes observations of fainter stars can be recorded while nearby brighter stars go unseen. An example of this is several observations of TX Pisces at mag 5.8 (Aug-Sep ‘03) having been recorded while a nearby 5.7 mag star remained unobserved. Color index may be contributing to this condition. (However, more than likely, TX, a variable, may not have been 5.8 at the time. TX is red).

The color of a star will have an impact on LM. For two stars of equal magnitude, a red star will appear fainter, a blue star will appear brighter. The observer would reach a deeper LM if observing a field of blue stars. A review of the article "Telescopic Limiting Magnitudes" by Bradley Schaefer may provide a possible explanation for this phenomenon. " ...the reported magnitudes of stars are in the V magnitude system... consider the case of two stars with equal V magnitude but different color. ...if night vision were being used the redder of the two stars would appear fainter."

NELM should be considered as that faintest star that can be confirmed whether seen direct, averted or even glimpsed. The key is that it is confirmed. It is much easier to determine the NELM in a transparent sky than in a washed out moonlit or hazy sky. Observation is made more difficult when contrast between a brightly lit sky and a star is so much reduced.

edz

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