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Re: (meteorobs) Bob Lunsford's Suggestions/Help & Nov. 19/20 Correction
Hello,
This is a very interesting topic! The limiting magnitude has a significant
influence on the ZHR, but may be a weak link. Over the years, I have noted
remarkable differences in Lm-values between observers at identical
locations & dates (differences up to Lm ~0.5-0.6 in a group of ~15
observers)! The strange thing is; a better Lm not always implies an higher
hourly rate! However, this is just a personal impression -- it should be
carefully studied first on our complete dataset.
Fields in the milky way (e.g. 14) usually don't deliver divergent Lm-values
to me, but one should note that the light pollution in our country is a
bigger influence than the milky way ;-). Because of this light pollution,
and therefore low Lm-value (~5.0 to 5.5), we always take the 'best' Lm
count from at least 3 or 4 areas. This avoids the "gap" problem, as Rainer
noted.
Furthermore, I have noted that our new observers systematically get lower
Lm's. This may be caused by the "memorisation"-effect. For example; I have
counted the stars in areas 1, 6, 7, 14 or 18 at least hundreds of times in
my life, and I can see these stars in front of me with my eyes closed..
This "memorisation" may result in higher countings, because one knows where
to look for dim stars.... Thus, it may not be a good idea to use the same
counting areas year in year out.
However, we studied this effect last year by performing additional
countings in different fields (including some new "personally invented"
areas). We found that there was little or no difference compared to the
"classical" fields. This could be studied more carefully as well though...
Kind regards,
Geert
PS: New observers also have lower hourly rates, so it's ok for them to have
lower Lm-values :-)
At 23:15 28-11-2003, you wrote:
>While I think some of the count areas are difficult to use (involvement
>with the Milky Way for example) the count area method seems to work for
>me. Since I observe from the same location nearly every time I have a
>pretty good "general-impression" of the limiting magnitude when I
>arrive. I do the count areas to see if the counts agree or disagree with
>my general impression. I don't spend a lot of time with this and I don't
>agonize over the exactness of the counts - I have severe astigmatism so
>faint stars often just look like dim smudges to me. Altho I sometimes have
>it happen that my first area count before I am completely dark-adapted
>shows a much better limiting magnitude than any later count.
>
>One problem with a selected series is that it may have to be a different
>series as the night goes on if one is observing for several hours. The
>selected series should be high enough to avoid any atmospheric dimming
>effects - a series that works fine at 8 pm may well be too low by 11
>pm. One would almost have to make a series of "custom" series for his
>particular latitude to use as the night progresses.
>
>When it comes to nights with bright Moonlight I think accuracy to a tenth
>of a magnitude is wishful thinking. Depending on haze, cirrus, dust, fog,
>etc the sky is much more unevenly lit (or unevenly dark) when the Moon is
>up. Often in Iowa there is horizon-haze so the sky is extra bright quite a
>ways up all around the horizon when the Moon is shining. The darkest part
>of the sky is usually opposite the Moon but somtimes doesn't reach up to
>the zenith. So where do you do any sort of area count or star series
>count? I think the error bars are large on Moonlit nights not only
>because of the brightness of the sky, but also because of the difficulty
>in determining what the limiting magnitude "really" is.
>
>Paul Martsching
>
>
>Michael Linnolt wrote:
>>This is definitely a shortcoming of the current count-area based LM
>>estimating system suggested by IMO. Several other problems with it are:
>>(1) Difficulty of determining if a faint star near the edge is inside or
>>outside of the artificial boundary of the area, especially using averted
>>vision (2) If you are under dark conditions, the number of faint stars
>>near your limit in a region is large and/or close spaced and it gets
>>difficult to make an accurate count, again particularly using averted
>>vision. (3) There is no smooth transition in LM, since each star counted
>>results in a step-wise change in LM, which is not realistic. (4) You
>>waste a lot of time to struggling with these difficulties every time you
>>need to re-evaluate the LM during a session, which may discourage people
>>from doing that and be detrimental under changing conditions.
>>I use the simpler system of just finding a sequence of stars covering the
>>range of LM expected, in 0.2 mag steps. I use Tycho2+Bessell corrections,
>>the most accurate available for brighter stars. I need just a half dozen
>>stars or so, which are easy to keep track of, and I get a very accurate
>>and continuous estimate based on which star I can just see with moderate
>>effort by averted vision. I can re-check the LM frequently and
>>effortlessly during my session without interference of even counting the
>>meteors.
>>I would highly recommend IMO discard the count-area based system with a
>>simple one like I use.
>>Mike Linnolt (LINMI)
>>--- RainerArlt <rarlt@aipdot de> wrote:
>>
>>>At low limiting magnitudes, there are severe gaps in
>>>the
>>>conversion tables from star count to LM. This is of
>>>course
>>>since not every tenth of a magnitude is 'occupied'
>>>by a star in the count area. You will often encounter a
>>>significantly
>>>lower LM in one of the counts made the same time. A
>>>count in area #4 of 7 stars may be 5.1, but since the next
>>>star is at
>>>something like 5.8, the LM could have been also 5.2,
>>>5.3, 5.4,
>>>... 5.7 with the same count in #4.
>>>
>>>I'd suggest to use the highest count for each time
>>>and average
>>>these (unless LM better than say +6.0). In your
>>>example, this
>>>would come down to
>>>
>>> 07:06 5.5
>>> 07:27 5.9
>>> 07:50 5.3
>>>
>>>AVERAGE 5.57
>>>
>>>The method of course requires that enough star
>>>counts are made
>>>during the observation and that -- most critically
>>>-- several
>>>areas are counted.
>>>
>>>Please note also that updated LM conversion tables
>>>based on
>>>the Tycho catalogue are available at
>>>
>>> http://www.imodot net/visual/lm.html or
>>>
>>>http://www.amsmeteors.org/imo-mirror/visual/lm.html
>>>
>>>
>>>Best wishes,
>>>Rainer
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>
>
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>
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