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  1. #1
    Tony Flanders's Avatar
    Tony Flanders Guest

    Default NGC1647 Open cluster - Help in field testing a draft cluster magnitude chart (PrisNo6) wrote in message news:< com>...

    It is rather late, but I finally got around to field testing your chart
    the night before last. It was just the thing for a busy night with nearly
    full Moon and obviously poor seeing, when a full-fledged observing session
    would have been more trouble than it was worth. Instead, I viewed this
    lovely cluster through my 70mm refractor looking out my dining-room
    window. Thanks for a very pleasant half hour!

    The results have few surprises; broadly, they confirm the figures that
    I have already posted at
    The transparency was good but not great, it was early in the evening
    when the light pollution is worse, and the 4-day-before-full Moon
    was nearly at the zenith. The cluster was about 45 degrees above the
    horizon. I estimate the sky brightness at the cluster as mag 17.0
    per square acrsecond.

    At my lowest power of 16X, the cluster was just a tantalizing ghost;
    five stars clearly visible and hints of many more. Raising the power
    to 60X brought large numbers of stars into visibility. The faintest
    stars seen at various powers were:

    16X - #102 [mag 9.4]
    20X - #099 [10.0]
    60X - #031 [10.6], maybe #048 [10.7], but hard to split from #049 [10.3]

    Visibility tended to follow the listed magnitudes except for a clear
    bias effect that stars are harder to see in crowded sections of the
    cluster than in isolation. Thus, at 16X, the most obvious stars were
    #022 [9.1] and #084 [9.2] on the edge of the cluster, more obvious
    than mag 8.7 stars in the heart of the cluster.

    In a few cases, I was surprised that stars with fairly different listed
    mags seemed about equally easy (or hard) to see. Notably, at 20X,
    #094 [9.7] seemed no brighter than #099 [10.0], and at 60X, #065 [9.6]
    seemed little brighter than #066 [10.3].

    The chart would be easier to use if the dot sizes were significantly
    smaller. Correlating the star numbers on the correct-image chart with
    the magnitudes on the mirror-reversed chart was a minor nuisance.

    - Tony Flanders

  2. #2
    PrisNo6's Avatar
    PrisNo6 Guest

    Default NGC1647 Open cluster - Help in field testing a draft cluster magnitude chart (Tony Flanders) wrote in message news:< m>...

    Tony -

    Thanks again for taking the time to review the NGC1647 magnitude
    chart. At 41 deg N, I'm still socked in under the jet stream and
    cloud covered skies. I'm glad that you found the chart enchanced your
    enjoyment of a half-hour of evening viewing. That is part of the
    intent of the chart. For the role of this cluster in astronomical
    history, you might want to take a look at Hertzsprung (1915).
    Hertzsprung, E. 1915. Effective wavelengths of 184 stars in the
    cluster N.G.C. 1647. ApJ 42:92H

    You noted the following probable magnitude errors in the chart:

    In response to your notes I adopted the following correction procedure
    before adopting a manual adjustment to the magnitude charts.

    1) Is the difference outside the expected range of observing error?
    2) Check the plotted data.
    2) Recheck the observed magnitudes in less light polluted skies.

    The magnitude differences for these stars are outside the expected
    range of observing error, generally +- 0.2 mags for an "average
    observer" and +-0.1 mags for an experienced observer such as yourself.

    I checked my photometry data for these four stars. That data is
    assembled as an appendix at the end of this note. Turner's
    photoelectric data are used for stars 94 and 99; Tycho 2 data for 65
    and 66. I rechecked the V and B-V data against the source Turner
    article and the Tycho-2 catalogue.

    For Star 99, I made a transcription error for the B-V data for star
    99. It's recomputed magnitdue should be 10.1, and not 10.0. This only
    exacerbates the discrepancy between the plotted and observed
    magnitudes that you identified.

    With respect to Stars 65 and 66, Star 65 is a spectral class G0 star
    that I added back in in order to have more complete sequence of stars
    in tenths of a magnitude. The Tycho-2 color index for this star is
    high - 0.6 (10.249 B_tycho - 9.636 V_tycho = 0.6 CI_tycho). It looks
    like I should pull this one from the chart because its reddening makes
    a discrepancy from between the Johnson V plotted value and what is
    seen by the human eye. Or maybe I'll leave it in and just note it as
    an example of how star reddening causes a difference between Johnson V
    magnitudes and those seen by the human eye.

    These checks did not fully resolve the problems you identified.

    You noted that:

    and that:

    Your observing was done in poor urban skies at 17.0 mags per square
    arcsecond. From our usenet conversations during the fall of 2003, I
    believe that translates into an NELM of about 3.5 to 4.0 mags - a
    typical urban light polluted sky. The four star discrepancies
    identified occurred when you are viewing within 1.0 mags of the
    magnitude observing limit of your 70mm refractor at 20x (v10.0 for
    stars 64 and 65) and at 60x (v10.3-10.7 for stars 94 and 99).

    Since the discrepancy stars are near or at the NELM limit of your
    light polluted session, my instinct is to wait and recheck their
    observed magnitudes in ZLM or NELM v5.5-6.0 skies. I'm thinking that
    there might be some kind of background sky contrast problem that
    effects visual perception. I would like to rule that out before
    making a manual adjustment to the chart.

    If stars 48 and 49 cannot be split at 60x, looks like those two should
    be removed from the chart as unsuitable for binoculars and small
    scopes. That will result in gaps in the magnitude range by tenths of
    a magnitude.

    Any recommendations?

    Okay, I admit it - -- I crapped out on doing the star numbering
    system charts in even and odd reversals because the labels have to be

    Which do you think would be the most useful to the most number of
    amateur observers - the even (mirror) or odd number of reflections?

    Any ideas on software that would plot these kinds of charts
    automatically? Am I doing this the hard way?

    I will update the web posted charts reducing the star sizes to about
    40% of their current area, sometime in the next week.

    In general, it sounds like for the your observed range of v6.5 to
    about 9.0, the NGC1647 chart accurately reflects the order of the
    magnitudes as seen by the human eye through a telescope.

    That leaves about v9.0 to v13.0 still to check. Again, if other
    lurkers are interested, any help you can provide would be appreciated.

    Shortly, I will update the project site with a similar NELM estimating
    chart for v4.8 to 7.0 , using low-variance photometry stars in the B,A
    and F spectral classes, for the area around NGC1647 and the Haydes
    stream. Preliminary charts are available now. That chart supplements
    the existing International Meteor Organization Limiting Magnitude
    Chart Area No. 8 which already includes NGC1647.

    My observing notes page for this activity is at:

    Thanks again for taking the time to look at this. - Kurt

    Photometry Appendix for discrepancy stars 99, 94, 65 and 66
    Webda_id 0099* 0094 0066 0065
    X 1.05 1.96 -13.93 -13.05
    Y 7.57 4.66 -3.77 -2.45
    RA_J2000 4 46 16.1 4 46 22.3 4 44 30.9 4 44 37.2
    Dec_J2000 +19 17 06.8 +19 12 18.4 +18 58 26.1 +19 00 34.6
    HD_id HD 284840 HD 284839 HD 286010 HD 286009
    Tycho_id 1275-1826-1 1275-1672-1 1275-765-1 1275-1183-1
    Spec B9 B7 B8 G0
    V_xy 10.16 9.68 10.4 9.83
    HIP_id HIP 22185
    V_Tycho 10.194 9.708 10.296 9.636
    V_Tycho_se 0.041 0.029 0.051 0.031
    B_Tycho 10.55 9.936 10.77 10.249
    B_Tycho_se 0.041 0.028 0.054 0.038
    V_TurnerPE 10.09 9.69
    V_TurnerPE_se 0.01 0.01
    B-V_TurnerPE 0.41 0.23
    B_TurnerPE_cp 10.5 9.92
    B_TurnerPE_se 0.02 0.02
    B_TurnerPH_se 10.29
    Photomty_type Turner PE Turner PE Tycho2 Tycho2
    Johnson_V 10.09 9.69 10.29 9.63
    m_V_computed 10.17 9.73 10.3 9.6
    m_V_se 0.24 0.24 0.3 0.2
    B-V_used 0.41 0.23 0.47 0.61
    m_V_Diff 0.08 0.04 0.01 -0.03

    * - Star 99's estimated magnitude is corrected here for a
    transcription error in the B-V value. The B-V_TurnerPE value of 0.41
    was transcribed originally at an incorrect value of -0.41.

  3. #3
    Tony Flanders's Avatar
    Tony Flanders Guest

    Default NGC1647 Open cluster - Help in field testing a draft cluster magnitude chart (PrisNo6) wrote in message news:< com>...

    Heavens, just because my crude guesses disagree with the charts, don't
    assume that the charts are wrong! I'm no variable-star observer,
    accustomed to estimating magnitudes with an accuracy of 0.05! I am
    happy to chalk up the 0.3 mag difference between #094 and #099 to
    my own incompetence, or to random variation. I was a little more
    baffled by the 0.6 discrepancy between #65 and #66, and I am glad
    to have it explained by color.

    Whenever I have done limiting-magnitude experiments, I have *always*
    had the experience of being able to see stars quite clearly that
    were 0.2 or 0.3 magnitudes fainter than other stars which I cannot
    see, no matter how hard I try. I have always suspected that star
    color plays a big role in this, but I have never tried to analyze
    it systematically. And aside from the question of varying sensitivity
    at different wavelengths -- which also varies markedly from one
    individual to another -- let me throw another monkey-wrench into
    the works. In comparing urban light pollution against skyglow from
    the Moon, I have noted that artificial light pollution is (not
    surprisingly) much redder than scattered moonlight. Perhaps blue
    stars are more visible in urban light pollution, and red stars
    under moonlight, because of color contrast against the background?
    Just a thought.

    But the other huge bias, which is unavoidable when looking at
    star clusters, is the effect of proximity to other stars. I
    find that a bright star casts a surprisingly big "shadow" where
    it is impossible to see fainter stars that would be readily
    visible on their own. I'm sure that this effect is bigger for
    me than for most people; I know that my eyes have very bad
    internal glare. But I am also sure that it exists to some extent
    for everyone. Conversely, two faint stars just below the threshhold
    of vision may add up to a single faint fuzzy if they are near the
    limit of split-ability.

    So although clusters are good for limiting-magnitude studies
    because of having lots of stars closely spaced in magnitude,
    they are also bad because of the proximity effect. I suspect
    that looking at NGC 1647 with hand-held binoculars, the
    proximity effect would be dominant -- many or most of the
    stars would be hard to split from each other. I'll let you
    know if the weather ever clears up again during a moonless
    time of night.

    As for the charts, they are basically just fine. Do you suppose
    it would be possible to print *both* the star numbers *and* the
    magnitudes on a single chart? That would save a lot of work!

    - Tony Flanders



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