[vsnet-chat 5383] Re: Determining Limiting Magnitude

[Brian Skiff <Brian.Skiff@lowell.edu>]

>> The magnitudes comes directly from the GSC catalog (fainter stars) and the
>> Tycho Catalog.

     It would probably be useful to specify this on your chart Web page.
     My concern derives from having spent a long time on this list and
elsewhere getting folks to use standard V magnitudes for variable-star
work.  This is simply to avoid ambiguity in reporting magnitudes from the
many individual observers.  Though I didn't have anything to do with it, the
AAVSO and other variable-star groups have now pretty much agreed to adopt V
for all visual comparison stars, and work is underway to produce a uniform set
of charts for variables.
     If the zero-point offset in the GSC magnitudes in the Lyra field is
similar to the nearby M57 region, then you will be misleading folks who
use your chart, since the GSC here is about 0.5 mag. too bright compared to V.
In other words, folks will actually be observing to fainter limits than
indicated by the magnitudes you show.  As you perhaps know, the GSC magnitudes
vary considerably from place-to-place on the sky (even plate-to-plate) because
of the quite heterogeneous plate collection that was scanned; calibration was
a bit dodgy in places as well.  Only in a narrow strip along the southern
galactic plane are the magnitudes systematically close to standard V.
     By the same token, if the Tycho magnitudes are the original "VT"
magnitudes from that catalogue, then the values will be misleading (though
by a smaller amount) in the other direction, since "VT" is too faint compared
to standard V as a result of a substantial color term in the data.  Thus an
observer using those data will not be seeing quite as faint as indicated
by your chart.  You used Tycho-2, not Tycho-1, yes?  (Tycho-1 has a scale error
that makes things even more problematic.)

>> For the record, most amateur telescopes...will not reach magnitude 20
>> visually...

     Agreed.  Anticipating that, the M57 sequence in S&T covers the range
11.1 < V < 19.8, so is suitable for use with just about everything from
small telescopes visually up to modest apertures with CCD.  The original
sequence included stars a couple of magnitudes brighter---an 11-magnitude
range---so can be used even with small binoculars (for details see: 
http://c3po.cochise.cc.az.us/astro/deepsky02.htm).

\Brian

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