[vsnet-chat 5384] Re: Determining Limiting Magnitude

["Clay Sherrod" <sherrodc@ipa.net>]

Good points and well taken, Brian.  I appreciate this detailed and valued
input.

Thanks!  I am off on a 3-week public forum lecture tour in the Southeast and
will work with the webfolks on this; your points are absolutely
correct....thanks again.

----- Original Message -----
From: "Brian Skiff" <Brian.Skiff@lowell.edu>
To: <sherrodc@ipa.net>; <vsnet-chat@ooruri.kusastro.kyoto-u.ac.jp>
Sent: Saturday, July 13, 2002 11:13 PM
Subject: Re: Determining Limiting Magnitude


> >> 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

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp