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(meteorobs) FWD: Magnitude limits, continued




[
 Brian Skiff made a fairly notable comment regarding the IMO visual LM method, which I thought it worthwhile forwarding here... The gist: the IMO LM areas may be based on poorly calibrated (or spectrally inappropriate) stellar-magnitude sequences! Of course for some of IMO's analysis - involving as it does relative rather than absolute magnitude comparisons - such discrepancies may be unimportant. But certainly for magnitude-distributions, at least, these poor baselines for LM estimates may be a real problem!
 So do any representatives of IMO have comments on this? Have the IMO star areas currently in use for LM estimates been recalibrated with new V-band stellar magnitude sequences? Or is it an issue at all?
 Clear skies,
 Lew Gramer
]

------------------------------ FORWARDED MESSAGE ----------
>Date: Wed, 8 Jul 1998 16:20:12 -0700
>To: aavso-discussion@physics.mcmasterdot ca
>Subject: Magnitude limits, continued
>From: bas@lowelldot edu (Brian Skiff)
; charset=us-ascii
     Folks posted a lot of interesting items about naked-eye and telescopic
magnitude limits.
     Berto remains incredulous about claims of various of us seeing quite
faint with quite small apertures.  Admittedly, these are not 'blind' tests,
as Arne has argued they be, but they are nevertheless real detections.  Since
I do not doubt that he is a reliable observer, I think that Berto merely needs
to do some observing at a first-class site---a trip this winter to the SAAO
Sutherland Station?  From our earlier correspondence, it sounds like his
Pretoria backyard is not terribly different from that of most folks in North
America, which is pretty crummy compared to remote mountain sites in the US
Southwest or northern Chile.
     Berto also intimates that limits in Milky Way fields are worse (i.e.
brighter), and I agree---the brighter background of unresolved stars makes
a big difference in contrast, which is practically all that matters for visual
detection.  I find it much easier to see those mag. 14 stars for instance in
the field of the high-latitude globular NGC 2419 (using the chart in Luginbuhl
& Skiff) than near M57.  Lew Gramer correctly noted that there aren't any
far-southern sequences in our book, although the one for NGC 2204 is plenty
far south enough for Berto (but available only early in the year).
     Lew mentioned the IMO star-count fields.  Last time I saw the
documentation for these, the star magnitudes came directly from the SAO
catalogue, which means they're basically wrong.  Have these been fixed with
photoelectric V magnitudes?
     As he also mentions, naked-eye limits for different people standing/lying
side-by-side will also differ considerably depending on acuity and skill
(broadly "experience").  That's why high-power telescopic limits are a better
comparison short of making actual sky-brightness and extinction measurements.
Small exit pupils are involved, greatly reducing acuity problems, and working
with dark eyepiece backgrounds, cloth shielding your head from stray light,
etc. (all the tricks), means you can do a lot better than you think.
     I'll also mention a true "double-blind" test performed by Dave Nash at
the Nebraska Star Party a few years ago.  He drew stars in an area near
the head of Draco without reference to charts.  Not only did he not know where
the stars were, but he could not know their magnitudes, since at the time there
was _no_published_photometry_ for them.  He sent me the list of star IDs and I
later measured them at the Lowell 21-inch.  The faintest star he saw was V
mag. 8.2.  The magnitudes in the SAO catalogue (mostly from the HD) for the
stars turned out to be up to a half magnitude off.  It was clear that he was a
careful observer from his posts to sci.astro.amateur, and this confirmed it.
Since you can make the sky background so much darker with magnification in a
telescope, the limits using a telescope do not scale strictly with aperture
compared to the eye, but are actually quite a bit better.
     As far as faint southern photometric sequences, I'll mention that I've
added a large number of BVRI sequences to my calibration-star file at:

ftp://ftp.lowelldot edu/pub/bas/starcats/loneos.stds

The relevant southern ones are mostly around recent supernovae found during
the Tololo/Calan survey, often at -50 or -70 Dec.  Many of these go quite faint
(18th-20th mag.).  Since the file includes only positions, for visual use
you'll have to get a DSS image from SkyView with a coordinate-grid overlain to
identify the stars.  Another possibility is using the source finder charts,
which are all available via the ADS article service.

\Brian