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[vsnet-campaign-be 111] Re: [vsnet-chat 4331] Of comp stars, extinction, and colors...
Hi Thom and all,
Your comments and calculations about extinction are very interesting. I'd
made comments about these on occasion but was too lazy to actually do the
calculations. However, I've seen the results of low altitude (high ZD)
visual measures of brighter stars where the comparisons are necessarily some
distance away. We'd made UBV measures of eta Carinae at 9 or 11 airmasses
(who knows if the Bemporad equation is valid at that level) but the
extinction increased almost by the minute!
Presumably you're not measuring from Tucson itself but we've noticed some
other extinction effects in Auckland where the observatory is well inside
the city boundaries. The first is that there is a marked azimuth
dependence - the industrial areas are more opaque - and the practical zenith
is displaced away from the city centre by about 10 degrees. The other is
that extinction is not exactly a function of the air column length but
performs as if there was a discus shaped region of extra opacity centred on
the city. This causes some small but odd effects.
The values for kV also fall into two groups - about 0.15 for very clear
nights and about 0.3 on nights where there is a pronounced inversion layer.
Auckland has reasonably effective clean air regulations but a million or
more people still produce a lot of crud.
None of these change your comments but suggest that for city observers even
more care is needed. Not a problem for me these days - I'm at Waiharara
where the nearest street light is 5 miles away and the next 10 miles.
Regards,
Stan
----- Original Message -----
From: Thom Gandet <tgandet@mindspring.com>
To: <vsnet-be@kusastro.kyoto-u.ac.jp>;
<vsnet-campaign-be@kusastro.kyoto-u.ac.jp>;
<vsnet-chat@kusastro.kyoto-u.ac.jp>
Sent: Thursday, April 26, 2001 7:07 AM
Subject: [vsnet-chat 4331] Of comp stars, extinction, and colors...
> ...but not of cabbages or kings!
>
> I've been asked what stars I would choose as comparison stars for
> Theta CrB, and also about possible zero-point errors when comparison
> stars of widely different colors are used. These issues are closely
> bound to achieving more precise visual estimates.
>
> Iot Ser and Eps Her are the two primary comparison stars I plan to
> use, and Eta CrB and Chi Boo will be the check stars. Here are my
> reasons for choosing those stars. The logic of my reasons have
> applicability to choosing comparison stars in general, especially
> for blue variables.
>
> Only Eps Her and Bet CrB are brighter than Theta CrB at maximum.
> Of course, at least one comp star brighter and one comp star fainter
> than the variable must be used always. But because of the large
> zenith distance between Eps Her and Theta CrB, the magnitudes must
> be corrected for differential extinction (see below). As for Bet
> CrB, I don't trust its future behavior despite its recent
> quiescence, and my own practice is to eliminate known variables
> where the known range in V is >~ 0.03 mags; an Alpha CVn
> variable is more or less predictable, but sorting that out seems to
> me additional work that is made unnecessary choosing more constant
> comparison stars whenever possible.
>
> Of the remaining stars, Iot Ser has the advantage over the rest:
> it is closest to Theta CrB in ZD and in color. However, to keep the
> extinction correction <0.05 mag., zenith distances must be kept
> <~40for Iot Ser.
>
> Because I always aim to have two comparison stars and two check
> stars, I chose Eta CrB to compare with Chi CrB and Iot Ser - Eta
> CrB is bracketed in magnitude by Chi Boo and Iot Ser. I chose
> Eta CrB for its (relatively) close color; this compromise in
> the color difference requirement is acceptable here only for
> check stars and only if the color difference is <~0.5 mag.
>
> Applying extinction corrections, for visual observers, doesn't
> have to be time consuming or complex. Extinction corrections are
> applied to all observations made here. For ZD differences under
> ~1 extinction corrections should always applied for ZDs > 60
> For example, at ZD=~75 the two stars must be <~2apart in ZD
> to result in differential extinction <=0.05 mag.
>
> As an example of the large combined error that results from
> large color differences and differential extinction, I assumed
> that Theta CrB is at V=4.23 (v=4.20). The following cases use Eps
> Crb and Eps Her as comparison stars in and assume the step values
> are what an "average" observer can reasonably be expected to make:
>
> 1) If V magnitudes uncorrected for extinction are used and the
> observations are made at the minimum ZD of Theta Crb (~11at
> Tucson), the V magnitude of Theta Crb will be estimated as
> approximately 0.07 magnitudes too bright; this is due entirely to
> the color difference between Eps CrB and Theta CrB. If extinction
> is taken into account Theta CrB will be estimated as only 0.03 mag.
> too bright.
>
> 2) If uncorrected v magnitudes are used (again, at ZD of ~11,
> then Theta CrB will be estimated as approximately 0.03 mags. too
> bright (in the v system); converting the v magnitude* to the V
> system, the V magnitude is correct (in the V system).
>
> 3) Still using uncorrected v magnitudes but a ZD of 45 then
> Theta CrB will be estimated as 0.6 magnitudes too faint (in the v
> system); converting the v magnitude to the V system, the V magnitude
> will be 0.63 magnitudes too faint. The contribution to the error
> is almost entirely due to not correcting for extinction.
>
> [* The v magnitudes have been calculated using the formula:
> v = V+0.21*(B-V), which comes from an AAVSO study of the response
> of the "average" observer; other studies (thanks John for the info
> on that from the UK!) have determined color coefficients as low as
> 0.15. These values are approximate and empirical, and the AAVSO
> study showed that the color coefficient was significantly different
> for different observers. In fact, on theoretical grounds alone, v
> (and V, speaking somewhat loosely) are dependent on (U-B) color as
> well, so that the formula would look something like:
> v = V + a*(B-v) + b*(U-B).]
>
> In fact, because of the strong color dependency, the AAVSO uses
> only comparison stars with (B-V)<~1.0 for blue variables (if my
> memory is right) whenever possible. (For the sake of completeness,
> I believe, but am not certain, that a later AAVSO study showed
> that the color coefficient was closer to 0.20.)
>
> The above calculations are based on an extinction coefficient,
> k, of about 0.25, which is ok for a middling average site. They
> assume the extinction coefficient is constant throughout the year
> and has no wavelength dependence. In fact, k changes seasonally
> and, at low levels, throughout the night; it is also wavelength
> dependent. The variation in k may prove to be another source of
> error in visual observations that could be beaten down.
>
> I have a paper in preparation for the Journal of the AAVSO on
> making more precise visual observations, but the main points of
> that paper are presented here.
>
> Cordially,
> Thom Gandet
>
> --
> ************************************************************************
> Lizard Hollow Observatory
> Thomas L. Gandet, Director
> PO Box 77021
> Tucson, AZ 85703-7021 USA
> ************************************************************************
>
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