Sky and Telescope had a nice article on Delta Scorpii. In the summer time II spend a lot of time observing in the southern Milky Way and though that I should add this star to my observing list. I periodically make magnitude estimates for Alpha Orionis during the late winter early spring. The Sky and Telescope article recommends using Beta Scropii (Mag 2.6) and Antares 1.1) as comparison star. This is a pretty wide magnitude range. Is there a third star that falls between these two stars?
That was an awful choice of comparison stars.
Delta Scorpii is about V= 1.8 nowadays and using a red (and variable!) star to estimate it is not a good recommendation in any way. Also beta Scorpii's true naked eye combined magnitude is 2.50 not 2.6 since it is an unresolved multiple star. (Antares mean magnitude is also 1.0 not 1.1).
Forget abut the two.
Use the following ones:
lam Sco = 1.62 (slightly variable but ok for visual purposes)
eps Sgr = 1.84
the Sco = 1.86
alp Pav = 1.93
alp Oph = 2.08
sig Sgr = 2.09
For additional stars check out this chart:
Thanks for the information. I find that the Sky & Telescope articles are fairly decent, but I wondered about the comparison stars. It is not a biggy on my list, but as I stated I do a fair amount of viewing in part of the sky and though that I would add it to my viewing list.
Cheers and clear skies,
Also, if you are in the northern hemisphere, there's the serious problem of atmospheric extinction, which will strongly affect these widely scattered comp stars. At my latitude of +40, delta is itself pretty far south. I keep an eye on delta, but I don't clutter the database with poor estimates.
I forgot to mentioned that but it is a serious problem for Northern hemisphere observers.
Delta Sco amplitude is very small, it only changes more than 0.2 or 0.3 mag. in the long term so the advice is only observing it when the conditions are more or less acceptable.
Alp Oph would be the best help for Northern Hemisphere observers but you are missing the best part of the sequence.
Delta Sco has been active for the last 4 years due to a new periastron passage (it is a 10.8 yr. binary) but the extreme range has been V= 1.66 - 1.93, less than 0.3 mag. If a poor sequence and differential extinction may cause your estimate to have a large error, it will probably be of small value...
Furthermore, I follow it as often as I can and I have only been detecting 0.1 mag. changes this season.
Anyway, if the current cycle follows the previous one, the star could be undergoing a fading soon (10 years ago it dropped from 1.7 to 2.3 in ~10 months).
For more information on its nature you can read this VSOTS article.