[Aavso-photometry] CI CYG Time Series Interval

[Wolfgang Renz]

Hi Rich

In general:

With vis obs of short periodic vars one uses usually about constant
intervals that depend on the rate of change of the var to ensure that
one can detect a difference visually.

But if you stay on a single target during a longer time (TS) with a ca-
mera anyway, why don't you take as much data as possible (= with
out gaps) ?
This will ensure that you have covered the event as good as possible.

Some observers with cameras also use gaps. Some just to not to
take too many images that have to be saved on the HD and be eva-
luated (IMO not a good reason anymore nowadays), some because
they image something else in the time in between (e.g. monitoring
of several not too fast changing vars in parallel) and some because
the rate of change of the var is really slow (then thats OK).

Choose the exposure time long enough to get a good SNR and short
enough to get a resonable time resolution (just important with short
and short periodic events). But always ensure that all stars of interest
(vars and comps) stay in the linear range of the camera.

If you do the TS unsupervised, you should choose the exposure time
short enough that the stars of interest do not leave the linear range
when they are at their highest point when they are still rising in the
sky. Then you'll have also to consider that the pulsators usually get
brighter when you observe a maximum and its brightness might even
exceed the one of the brightest comp star you were considering.

If you do the TS supervised or check it every hour or two, you could
adapt the exposure time if required to account for changes in the at-
mospheric transmission and for changes in airmass. Then you might
want to consider also that the eclipsers (a few have an amplitude of
2-3 mag) will get fainter when you observe a minimum and its bright-
ness might fall below the one of the brightest comp star you were
considering and you could increase the exposure time to get a better
SNR for the var. In the case you use gaps in between the exposures,
you could also shorten the gaps during the time when the eclipser is
faint and then stack two images during the evaluation to increase the
SNR.

On CI Cyg specially:

Its currently on the rise to an other Z And maximum. Its EA (GCVS:
P=855d, D=167d, d=72d) and the Z And (every few to a couple of
years) variations are pretty slow. This would usually require just one
obs per night. But one never knows. If nobody makes higher speed
TS on it, faster variations couldn't be detected if it would have some.

You can compare your TS results with the recent ones of Kenneth
Thomas Menzies which shows some minor variations during a 6.5
h run:
<http://www.aavso.org/cgi-bin/lcg.pl?name=CI+Cyg&lastdays=0.28&start=&stop=2454710.81&grid=on&pointsize=1&width=800&height=600&mag1=9.84&mag2=9.96&mean=0.00694&v=on>
If you need more details on his obs, download the CI Cyg data via:
http://www.aavso.org/data/download/
and look at his obs.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Richard Harvan" <tkdvarstar at yahoo.com>
To: "AAVSO Photometry" <aavso-photometry at mira.aavso.org>
Sent: Monday, September 01, 2008 6:26 PM
Subject: [Aavso-photometry] CI CYG Time Series Interval

Dear All,
I obtained a time series of CI CYG last night.
What is a good interval between measurements?
I have seen on this discussion group that 10 minutes is fine for a time
series. I can go significantly shorter, down to 2 minute interval, which I
have done for RR LYR/DSCT types.

Clear skies,
Rich Harvan HAV