time series photometry using iTelescope

Hi Chris:

As a prototype of a dwarf nova, U Gem is certainly interesting with its outburst about every 100 days. Many observers keep track of such outbursts and some collect time series. Take a look at its light curve in the LCG. At quiescence, the magnitude is about 14.5 (V) and up to 9 in outburst. Dwarf nova are certainly interesting to monitor and to follow with time series to learn more about the astrophysics of the system.

Usually a time series is collected with a cadence that gives you at least 100 images over the period of the system. Since much of interest with such systems happens over orbital periods of a few hours, you need to take frequent images (e.g., 5 hours/100 = 0.05 hours = 3 minutes). The limiting factor is thus the magnitude of the object and the size of the telescope. iTelescope has some moderately large scopes so this is not a problem.

I do wish to mention one issue with running time series with iTelescope. Note that I am an iTelescope member and not opposed to free enterprise, nor opposed to beginner's enthusiasm but time series with iTelescope over a few hours gets expensive! I usually stick to infrequent images to monitor outbursts with such systems and use my own scope for time series. If this is not an issue, great but just thought I should mention it. There are lots of other objects that require less frequent imaging.

HTH, Ken

Chris,

Foremost welcome to the AAVSO and Variable Star Observing.

Not sure that this initial reply will directly answer your main question but it will provide you with a number of informational tools that you are going to need to help you develop both an understanding of some of the elements of Variable Star Observing as well as where to locate resources.

1) Frequency of Observations:

Check this AAVSO web page: https://www.aavso.org/obs-frequency

(which is mostly intended for visual observers)

I don’t have a link to this post by Arne Henden (our previous director) so I will post his remarks as part of this:

Posted: June 23, 2012 - 6:54am

The recommended observing frequency is listed in Table 6.2 of the Visual Observing Manual; for LPVs like Miras, it recommends once per week.  I think once/10days may have been fine in the years when we had lots of active visual observers.  Today, I think a higher frequency is needed, if nothing else than to ensure that the Legacy stars at least get observed by someone, even if the light curve gets defined by only a handful of observers.  Once/week also seems like something easier to remember.

The traditional cadence in the digital world is 1/100 period for periodic variables so that you get a well-sampled light curve.  I think it is important to consider using this cadence for visual observing as well.  The Bright Star Monitor, for example, has found that many Miras have bumps and wiggles in their light curves that may be missed with longer cadence by visual observers (also noted in the LPV humps campaign), and LPVs come in many flavors, from about 100day periods to over 1000days.  Using a 10-day cadence on a 100-day variable means you aren't sampling it very often.  So a general rule may be good for a starting point, but I'd suggest adjusting as needed.  I'd like to hear from the expert visual observers like John regarding their cadence on the short-period stars.

The other extreme is observing too often.  Observing every 5 minutes on a Mira doesn't have any known astrophysical value - high frequency oscillations are not predicted for these big stars; planetary transits take many days since the orbit has to be outside the stellar surface; flares are not known on these big stars.  Over-observing a star means you have to under-observe something else on the Legacy list; I'd rather those stars get well-covered.  If you are a CV observer on the other hand, high-cadence observations have value since these small bodies can flicker with quite short periods.  CVs can benefit from almost all cadences - the once/day measures can catch new outbursts, the 5-second digital estimates can measure the eclipses, etc.

Arne

You will note that Arne suggests that CV’s can benefit from all cadences and CV’s are excellent targets for CCD observing (the bold type and underling were added by me)

2) VSX:  https://www.aavso.org/vsx/

The VSX data base contains information on the variable stars included in our data base.  Click on Search, as an example and enter U Gem in the name box then click the search box.  The second line (AAVSO UID) shows 187,777 observations as of date. About ½ way down you will note the Variability type:  UGSS+E and two lines after this you will note the V magnitude range of 8.2-14.9V.  This is all important information when considering observing a target.

3) Ok, the next thing you would need to work out is just what does UGSS+ E really mean as to the type of star, which is important when making cadence decisions:

The quick look data base for this is located outside the AAVSO: 

https://www.assa.org.au/sig/variables/classifications

A more extensive listing which contains all the elaborate listings within the VSX is located here:

https://www.aavso.org/vsx/index.php?view=about.vartypes

Looking up UGSS in either data base will lead you to the Cataclysmic Variable classification which further shows the target to be a SS Cyg type of variable with other info.  Now you need to find what the +E signifies.  That answer is discovered under the Eclipsing classification and simply shows that U gem is part of a binary system and one component periodically passes in front of the other.

4) As mentioned by someone already in response, it is also worthwhile to check the light curve generator for a target of interest…. I would suggest entering a 1,000 days or so if the VSX shows a lot of observations:  https://www.aavso.org/data/lcg

5) Yes, you can process a time series using VPOT:

https://www.aavso.org/vphot

Also, there is a reference page to the AAVSO on one of the itelescope pages:

http://support.itelescope.net/support/solutions/articles/143059-how-can-i-use-vphot-with-itelescope-

6)  Ken (MZK) gave you some great information and I would add to that a small caution, in that your images could potentially saturate when U Gem goes into outburst… there again this is a variable of the scope size and the exposure time.

U Gem does appear to be a target that has been and continues to be well studied; by the way, this is not to suggest that you should try and find a target with no observations (there can be a lot of good reasons why the target has not been observed).

My suggestion would be to begin your journey by making a number of single star observations, especially when the cost of a adequate time series is considered (2 hours plus?).

If you send me your email address, I will send you a document that I have previously prepared for new observers that contains a number of suggested observing options to be considered.

I would also suggest that you take a look at the many observing sections that the AAVSO offers:

https://www.aavso.org/observers#sections

FYI, there is no such thing as a dumb question!

Tim R Crawford, CTX

tcarchcape@yahoo.com

Hi Chris!

1) Are you already on Education Plan? Remember - there's no Moon discount on it, so for example Plan-290 is more advantageous when the Moon is above 75%. But Education Plan is really better when there's no Moon in the sky.

2) Exposures shorter than 60 sec are not practical, cause you are being charged for 1 minute of telescope time even if your exposure time was 30 or 10 sec. Thus try to avoid shooting very bright stars, or use filters to avoid saturation.

3) 40 hours at iTelescope would cost you approximately the same as a new CCD. Hence take the best from the very start of your iTelescope usage! Spend your time on the objects you can *not* reach from Sweden. Use T32, T31 or even T27 at Siding Spring.

4) Don't waste your time on U Gem which is a) too bright; b) over-observed; c) visible from Northern hemisphere; d) too long-period for a CV. Better choose some stars in 15-17th magnitude range with a short period which are in the need for observations. I would select poorly studied AM CVn (UG/IBWD in VSX classification), newly detected UGSU/UGWZ dwarf novae in outburst (check out vsnet-alert for those) or X-ray selected CVs of AM Her type aka polars (contact me, I can give you a couple of hot targets for joint analysis).

Remember that the goal of time series photometry is to determine the period (and to follow the changes of phased light curve over the time). To do that, you need to cover several orbital periods. At least, three orbital periods. At the very-very least, two. And 3*P_orb(U Gem)=12.7 hr! In those 12 hours of observing time you can get 9 (nine!) orbital periods of any UGWZ-type star with P=0.055d, 8 revolutions for any polar with P=0.06d and 20 (twenty!!!) revolutions for ultra-short period star like NSV 1440.

5) Try to kill two or three "birds" with one shot. That is, select the targets in potentially rewarding areas where the by-product can be more valuable for you and/or for science than the time series you were going for. Find the objects with lots of galaxies around and check the images for Supernovae by blinking them against DSS plates. Look for moving objects on your images in hope for finding new comet or near-Earth asteroid. Finally, use C-Munipack or some other variability search software for finding the new variables on your images.

Good luck!

Denis in Moscow (a place at 55.7N with ~60 clear nights in a year, including June-July)