Sun, 12/27/2015 - 08:13
I was looking at the lightcurve of EF Peg and there is wrong photometry in the lightcurve. There are 3 ccd observers whom are constantly measuring the close companion of EF Peg at magnitude 12.6. I wonder why no one has ever noticed this? This is going on for years. Don't they check the charts and theirs images?
While I haven't yet observed EF PEG, looking at the "g" chart (7.5 racking) it shows virtually no separation between the star and the Comp. I wonder if the chart team could point this out for future observers. It's unfortunate that it too so long for someone to point this out!
SEASONS GREETINGS AND CLEAR SKIES
Mike
NMI
While I haven't yet observed EF PEG, looking at the "g" chart (7.5 racking) it shows virtually no separation between the star and the Comp. I wonder if the chart team could point this out for future observers. It's unfortunate that it took so long for someone to point this out!
SEASONS GREETINGS AND CLEAR SKIES
Mike
NMI
I have not observed this star, but I did pull up a F and G chart for this object. Both have a note in the lower left hand corner that warns that EF Peg and the 126 are separated by only 5 arc seconds.
I am curious what magnification and fov are being used by visual observers to measure this object?
WGR
[quote=WGR]
I am curious what magnification and fov are being used by visual observers to measure this object?
[/quote]
I don't observe this star regularly, but others of similar separation are on my list. I generally use 500x with my large scope for most estimations, unless its a really bright star and I need to see comps far away. The difficulty faced, depends not only on the separation of the companion, but also on the relative brightness of both. Using a large scope at this high magnification works fairly well for something of the brightness and separation of EF Peg. At mag 12.6, the companion is not so bright that it overwhelms detecting the faint variable, nor too faint that its hard to tell which star you are seeing.
An even more difficult one for me is BT Mon, which has a 15.3 mag companion about the same separation ~7 arcsec. Its a bit tough to see both this companion, and BT (which commonly are similar brightness) separately with direct vision in a 20" scope. Especially, to estimate BT accurately with such small separation and being near the detection by direct vision, I have to alternate between direct and averted vision, tough to resolve them this way. V838 Mon is another fairly hard one visually, with a number of similarly faint stars in the immediate vicinity.
In general, 500x is sufficient magnification for just about any pair or multiple of stars, since a 5 arcsec separation then has an apparent separation over a full moon's width by naked eye. If conditions are poor - bad seeing, bright background, etc., many times the best I can do is just say the variable is "fainter-than" the companion, within one magnitude fainter, since then the effect of the variable on the combined light of both would become too small to reliably detect.
Mike
[quote=hhu]
I was looking at the lightcurve of EF Peg and there is wrong photometry in the lightcurve. There are 3 ccd observers whom are constantly measuring the close companion of EF Peg at magnitude 12.6. I wonder why no one has ever noticed this? This is going on for years. Don't they check the charts and theirs images?
[/quote]
This kind of careless "systematic" error is indeed a big problem! And that nobody has reported it or marked them in "Zapper" for so many years too. A legitimate outburst of EF Peg may have been caught in Dec 2009, but by some other than the observer in question.
Mike
In the April 2014 Newsletter, Sebastian Otero gave a list of stars that had obvious problems due to close companions. This list can be found here:
https://www.aavso.org/variable-stars-companions
He has been diligent in marking stars with companions in VSX/VSP, so that there is usually a comment at the bottom of charts for such problem targets. EF Peg has been known as a difficult target for quite some time. I'm frankly amazed at the number of visual observers who are able to give "fainter than" observations of this close pair (and you CCD observers should take some pointers from them)! Submitting data of the combined light for the variable and its companion is not a great idea, but can be split by software in the future (such as with Lew Cook's Nemesis spreadsheet). What we asked in that article was that observers who submit such blended objects carefully note the problem in the observation comment field.
There are dozens of variable stars with close companions, and I personally avoid them in my own program unless there is some specific scientific reason (such as a campaign). At the same time, they can be a fun challenge, so I understand the draw to observe them. Sometimes both the main variable and its companion are both variable (such as for U Men)! Just be aware of the problems and make sure your observation contains a notice as to the circumstances under which it was made.
Arne
How does the CCD chart figure out the 2 hour period?
Ray TRE
Thank you for bringing this to our attention Hubert.
I have contacted the observers and notified them of the problem. The blended observations will be removed from the database soon.
It is good to discuss issues like this once in awhile because it serves as a reminder for everyone to:
Remember that the AAVSO always appreciates quality over quantity of observations. Please take the time to check things carefully and let us know when you see problems via email, Zapper, or posts like this.
Happy new year!
Sara
Hello
An additional source of error in cases like this happens when you designate the correct star on the first of several images, and then the software propogates the aperture and sky anulus, and it jumps to the brighter star, giving an error in the remaining images. Something else to look for.
Gary