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XZ Cet revisited

HQA's picture
HQA
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As I discussed in the Director's Report from the SAS/AAVSO meeting, XZ Cet is an interesting star.  Available photometry, for example, shows considerable variation at the peak brightness, yet the rest of the light curve falls pretty much into place.  That would imply to me some emission mechanism occurring near max.  Doug Welch also called for continued monitoring of the candidate anomalous cepheid.

Therefore, I ask observers to either continue their observations of XZ Cet, or to begin observing this star.  Details are given in a prior post.  Also, I expect to continue my public discussion of each observer so that we can all learn from each other's mistakes or excellent techniques.

If you do a single nightly visit to XZ Cet, then I ask that you (a) use at least two filters; (b) do at least 3 sets of observations (for example, BVBVBV), take the average and standard deviation, and report that rather than the 3 individual sets.  A new Windows program called Boxster is available for download on the transformation page; you should try this out or develop your own technique.

If you do time series, the best uncertainty estimator is (K-C).  You can do a running average and report the standard deviation as the uncertainty for the target.

If you can transform, please do so.  Again on the transformation page:

http://www.aavso.org/transforms-everything-you-need-transform-your-ccd-o...

are programs to download that both calculate your coefficients and apply your coefficients.

I recommend that you use a single comparison star for now, and not ensemble, so that we can learn about transformation and apply it easiest.  You can always go back and update your submissions with ensemble results if you want to later.

I will also be monitoring the EE Cep campaign, soon to be announced.  Let's get some really excellent photometry this year!

Arne

Use same comp star
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You suggested that folks use a single comp star.  Why not pick one, so that we can all use the same one, to eliminate that variable from the equation?  Just tell us which one.

 

Gary
 

EE Cep
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Arne wrote:

" I will be monitoring the EE Cep campaign, soon to be announced..."

Arne, I'm sending data to Piotr Wychudzki at the Nicolaus Copernicus Astronomical center in Poland, is this the campaign you're refering to, or will the AAVSO be running it's own campaign?

Douglas.

 

XZ Cet comp star
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HQA
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WGR wrote:

You suggested that folks use a single comp star.  Why not pick one, so that we can all use the same one, to eliminate that variable from the equation?  Just tell us which one.

Gary

Hi Gary,

I would suggest the closest comparison star that is reasonably bright: 000-BLF-059 at 10.7V, about 10arcmin to the NE.  For those with small fields of view, try to offset your field center so that you include both XZ Cet and this comparison star.  That automatically gives you the 116 comparison as well, so we can use that star as the check star.

Arne

XZ Cet Comp/Check
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Arne:

Just to help others avoid an unnecessary question. There are two 107 comps at about the same radius in the FOV. The one near the 99 comp is not the correct comp star you selected. The correct comp is NNE of the target.

BTW, the magnitudes on the VSP/VSD chart are not the same as I got previously from APASS. They differ  by a few 0.01 mags! Too bright for APASS?? All this is a good reason to select the comp for this campaign so we use the same comp and same mags! However, can you look at the wright scope and apass comps to confirm mags?

Ken

107 comp
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HQA
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Hi Ken,

You are right; the comparison star I was suggesting is the closest of the 107 stars, located NE of the variable and not next to the 99 star.

I'll get the comparison star magnitudes updated with what I think is the best set of merged values in the next week or so.

Arne

EE Cep
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HQA
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Hi Dougles,

The AAVSO will be issuing an Alert Notice shortly regarding observing EE Cep.  While we will be requesting observations to be sent to the AAVSO International Database, we are also working with the Polish team on their specific campaign.  So sending observations to both places is ok, or just to the AAVSO and let them retrieve them from our database, or whatever.

Arne

XZ Cet
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Just curious about a couple of things re the XZ Cet comps:

1. The magnitude starndard deviations for 000-BLF-059 in B&V are larger than those for 000-BLF-055 (99 label star) or 000-BLF-056 (the other 107 label star). Is the new APASS release likely to reduce the  STDEVs for  the 000-BLF-059 magnitudes?

2. Is the new APASS release going to include Rc & Ic data for 000-BLF-055 and 000-BLF-056?

Brad Walter

EE Cep chart and sequence
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The Nicolaus Copernicus Astronomical Center suggest to use the following chart and sequence for the EE Cep campaign:
https://sites.google.com/site/eecep2014campaign/for-observers

Btw.: should'nt we move this topic to a separete thread?

EE Cep
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Thanks Robert, Ill start a new thread.

 

XZ Cet
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Mark Blackford
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Hi Arne,

I plan to observe XZ Cet when it is conveniently placed later in the year. In a July 3rd post you said:

"If you do time series, the best uncertainty estimator is (K-C).  You can do a running average and report the standard deviation as the uncertainty for the target."

How many observations should be used for the running average, would 5 be sufficient (i.e. the current observation plus 2 before and 2 after)?

You also suggested using only one Comp star initially. I use a variation of the Citizen Sky Intermediate Spreadsheet to analyse DSLR time series. My spreadsheet requires an ensemble of at least 6 Comp stars to work out transformation and extinction corrections.

There are not enough bright comps in the photometry table for XZ Cet, even for my 6x4 degree FoV. So I plan to use V and B-V values from Tycho 2 for six bright, non-variable, stars for the ensemble. I see that different souces of Tycho 2 data have slightly different catalogue values for the same star. The difference is usually only a few millimag and shouldn't make a significant diference at the 0.01 mag accuracy you are looking for in the XZ Cet project. But which source would you recommend for Tycho 2 data? Cheers,

Mark

running average
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HQA
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Hi Mark,

I like 5 points for the running average.  Those points are therefore close in time to the current observation, and so reflect well the current uncertainty.  More data points tend to average out transparency fluctuations; fewer points don't measure the underlying "population" in a statistical sense.  You can use 3, or 9, or whatever; 5 is just a general guideline.  Usually you pick an odd number of points to average so that the current observation is centered.

The reason I asked for a single comparison star, at least at the beginning, was to make it easier to compare one observer with another; find and correct discrepancies.  Ensembles work fine, but they include more parameters that need to be checked, such as the underlying published photometry, how you combine the ensemble stars, how you transform each one, how you handle first and second order extinction, what affect your aperture size or sky annulus size makes, etc.  That makes it more difficult to compare apples and apples.  However, I'm happy to look at ANY set of data and make (hopefully) useful comments!

Arne

running average for ensemble of Comp stars?
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Mark Blackford
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Hi Arne,

calculating the standard deviation of the running average of K-C is easy for the situation where one Check and one Comp star are used. But how do I do it with an ensemble of Comp stars? I thought perhaps the following would be ok.

Calculate the K-C standard deviation for each Comp star:

STDEV(K-C1),  STDEV(K-C2), ...,STDEV(K-Cn)

K-C standard deviation for the Comp star ensemble would then be:

STDEV(K-Ce) =SQRT(STDEV(K-C1)^2,  STDEV(K-C2)^2, ...,STDEV(K-Cn)^2)

Does that make sense? Cheers,

Mark

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484