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V467 Dra - 5hrs of data


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Another HADS star from last night, V467 Dra - this one has a period of 4.74hrs, so I let the scope chug away. Guiding was working really well as well.

This is from 369 separate 30sec exposures over almost 5hrs, through 350mm f4.6 newt, V filter, ST2000XM. Transparency not so great and the y-axis is quite expanded compared to some other HADS plots as there's only 0.35 mags variation for this one.

Measurements.png.2e48b0a78ead3118788252b686c181e2.png

Edited by coatesg
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  • 2 months later...

Looks good. I noticed that the variation (noise) in the measurement values is about 0.02 = 20 mmag. That I see also in my measurements.  That seems to be the minimum for observations just above sea level. Does it ever get better then this?

Han

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Well, part of it is governed by Poisson statistics and the inherent shot noise arising from the star's signal and the background, and thermal + read noise contributions. 

I think the plot above is considering the first two largely. The BAA spreadsheet also takes into account the error in reference star magnitudes as I understand it.

Error bars of the order of about 20 mmag are about the best I get too. To get better, expose for longer (avoiding saturation), go somewhere darker (though may not make a huge difference for bright objects) or stack images (which is not ideal for some time resolved work).

Edited by coatesg
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15 hours ago, coatesg said:

 To get better, expose for longer (avoiding saturation), go somewhere darker (though may not make a huge difference for bright objects) or stack images (which is not ideal for some time resolved work).

With respect to timing an object in a stacked image, If the stacking program keeps a record of the start exposure times and fills the final stack with the correct earliest start exposure time and correct summed exposure time this "timing problem" should be no problem.  I assume an equivalent solution would be to draw a trend through an abundant amount of measurements. All according the same principle noise reduces with more samples.

Just noted the time is geocentric. Is helio-centric not more used?

 

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53 minutes ago, han59 said:

With respect to timing an object in a stacked image, If the stacking program keeps a record of the start exposure times and fills the final stack with the correct earliest start exposure time and correct summed exposure time this "timing problem" should be no problem.  I assume an equivalent solution would be to draw a trend through an abundant amount of measurements. All according the same principle noise reduces with more samples.

Just noted the time is geocentric. Is helio-centric not more used?

Stacking (and trends) - yes, correct, but if you're looking for changes of the order of a few minutes (eg cataclysmic variables), and stack up (eg) 10x2min exposures, you'll average it away... for faint targets, you have to choose between time-resolution and signal-noise...

JD vs HJD : correct, but both the BAA and AAVSO databases expect JD on submitted data so I just tend to present it like that. A researcher can always do the JD->HJD conversion themselves if they want to use the data. As long as it's clear which one it is...!

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23 hours ago, han59 said:

With respect to timing an object in a stacked image, If the stacking program keeps a record of the start exposure times and fills the final stack with the correct earliest start exposure time and correct summed exposure time this "timing problem" should be no problem.  I assume an equivalent solution would be to draw a trend through an abundant amount of measurements. All according the same principle noise reduces with more samples.

Just noted the time is geocentric. Is helio-centric not more used?

 

If you are imaging an object that has repeatable cycles (e.g. a transit) it is best to observe multiple periods and then phase fold the data onto the identified period.  This way you combine the data in phase to get a better signal to noise.  However that can be more tricky in the UK because of weather and when a transit might occur.

Yes heliocentric (HJD) is much better to use. Barycentric data is even better really (although there isn't yet a standard determination for BJD) although is only really important for very time sensitive observations such as pulsars.  I'm not sure why anyone would want data in JD for anything that might be periodic.  It means you can be up to about 15 minutes out depending on the time of year, your observing location and where the object is located. If you want to compare observation over long time frames (e.g. if there are subtle changes over time) then using JD will give erroneous timing issues.  

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