How many variable stars are detectable to NSL cameras?

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RJN
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How many variable stars are detectable to NSL cameras?

Post by RJN » Thu Jul 29, 2004 2:45 am

This question has been lingering on my mind for a while and came up explicitely in discussions today: how many variable stars are really detectable to NSL cameras? On one hand, every star is variable at some level. On the other hand, most stars that are not famous variables are assumed to be constant. Few stars are actually cited as specifically constant in the Hipparcos catalog, though.

Now CONCAMs can see down to past visual magnitude 6, thousands of stars are detected. But known variable stars might vary by too little to be detectable between two CONCAM frames. We might assume that 10 percent variability is the limit between two frames. Given this, perhaps the AAVSO has complete data on bright stars that variabie.

Clearly variability of bright stars can be detected at a lower level than dim stars. Even this situation becomes more complex when frames are added
and smaller fluctuations are visible on longer time scales.

Anyway, hard numbers, real analyses, and good comments are encouraged about this matter.

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Re: How many variable stars are detectable to NSL cameras?

Post by tilvi » Wed Aug 04, 2004 4:04 am

RJN wrote:This question has been lingering on my mind for a while and came up explicitely in discussions today: how many variable stars are really detectable to NSL cameras? On one hand, every star is variable at some level. On the other hand, most stars that are not famous variables are assumed to be constant. Few stars are actually cited as specifically constant in the Hipparcos catalog, though.
Anyway, hard numbers, real analyses, and good comments are encouraged about this matter.
Little analysis on data for a single star (spica, Mag 0.8 ) at MK station reveals that MK CONCAM can actually pick up variability of such stars easily. Although one needs to be little careful while processing data of such bright stars as the pixels can saturate at such higher brightness and infact Dan's work on the same (spica) star at SA station shows such pixel saturation.

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RJN
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Post by RJN » Fri Aug 06, 2004 2:44 am

Here is a crude estimate of the fraction of stars that might show variability. The list of the brightest 25 stars found here indicates that 8 of these are "visibly variable," as indicated by a (var.) designation in the Magnitude column. I guess that this means that these stars have variability detectable by unaided human observation. This might translate to about 10 percent brightness change, a change that should be detectable also on CONCAM images. Of course the amount of detectable variability depends on apparent star brightness. Given the hundreds of stars that are bright enough for automated photometry by CONCAM3s, though, perhaps 1/3 of them -- over 100 -- will turn out to be detectably variable.

- RJN

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Post by tilvi » Fri Aug 06, 2004 2:57 am

RJN wrote:Here is a crude estimate of the fraction of stars that might show variability. The list of the brightest 25 stars found here indicates that 8 of these are "visibly variable," as indicated by a (var.) designation in the Magnitude column. I guess that this means that these stars have variability detectable by unaided human observation. This might translate to about 10 percent brightness change, a change that should be detectable also on CONCAM images. Of course the amount of detectable variability depends on apparent star brightness. Given the hundreds of stars that are bright enough for automated photometry by CONCAM3s, though, perhaps 1/3 of them -- over 100 -- will turn out to be detectably variable.

- RJN
Infact, I also used this list to find the bright variables. But, in this list most of the variable stars are brighter then mag 0.8. And i guess most of the pixels will get saturated at such brightness. But, as you suggested, we can try taking the data close to horizon in order to avoid the saturation and can play the trick. This should work, I suppose, atleast for periods larger than 2 days.

-Tilvi
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Post by Vic Muzzin » Fri Aug 06, 2004 4:37 am

Infact, I also used this list to find the bright variables. But, in this list most of the variable stars are brighter then mag 0.8. And i guess most of the pixels will get saturated at such brightness. But, as you suggested, we can try taking the data close to horizon in order to avoid the saturation and can play the trick. This should work, I suppose, atleast for periods larger than 2 days.


In studying Beta Lyrae I have used Vega (5?th brightest star) as one of my comparison stars. Using the Canary Islands CONCAM, I have never once seen it saturate. I do not have C1 listed in my data, but C5-B stays consistenly near 30,000 and having looked at many FITS images I have never seen a bad pixel near Vega. This would suggest that Canary Islands images, in general, have lower counts (Vega saturates quite often on the Mauna Kea cam), and we may not see as many of the dimmer stars on that CONCAM. A quick look seems to indicate less stars collected in photometry files for Canary Islands? However, for analysis of bright variables the Canary Islands camera may be a good resource, provided of course that the star of interest is visible from there.

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Post by RJN » Sat Aug 07, 2004 1:29 am

Photometry for the brightest stars can be tracked by CONCAM2s which rarely saturate, while all other detectable variables can have their photometry tracked by the CONCAM3s. Currently, the brightest stars can also be tracked by the CONCAM3s when the Moon is up, as the exposure then is only 20 seconds, avoiding saturation.
- RJN