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Exposure with an Ic filter

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Exposure with an Ic filter

Being new to image through an Ic filter I wonder how long exposure will likely be compared to a V filter. Both are colored glass filters. I can see that many red stars that I follow (LPVs, SRs..) appear brighter through an Ic filter but I don't know if exposures are to be increased compared to V.

HQA's picture
Ic exposures

Hi Gianluca,

With most CCDs, LPVs will be MUCH brighter at Ic.  Typically, for a normal-colored star (say V-Ic = 0.7), Ic has about the same throughput as V, up to perhaps 30% longer.  For LPVs, however, they are typically (V-Ic) = 3 or more, which means the exposures are 10x shorter.  For red stars, I'd start with such an exposure difference (if V was 30 seconds, try a 3-second Ic exposure).  Then adjust upwards or downwards for your particular star until you get the signal/noise that you want.

What this also means is that the comparison stars you used for your V-band exposure may be underexposed for the Ic exposure.  You can either choose different comparison stars, depending on your filter, or you can stack multiple short-exposure Ic images so that you can increase the signal/noise of the comparisons without saturating the target star.

Red pulsating stars are interesting to measure in Ic, as their amplitude and light curve shape will be very different.  You are closer to the peak of the blackbody curve, and not as wildly dependent on the blue end of the emission curve shape.  At the same time, molecular bands become strong, and the differences between filters will make it harder to compare one observer against another (and even to properly transform your data).  Adopt your stars and monitor them for years, if possible, so that there will be overlap between your observations and those from other observers.  Good luck!


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