Question about optimal comp star selection

Technology VPhot
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I’m not sure if this is the optimal category for this query as it relates not just to VPhot but to any similar photometry tool one might be using but I don’t see an obviously better place for it so I’ll post it here. Recommendations I’ve seen for choosing a comp star (or stars if using an ensemble) say that the comp star should ideally be close to the target star in magnitude and should also have a decent SNR. Is it possible in general to say which of these criteria is more important?

Specifically, I have been observing the RCB-type variable AO Herculis for a while. It’s typically around magnitude 11.5 in V and I have been using a comp star that is approximately magnitude 12. With my setup I can usually get an SNR of around 120 for this comp star. But AO Her has started fading recently and is now down somewhere around magnitude 14.3. An alternative comp star is available at a similar magnitude and in my current observations comes in at an SNR of around 40 or 45. I did estimates both ways and came up with about a 0.07 magnitude difference, but have no reason to favor on result over the other.

Assuming both comp stars are otherwise similar in suitability (in terms of B-V values, proximity to the target star, etc.) is it possible to lean toward one choice or the other as being preferred?

Brian (SBQ)

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…and color :slight_smile:

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Yes, and color, I forgot to add. In my case the two possible comp stars are reasonably close to the target in color and very close to each other.

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Hello Brian,

However, be careful with red stars, since they tend to be dimmer and CMOS sensors are not too sensitive in the red part of the spectrum. Your comp stars should be red-ish, but not so much so that they have a low SNR.

I normally tend not to choose comp stars above (B-V)~0.950.

Cheers,

Enrique
(BETB)

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