# V Aql - the difference a chart can make

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tiburd
V Aql - the difference a chart can make

One of my favorite binocular targets is V Aql, an AAVSO Legacy star for which the Target Tool requests observations at 10-day intervals.  It sits near the prominent five-star asterism located just above and to the left of M11 and R Sct.

Let’s suppose we want to observe it using a chart such as X15386FWJ, which has a limiting magnitude of 7.9.  This chart shows V Aql as the left-most star in a distinctively shaped group of three, with a 69 comp to its right.  Easy.

So we look in the sky and see the three stars, and notice that the left-hand star is brighter than the middle star of the three.  That means V Aql is brighter than 69, right?

But wait… what’s that extra star I’m seeing in the sky, just a bit to the left of the group of three?  That one doesn’t seem to show up on the chart.  What’s going on?  Is it missing because it’s fainter than the chart’s limiting magnitude?  That doesn’t seem right – after all, the chart goes down to 7.9 but that extra star seems brighter than the 75 comp, at least.

So let’s try a different chart with a fainter limiting magnitude, such as X21531LJ.  Oops – now we’re seeing something different:  There’s that familiar three-star asterism again, but this time with a difference:  The middle star of these three is a labeled comp at 80, the 69 is the left-most star of the three, and V Aql is sitting off to the left of the three, just where I saw the “extra” star in the sky.  So did we mistake the 69 for the target when using the other chart?

Let’s check a bit further.  Both charts show the 69 as sitting almost in line between the two brighter stars 34 and 40, with V Aql distinctly below and to the left of that line.  That’s what I’m seeing in the sky.  So, yes, it appears that using the first chart (or any chart that fails to show the 80 comp) would likely mislead an observer into thinking that the 69 comp is actually the target V Aql.

How would this confusion affect our estimates?  Well, the 69 is brighter than its 80 companion to the right in the second chart, which implies that we would have concluded that V Aql is brighter than 69 if we were only using the first chart.  But (lately at least) the “extra” star is fainter than the 69 and, in fact, has been closer to (or even fainter than) the 73 that sits a few stars farther right, between the 40 and 48.  Wow.

Consistent with this, the light curve for V Aql over the past two or three years shows that most visual observations have been reported as fainter than 69, or even fainter than VSX’s “official” range down to 7.22 (which, by the way, is not unusual and happens with many stars in the database).

So, in this case, anyone reporting V Aql as brighter than 69 these days should check to make sure they are using an adequate chart and not targeting the wrong star. The more general lesson is that, for any target, using a chart that fails to display all the stars prominently visible in the observer's instrument may have the potential to mislead.

tiburd
V Aql - the difference a chart can make

Here, for convenient reference, are the two charts that my original note mentioned.

Sebastian Otero
V Aql magnitudes and its very red color

Hi Sherrill,

I can talk about the VSX ranges. You wrote:

>>> Consistent with this, the light curve for V Aql over the past two or three years shows that most visual observations have been reported as fainter than 69, or even fainter than VSX’s “official” range down to 7.22 (which, by the way, is not unusual and happens with many stars in the database). <<<

V Aql is a carbon star and most visual observers are not red-sensitive or use technics (like averted vision) that make very red stars to appear dimmer than their V magnitudes. VSX uses standard V magnitudes for the displayed ranges. That's where the differences you see come from most of the times.
The amplitudes of carbon stars will be larger for the visual observers than actual V. When the star gets dimmer, it becomes more difficult to detect visually and when it becomes brighter its strong color causes the opposite effect stimulating the eye cones.
Unfortunately, even the V data in the AID has systematic errors (0.4 mag. fainter in 2003 compared to contemporaneous ASAS-3 data, you can see a historic V light curve here) and random errors for the most recent data (a spread of 0.7 mag. for a star with an amplitude of 0.55 mag...). Its very red color is obviously causing problems to all kinds of detectors...

Cheers,
Sebastian