The CH Cyg observing campaign has been continued through 2012. For details please see AAVSO Special Notice #294. Thank you for your contributions! Good observing, Elizabeth Waagen
For a well known star such as this, the sequence for visual estimation is not "up to par", so to speak. Specifically:
1. Large gaps in sequence (65-81 the worst, 84-92, 100-107). These lie right within the typically range of this variable.
2. The 109 comp is extremely red B-V>+2.5 and should not be shown, especially since there is the 107 comp right beside it.
I have submitted a CHET report with some suggested Tycho stars to fill the largest gap.
For a well known star such as this, the sequence for visual estimation is not "up to par", so to speak.
Surprisingly, the sequence for CH Cyg has had notable problems for at least as long as I've been a member of the AAVSO (half a century), with at least portions of the comp star sequence having been replaced, or revised, more times than I can count. Even the latest version has some obvious problems with the sequence accuracy from a visual standpoint.
Addressing at least the "gap" in the bright end of the sequence, there is a magnitude 7.3 comp star included on wide scale AAVSO charts that is situated a bit more than a degere west of CH. It closes the gap between the 6.5 and the 8.1 nicely. Its distance from CH should not be a disadvantage since binoculars should be employed to observe CH Cyg when it is brighter than 8.0 anyway.
In AAVSO Special Notice #294 I posted September 11, I did not mean to imply that Dr. Karovska no longer wants R data. She does indeed still want R data, it is just that V and particularly B have priority.
If you are able to continue observing CH Cyg, it would be great if you could include R observations.
Many thanks, and good observing, Elizabeth
Thanks for the historical info on this issue, John. Not sure why finding good comps for this star is so difficult, being in the milky way with lots of stars around...
Yes, while estimating it recently, I found the 81 looks very close to the 84 in brightness. I checked their colours from Tycho, and the 81 is B-V~+0.6 and the 84 is near zero, so the Stanton adjustment alone will pull the 81 down fainter better than a tenth.
I think comps should be chosen with similar colors if they are close in brightness, to avoid this kind of skewed appearance visually, which can be especially troublesome for newer observers, who may not be able to figure out what the problem is.
I would like to call attention to another anomally regarding this star, in this instance its plotted recent lightcurve from AAVSO data. Even a casual glance shows the mean CCD and visual lightcurves are well separated, consistently paralleling one another by around 0.7 magnitudes. However, what I particularly take notice of is that even the CCD values do not agree by any means. In fact, within these data there are clearly two paralleling lightcurves which differ by about 0.3-0.4 magnitudes! What in the world is with that? And this situation is not new either, being traceable back to at least May of this year.
Now offsets between AAVSO CCD and visual data are nothing new and attempts to consolidate lightcurves made using different methods is an eventual goal. But shouldn't at least the supposed ultra accurate CCD values concur with each other? Obviously we have a situation here where the parties taking these readings need to be called together and make sure that all are employing the same set of CCD comp stars, or reduction methods.
I totally agree. Something is going on with these observations. It is probably an observing technique, reduction problem or someone may be going into the non-linear range of their camera.
It looks like the two parallel curves are generated primarily by two individual observers. The first thing that I noticed is that neither one has transformed their observations to the standard system. So, the difference in their instrumentation is not being taken into account.
The second thing I noticed is that they are using two different comparison stars. Although they both are Tycho-2 based magnitudes, the errors on the sequence listing are on the order of +/-0.03. I don’t know if this is a standard deviation but this is pretty large for CCD comparison stars. In this case we are lucky because they came from the same source. I’ve run across sequences that are a mish-mash of different sources and have differences of 0.2 or 0.3 magnitude which is huge in the CCD world.
CCD’s can be precise compared to visual observations with the right care and techniques. There are different levels of skill and care among CCD observers just as there are among visual observers. I think that you are correct in that the perception is that when one sees a CCD observation, it implies a certain level of precision and perhaps accuracy. Like with visual observers, it is the responsibility of the CCD observers to use proper care to make sure that the observations reported are of the best quality sufficient to carry out the scientific programs.
I’m rambling again but I think that there is a perception that CCD observing is ”easy”. Done properly, it is definitely harder than it seems!
I've posted the remainder of last season's photometry of CH Cyg using the Bright Star Monitor. This coverage is dense enough at BVRI that it can be used as a comparison for other observers. If you want to see the data, highlight observations by HQA over, say, the last 250 days. We are just starting to observe again from New Mexico as the monsoon is winding down.
BSM uses Bessell-prescription glass filters, and so is a good system for comparison. No red leaks, good transformation (and all of the data is transformed). I use an ensemble of 14 stars from 7th to 9th magnitude, using a wider color range than others might do (since I'm transforming); the stars are spread over the 2.2x1.4 degree field of view. However, the BSM pixels are large, and the apertures are typically 20-30arcsec diameter, so some blending will happen in fields like Cygnus.
There are several features that show up. First, as is often the case with red stars, there is an offset between visual and V. These are really two different bandpasses, so offsets are to be expected. Dick Stanton came up with the transformation equation between visual and V; someone ought to apply it and see if the two curves come closer together. Next, there are a number of Rc datapoints in the March/April timeframe that are about 0.7mag too faint. This is most likely due to an incorrect value for a comparison star. There is a B time series in March and a V time series in June that have too wide a scatter. Most likely these were done over a large airmass without extinction correction. There are some systematic differences between CCD observers - some of this could be eliminated by transformation, as this is a pretty red star. Finally, note that BSM quit taking data on 6/28 as the monsoon began, yet there are continuous BVR observations over the past 3 months from other sites. This shows the value of geographical diversity - just because Arizona/New Mexico are well known for clear weather does not mean every telescope should be sited there!
I think this would an interesting star to investigate observer differences. It is bright, so you get plenty of signal (though you also have to worry about saturation); it is red, so differences between untransformed filter systems are obvious; it didn't have the best sequence, so differences between comp star values can be tracked.
We need to be doing a better job as observers. Let me look at this dataset more carefully and see if I can make some recommendations.
In order to keep discussion in one place, please visit this thread if you would like to further discuss the photometry and sequencing of CH Cyg. Thank you!