Hello everyone,
I hope you'll forgive a beginner's question. I was observing R Aqr last night using the AAVSO chart X29437RK. I later looked up more information about the star in my out-of-date yet still beloved Burnham's Celestial Handbook (1978). In volume I (page 180), Burnham provides a finder chart based on a photographic plate from the Lowell Observatory, but that lists comparison magnitudes provided by the AAVSO. So my question is: why are the comparison magnitudes different from my AAVSO chart? I understand that a photographic plate will show higher or lower sensitivity based on the color index, but I believe the comparison magnitudes that Burnham provided were visual magnitudes. He wrote: "Throughout this "Celestial Handbook" all magnitudes given are visual, unless otherwise noted. (p. 70)." For example
5.3 (AAVSO) vs 5.6 (Burnham)
12.0 (AAVSO) vs. 11.4 (Burnham)
11.0 (AAVSO) vs. 10.7 (Burnham)
etc. Note the directionality and magnitude of the change varies. The software program, Stellarium, also shows differences but I'm not sure the magnitudes provided are visual magnitudes or corrected in some other way. So why the differences / changes in visual magnitudes for comparison (non-variable) stars? Thanks in advance - I'm a recently returned member after a long hiatus and hoping to get more involved in citizen science and astronomical data mining in the years to come. (I teach CS and DS at Northeastern.)
John
Answering my own question. From Percy J.R. (2007). Understanding Variable Stars, Cambridge University Press:
"For continuity, the AAVSO has used the same charts and comparison stars for decades, and has used the same comparison star magnitudes, even though more accurate photoelectric magnitudes are available. Other variable star observing organizations may have used other charts and magnitudes. As of 2005, the AAVSO is engaged in a massive project to revise their charts and comparison star magnitudes, while making every effort to maintain the long-term continuity of their data." (p. 52).
So I gather the comparison magnitude changes I see between Burnham's Celestial Handbook and more recent charts is the result of this chart-revising initiative mentioned above.
Yes, John.
Nowdays, all visual charts list Johnson's V band magnitudes. The old AAVSO charts listed visual magnitudes, a visual scale was in use, not standard but based on the typical human eye response (using rod vision).
The problem I see is that eye response varies between individuals and also with age. There were several papers published in JAAVSO a couple of decades ago about this issue.
Usually red stars are brighter in V and fainter in the visual (again, if you use averted -rod- vision) and that is why the 53 (from memory I thin it was rather red, I used to observe R Aqr 20 years ago).
But again, that is if you use averted vision, and it depends on how bright the star is. I used direct (cone vision), which is color sensitive, so I did not have that color term in my data. So determining an independent visual scale was not easy, our eyes are an incredible machine but not as predictible as an electronic device...
Right now, v (visual) and V are taken as the same thing. Individual response will vary between individuals but using the same comparison stars with V magnitudes, trying to keep them within an acceptable color range, wil give satisfactory results.
About the differences for the 12th and 11th mag. stars, that is different. There weren't many good photometric sources for faint stars back in the day. So I consider those differences more like a random thing. Some of the magnitudes in AAVSO charts for faint stars were visual estimates themselves!
Cheers,
Sebastian