American Association of Variable Star Observers (AAVSO)
Sat, 03/07/2020 - 21:37


Minor issue ... on the chart (attached) for ome CMa, there is a stray #, "17".  I'm new so if this is not a error, please let me know what it means.  The "5918" just NW of ome CMa is a 5.9 magnitude star and Wezen is 1.8; the two magnitudes simply run together appearing as 5918.  I don't see any 1.7 magnitude star anywhere near that 17 so I assume it is a stray number and certainly not near the magnitude of Wezen.

Re the comp stars, I know there is a place to address this next question (I think it's called CHET) ... the high mag comp star at 35 is sig Cma but it is a variable star.  The low mag comp star at 44 tau CMa is also a variable.  I suppose this is simply the best that can be done since if there are no constant comp stars within 1200 arc min at the best magnitudes for the range, there are none there, simple as that.  It would be nice if there was a 42 constant comp star but again, that's just the way it is.  I'm new so this likely is not unusual.

The mag range is listed as only .06 for tau CMa and .10 for sig CMa.  Pretty slim.  I guess as a new observer, getting to understand that many of the stars are variables is a first lesson learned and you can't always have pure constant comparisons. 

Wishing the galaxy had more stars, :)



Royal Astronomical Society of New Zealand, Variable Star Section (RASNZ-VSS)

Hi Bob.

Just go to CHET and fill in the details. There is an excellent team that responds to queries like yours :-) (Good spotting BTW).

Stephen [HSP]

American Association of Variable Star Observers (AAVSO)
Large FOV Chart Issues


Welcome to the AAVSO as a newer observer.

FYI, there will be exceptions to much of what I convey below but in general the background information, herein, is intended to help provide you with some foundation upon which to understand this and other chart issues.

Prior to about 1990 most all areported observations were visual (exceptions being PEP); the best accuracy an experienced visual observer typically manages is about 0.100 V.  For VS within this range of variability it has been the historical practice to use those minimally variable stars as comp stars for visual observers  and which typically has only occurred for stars brighter than about 6.0 V with a few exceptions out to about 9.0 V,

Since CCD observers can achieve an accuracy of .001V it has been the modern practice to label any such occurring existing VS comps as being for Visual Use Only (Comment field of the photometry chart).  I suspect that the majority of the brighter comps in your large fov were created well before CCD equipment became available and therefore lack the Visual Use Only comment as well as being generally to bright for typical CCD equipment (excpeting PEP equipment).

It would be an extremely rare occurrence that the Sequence Team, since about 2000, would, for any reason select a visual use only comp star.

There does not exist any software to enter the data base and mark any VS used as a comp with a Visual Use Only comment in the photometry table.  The Sequence Team will do this when brought to our attention.

Your supplied FOV is enormous covering some 20 degrees of the sky (1200 arcmin)  with a limiting magnitude of 7.0V.  This is the largest chart the AAVSO software can generate.  Only some binoculars and  the smallest of telescopes (depending upon eyepiece data) can encompass such a large fov.

There are probably an excess of 6 dozen variable stars within this fov that are brighter than 7.0V; fainter VS number in the hundreds if not thousands within this fov.

Typically, stars brighter than about 10 V are to bright for CCD equipment, although in recent years observers using CMOS cameras have been able to go somewhat brighter.  In addition, CCD equipment (dependent upon the nature of the CCD as well as the nature of the scope used) observes a fov of anywhere from about 15 arc seconds to one degree (some CMOS observers can image a fov as large as 2 degrees). 

I have taken the liberty of adding the comment, Visual Use Only, to both Tau CMa & Sig CMa Variable comp stars.

The 5918 values crowded together is a limitation of the software in this specific instance.  A different LM or a smaller fov will show these values separated.  You might find some value in selecting the VSP option Would You Like All Magnitude Labels To Have Lines?.

As to the 17 star, I am unable to resolve this issue at the moment.  There appears to be several issues possibly occurring at the same time.  It will remain on my priority list to re-visit.

I hope some of this will help you understand a bit more about the issues you have raised.

By the way, have you read the visual observing Manual?

Or if appropriate the CCD observing Manual

Keep asking questions… there is no such thing as a dumb one.

Also, you can request a mentor if you would like to:

Ad Astra,

Tim Crawford, Sequence Team

American Association of Variable Star Observers (AAVSO)
thank you for reply

Thank you Tim.

The reason for the 1200 arcmin chart is that I kept making wider and wider area charts trying to find a comparison star for the 4.18 low mag range of ome CMa.  (I use a 400 arcmin chart for this star after discovering wider areas didn't include a closer comparison).  I thought the 4.4 comparison star could be bettered with a 4.3 or 4.2 star if I widened the area.  Unless I overlooked a star, I just don't see a closer comparison star.  I'm learning quickly that my thoughts of so many megabillions of stars doesn't mean that all variable stars are going to have "perfect" comparison stars and that many comparison stars have some small range variance.

I did find the link for CHET and submitted a report before I saw your reply.  The one thing about sig CMa I mentioned in the CHET report is that it is a LC type and VSX states LCs can have a 1 magnitude change.  That's why I was curious about it being a comparison star but I have to assume that those that do the charts believe sig CMa has not historically had that big of a change and is a solid comparison star.

I have read the manual and do have a mentor.  Everything has been great so far.  Thank you for the great information here.  It's nice getting to know some of the people. 

Bob Stalnaker