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starbux
sage
Reged: 02/08/06
Loc: Sunnyvale, CA
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Small range variables usless as comparison stars?
#5349046 - 08/02/12 04:56 PM
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When looking up a comparison chart for Alpha Hercules, there really aren't any nearby stars suited for comparison at the brighter end of the range. The closest one that would SEEM to fit is Alpha Ophiuchi but the AAVSO no longer has it as a comparison star apparently because it has a small variation (about 0.1 mag).
If there is a dearth of suitable comparison stars on the brighter end of the range could it be acceptable for one to conceivably use Alpha Oph even though it would introduce a possible small error, as long as the observer made a note of it with the assumed magnitude?
And while I'm on the subject of bright variables there is Betelgeuse. Try to get an AAVSO chart with designated comparison stars for this particular star is near impossible (the automatic chart generator outputs something unrecognizable and with no star magnitude marked). If one were using the usual comparison stars such as Rigel, Procyon, or Aldebaran, would one have to cite the source of the magnitudes since there are differences in sources?
I would be interested in knowing which star catalogs whose listed magnitudes are considered reliable enough by the AAVSO for use in a comparison star sequence? Not just for bright stars, but clearly the brightest ones are the most problematic.
Point of info. I have the current Cartes du Ciel with all the regular catalogs as well the GCVS and New Suspected Variables and other supplemental catalogs. For the purpose of best magnitude listing for example would I prefer Tycho 2 or Extended Hipparcos, or a third party catalog?
Thank you for entertaining these questions.
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brianb11213
Postmaster
Reged: 02/25/09
Loc: 55.215N 6.554W
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Re: Small range variables usless as comparison stars?
[Re: starbux]
#5349141 - 08/02/12 06:04 PM
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I think we have to face up to the fact that some of the bright semiregulars are not really suited to visual observation. For a start differential atmospheric absorbtion almost always needs to be allowed for. Secondly - unless you're only interested in the times of minimum & maximum but not the amplitude - there are issues such as the sparsity of comparison stars which you note.
These variables are IMHO much better dealt with photometrically. Using the green layer of the image from a DSLR, using an ordinary lens an a short exposure which doesn't require a driven mount, is quite capable of getting a pseudo-V magnitude accurate to at least 0.01 mag and does not require comparison stars to bracket the range of the variable, just be within a couple of magnitudes. Also the relative brightness of the comparison stars comes out in the wash so that previously undetected variations can be found; as several comparison stars are normally used, the effect of one being disqualified is not a serious matter.
Semi regular variables, with their intense red colour and small ranges, are difficult visual targets at the best of times. Position angle error, sky brightness over the lunation and the Purkinje Effect often exceed the actual variation range.
Edited by brianb11213 (08/02/12 06:07 PM)
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RLTYS
Post Laureate
Reged: 12/18/04
Loc: New York (Long Island)
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Re: Small range variables usless as comparison stars?
[Re: starbux]
#5349831 - 08/03/12 07:55 AM
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You might try makeing your own chart using a good planetarium program to get magnitudes of surrounding comp stars. This would give you something to work with until something better comes along.
Rich (RLTYS)
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starbux
sage
Reged: 02/08/06
Loc: Sunnyvale, CA
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Re: Small range variables usless as comparison stars?
[Re: RLTYS]
#5350397 - 08/03/12 03:40 PM
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That was pretty much what I had in mind. The question at this point is which star catalog has more reliable (preferably visual) photometric data as far as the AAVSO and variable comparison goes. CdC has the option of all the star labels being the magnitudes. But is Hipparcos or Tycho2 considered more reliable (small as the differences might be)?
Added: I noticed on the International Variable Star Index that for Beta Orionis the magnitude listing has an "Hp" following the magnitude.
Mag. range 0.17 - 0.22 Hp
Would "Hp" stand for Hipparcos catalog, or is the "p" photographic mag and the "H" something I don't know about?
Edited by starbux (08/03/12 03:54 PM)
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RAKing
Postmaster
Reged: 12/28/07
Loc: West of the D.C. Nebula
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Re: Small range variables usless as comparison stars?
[Re: RLTYS]
#5356251 - 08/07/12 10:35 AM
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Quote:
You might try makeing your own chart using a good planetarium program to get magnitudes of surrounding comp stars. This would give you something to work with until something better comes along.
Rich (RLTYS)
To follow up with Rich's suggestion: If you do find some 'home made' comp stars -- do a series of observations, then send your results to AAVSO. They could then vet your results and possibly include them in the catalog. Your efforts could help us all.
It never hurts to try something like this.
Cheers,
Ron
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