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Transformation Stars

HPO's picture
HPO
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Joined: 2010-07-23

 

Since I am no longer doning photomery and concentrating on spectroscopy I am posting this to the photometry forum in hopes someonce can help. I recieved the following question.

I have a question pertaining to selecting transformation stars. I know different spectral types should be chosen and that they must not be variable stars. But it is hard to find stars that are  included in Cousins database that are not double stars. I noticed that Jerry Persha used some double stars in his sample transformation star database. Is this ok?

Anyone have suggestions or comments? Thanks!

Keff

Transformation Stars
roe's picture
roe
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Joined: 2010-07-25

Many of us use M67 or NGC 7790 for which there are good spreads of color (B-V, V-I) and good calibrations (available in VPHOT, too).  Arne also recommends the field of SA110-503.

Jim Roe [ROE]

transformation stars
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HQA
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Hi Jeff,

My assumption, since the question pertains to the Cousins standards, is that this observer is in the southern hemisphere.  Cousins used a standard photoelectric photometer when measuring his stars, which meant a fairly large entrance aperture (typically 30-60arcsec).  Therefore, any close double would have been measured as if it were a single star.  You can use these close doubles for your analysis, as long as you use a sufficiently large aperture to fully include both stars.  Note that most software will select one star or the other as the "centroid", so be sure to take the offset into account when deciding the aperture size.

Of course, it is preferable to use single, isolated stars for standards.  Landolt did a good job of selecting such stars, so if you can work with equatorial standards, I highly recommend using Landolt's stars rather than Cousins.

Arne

Transformation Stars
HPO's picture
HPO
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Joined: 2010-07-23

 

Thanks Jim and Arne. I will pass on the information.

Jeff

Transformation Stars
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Ken Sikes
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Joined: 2013-01-27

I am the one that originally asked Jeff the question concerning using double stars measurements for standards. I meant to say Johnson and not Cousins. I was refering to the "UBVRIJKL Photometry of the Bright Stars" by Johnson , Mitchell, Iriate and Wisniewski. I recently purchased a SSP-3a photometer from Optec and the software seemed to be based around this catalogue. I am choosing stars from the catalogue, but many are also listed as double stars. Would the answer be the same as far as selecting stars ?

Thanks

Ken Sikes

Johnson standards
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HQA
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Hi Ken,

What filters did you get with the SSP-3a?

Usually, I don't recommend using the Johnson/SkyTel set of stars, as they were not observed sufficiently to be classified as standards.  On the other hand, there are two cases why you might use them:

1) they are bright, and you may not be able to work faint enough to use Landolt standards

2) they are true Johnson UBVRI.  Optec's R/I filters are Johnson, as opposed to every other vendor's choice of Rc/Ic = Cousins filters.  These two standards differ, so if you have Johnson R/I, then the Johnson list is your best bet to calibrate these two passbands.

Arne

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