RR Lyr reference stars

RR Lyr reference stars

Postby Keith Graham » Fri Jun 07, 2013 8:04 pm

Hi,

I have been looking at reference star candidates for the RR Lyr campaign. Christian suggested HD 192640, an A2V star with mag 4.93. I noticed that this star is about 46 minutes in RA and 12 degrees dec from the target. I do not know enough about this to know if this would have adverse effects on atmospheric extinction, but I did find HD 184875, also an A2V star with mag 5.34. This star is much closer to RR Lyr and is very close in air mass. Is there any reason why I should not use this star as the reference star or why HD 192640 might be the better star to use for reference?

Thanks,

Keith Graham
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Re: RR Lyr reference stars

Postby Christian Buil » Fri Jun 07, 2013 9:07 pm

You can select two strategy:

(1) If the goal is to measure absolute atmospheric transmission, the relative position of target object (RR Lyr) and star used for evaluate atmopsheric transmission (or extinction) is not very important (the two are independant). The critical point is : the "reference" is observed at a large air mass difference at the beginning and the end of the session. HD192640 is usefull because it is a moderate brithness star for quick and regular check observations during the session time if necessary. During data reduction use the absolute atomospheric transmission converted to the actual air mass of RR Lyr.

(2) HD184875 is also a good candidate for reference (I have used), but less bright (but do not use Vega !). HD184875 is usefull for evaluate the (instrumental + atmo. transmission) response of the system by computing the ratio : "Response" = (HD184875 observed spectral profil) / (HD184875 MILES spectral profile).
Now, for correct you RR Lyr actual observation compute : "True RR Lyr profile" = "Observed RR Lyr profile" / "Response"
(but.... note that the "Response" profile is continuousy variable during session, because effective atmospheric transmission change !)).

Method (2) is the standard. Method (1) is not very standard actually but it is better if night is photometric and less time consuming during session (important time resolution work).

Not very easy to explain with words, very sorry.

Christian
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Re: RR Lyr reference stars

Postby Keith Graham » Fri Jun 07, 2013 11:56 pm

Hi Christian,

First, there is absolutely no need for you to be sorry. I cannot tell you how much I appreciate your input and help. I, like many others,strive for accurate results. Thanks to your continued assistance and advise, this is a very possible goal.

Actually, Robin did offer similar explanations for both methods. And,like you, he stated that method 2 is easier. I fully understand method 2, but method one still has my head spinning a bit. So, it would appear I will stick with method 2 for now.

I would like to ask one more question for clarification. You mentioned in an earlier post that it is a good idea to take periodic reference star images throughout the observing session. If I were to take say 5 reference star images spaced out throughout the observing session and process them in ISIS the same way I would process the all RR Lyr target images, would all of the atmospheric changes be averaged out in the resulting response profile that would be used to correct the observed RR Lyr profile?

Thanks,

Keith
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Re: RR Lyr reference stars

Postby Andrew Smith » Sat Jun 08, 2013 9:54 am

Keith - Not sure if this will help but as I see it method one is like all sky photometry while method two is like differential photometry. For method one to be accurate you need good stable sky conditions and hence the recommendation to check the reference star during the period of observation.

You mentioned in a different post you would be doing photometry at the same time so I would propose you plot and extinction curve for a photometry reference star. If the plot of the log of intensity (or instrumental magnitude) v air mass is smooth and a straight line then your skies are ok if not then method two would be better for correcting the spectra.

I am not sure if Christian would agree but I would recommend anyone proposing to use method one to plot an extinction curve to verify the sky conditions. You don't need any special filters and the images from the guide camera should be good enough as long as care is taken to ensure size of the measuring aperture covers the whole of the star.

Regards Andrew
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Re: RR Lyr reference stars

Postby Keith Graham » Sat Jun 08, 2013 9:45 pm

Hi Andrew,

Thanks for your input. After reading Christian's post, I pretty much made up my mind to go with method 2. Your comments simply reinforced that thinking. I have found a reference star very close to RR Lyr that will work, and method 2 appears to be a lot less of a hastle. My only question still remains - does processing a series of reference star images within a given observing session average out the atmospheric changes? My mind says this is what happens because I think that is also what is happening when the target images are processed. Therefore when the IR profile is applied to the target profile, the atmosphere changes will be cancelled out. At least that is what I surmise from a comment you made in response to my question in a previous topic. I am just trying to understand why this works.

Cheers,

Keith
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Re: RR Lyr reference stars

Postby Robin Leadbeater » Sun Jun 09, 2013 12:56 am

Hi Keith,

With method 2 you are trying to match the atmospheric effects on target and reference star observations as closely as possible. ie the reference star spectra should match as closely as possible in terms of time and altitude the target star observations you are correcting. If the observations are made over a significant amount of time and by inference also with changing elevation then you need several spectra of the reference star to match your observations of the target. (How many you take and how you allocate them to the target star observations is to some extent a matter of judgement and practicality. At one extreme, you would ideally alternate reference and target star observations. At the other, where perhaps you are just averaging a series of target observations to produce one spectrum, or the air mass is not changing much during the run, a reference taken at the start and end and averaged or even a single reference spectrum for a short observation) could be sufficient.

I dont know the particular requirements of this campaign but when attempting to contribute to a particular campaign, I try to look at the requirements on a case by case basis. In this case I would be asking the question "Why is an acccurate continuum required for this campaign and if it is, how accurate does it need to be? " Armed with this knowledge you can then consider what you need to measure or indeed if a suitable measurement can be made at all. In many cases though precise knowledge of of the continuum shape is not a requirement.

Cheers
Robin
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Re: RR Lyr reference stars

Postby Christian Buil » Sun Jun 09, 2013 3:02 am

For the present campaign and for low resloution spectrograph (LISA, Alpy, ...) the continuum restitution is important for evaluate the surface temperature during a cycle and for many cycle. Temperature is an important physical parameter for understand phenomena concerning RR Lyrae.

Christian
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Re: RR Lyr reference stars

Postby Keith Graham » Sun Jun 09, 2013 2:07 pm

Hi Robin, Christian, Ken, and Andrew,

This has been one more chapter in my spectroscopy learning curve. With the LHires and Be stars I had never used a reference star, I understand the reason for including them in an observing session, and I certainly did not understand how to apply them. Since I am now entering the world of low res spectroscopy, it does appear that I will need to understand their value and how to use them. Thanks to the posts from all of you you, I now believe I have enough info for reference star acquisition and application. Many thanks to all of you.

Cheers,

Keith
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