RR Lyr campaign questions

[Keith Graham]

Hi,

I am getting ready to begin observations for the RR Lyr Campaign 2013, and I will be doing simultaneous photometric/ low res spectroscopic observations. In earlier communications with Jean-Francois, we agreed I will be doing VRI photometry on a 20 cm scope and low res spectroscopy using the Alpy600 on a 25 cm scope @f/6.3

I have written 2 emails to Jean-Francois recently asking for some clarification on exactly what he needs, but I have received no replies. He is apparently extremely busy, so I am hoping someone here can give me some guidance as to what I need to submit.

My questions:

1. For photometric measurements I have the chart placed on line, but all of the comp and check stars will be well out of my fov. Would it be possible for me to use the following 2 stars as comp and check that are within my fov?
Comp GSC 3142:0068 V mag 8.6
Check GSC 3142:1114 V mag 10.3

2. For spectroscopy, in what form should the data be submitted? Should only the raw images of the target and the standard star (Vega)be submitted or should I submit ISIS processed profiles?

3. If ISIS processed profiles are preferred would should they corrected for atmosphere and instrument response, or will this be done by Jean-Francois?

4. I am uncertain as to whether all of the processing will be done by Jean-Francois using just the raw images. If so, I assume he will also need a copy of the flat, dark, and bias frames in addition to the target and standard star frames. So what degree of processing should I do and what should be left to Jean-Francois?

Thanks for any help you can offer. And if I have missed something, please let me know what else I should include when submitting data.

Cheers,

Keith Graham

[Christian Buil]

Hello Keith,

Do not panic, François is often traveling!

I can answer to questions 2, 3 ...

2 - you should process your proper data, in the state of the art as possible. You only send the processed data by yourselfe
(but with all the necessary information: the UT time with a one minute of better precision, description of the instrumentation).

3 - It is your responsibility to find atmopheric transmission and instrumental response. Only you
can make good observations and verify that the result is correct.

I now recommend to observe the Milles star HD192640 as reference. The RR Lyrae observation is potential a long duration run
and the elevation of the star is strongly modified. I recommend observation of HD192640 at the beginning
of the session (typical 6 x 60 sec. Exposure). Observe the reference quickly again in mid-session (for checking).
Finally observe the reference star at the end of the session. It is assumed that the first and last observations
are made ​​at a large difference in air mass, which makes finding the atmospheric transmission (and instrumental response).

Never use Vega for low resolution (too bright!!!).

4 - There is no need to send the bias, flat ... in a fisrt time because the processing is already done.

Test the protocol for a first RR lyrae observation (is possible with a temporal resolution of 5 minutes), and send
your data (Fits profiles) for an analysis and correction if necessary. After one or two iterations the observation
is under control

I can help you to use ISIS for this type of observation (a new version is coming with better automation for compute
atmospheric transmission, but you can work presently with 5.2.0).

Christian

[Keith Graham]

Hi Christian,

Thanks for the good information.

So if I understand correctly, I should process my spectra in ISIS including instrument response and atmosphere transmission - correct?

If the above is correct, I am not sure why I should take reference star spectra. What do I do with that data. Sorry for this question, but I know a reference star should be taken for this campaign, but I do not understand what I should be doing with the reference star spectra. Could you please explain this a little further? Also, I mentioned Vega only because Jean-Francois suggested I use it. However, I will use HD192640 at your suggestion.

Also, I am not sure what you mean by

>Test the protocol for a first RR lyrae observation (is possible with a temporal resolution of 5 minutes), and send
>your data (Fits profiles) for an analysis and correction if necessary. After one or two iterations the observation
>is under control

Are you saying I should take take a 5 minute spectrum or RR Lyrae, process it in ISIS including IR and atmosphere transmission, and then send it to Francois to see if this would be acceptable?

I look forward to the new version of ISIS. BTW-speaking of V5.2, I see that there is a tab for the Alpy600,however I do not see what choosing that tab will do in the processing. Can I not simply start with the Image tab and progress through the General,Calibration, and GO tabs like I do with the LHires to get the final results with the ALpy?

Cheers,

Keith

[Robin Leadbeater]

Hi Keith,

The reference star is the one you use to calculate instrument response (and atmospheric extinction) The one suggested has a reliable actual spectrum from the Miles database to use as a reference

Cheers
Robin

[Keith Graham]

Hi Robin

Thanks for the reply. You and Christian have stirred up some issues I have wondered about. I think I have reached another plateau in my spectroscopic learning curve, but I want to be certain I have it correct.

I have created and have been using an instrument response profile for my Lhires (2400 grating) using the profile of a reference star I took and same star class profile from Elodie. Somehow I was under the impression that this IR profile would be good to use for some time. If I understand you correctly, I should be taking a reference star spectrum close to the target for each imaging session. I would then create a new IR profile each night for this target. Is this correct? If so:
1. Should a separate reference star be taken for each target for any given night?
2. Should I be creating a new IR profile for each observing session in high res as well as low res spectroscopy, or does this apply only to low res. I do understand that IR correction is more critical for low res spectroscopy but can the IR profile taken with hi res be used for subsequent observing sessions as I have been doing?

For atmospheric extinction, ISIS has an Auto Atmos. function. My understanding was that ISIS automatically corrects for this once the suggested AOD coefficient(from the Misc/Atmosphere tab is placed in the AOD box in the General tab. I do believe Christian will introduce a more elaborate and accurate method for this in the new ISIS update. BUT-I still do not understand how the reference star is used for atmospheric extinction. Christian suggested that I take a few reference star spectra each session and use these to derive the IR. Would the atmospheric extinction be accounted for in the resulting IR profile since the reference star is in proximity to the target?

Cheers,

Keith

[Robin Leadbeater]

Hi Keith,

Christian might have a different view but for high resolution work with the LHIRES, I do not normally worry about atmospheric extinction correction (which is going to change by a small amount generally across the covered wavelength) and just use a bright star measurement (Altair Vega Regulus etc) each observing run for response correction.

With wide wavelength coverage eg at low resolution then atmospheric extinction needs to be taken into account if the continuum shape is to be correct. This needs either :-

A known standard close to the target in position, time and elevation (The method I currently like to try to use for its speed and simplicity)

Or a series of measurements of a known standard at different elevations and stable atmospheric conditions to enter into the atmospheric model and allow an estimate the extinction for the target elevation (which is what LISA calculates.)

The second method is potentially more accurate and does not need the standard to be nearby but takes more observing time and planning during which conditions can change.

Cheers
Robin

[Keith Graham]

Hi Robin,

So, if I take several images of the reference star close to the target with the Alpy as Christian suggested, I then process them in ISIS as I would the target. The resulting profile is then run through the Instrumental Response algorithm using a same class library star to get the IR profile. What I am still uncertain about is how the reference star profile is used to get the atmospheric extinction. Will the atmospheric extinction be corrected in the IR profile that is applied to the target? Or is there something else I need to do besides entering the AOD coefficient into ISIS?

Cheers,

Keith

[Andrew Smith]

Keith - If your reference and target are at similar air mass then they will have the suffered the same atmospheric extinction. So when you create your IR it will divide out the effect of the atmospheric extinction. So you don't need to use the AOD correction in ISIS.

Regards Andrew

[Robin Leadbeater]

I have not used the ISIS feature but if you have a good atmospheric extinction model with the coefficients for the current conditions you can correct your raw spectra of target and reference to the top of the atmosphere. You can then calculate the true instrument response using the extinction corrected reference star and the published spectrum (ie without atmospheric effects) If you then apply this to the extinction corrected target star spectrum you get the fully corrected spectrum of your target. Unlike the simple method This should compensate for the difference in extinction due to any difference in altitude between target and reference.
(In addition, measurements of the reference at different altitudes can be used to fit the coefficients needed for the atmospheric extinction model)

Cheers
Robin

[Keith Graham]

Andrew and Robin,

Thanks for your replies. I think I now have a better idea of how to apply IR and atmospheric. It does sound like if I use a reference star close to the target and develop the IR profile from that, I will have both the IR and atmospheric correction when that IR profile is applied in ISIS. I think I have found a good reference star(HD184875) close to RR Lyr that i can use. I am anxious to see what Christian has in mind for atmospheric correction in the new ISIS version.

Cheers,

Keith