How I go about planning the acquisition and logging of a stellar spectra ?

By Forrest Sims

The LISA spectrograph mount on the CDK 14′
Photo : Forrest Sims

I have a Shelyak LISA spectrograph. I am fortunate in that this instrument allows me to investigate a wide range of targets with magnitudes typically in a range between 4 and 15. I use the software package AstroPlanner by iLanga, Inc. for my initial planning. I have added approximately 75 stars which contains all the information I need to plan the acquisition. AstroPlanner allows in most cases adding a new star by simply entering the star’s identifier and clicking on a button to import its information from SIMBAD. In addition to numerous standard fields such as RA, DEC, Magnitude, I have added DateLastObserved, Cadence and a few others. When given a date and time, AstroPlanner provides target Azimuth, Altitude, Rise, Transit and Set times, Angle from the Moon and other bits of useful information.

Fig-1 : screen shot of AstroPlanner software

The next thing I do is to identify a Reference Star that is preferably of Type A/B at a very similar airmass, close in azimuth if possible, to the target star and has an E(B-V) less than 0.05 to use for Atmospheric and Response correction of the spectra. To do this I use either of two excellent Excel spreadsheet tools. François Teyssier’s ReferenceStarFinder.xlsm or the Miles_Search_V1 spreadsheet tool from Paolo Berardi, et. al. In this selection process, preference is given to choosing the same star I used last time on this target if for the time of night, it satisfies the above requirements.

Once I have selected my target, I begin filling out my “Spectroscopy Acquisition/Processing Log” sheet. See Figure 2. This sheet works both for recording plan information but also and importantly for logging information about what I actually did and the summary results from processing the acquired data in ISIS. It is also very useful for planning when you have previously worked the same target. The reason is that it gives you initial values for the possible Reference star to use as described earlier, as well as science camera exposure times for the stars and guider exposure settings.

When the time comes, I open my roll-off roof on the observatory an hour or two before the planned acquisition time. The Observatory, camera’s, rotator, cooler, etc. are powered up and with the plan in hand I am ready to slew to the Reference star. It is at this position in the sky that I take a set of Flats using the LISA Calibration Module Flat lamp. After centering the star on the slit I take a Neon/Argon calibration spectrum. With the Reference star centered on the slit autoguiding is initiated and a minimum of 7 spectra are acquired. Then I slew to the target star and again take a Neon/Argon calibration lamp spectrum followed by a minimum of 7 target spectra. These are usually much longer exposures up to as long as two hours of total exposure time. I always plan to take an additional Neon/Argon calibration lamp spectrum afterward to bracket the target data. Further I record in the Notes field anything that might be important later such as did I rotate the slit or not, cloud, wind anomalies, etc.

I won’t go further into the details of the acquisition but move onto using the Log sheet to record the ISIS processing results. I move the raw data files for the Observation from the observatory computer to a network attached storage device with a RAID5 array into my office. From here I process the data in ISIS referring to the Log sheet. It has the star names that I need to enter into ISIS.

I record the Tilt, Slant (Smile in case of an Alpy) and VC (Vertical coordinate of the spectrum), A (The X-axis) position of the spectrum and the FWHM of a calibration line. In addition, I record the calculated R and RMS values that I get from the Go processing tab in ISIS.  I do this for both the Reference and Target stars. Recording and retaining this information provides me with important hints on drift in spectrograph focus (FWHM increases, primarily due to temperature changes) and alerts me if I suddenly see changes in Tilt or Slant for example.

When the spectral data processing is complete, I upload it to one or more databases and record this on the log sheet.

Fig-3 : the completed log sheet for AG Dra
Credit : Forrest Sims

Here is an example of a completed log sheet for a recent Observation of AG Dra. I file these log sheets by target, so they are readily at hand the next time.

I hope that this may be of some help to someone just getting started and trying to figure out where to start.