MMT/Hectospec: data reduction and common problems

Benjamin Weiner, MMT Observatory

The Hectospec fiber spectrograph on the MMT telescope obtains 300 fiber spectra at a time over a 1 degree field of view. This page is intended to compile some notes on data reduction and issues that occasionally cause problems in reduction.

Resources:

How to run HSRED

See the HSRED page for how to run HSRED v2 using a wrapper to run all the reduction steps. I also strongly recommend reading the older HSRED instructions. These tell you what the individual steps are doing.

Common problems

Fits files that don't belong

HSRED will usually attempt to reduce all the *.fits files in your directory. If there are fits files that it should not touch, it will run into problems. Generally, every *.fits data file should have an accompanying *_map file, a text file that describes the fiber assignments. If there is no map file, an error will occur in HS_MAPTOPLUG.

Mix of 270 and 600-line grating data

Hectospec has two gratings, 270-line and 600-line. The 270 is more commonly used. If your directory has a mix of 270 and 600 data, the reduction will run and may complete, but the data will look terrible. The most common issue is that there are some 600-line comps (arc lamps) or flats mixed in with your comps and flats.

If you don't have pyraf, you can use anything that looks at the fits header of extension 1 or 2 for the DISPERSE keyword to find the files with 600_gpm. If you are reducing 600-line data and need to get rid of the 270-line files, just change all the 600 references to 270.

Note that if there are data with multiple wavelength settings (only likely with the 600-line grating), you also need to sort those into one wavelength setting per reduction directory.

Missing skyflats

If there are no skyflats, HSRED will balk and issue an error partway through the reduction. Sometimes there may not be any skyflats named sflat.*.fits in your data directory, if for example the weather was bad at sunset.

No standard stars on config

If you try to reduce a configuration with the /uberextract option to get flux calibration, and you don't have any standard stars on the config, HSRED will complain. If you did put F stars on the config, you need to add them and their magnitudes to HSRED's list of standard stars in $HSRED_DIR/etc/standardstars.dat, described at the older HSRED instructions.

Restarting a reduction halfway through

I find that if the pipeline crashes for some reason, you fix the problem, and try to run it again without making changes, the end data are often garbled - this may include bad sky subtractions, or a SNR vs magnitude plot with a lot of scatter. One issue is that HSRED detects whether files have been processed and doesn't redo them, e.g. if you've run HS_CALIBPROC and made all of the files in the calibration subdirectory, running HS_CALIBPROC again will not reprocess them. To avoid this, start over from the beginning with a new rerun number. Your first run will probably be number 0000. You can start over by giving the optional command rerun='0100' (or 0200, 0300, etc) to commands such as HS_PIPELINE_WRAP, HS_CALIBPROC, HS_EXTRACT, HS_REDUCE1D.

Detecting completed but problematic data

One tool for checking whether your data are sensible is to make a SNR vs magnitude plot. You can do this by running HS_SNR on the spHect output file. In IDL, run commands like these:

fname = 'reduction/0000/spHect-2016.1001_1.fits'
lam = mrdfits(fname,0)
object =  mrdfits(fname,1)
plugmap = mrdfits(fname,5)
hs_snr, lam, object, plugmap, snr=snr

This should pop up a plot with continuum SNR measured from the spectra vs. magnitudes from your input catalog. If your magnitudes are good, there should be a decent correlation with modest scatter. For galaxies with SDSS photometry, I typically see a number like SNR ~ 2-3 at r=21 in about 1-2 hours of Hectospec exposure. If there are a few outliers, it may indicate problems with some of your targets - for example, low surface brightness objects will be low SNR outliers for their magnitude. If it's a total scatterplot with no correlation, then something may have gone wrong in the reduction. This has happened to me a couple of times with the mixed 270 and 600-line files, or with a reduction that crashed halfway through and was restarted without a new rerun.

Other problems?

If you have a Hectospec data reduction issue that isn't covered here, please contact me at bjw@mmto.org. If you have suggestions to add to this page, contributions are welcome!


Benjamin Weiner, bjw@mmto.org
Last modified: Wed Apr 12 17:26:46 MST 2017