Hectospec Run Preparation
Detailed information about hectospec can be found at the
page, at Doug
Mink's hectospec page. The published description of the
instrument by Fabricant et al. can be found at
- First and foremost be sure that Guide Stars and objects are in the
same astrometric reference frame. The fibers cover 1.5" and an RMS
error < 0.3" is extremely advisable. Fibers collide at distances
of about 20".
- The magnitudes of guide stars must be brighter than R=15.5 according
to hectospec documentation. Too faint means loss of guiding or
goofing up the instrument rotation.
Another recommendation is that these stars should not range by
more than 1 magnitude. Guide stars can be
selected from the
Guide Star Catalogue. Otherwise the SDSS can be used, which
has the advantage of good magnitudes and colors at the expense
of smaller coverage on the sky.
In either case, make sure to use either catalogue to find offsets
between science objects and guide stars.
One recommendation by D. Eisenstein is that the guide stars should
not be very red, since these could be M dwarfs with large proper
- According to documentation of Hectospec, the safe detection limit
for absorption line spectra in 60 minutes is about R = 21.5.
For 22.0 <= R <= 22.5 the success rate depends on color.
Successful detection in 1 hour integration is possible for
sources whose "I-band central surface brightness ~ 10 % night sky
brightness". I could not find in the MMT documentation what is
the typical sky brightness in I band at the MMT.
- Select Flux calibration stars. For fields with SDSS photometry this
can be done by querying the
SDSS Query/CasJobs site. This was the query used to select
Spectro-photometry calibration stars.
These are typically F stars whose fluxes can be approximately fitted
using the multi-band photometry. The stars were selected to be in
the range 15 < g <18. F stars will have colors
0.1 <= g-r <= 0.3. For the EGS run,
stars selected for fiber
placement were constrained to 0.1 <= g-r <= 0.5,
-0.1 <= r-i <= 0.2 and 0.9 <= u-g <= 1.4, containing
the range from A5 to G5.
- When setting priority to sources, be aware that the rank goes
from 1.0 (high) to N > 1 (low). xfitfibs will only place fibers on
rank = 2 objects after it has finished placing fibers on
rank = 1.0000-1.9999.
However the manual is not clear how fibers are allocated within the
1.0000-1.9999 rank interval.
- When preparing the catalogue ensure that enough
spectro-photometric calibration stars are placed in each field.
Write each one out the number of times corresponding to the
configurations that will be used. Example - if the field will
be covered by 16 configurations, write out each spec-ph star 16 times
with rank = 1.
- The xfitfibs manual states that sky positions (given by the
user, not those configured by the programme) should be given
lowest rank possible. However, since these will be the
last fibers to be placed, it is very likely that only a few will be
used (it depends on the total number of objects in higher
ranks). If the user's positions for sky fibers must be used,
they cannot be the lowest priority objects.On the other hand,
xfitfibs seems to do a good job in placing the sky fibers.
The code used to place fibers is downloaded from the hectospec
assignments page. Follow instructions on that page to download the
code and making it executable. Some information on how to prepare data
is available on the
assignments page. The xfitfibs manual itself is only accessible by
- First of all find out the dates of observation and times when the field
transits. Since hectospec is operated on queue scheduling, any time
while the field is below 1.5 airmasses is valid. However, note that
since the MMT has an alt-az mounting, fields that go close to
the zenith cannot be observed while transiting because of the
very quick change in instrument rotation.
- Create the field catalogue which will be used to determine the field
coordinates. These must be in the "starbase" format where columns
are separated by tabs, not blanks. See the example perl
scripts for the field center and the
object catalog . Make
sure the field (*.fld) and catalogue (*.cat) have the same name.
Notice that the test field name has an '.orig' appended. It must
be copied as '.fld' so xfitfibs recognizes it. The 'fld' file
will be overwritten by xfitfibs.
If the field and catalogue do not follow
the convention above (2 line header etc), xfitfibs will not
run . The times that were found in item 1 are placed in the fld
file under variable '$start'.
- Using a catalogue with the guide stars only, use the default parameters
under the parameters tab of xfitfibs (the radio boxes with red
colors). These specify the parameters used by xfitfibs to
classify the guide stars, interacting directly with the Guide Star
Catalogue and DSS (at the CfA presumably).
- Press the "Fit Guides" tab. If the radio box just before the RA column
is red, this means that the field position may be tweaked, using
the grid specified in the tab (default 2 arc minutes at
0.25 arc minutes intervals) and the final result written to
the "fld" file. The grid parameters can be
changed ad libitum. Often it is easier to tweak by hand,
toggling the guide stars and guide annuli, placing
the mouse on the ring representing the hectospec field of
view and moving it.
Before doing this, unset the red button next to the field RA
column so that changes will not affect the "fld" file.
Move the field around examining when stars fall inside the
guide annuli. The boxes with r0, r1, r2 show the rotation angle for
beginning, middle and end of exposures and should be green.
If any box is yellow or red, the field may need to be rotated
so all three boxes are green.
To do this toggle on the guide annuli and place the mouse on the
red circle at the ends.
- Once candidate stars are selected, they can be inspected with ds9.
They must also be classified by pressing the
"Classify Candidate Guide Stars" button. This will
retrieve and analyse DSS images, deciding if the candidates are worthy
enough to iluminate the guide probes. Stars that are in legal
positions have blue colors in the field plot, while stars that will
be monitored by the guide probes have green colors. The last box
in the field table will show how many guide stars are available. If
it is red that is bad news - a new position has to be searched for.
The bottom line is that there must be at least two probes which can
- To make life easier configure guide stars for one field at a
time (i.e., a separate file for each configuration. xfitfibs
updates these files.
- The ensemble of configured files can then be "glued" together:
head -2 egs_field_01.fld > egs_final.fld ; grep '14:' egs_field_*.fld >> egs_final.fld
- Edit the rank file (in this case junk.rnk) writing the minimum and
maximum number of objects of a given rank should appear in
each configuration. In order to have 6 spec-ph stars, write 6 in both
the minfield and maxfield columns.
- Now that the ensemble of field centers are ready run xfitfibs again.
If the configurations were done correctly for the individual fields,
fitting guide stars and classifying them will be quick and easy.
- In the parameters tab set the minimum and maximum number of sky
fibers and how they will be chosen.These numbers refer to the
sky positions "calculated" (somehow) by xfitfibs, not those
indicated in the catalogue file by the user.
- Press the "Fit Fibers" tab, set the number of rank levels ("depth")
set the rank button (it becomes red) and press "begin fit".
This will fit all configurations and place sky fibers at the end.
Depending on the number of ranks this is a long process. Unfortunately
the present incarnation of xfitfibs does not tell you which rank
it is fitting. I have found out that if there are too many rank
levels (I tried 10) it crashes with "segmentation volation".
This was solved by limiting the number of ranks to 6 or 7.
Of course this need further checking. Once all ranks are exhausted,
xfitfibs will place the sky fibers at sky positions it calculates.
- xfitfibs outputs the final configurations in a '*.cfg' file. This will
contain the fiber number, RA, DEC, plate X, plate Y position,
catalogue number and rank. If the number is -1, this is a sky
position chosen by xfitfibs. A zero means that the fiber is parked
(unused); otherwise it corresponds to the entry number in the input
catalogue. The sky Ra and Dec seem bogus.
- Once finished press the "Send" tab, pickout the project number. If
it is not there, let the hectospec team know so they can update
the code. After pressing send several tests will be carried out.
If there are no problems the files are shipped to the CfA.
Viewing configured objects
To check that all is sane, I wrote a perl script
that reads the input and configuration files, generating regions files
that can be fed to DS9 for each instrumental setup.This also calculates
correct sky fiber coordinates. In order for this to work the following
csh script setting IRAF up must be edited and
Last modified: Fri Apr 27 16:27:28 MST 2007