Step-by-Step procedure for transferring SNAP calibration
Jan 15, 2002
Jan 31, 2002
Assumptions
We assume that
- the SNAP field is the North Ecliptic Pole
(we ignore the southern SNAP field for now)
- the SNAP field is one square degree
- the primary standard(s) have been chosen
- the primary standard(s) are within 10 degrees of the SNAP field
- the primary standards have been measured with an accurate
spectrometer
Pick field(s)
The actual SNAP field should not have bright stars in it;
they will contaminate measurements of the faint, V=25 SNe.
We assume that no stars brighter than V=11 fall within the
SNAP field itself.
We expect to find roughly 10 K giants with V<15
in the SNAP field.
Step 1: Transferring Primary (outside SNAP field) to Secondary (outside)
- Comparing stars of approx V=6 to stars of approx V=10
- Stars should all be K giants
- Must worry a lot about extinction, since the stars are likely
to be 5-15 degrees apart
- requires all-sky photometry
- must observe stars
at different airmasses interspersed with SNAP standards
- requires absolutely clear skies
- skies may be bright, since there will be plenty of photons
- should repeat measurements on at least 2 different nights
within a few weeks; 3 nights would be better
- if can repeat N times, will beat down some significant random
errors in extinction
- can be done easily on 1-m class telescope with aperture mask
- can be done with difficulty on 4-m class telescope,
because aperture mask would be massive
- necessary to use aperture masks (one big, one small) to
double-check shutter effects
Step 2: Transferring Secondary (outside SNAP field) to Tertiary (inside)
- Comparing stars of approx V=10 to stars of approx V=15
- Stars should all be K giants
- Stars will be within 2 or 3 degrees of each other, so
corrections for extinction are less critical
- number of auxiliary observations of stars at different
airmasses may be smaller
- possibly could carry out observations though thin clouds
- must pay attention to scattered light, since will be looking
at bright stars close to (but outside) the field of view
- can be done easily with 1-m class telescope, without aperture mask
- may possibly be done on WIYN 3.5-m telescope, even without
aperture mask (depends on shutter accuracy)
- aperture mask(s) may not be necessary (on 1-m telescope)
Step 3: Transferring Tertiary (inside SNAP field) to Quaternary (inside)
- Comparing stars of approx V=15 to stars of approx V=20
- Stars should all be K giants
- Stars are in the same field, so extinction corrections are easy
- Should concentrate on stars of the same color and spectral class
as primary standards
- is necessary to use telescope larger than 1-m class,
in order to get adequate signal-to-noise
- exposure times on 3.5-m are more than 100 seconds, so shutter
probably accurate enough; no need for aperture mask
except as sanity check
Step 4: Transferring Quaternary (inside SNAP field) to Final (inside)
- Comparing stars of approx V=20 to stars of approx V=25
- Stars are in the same field, so extinction corrections are easy
- will have to choose stars of a range of colors, since SNe
will have different colors as they age
- is necessary to use telescope larger of 8-m class
in order to get adequate signal-to-noise
Internal checks
We can very easily build two aperture masks for the
WIYN 0.9-m telescope: one with a diameter about 0.9 meters,
one with a diameter about 0.2 meters.
These will allow us to check the shutter motion
and the linearity of the CCD.
They will also permit us to take images of
stars as bright as V = 6 with 10 second exposures.
If necessary, the smaller mask could be made 0.15 or 0.10 meters
in diameter to accomodate the primary standards.
Building an aperture mask for the WIYN 3.5-m telescope
is not so easy, because the darn structure is too large,
and sits far above the ground.
It might be relatively easy to place a mask of some
sort into the light beam at the Nasmythe focus.
However, the beam has converged a lot by that point,
and a simple circular hole will not dim all stars
in the field by the same amount.
I have been thinking about various kinds of obstructions
(wire mesh screens, plates with many holes drilled into them),
and haven't found one that will meet our needs yet.
It is possible that we might use a neutral density filter
at this point to perform certain internal checks,
even if we don't use it in calibration observations.
The narrower our filters, the better the neutral density
filter will work for us.
In any case, we should do at least
the following tasks to verify our ground-based calibration.
- measure the shutter motion of the all cameras we use
(MOSAIC on 0.9-m, miniMosaic on 3.5-m)
- measure the CCD linearity of all the cameras we use
- compare relative magnitudes derived from WIYN 0.9-m
to relative magnitudes derived from WIYN 3.5-m,
for at least the Secondary-Tertiary step
- place limits on the variability of all secondary, etc.
standard stars
- requires observations for at least 2 years
- 0.9-m okay except maybe for quaternaries and finals
- acquire spectra of all secondary, etc. standard stars
- requires moderate telescope + spectrograph
Passbands
Which passbands should we choose when comparing one set of
stars to another?
The choices are:
- wide: approx 1000 Angstroms (Johnson-Cousins BVRI, JHK).
Advantages:
- requires less exposure time, smaller telescope for faint stars
- less sensitive to narrow spectral features
- same filters used by astronomers in many disciplines
- narrow: approx 70 Angstroms
Advantages:
- requires more exposure time or a larger telescope, can help
when looking at very bright stars
- less sensitive to color-dependent atmospheric extinction
- less sensitive to color differences between stars
I believe we should use a set of narrow-band filters for the
first transfer of primary-to-secondary at least.
The narrow filters will allow us to use large telescopes
and reasonable exposure times to look at stars as
bright as V = 10.
Some of the filters which are available for the WIYN telescopes
and appear to suit our needs are shown below.
WIYN_Imager_Filter_List
wiyn # cwl fwhm %T maxsize name comments date_measured age
16 6618 72 874x4 UWisc 1-97 1997
17 6725 70 874x4 UWisc 1-97 1997
19 4063 56 634x4 comet set 2 C3 3-97 1997
20 4448 62 744x4 comet set 2 Blue Cont 3-97 1997
25 7026 190 874x4 comet set 2 H2O+ 3-97 1997
26 7121 60 874x4 comet set 2 Red Cont 3-97 1997
I have checked to make sure that the filters above do not
fall across any strong spectral features in K giants.
The filter number 15 is close to H-alpha, but may be okay.
The result
After all the photometric observations and reductions,
we end up with the following information:
primary standards: know relative intensity as a function of wavelength
from visible to near-IR, due to balloon-borne
spectrograph
all other standards: know the flux integrated over several passbands
relative to the flux of the primaries,
integrated over the same passbands
know the flux of each star, integrated over several
passbands, relative to the flux of itself,
integrated over other passbands
In other words, for the secondary standards, we do NOT know the
ratio (for example)
flux at 5000 Angstroms
----------------------
flux at 14000 Angstroms
Instead, we know only these ratios (where V and J are examples
of the several passbands we may choose to use for calibration):
flux of self integrated over V band
-----------------------------------
flux of self integrated over J band
and
flux of self integrated over V band
----------------------------------
flux of primary integrated over V band
The narrower the passbands we use for calibration,
the smaller the uncertainty in these ratios (I claim).
In other words, if we pick a set of narrow-band
filters and observe all standards through them,
we will end up with known ratios of flux at a number
of wavelengths. We can then calibrate an observed spectrum
by forcing it to pass through these points.
In order to perform cosmological tests of nearby SNe versus
distant SNe, one must know
- the shape of the spectra of SNe
- the shape of the passbands used to calibrate the SNAP standards
- the shape of the passbands used to observe the nearby SNe
- the shape of the passbands used to observe the distant SNe
Note that the passbands in items 2 and 4 above
WILL NOT BE IDENTICAL,
because the instruments on board the SNAP spacecraft will not
be the same ones used to calibrate the standard stars.