Re: calibration

[Christian Veillet <veillet@cfht.hawaii.edu>]

Ray,

Thanks for your comments on calibration.

> Christian - is there an explicit calibration plan? This is easy for the ultra
> deep, feasible for the wide deep, but a real issue for the very wide.

No explicit calibration plan for the CFHLS itself. The CFHLS is not different
from other programs. There is a calibation scheme for QSO though...

> An idea is that the survey itself build an explicit calibration plan, largely
> based on the 15% of clear, but poor seeing time (some fraction of that will
> be photometric). Of  course that time wouldn't be added to the cost of the
> survey--if done efficiently it may well represent a time saving for CFHT.

I am not sure we need to develop something specific for the CFHLS various
sets of observation. What we need however, not now but definitely
some time soon, is what the requirements of the various images are...
I agree that for the very deep/SNe fields, we shouldn't have any
problem to have photometric calibrations. For the wide survey, it will be
a bit time consuming, but it is part of the calibration scheme done for queue.
As for the very wide, the surface covered is huge. To come back to the fields
if they were observed under non photometric conditions is going to cost
some time, slightly more than for other programs due to the short exposures.
We can ask to have that survey done only under photometric conditions,
at least for the z' part where the ratio between taking the images and
coming back to calibrate them is too high to be a good use of telescope
time.

> An interesting technical suggestion is that the calibrations might be done
> with a "window pane" which would include panels of the 5 SDSS filters.
> Whether this would work depends on the projection of filter plane on to the
> detector plane. That would allow each field to be calibrated in a single
> exposure, instead of 5 of them. Of course we would need to zero point this
> calibration to the true filters, but that is a one time only operation.

Well, a composite filter could be an interesting way to save time on 
calibration sequences. I would like to advertise a bit on the work
done here with Skyprobe, a camera on top of the prime focus cage looking 
at the sky in front of the telescope. We think we can get from it a value
of the sky transparency at a 1% level, based on what we get now. In a 
month or two, we will have more statistics to demonstrate that. To have
five cameras instead of one checking the sky transparency with each SDSS
filter in real time for MegaPrime is easy and cheap (compared to the cost 
of MegaPrime  and the price of one observing night at CFHT). 

> Calibration of 0.03-0.05 mag is fine for galaxies, but definitely not the
> standard for stellar objects, which will be much of the interest of the
> ultra-wide.

We didn't see many programs claiming a 1% accuracy with a wide field
camera... though it is definitely something we think we can achieve!
>From our experience, the large scale effects are dominating the
remaining errors (sky or Moon scattering, ...) and a lot of attention 
should be paid to flat fielding!

> In any case, some discussion of the calibration procedures and expected
> precision should be included in the plan.

I agree that calibration will have to be discussed, but on a more general
scope than just the CFHLS. We could add in our report what we request for 
the photometric accuracy of the very wide survey, with reasons for these 
requirements. However, the goal of CFHT is to do the best we can, no just 
for the cfhls but for all our users, and our request should be best we can 
get! These discussions on the calibration plans within QSO will have to take 
place with all those interested, which means all those involved in the 
CFHLS anyway! By the end of the year, we should have a pretty good idea
on how skyprobe is doing in comparison with calibrations series done
directly with the camera. It will be a key information to prepare any new
calibration plan for the MegaPrime era.

Have a look at
http://www.cfht.hawaii.edu/Instruments/Skyprobe/

Aloha

Christian

-- 
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Dr. Christian Veillet,       CFHT Senior Resident Astronomer
Phone: (808) 885-3161   http://www.cfht.hawaii.edu/~veillet/
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