[Comm2011] flatfielding/illumination correction

[Gijs Verdoes Kleijn]

Dear all,

Outcome of QuickIllumination analysis for g-band for SA110 is consistent 
with notions below:
http://wiki.astro-wise.org/projects:omegacam:commissioning:illumination

Best wishes, Gijs

Edwin A. Valentyn wrote:
> Dear all,
>
> The first inspects and analysis of our flatfields and 'quick'  
> illumination correction evaluation recipes _indicate_ the following:
>
> - in u everything is dominated by poor output of the lamp and massive 
> straylight gradients amplified by the angular sensitivity of the u 
> interference filters  leading to very complex results
>
> for the other bands: g,r,i,z, B, V:
>
> - the illumination of the lamps on the screen appears quite flat, better 
> than 5% in general (surprising)
>
> - the gradients in detected straylight on the sky are substantial, and 
> worse in interference filters,  and can amount up to 15% over the fov; 
> in g its circular symmetric, while in r, i, z more stucture is detected, 
> which can not be modelled with a simple axisymmetric low order 2-d polynome
>
> With these notions in mind the set of divided dome/sky flats are 
> revealing. Note, also the complex behaviour of the vignetting of the 
> crosses in B,V!
>
> These notions imply that in principle it is justified to construct and 
> apply our masterflats in the way we planned; i.e a combined dome+sky 
> flat with using small scale info from dome and large scale info from 
> sky, and normalise these per chip. We then have to compute the 
> appropriate Illum. corr, to compensate for the staylight gradients 
> affecting the sky flats.
> (Indeed if we would only use dome flats, and ignore sky flats we would 
> probably get a 5% accuracy result- and this is why some parties prefer 
> dome flats).
>
> The main conclusion is that we can apply our software in the way we 
> planned, with the notion that higher order polynomes are required for 
> the IC fitting.
>
> But we want to do better and characterize the gradients in straylight in 
> the sky flats.
> The only way to do this unambigously is to reduce an observation with a 
> set of stars put on each chip. This is planned in COM2, but I made one 
> on SA110 in g at our last night of COM1b.
>
> We should start that reduction now straightaway, with our standard 
> pipeline i.e.:
> -  make combined dome flat + sky flat normalized (Masterdomeflat) in the 
> standard way
> - for the field with SA110 in the centre compute the set of 32 
> zeropoints in the standard way (or even another standard field is 
> allowed, when we don''t detect enough stars on each chip)
> - apply these to 32 zeropoints to each of the 32 offset observations of 
> SA110 in the standard way
> - run sextractor on these and make the catalogues in the standard way, 
> but note we need here not only the x-y position of the stars in the chip 
> but also the x-y in the overall focal- If that's not already there  John 
> could help.
> - from here on we make a small interactive analysis;
> - Associate the cats with reference cats and produce table
> star1-ccd1- x,y_focalplane- Delta m
> star1-ccd2, x,y,_focalplane-Delta_m
> ...
> star1 ccd2, x,_focalplane-Delta-m
> star2
> ...
> star2
> star3
> ...
> star3
> etc
>
> Then we can fit a higher order-2-d poly and we are done.
>
> So it's not so difficult and is essentially running our standard 
> pipeline, as we planned.
>
> One problem is that S110 does not have many stars on all chips, required 
> for the delta_m and astrom for assocation.
> So let's first focus on getting that right.
> In the worse case there are two fall-backs, which require more work:
> - ask Steffen to run the same OB with longer integrations
> - make our own secondary standards base on these observations- but that 
> seems a lot of work.
>
> - alternatively, if we want to push for a throughput evaluaton,  we 
> could do a reduction with the dome flats only to get a probably 5% 
> accuracy zeropoint result, but without internal verifcation of our 
> interpretations are correct
>
> Koen: about ESO's desire for zeropoints:
> Formally this was not planned as the outcome of OCAM1, but for OCAM2.  
> Also in OCAM1 we were faced with continuously changing det/ampl settings.
> In OCAM1 we would inspect anomalies, which we did find in the straylight 
> gradients.
> Any update on the throughputs of the instrument would have to wait until 
> the careful analysis is done, otherwise its pretty meaningless- not also 
> we will find a complex map of throughtput over the fov. and we have to 
> decide which number makes sense (worset?, average?) I expect these to 
> lay apart by 20 %
> Of course we are working hard to make a first assesment now, we most 
> likely will need the 32 chip measurements for the other filters, as we 
> always have planned in OCAM2.
>
> Please note, we have to do a lot of bootstrapping- I'm glad we know now 
> how- , and our experience is that with 32 CCD's (and two FIERRA's) there 
> are always 1 or 2 which behave strange- we have to gain a lot of 
> experience to judge when to ignore this and when to further pay 
> attentions to these anomalies.
> I leave it up to you how you want to communicate this to ESO.
>
> Coming week this will be our focal point to do the reduction of this, 
> all other things being well under control (3 test astom fields are well, 
> and PR images done).
>
> kind regards
>
> Edwin
>
>   

-- 
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|dr Gijs Verdoes Kleijn         |   astronomer                     |
|e-mail: verdoes at astro.rug.nl   |   OmegaCEN / Kapteyn Institute / |
|www: www.astro.rug.nl/~verdoes |   Target                         |
|tel: +31-50-3638326            |   University of Groningen        |
|mobile: +31-654658050          |   postal address:                |
|                               |   Kapteyn Astronomical Institute |
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