[Comm2011] astrometric solution

Sun Aug 7 07:20:45 CEST 2011

Hi Koen,

Here is the new solution of the CRPIXn and CDn_m values for all chips based 
on only the one 34 CCD 'exposure' you indicated:

chip name   [CRPIX1, CRPIX2]
----------------------------
ESO_CCD_#65 [ 8572,    8584]
ESO_CCD_#66 [ 6423,    8591]
ESO_CCD_#67 [ 4281,    8589]
ESO_CCD_#68 [ 2148,    8587]
ESO_CCD_#69 [    7,    8581]
ESO_CCD_#70 [-2147,    8583]
ESO_CCD_#71 [-4270,    8594]
ESO_CCD_#72 [-6412,    8598]
ESO_CCD_#73 [ 8565,    4131]
ESO_CCD_#74 [ 6428,    4123]
ESO_CCD_#75 [ 4288,    4121]
ESO_CCD_#76 [ 2148,    4124]
ESO_CCD_#77 [    4,    4126]
ESO_CCD_#78 [-2142,    4122]
ESO_CCD_#79 [-4280,    4135]
ESO_CCD_#80 [-6415,    4137]
ESO_CCD_#81 [ 8571,      84]
ESO_CCD_#82 [ 6431,      68]
ESO_CCD_#83 [ 4286,      78]
ESO_CCD_#84 [ 2144,      76]
ESO_CCD_#85 [    6,      78]
ESO_CCD_#86 [-2140,      83]
ESO_CCD_#87 [-4284,      79]
ESO_CCD_#88 [-6418,      68]
ESO_CCD_#89 [ 8575,   -4379]
ESO_CCD_#90 [ 6428,   -4386]
ESO_CCD_#91 [ 4282,   -4388]
ESO_CCD_#92 [ 2138,   -4390]
ESO_CCD_#93 [    0,   -4388]
ESO_CCD_#94 [-2147,   -4378]
ESO_CCD_#95 [-4283,   -4381]
ESO_CCD_#96 [-6428,   -4382]
ESO_CCD_#G1 [-9769,     -10]
ESO_CCD_#G2 [11727,    4219]

chip name   [CD1_1,                CD1_2, CD2_1,                CD2_2]
----------------------------
ESO_CCD_#65 [5.921335637810671e-05, -0.0, -0.0, 5.925764153178001e-05]
ESO_CCD_#66 [5.923168600720749e-05, -0.0, -0.0, 5.9249287927878e-05]
ESO_CCD_#67 [5.926354119996733e-05, -0.0, -0.0, 5.923879472804842e-05]
ESO_CCD_#68 [5.91969942963964e-05,   0.0,  0.0, 5.925265620265505e-05]
ESO_CCD_#69 [5.927946879634725e-05,  0.0,  0.0, 5.927948941890834e-05]
ESO_CCD_#70 [5.922451170065446e-05, -0.0, -0.0, 5.9252868043836e-05]
ESO_CCD_#71 [5.929128114549217e-05,  0.0,  0.0, 5.924793214431994e-05]
ESO_CCD_#72 [5.926349527875355e-05,  0.0,  0.0, 5.926428628348905e-05]
ESO_CCD_#73 [5.925503871061571e-05, -0.0, -0.0, 5.923141559357877e-05]
ESO_CCD_#74 [5.921873622492122e-05, -0.0,  0.0, 5.920316657209949e-05]
ESO_CCD_#75 [5.921573721642121e-05,  0.0,  0.0, 5.924205708223501e-05]
ESO_CCD_#76 [5.920055142426391e-05, -0.0, -0.0, 5.923652096603959e-05]
ESO_CCD_#77 [5.924448742864923e-05,  0.0,  0.0, 5.923779907449797e-05]
ESO_CCD_#78 [5.920967914860317e-05, -0.0, -0.0, 5.92424737032242e-05]
ESO_CCD_#79 [5.925118839346021e-05,  0.0,  0.0, 5.924147098830106e-05]
ESO_CCD_#80 [5.928573527582784e-05,  0.0,  0.0, 5.924623741487236e-05]
ESO_CCD_#81 [5.923401385950609e-05, -0.0, -0.0, 5.923256659732858e-05]
ESO_CCD_#82 [5.920442293582576e-05,  0.0,  0.0, 5.920942294831012e-05]
ESO_CCD_#83 [5.925243179863336e-05, -0.0, -0.0, 5.924047533475061e-05]
ESO_CCD_#84 [5.924925617008035e-05, -0.0, -0.0, 5.923063178120926e-05]
ESO_CCD_#85 [5.925487622016694e-05, -0.0, -0.0, 5.922713993240999e-05]
ESO_CCD_#86 [5.923738023771634e-05,  0.0,  0.0, 5.923494627992789e-05]
ESO_CCD_#87 [5.924171449381707e-05,  0.0,  0.0, 5.926062849243136e-05]
ESO_CCD_#88 [5.927371804742143e-05, -0.0, -0.0, 5.924575724152888e-05]
ESO_CCD_#89 [5.922352616075869e-05, -0.0, -0.0, 5.925341176953377e-05]
ESO_CCD_#90 [5.919921617666319e-05,  0.0, -0.0, 5.925339764678837e-05]
ESO_CCD_#91 [5.924022735297078e-05,  0.0,  0.0, 5.926055081733168e-05]
ESO_CCD_#92 [5.921899409019861e-05,  0.0,  0.0, 5.921925237910606e-05]
ESO_CCD_#93 [5.928503586041794e-05,  0.0, -0.0, 5.922663504426207e-05]
ESO_CCD_#94 [5.926288064096909e-05,  0.0,  0.0, 5.924298918343118e-05]
ESO_CCD_#95 [5.925524358987719e-05,  0.0,  0.0, 5.924188054791755e-05]
ESO_CCD_#96 [5.923313782404319e-05,  0.0,  0.0, 5.925589031135085e-05]
ESO_CCD_#G1 [5.927801232816358e-05,  0.0,  0.0, 5.925234914150914e-05]
ESO_CCD_#G2 [5.923450796976645e-05, -0.0,  0.0, 5.925509273112681e-05]

The precision of the CRPIXn values has been reduced to 1 pixel, but the 
precision of the CDn_m values has been left as stored in the headers by the 
correction routine.  It is LDAC.preastrom that has created these values. 
It is certain that the CD1_2, CD2_1 values are non-zero, but preastrom 
apparently sees these values as too small to be anything else.  This 
hopefully will not hamper the correct determination of the plate scale.

Please test these values however you and Andrea see fit and let me know if 
you need any more work on this.  I expect the best thing would be to have 
multiple measurements averaged, provided they are consistent.

Cheers,

-=John

On Fri, 5 Aug 2011, Konrad Kuijken wrote:

> Now that the rotator issue has been traced to the software that combined the information from the encoders of the rotator, we have a workaround (only use one encoder instead of the four existing ones) that seems to work fine. No more backlash, and the angular preset is now correct and predictable.
>
> We're close to having an astrometric solution that is good enough for automatic guide star acquisition - that will be a nice milestone.
> The WCS that is in the image headers at the moment is good to about 25 pix all over the field, including the aux.CCDs.
>
> If you overplot USNO stars over a full science array in skycat you still see a radial pattern of residuals, up to 25 pix in the outskirts. This implies that the plate scale is wrong by about 50/16000, ~ 0.3%.
> [NB there was a bug in the header writing up to now - the CD values only had 3 significant figures. Not quite enough to explain this effect though].
>
> However, on individual chips the residuals change by about 5 pixels over the length of the CCD, implying the plate scale is wrong by 5/4000, ~ 0.12% .
> So apparently there is an issue both with the crpix values and with the CD matrix.
>
> Could someone look into this and provide updated values? I think we can assume here that the plate scale is constant over the full array, but it needs to be determined together with the proper crpix's for each CCD. Once this is set we are in business. (science frame and AG stars).
>
> I put a frame (OBS217_0011) and a simulataneous full-frame readout of the AG CCDs in
>
> ftp://ftp.strw.leidenuniv.nl/pub/kuijken/ocmwcs/
>
> Thanks
>
> Koen
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