Re: [Fwd: Plate Scale for Hinode/XRT]

[Leon Golub <golub@head.cfa.harvard.edu>]

Dear Ishibashi-san,

As far as I know, there has not been any definitive work done on the plate
scale question. There is, as you note, the value provided by Shimizu-san
from the Mercury transit, but the value we have entered so far is only a
placeholder pending further work and also waiting for the outcome of the
orbital drift analysis.

Regards,

Leon


Bish ishibashi wrote:
> Hello, I am resending the email I sent out this morning (03MDT) since
> it was too long and I am told that it was rejected by the mail list.
> In any case, my apologies if you have somehow received twice.
> 
> ----------------------------------------------------------------------
> 
> 
> Hello, I realize that there has got to be a better thing
> to think on Friday night. But it has been bothering me
> a bit and I would like to know what others know about the
> question on the proper plate scale used for Hinode/XRT.
> 
> In a XRT FITS header, the keyword PLATESCL is generally
> set to 1.032"/pix. But is it really accurate to, say, a
> tenth of a percent?
> 
> As many of you are aware, co-alignment of an XRT image
> with an image taken with another instrument is proven to
> be rather challenging. A chosen co-alignment method of
> ours is to pick a SOHO/EIT 284AA (Fe XV, logT ~ 6.3) image
> matching with an XRT image taken nearly at the same time,
> and then co-align discrete XBP features (while avoiding
> any major AR features) in these images spatially. While
> doing so, I have noted that the plate scale of 1.032"/pix
> does not work well. Say for instance if I am to pick one
> discrete feature to co-align the two images and blink them
> to see the result, I see decent co-alignment with the
> discrete feature selected for spatial correlation, whereas
> other discrete features on the XRT image radially move
> toward the selected feature. This is a classic case of an
> underestimated plate scale. Please see an animated gif
> image (plscl1032.gif) below that demonstrates this point.
> 
> [the image is found at:
> 
>    http://space.mit.edu/home/bish/TMP/plscl1032.gif
> 
> ]
> 
> So I have, via trials and errors, tuned the plate scale
> to identify what correlates best with an SOHO/EIT 284AA
> image. I should note here that the plate scale of an EIT
> image is known to be 2.629+/-0.001"/pix (Auchere, DeForest,
> and Artzner, 2000, ApJ, L529, 115). And that is what I
> use for my coalignment program (I do fix all FITS/WCS
> keywords in an EIT image according to the guideline given
> by the FITSIO protocol). Anyway, the derived plate scale
> via comparing an EIT 284AA image and an XRT (Al/Poly+Open)
> image is 1.0417 ± 0.0005"/pix. The uncertainty is not
> based on a formal statistical error; rather it is meant
> to be the maximum probable inaccuracy associated with
> spatial correlation [1]. Please see another animated gif
> image for an improved co-alignment with the plate scale
> of 1.0417"/pix below (plscl1042.gif).
> 
> [this image is found at:
> 
>      http://space.mit.edu/home/bish/TMP/plscl1042.gif
> 
> ]
> 
> [1] the blinking method can usually tell if a discrete
>     feature is shifted by a quarter of a pixel. So
>     consider that as a total offset error accumulated
>     across the FOV of an image. In this example the FOV
>     of the XRT image used is 512x512, so the maximum
>     error is 1.042" * 0.25/512 ~ 0.0005"/pix.
> 
> 
> Having said all that, what is the most current value of
> the plate scale for Hinode/XRT? I am aware that the
> transit of Mercury event was used to measure a tentative
> plate scale of Hinode/XRT (1.056"/pix, if I recall).
> But I am not sure what details have been taken into
> account (i.e., Hinode's orbital solution, etc) to derive
> the value. I am not aware of the error value associated
> with the measurement, either.
> 
> Could someone please share insights on the plate scale,
> please?
> 
> Bish Ishibashi
> 
> 
> PS. the EIT and XRT datasets used here were taken at 0106UT
>     on 02-Feb-2007. I've done the same analysis on the XRT
>     and EIT images taken at also 0106UT on 08-Nov-2006, which
>     resulted in the same outcome.
> 
> 


-- 


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Dr. Leon Golub; Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Cambridge MA 02138
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e-mail: lgolub@cfa.harvard.edu
http://hea-www.harvard.edu/~golub/HomePage.html
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