[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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


Kathy did some work aligning the XRT and RHESSI data, and came up  
with a platescale of ~1.02", if I recall correctly.


On Apr 23, 2007, at 8:22 AM, Leon Golub wrote:

> 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.
> -- 
> ***********************************************************
> Dr. Leon Golub; Harvard-Smithsonian Center for Astrophysics
> 60 Garden Street, Cambridge MA 02138
> Ph: 617 495 7177; Fax: 617 496 7577
> e-mail: lgolub@cfa.harvard.edu
> http://hea-www.harvard.edu/~golub/HomePage.html
> ***********************************************************