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[fitsbits] New DUMP FITS extension
One of the proposed changes to the FITS Standard is a complete rewrite
of the Appendix F (previously Appendix I) which lists the reserved FITS extension type names (i.e., the value of the XTENSION keyword). One of the registered extension types listed in that appendix is as follows: 'DUMP ' - Suggested extension name for storing a stream of binary data (such as a telemetry stream) in a FITS file. This extension type was never implemented, but the FOREIGN extension type serves a similar purpose. This definition may now need to be updated because I recently learned that the FITS data from Solar Optical Telescope on the Hinode satellite (http://solar-b.nao.ac.jp/index_e.shtml), launched in late 2006, contains a DUMP extension with the following minimal set of header keywords: XTENSION= 'DUMP ' BITPIX = 8 NAXIS = 1 NAXIS1 = 300 PCOUNT = 0 GCOUNT = 1 END My understanding is that this extension contains the image header packet from the satellite telemetry. The NAXIS1 value may vary, but otherwise all the DUMP extensions look the same. The IAU FITS Working Group is charged with maintaining the list of all registered FITS extension types, and at least at some level, is responsible for defining how each type is supposed to be used. A number of questions regarding the DUMP extension would need to be resolved before writing such a definition document: 1. Is this how the DUMP extension was originally envisioned to be implemented? An alternate scheme, as used in the FOREIGN extension type, would be: XTENSION= 'DUMP ' BITPIX = 8 NAXIS = 0 PCOUNT = 300 GCOUNT = 1 END Is one of these 2 forms preferable over the other? 2. How does the DUMP extension as used in the Hinode data differ from an IMAGE or FOREIGN extension? What are the fundamental differences, if any, between the DUMP and IMAGE (or FOREIGN) extensions? What criteria should data providers use in deciding which extension type to use? These questions are particularly relevant now because the new draft FITS Standard in section 3.4.1 states: "New extension types should be created only when the organization of the information is such that it cannot be handled by one of the existing extension types." (This requirement comes from the original "Generalized Extensions" FITS paper published in 1987). Given that an IMAGE extension presumably could be used to store the same data, does the DUMP extension satisfy this requirement? 3. Can the DUMP extension have a BITPIX value other than 8, and a NAXIS value other than 1? Must PCOUNT always = 0 and GCOUNT always = 1? If any of these keywords can have different values, how does this affect the interpretation of the data? Presumably these questions, and more, should be answered in a short definition document that specifies exactly how the use DUMP extension should be used. At the moment however, it is not clear to me that there is any consensus on the answer to these questions. Bill Pence -- __________________________________________________ __________________ Dr. William Pence NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
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[fitsbits] New DUMP FITS extension
On Thu, 16 Aug 2007, William Pence wrote:
I recently learned that the FITS data from Solar Optical Telescope on the Hinode satellite [...] contains a DUMP extension with BITPIX = 8 NAXIS = 1 NAXIS1 = 300 PCOUNT = 0 GCOUNT = 1 .... which actually resembles a 1-d image ... My understanding is that this extension contains the image header packet from the satellite telemetry. .... if one wishes to encapsulate raw telemetry in a FITS file, the idea of using an ad-hoc extension is sound ... 1. Is this how the DUMP extension was originally envisioned to be implemented? WHO KNOWS ? Who did suggest the original name ? I think it was a mistake on our part to accept the registration of a name without an implementation proposal, without keeping track of who might have drafted such proposal, and without setting an expiry date. We should define a more formal procedure for extensions which either (or both) : - allows pre-registration of a name for a fixed time (6 months ?) after which the name is retired if no implementation proposal has come - allows direct registration of an existing implementation (probably contextually with a registration of an associated "convention") like for FOREIGN An alternate scheme, as used in the FOREIGN extension type, would be: BITPIX = 8 NAXIS = 0 PCOUNT = 300 GCOUNT = 1 Is one of these 2 forms preferable over the other? The second one WOULD BE preferable for me for the packing of an arbitrary blob of data into a FITS file. However if the arbitrary data is organized as a fixed record length file (anybody remembering IBM JCL ?) it might make sense using NAXIS1 and NAXIS2 But it is now too late to ask this question (see further below) ! 2. How does the DUMP extension as used in the Hinode data differ from an IMAGE or FOREIGN extension? For me the difference is that an IMAGE extension is an image (a 2-d or n-d array, not necessarily a spatial image), a FOREIGN extension is the encapsulation of some specific FILE structure (with some relation to actual "live" operating systems e.g. for things like file naming, or even directory trees, soft links etc (which may become obsolescent in the future). A DUMP will be just a blob of data with no implications as files at OS level (users will have freedom to use it in their OS). A further possibility I thought of in the recent past would be a MIME extension, similar to FOREIGN but with no "live" OS dependency, but just encapsulating a mime file with its content type and other header (which presumably is more general, less OS-related and more long lived). In a later mail you wrote : The Hinode satellite project is now publicly distributing FITS data files with this DUMP extension. They apparently saw the DUMP extension name in the list of proposed extensions in the appendix to the FITS standard and decided to use that extension name for their telemetry data. Even though there is no specification of what the DUMP extension keywords should look like Again, it was our fault that no such specification did exist, and still DUMP was in the namespace ! The only thing we can do now is asking the Hinode people to provide a documentation of their *convention*, enter it in the convention registry, and ideally "assign" to them the registered DUMP extension (like FOREIGN is "assigned" to NOAO). This assumes nobody else is using another DUMP variant. If so one could have different (hopefully not inconsistent) conventions : a Hinode DUMP convention, an Erewhon DUMP convention ... On Thu, 16 Aug 2007, Doug Tody wrote in two other messages: Since the convention is in use for actual data, we should probably follow our existing practice of merely documenting existing conventions, and describe the convention/extension as used by Hinode. I agree with Doug in the terms specified above (the Hinode people should provide the documentation). My personal view on the more general question is that Image should only be used for image data, and it is a trick to stuff arbitrary binary blobs in something called an Image. I also agree with this. It is not "clean" to store a blob as an 1xn image nor as an 1xn binary table. But what is then an image ? One answer would be that if it is array data of any dimension it is reasonable to store it as an "image" I also agree here. I see absolutely no difficulty in storing e.g. a chi-square grid vs spectral index and NH as an image. I have also seen people storing 1-d spectra or histograms as images, or even collections of 1-d arrays (typically spectra) packed in an image (e.g. gross, bkg, net and quality flag vs wavelength stored as 4 rows in an image). Although in mostt cases my preference would be to use a binary table, both usages are legal and probably nearly equally efficient. Lucio Chiappetti -- ---------------------------------------------------------------------- is a newsreading account used by more persons to avoid unwanted spam. Any mail returning to this address will be rejected. Users can disclose their e-mail address in the article if they wish so. |
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