Re: Satellite Photography

From: John A. Dormer 2 (
Date: Thu May 21 2009 - 02:01:39 UTC

  • Next message: Kevin Fetter: "Look at that nice extra star = Intelsat 1R and bright flashes from 23415"

    Technically, PNG's compression is "lossless" like the GIF type. JPEG
    compression is always lossy unless the tools are told to use 100% of the
    original pixels. Typically I have seen most tools use 70% (that is,
    reduce the image size on disk to 70% of the original, effectively).

    Greg Roberts wrote:
    > Morning all
    > Gerhard addressed the following comments to me:
    >> Greg
    >> - 640 x 427 pixels could then be the standard to set the camera to ?
    >> - .png format the standard ?
    >> - for your work will do at max telephoto.
    >> - will do stacking for party pictures.
    >> - on a alt/az tracker do you not think this
    >> will be a excellent gift to the wife? Your opinion will be apreciated or
    >> are there simpler stuff on the market ? your opinion please.
    > Sorry for the delay in replying - I observed last night till 1am and am
    > now busy
    > processing the results.
    > I just happened to use 640 x 427 -- all I did was reduce the size of
    > your original
    > image of 1600 x 1067 to 640 x "something" and maintaining the aspect
    > ratio ,so it
    > ended as 640 x 427 pixels so there is nothing "magic about the numbers
    > used. I was
    > a little surprised that it didnt come out 640 x 480 so it looks like
    > your camera
    > has its own format size. Anyway , for the present, I would not say its
    > necessary to
    > reduce your image size- although it will save you storage space. Scott
    > also has a
    > large image format from his CCD camera and 2x bins it which reduces the
    > size by a
    > factor of 2 and 4x (?) smaller in file size ( I think:-)).
    > Re using PNG - a similar type of answer - PIXY -which was the program I
    > did the
    > astrometry on your image, either wants a FIT or a PNG file- it was easy
    > enough to
    > convert your image to PNG format. I think PNG is actually superior to a
    > JPEG image
    > (in theory) since JPEG uses compression - whether the difference is
    > significant in
    > your case I dont know -- I wouldnt think there would really be much
    > difference.
    > Yes I would prefer you use a longer focal length-especially for the high
    > altitude
    > stuff.
    > Okay on your "party pictures" :-))
    > I had at look at the "Acuter All in One camera/spotter and telescope
    > mount". I have
    > not seen this before and know ZERO about it. I went to the SKYWATCHER
    > site and this
    > didnt give much more info - Ill do a search on the Internet and see if
    > the manual
    > can be located. It LOOKS useful but thats all I can say at this stage so
    > hang onto
    > your R3000 a bit longer and tell your wife she will have to wait -- do
    > you think I
    > really believe its for your wife ? :-)))  -- I must try that argument on
    > my wife!
    > Scott suggested the "cube" - this also looks nice but I think is quite a
    > lot more
    > expensive than the one you mention - I think its around $600 - I have
    > seen it
    > advertised here in South Africa.  Again I have NO real knowlegde of it -
    > I seem to
    > recall there was some disagreeable "flop" in it but I am open to
    > correction.
    > Re dead pixels - there is software available to map these pixels and
    > remove them
    > from the image, however I would not bother with this. If we eventually
    > use APEX
    > for your reductions APEX takes care of the problem internally so there
    > is no need for
    > flat fields, dark frames, dead pixel mapping etc- not even necessary to
    > do any image
    > processing at all as APEX does it all .
    > Okay back to measuring
    > Cheers
    > Greg
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