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). http://en.wikipedia.org/wiki/Portable_Network_Graphics John 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 >> http://shop.technopro.co.za/acuter-allinone-camera-spotter-telescope-mount-p-916.html >> >> 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 > > > > > > ------------------------------------------------------------------------- > Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L > archive: http://www.satobs.org/seesat/seesatindex.html > ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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