Re:first images

From: Greg Roberts (
Date: Tue May 19 2009 - 11:01:18 UTC

  • Next message: Marco Langbroek: "Re: first images"

    Afternoon Gerhard ( and other readers)
    Took me a while to master your image of Lacrosse 4 but eventually got what I 
    Original image 1600 x 1067 pixels was too big for some of the programs to handle 
    so had to reduce it and ended up reducing it 2.5x to  640 x 427 pixels. 
    Eventually got the freeware astrometric program PIXY to handle the image after 
    converting it to  .png format and got the following:
    centre of field of view  RA 07h48m16s   Dec -07d 48' 28"
    ( I bet you wont get that right again by accident- I did not believe that the RA 
    and Dec would have essentially the same digits but checked it in another 
    astrometric program and its correct!)
    Field of view  43.38 x 28.94 degrees
    pixel size 244.00075 arc sec/pixel on the 2.5x reduced image so the true scale 
    is 99".18 arc sec/pixel in raw format.
    The position of the end of Lacrosse 4 trail is RA 07h59m08s Dec +10d 42'04".
    Magnitude at brightest +3.3 and at end of trail +5.8    ( wouldnt put too much 
    faith in the magnitudes)
    limiting magnitude +7.6
    Okay now some comments:
    I do NOT suggest you use the 28mm lens - the field is far too wide and this 
    reduces the positional accuracy since a pixel then covers 99 arc seconds or 1.5 
    minutes of arc. I would suggest around 100mm lens minimum- scale would then be 
    100/28 = 3.57 x "better" or 99/3.57 = 27.7 arc seconds/pixel. Similarly a 200mm 
    focal length lens would give you  13.86 arc sec/pixel. Perhaps the only use for 
    a 28mm field is if you know the satellite is well "off" in time so the position 
    uncertainty is large, or in looking for strays. The star density is also 
    somewhat high with stars "smearing" into each other- a longer focal length would 
    give you better resolution on the fainter stars as well as magnitude 
    I think you used the lens at f/5 -- next time try opening the aperture one or 
    two stops more - it looks like you have a dark sky so you might be able to get 
    away with a faster system - it will also effectively give you a bigger aperture 
    and hence ability to go fainter in magnitudes - one "stop" is approximately 0.7 
    to 0.8 magnitudes gain or decrease-depending which way you go..
    For geostationary or other high altitude satellites you HAVE to have much better 
    resolution than the 28mm can offer - I suggest 200mm minimum - this will still 
    give you a field of view of approximately 200/28 = 7.14x smaller than your 
    original 43 x 29 degrees, so you will have  6.07 x 4.05 degrees which is a nice 
    size to work with- even for leo sats.
    Ill send you your original picture after I had converted it etc as well as a 
    list of goesats for you to try.
    Incidentally in normal positional work I would not recommend stacking images - 
    its fine for pretty pictures or other space objects, and sometimes geostationary 
    satellites but I would not encourage it.
    Hope this helps
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