Re:Using a DSLR

From: Greg Roberts (grr@telkomsa.net)
Date: Fri Dec 14 2012 - 20:33:15 UTC

  • Next message: Marco Langbroek: "Re: Using a DSLR"

    Evening all
    
    Will add my pennies worth to the discussion - I guess Ive had a lot of 
    "experience" with focussing for satellites since Im constantly changing my 
    optical setup and each one presents different problems.
    
    Have no problems with the discussion about "live focus" using modern DSLR 
    cameras. Ive just purchased a NIKON D3100 which has this feature - I focus on a 
    bright  star - its easy to see and brightness depends on the f/stop of the lens 
    used since you are actually viewing through the lens - then use the zoom feature 
    in "live view" - its actually just a magnified image of the initial image and 
    not a true zoom but there is no problem seeing where the image is sharpest.
    
    Having got this I stick some masking tape around the focus ring of the lens used 
    as this tends to move depending on the elevation of the camera system - the 
    focus ring slips under gravity so one needs to prevent this.
    
    For other camera's - eg the CCD camera focus is not so easy - Ive constructed a 
    crude system where I use a small DC motor mounted next to the camera lens and a 
    system so that as the DC motor shaft rotates this motion is translated to 
    rotating the lens focus mechanism - you could use rubber bands, gears etc - use 
    whats most convenient.      When observing I watch the image on the screen and 
    apply a DC voltage to the motor very momentary and this will cause the motor to 
    rotate the focus ring in very small increments.    By changing the polarity of 
    the DC voltage applied I can change the direction of rotation of the focus ring. 
    Once Ive found what looks like the best focus I again apply my masking tape to 
    secure the focus ring.
    
    For lenses where I dont have this its a case of trial and error -- more long 
    winded than complicated  - until I find a best focus. If I have a video camera 
    attached to the lens its easy to get best focus - use a miniature TV monitor and 
    take the video feed from the camera into the video socket on the TV monitor and 
    focus for best image.    Once done again apply the magic masking tape.
    
    As to the best lens to use -- I prefer a focal length of at least 50mm for leo 
    satellites. For higher altitude satellites  longer focal lengths as this gives a 
    better image scale and also makes it more easy to see which is the satellite and 
    which is a star-- the stars will trail whilst most high altitude satellites 
    around 37000 kms dont produce much movement in a few seconds exposure.
    
    Now to the f/ratio of the lens -- this depends on how bright your sky is. I live 
    in a heavily light polluted area so cannot do exposures long enough to produce a 
    decent satellite trail unless I use an optical system slower than about f/2.8 
    and I generally prefer around f/3.5 to f/4.5. I have found that there is not a 
    significant loss of light if one changes from say f/2.8 to f/3.5.   A loss of 
    light would be noticed on extended object sources but for stellar and satellite 
    sources the light intensity doesnt seem to vary that much.
    
    What ISO speed index to use on a DSLR - I havent dont much experimenting here 
    but a setting of 400 or 800 produces reasonable images. For me there is no sense 
    in going to 1200, 1600 or higher -- I end up with a white screen as the image is 
    "over exposed". For leo sats one might be able to use higher than 800 if your 
    exposures are short enough but for high altitude satellites and at a suitable 
    focal length one needs to expose for 10-20 seconds to get a suitable length star 
    trail.
    
    
    What size image format ? - I havent found it really worthwhile to use the RAW 
    format so go for the JPG ( in my case about 1:4 compression)option. The larger 
    the image format the better the image resolution but of course the very large 
    images produce big files which you need to consider depending on how 
    fast/powerful your PC is - my one pc actually battles to process very large 
    images. For CCD work I use 1 ,2 or 4 binning so image size ranges from about 1 
    Mbyte up to about 16 Mbytes. Unfortunately, unless you have a moderately 
    powerful computer with plenty of RAM, the computer can lock up with CPU 100% and 
    one has to crash the PC to get the computer back into operation whilst using the 
    CCD image capturing program - this is probably a program problem and a poor 
    excuse for a computer :-)).
    
    A lot depends on one's sky and what one wants to image - not everybody has the 
    identical set-up so there will be variations of methods.
    
    Obviously with a DSLR you MUST use the MANUAL options - autofocus etc does not 
    work when the illumination is too low.
    
    As a minimu lens setup I would favour a 50mm f/1.8 and then probably cut down an 
    f/stop or two -- this will make focus easier as focus is VERY critical at the 
    very fast lens f/ratios. Also lens aberations will also be reduced by "stopping 
    down". A fast lens, unless very expensive, will have pronounced field curvature 
    and non-linearity in image scale.
    
    Enug said
    Cheers
    Greg
    
    
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