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 _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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