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How big is a 100px anyway? and other AP thoughts...


ardbeg74

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Confronted with the knowledge that jupiter was going to be around 100px in a 640 x 480 image my natural reaction was to create a gap of around an inch with my thumb and forefinger and think, yea thats not too small. To be honest, with different screen resolutions on various devices i had no idea. Sure its about a fifth of the height of the image, so whats that... hmm 1 inch out of 5 inches?


What i needed was a frame of reference.... So created the chart below for some multiples of 25px against a 640 x 480 background. But like many things in life, the answer posed more questions...


On this formula for focal length for ccd imaging ( thanks JamesF :) )


    focal length required =  ( 206265 * ccd pixel size in mm * planet diameter in pixels ) / planet diameter in arc seconds


An ASI 120mm ccd imager with a 2.5 barlow should be produce about 200px worth of jupiter. This seems pretty big to be honest, at least based on my current AP, ahem, "method". ( A-focal, 2xbarlow, 10mm EP and  3x zoom on my point and click camera )


Even more interesting is that, using prime focus with the ASI, my 1200mm focal length is going to produce an 78px image, which is about the same as i get with the above set up. So afocal = adding a ton of glass for nothing.



Am i missing something here? Or does this sound about right?


Wondering about getting the ASI.


At the moment im just letting the target drift across the camera while recording at 640x480, and planning on doing the same with the ASI. With my current setup thats my only option. So my thinking is that the ASI would give me better images and be a lot easier to live with than messing around with brackets, zoom, alignment, battery life,small screen,low frame rate, etc etc.

    

And when the day comes to buy a tracking mount, i'll be comfortable with the camera / filter wheel / software and image processing.


What do you think? Will it work out good or will aligning/stacking the LRGB avi's turn into a mess?


Sorry for the ramble :)


Mark

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