Quote:
Originally Posted by Quopaz
Hi Andrew, using that formula of 5x (the video says even up to 6x or 7x) the pixel size for the 60D I get: 5x 4.3= f 21.5. With the natural focal length of 4.8, I should need a 4.5x barlow to acheive this. Why then am I getting better results with a 3x barlow than 4x (two 2x barlows joined)? Something to do with the magnification factor in movie crop mode? Or would a proper 4x barlow or powermate be better than joining two 2x barlows together?
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Firstly, are you sure you are getting 3x magnification from your 3x Barlow? Nearly all Barlows change their magnification with distance from the back of the Barlow (for instance, the graph on this page).
http://www.televue.com/engine/TV3b_p...=52&Tab=_photo
You can tell what focal length you are actually using by using the formula shown in the image below (from
here), simply measure the size of Jupiter's width in pixels (with the planet horizontal), type in the pixel size and the size of the planet in arcsec and you can work out the actual focal length you are capturing at. You might be closer than you think. When I was using my Canon DSLR and Celestron 2x Barlow, my effective magnification turned out to be 2.6x. A quick back of the envelope calculation from your images above shows you are capturing at about f/19 with your current setup, or about 4.4x the pixel size (which is pretty much bang on).
That being said, using just 1 Barlow would be far preferred over stacking two together.
Andrew