bendavidson

Thanks - yes, marginal difference in the end, but good to understand how you made the numbers work. With a 9.25" SCT and 60mm Altair guidescope and cam, I end up with a ratio of 8. I'll see how well it does and maybe add a Barlow lens. Adding a 6.3 focal reducer takes the ratio down to about 6... Once again thanks - really good article. The setting up walk through was excellent and I'm sure you're right it made more sense to do it indoors rather than outside in the cold, even if there weren't hitches...

What a brilliant beginner's guide - thank you so much Ian. A couple of questions re the pixel scale calculations. I have a Canon EOS 700d, similar to the one you used as an example here. Your EOS 500d has 15.5 megapixels (15.1 effective megapixels) and a chip size of 22.3 x 14.9mm. That's a chip size of 22,300µm x 14,900µm = 332,270,000µm2, divided by 15,500,000 pixels, comes to 21.44µm2 per pixel (or 22µm2 per pixel if you're only counting the effective ones). Yet you say the chip's pixels are 4.7µm2. 1/ Your number looks like the square root of mine. Is that right and, if so, why? 2/ Also if so, how come you are using the effective number of pixels rather than the total number? Thanks in anticipation. Ben