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Saturn May 21st with extra magnification

Peter Reader

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Hi all,

Managed to grab some more magnification from my 2X barlow by pulling the webcam adaptor out as far as it would go without falling out. Here are the results:

No idea how to tell how much magnification I have achieved though. If anyone could walk me through the maths of it that would be fantastic.





Comments on improvements welcome!

Edited by Peter Reader
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Hi Peter,

Not bad images.

The following formula isn't exactly what you were asking for, but it will give you a rough f ratio.

F = 206.265 x P x U



P is the size of the object in pixels as captured

U is the pixel size of the camera in microns

O is the true size of the object in arc-seconds

F will be the focal length as calculated from this formula, in mm.

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How do you measure the number of pixels an object takes up in an image?

How do you know the current apparent size of an object?

Thanks for the equation -whacked it into Microsoft Excel ready to use :)

Edited by Peter Reader
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Hi Peter

To work out how many pixels, I simply used an image and a ruler (and a calculator :p ) number of pixels = (640/width of image) x width of target, you need to bear in mind with Saturn whether you need to measure the rings or just the planet disc, from memory you need to measure the disc, however one answer will make sense and the other not.

The 206.265 is just a constant and I think it's a bit OTT to use 3 decimal places in light of the many other factors that are not well constrained, but if they are already in the spreadsheet :p

To find the apparent diameter I used "WinJUPOS" in "Tools>Ephemerides". But there are many Planetarium programs and Web sources (many report different values ??). These can vary by around 1% but I think that is good enough for this exercise, who really cares if it is f30 or f30.3 :)

Once you have the f ratio it is pretty easy to work out the focal length. In your (and my) case simply multiply by 200.

As for magnification calculations, I am not certain. I believe that it's not really possible in any meaningful way, it depends on the size of your monitor and how far you sit from it, likewise for prints:dontknow:

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