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Old 25-03-2013, 03:26 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,824
Planetary imaging FNo rule of thumb - which Barlow

If you want to do some planetary imaging, the following very straightforward rule of thumb will give you the best possible combination of resolution and sensitivity for any scope and camera that you choose.

FNumber = pixel size(microns) * 5

Use a Barlow to boost your FNo up to the number you get from the calculation - eg if you need f18 and have an f6 scope, use a 3x Barlow.

For 5.6 micron pixels (eg TIS618), aim for F28 - for 3.75 micron pixels (eg ASI120), aim for F19. Note that a Barlow will give you varying magnification depending on spacing to the camera focal plane - Powermates are telecentric Barlows with an extra lens group to compensate for the change in magnification with spacing (compensation is incomplete for the 5x) - except for the 5x, you cannot vary magnification significantly with powermates.

couldn't put equations here easily, so detailed derivation of the rule of thumb is in attached screen copy from my word processor, along with comments on how to use it.

regards Ray
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Last edited by Shiraz; 02-04-2013 at 01:01 PM.
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