Which F-ratio for planetary & lunar imaging?

[MarkS]

I did an experiment today to test out the rule of thumb that a 'scope will reach its limit of resolution at around F30-F40 for typical CCD imaging (approx 6-7 micron pixel spacing).

I used today's finance page of the Torygraph as a test target and adjusted it's distance from the camera as I changed the F-ratio to give the same magnification change you would get when added Barlows etc. in the optical chain of a 'scope.

On the test image below (taken with a x2 converter on a 300mm lens) you can see that as you increase from F11 to F32 the amount of detail that can be resolved goes up enormously.  The change between F22 and F32 is subtle.  Beyond F44, no more detail is resolved - the image just gets bigger.  Each sub image is a pixel per pixel crop of each original image.

So the experiment confirms the F30-F40 rule of thumb.  The rule of thumb holds true when your imaging is diffraction limited.  Unfortunately, here in the UK, it is the seeing that is the main limitation.



So I guess I'm going to be looking for a Televue x4 Powermate to go with my F10 scope!

Mark

[Daniel]

That's awesome stuff man, I've always wondered at which point I was pushing my optics too far, I've got a 4x Powermate you can borrow if you like before you go shelling out a load of cash on one, be very interested to see what you could do with it. I've only had a chance to try it on Jupiter so far and that was on a day with very bad seeing.

Daniel
:O)

[JohnP]

Yet again excellent work Mark. Any particular reason you focused on 'Barclays'.... Nice one,  John