Optics

[Stub Mandrel]

Making a broad assumption that taking a digital picture is like making a digital sample of a sound, the minimum sample rate for sound is twice the highest frequency (nyquist criterion).

I'd therefore guess that the minimum pixel density to get the best out of your scope is twice the optical resolution. Now I can find the pixel density of different cameras, but how do I find out what the resolution of my scope is if using it as a prime focus lens i.e. if a 6" x 1200 mm scope resolves down to 0.7 arc seconds, how far apart would two 'spots' spaced by 0.7 arc seconds be at the prime focus?

The reason I ask is that, if the spacing is less than two pixels apart, then i can use a more powerful barlow to increase the resolution. If its more than two pixels apart, I can get rid of the 2x barlow and get sharper, brighter, wider field pictures without losing resolution.

Hope this makes sense, I've underlined the main question so it doesn't get lost in the noise (apologies if you have to read this several times and stack it to figure out what I'm waffling about...)

[Stub Mandrel]

To start answering my own question, I found this: www.grelf.net/telecamera.html which means at least someone else worries about this!

[ronin]

Simple geometry says 4 micron, I think.

Sep = 1200 *2* Tan(0.7"/2)

Focal length times 2 times tan of half the subtended angle.

Should you not be requiring that seperation to be 3 pixels?

2 side by side would appear as one "lump" I would have thought that to resolve then there should be a "dark" pixel between.

Would also guess the figure is a bit meaningless as it would require perfect tracking, and no leaking from one pixel to another.

[Altais]

I agree that the separation is about 4 micrometers. But keep in mind that seeing might smear out the stars to several arcseconds, so the diffraction limited resolution of the scope usually isn't the real limiting factor. There's some useful information on this webpage that can help you find a good image scale for your system.

[Stub Mandrel]

Thanks Altais, that link tells me everything I wanted to know and a lot more besides!

I was interested to find this Nasa image on Wikipedia - seven frames stacked and resampled at 500% then colourized from ordinary frames. If NASA can cheta like that, I'm not proud...