thesis on high-res imaging

[bird]

I came across this paper this morning:

http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/node3.html

If you ignore the maths it still makes fascinating reading, especially the conclusions about the benefits (or otherwise) of larger apertures vs atmospheric turbulence.

regards, Bird

[Dennis]

Thanks Anthony - I had a go but ran out of steam! I'll return later and try nibbling away at it to digest it at a more leisurely pace.

Cheers

Dennis

[[1ponders]]

Me too Dennis. :scared: I think I'll go straight to the conclusion and see how the story ends

[[1ponders]]

Any chance of a laymans summary bird?

[John K]

Well it does look like a well references thesis al right! I has a quick speed read of this and like the rest of you moved quickly towards the summary pages.

So what he is saying is that with high speed low noise imaging (even with optics that are not 100%) you get very high resulution images with resampling of some sort and that the "Lucky" exposure technique works more cost effectively than adaptive optics. Is that right?

[bird]

There are a couple of parts that caught my attention - none of the maths, that's too hard these days :-)

- Depending on the seeing, there is an "ideal" mirror size that will maximise your resolution and minimise the effects of turbulence. It seems to be about 7 x the size of the "cells" of turbulence that are flowing overhead. Much larger than this and the effective strehl of the optical system + atmosphere starts to drop off significantly.

What size is this? Well it depends on the steadiness of the seeing, so it'll change from night to night. Best case scenario (very steady seeing, and good location a-la a professional observatory) he estimates the cells are around 20cm across, so that gives an ideal aperture of 1.4m. Going much larger than this actually *decreases* the image quality as the image breaks up.

I wondered about the ideal aperture for the sort of seeing we get in Australia, most of the time it's much less than perfect. I'm going to enquire locally with some meteorologically connected friends and see if I can find out this "cell size", maybe it's something that's measured and recorded somewhere...?

If you knew the average size of these air cells for your local area then it would let you work out the largest mirror that would be all-round effective. Nothing stops you getting something larger, but you might find fewer nights where it goves a good result.

cheers, Bird

[Satchmo]

I digress: Way back in 1988 I joined the CSIRO Division of Applied Physics Optical Workshop. My first three years were dedicated to polishing optics for the Sydney University Stellar Interferometer ( or SUSI for short ) . For SUSI I made a conglomeration of optics including a 22" F3.2 Cassegrain confocal beam reducing telescope, I polished many 1/40 wave flats from 80mm to 200mm which were used to study seeing conditions and ultimately from Narrabri , image surface details and distances of hundreds of the closest stars.

The process of stellar interometry also gave the astronomers a very good map of the seeing cells at the Narrabri site and at the site of the prototype, built just oppposite our workshop.

To cut a long story short, at Lindfield at least, they found the average nightly size of the seeing cells was about 150mm, which suggests from this paper that serious high resolution imaging could be done from down-town Sydney with up to a 1 metre aperture telescope.

Food for thought for you serious imagers :)

Mark

[bird]

Thanks Mark, I suspect that the seeing in the coastal areas is generally pretty good on teh east coast due to the laminar flow off the ocean. Especially brisbane seems to have its fair share of good nights, probably due to the stable temperatures and humidity.

Not so sure about my neck of the woods though...might be an interesting project to do, using different aperture masks to find the size where the image stabilises (only translational motion left).

cheers, Bird