Dragonfly, Powered by Canon Lenses

[Bdube]

I'm one of the astronomers who put the thing together - thanks for the interest!
We use standard SBIG off the shelf astronomical cameras as detectors. The lenses are fast enough, and the integration times long enough, that read noise is negligible even with only modest cooling of the detectors.

We've published a paper that describes the thing; this is the publicly available version, for those who are interested:
http://arxiv.org/abs/1401.5473
As you may notice we're mostly concerned with scattered light; we measured the point spread function out to ~1 degree (Fig 6) and found that it is amazingly well controlled (this was recently confirmed by another group, who compared our results to a wide range of other telescopes).

We've had some science results out this year, too - and we just put out a press release on the discovery of seven very faint galaxies (which might be why CR posted it today!). The reason why we went with Canon is that I'm a Canon shooter (with a special interest in dragonflies..) and I was aware of the quality of the updated lenses. I know lenses and Bob knows telescopes, so it all worked really well.

Other groups have used Canon lenses for astronomical purposes, but typically just to cover a wide area of sky - not to detect very low surface brightness emission, beyond the reach of reflecting telescopes.
Anyway, sorry for rambling on - it's a fun project!

Your results evaluating the lenses are quite interesting. I am not sure they are correct, however. I have measured 6 of these lenses under photopic light at an aperture of f/4, and even then they are nowhere near the diffraction limit. I have little to no doubt they are designed this well - but realizing such a performance in large scale manufacture, when I have measured to the contrary, leaves me skeptical.

I am not familiar with the technique you have used to measure the wavefront error, but judging by the use of defocused images, it seems to measure only wavefront curvature, not tip/tilt, yes? I am then confused how your synthesized interferogram is dominated by tilt, if the test would be unable to see tilt.

Further, your defocus is quite enormous, and I would suspect that the very large defocus term you have added to the zernike polynomial may be masking other aberrations. However, I am not familiar with the method, and perhaps it is robust with respect to that parameter. It seems that by increasing the size of the defocused spot, you will increase the spatial resolution of the reconstructed wavefront, but decrease the sensitivity of the test.

Are you able to describe in more detail the workings of this method? It intrigues me, but I have my doubts as to its accuracy.

 

[jrista]

Actually, surveying for nearby supernova remnants in H-alpha might be a pretty interesting project scientifically in itself for this Dragonfly.

Yes - the problem is that we'd have to get different detectors, with much lower read noise. With narrow band filters the read noise is no longer smaller than the noise from the sky background, and the setup is no longer competitive.

Any chance you guys have some forward knowledge of larger ultra-low-noise sensors coming out? Sony's newer ICX line are pretty nice, with very low dark current, and pretty low read noise (~5e-?). But the sensors are tiny. Really tiny, as in 1/3" or maybe 1/2", which is about half the size of a KAF-8300 and about 1/5th the size of a full-frame/KAF-11002 sized sensor. Would be really nice to know that Sony has some larger sensors based on their new low-noise technology coming out...

We are considering other projects to augment what we're doing now - particularly when the moon is up and our main science is on hold. We're also hoping to build a bigger array at some point in the future - with 50 lenses we'd effectively have a 400 mm f/0.4 lens, with a 1m aperture.

f/0.4 @ 1m...now that would really start to surpass, just in specs, some of the really large earth-based telescopes for sensitivity.

The gains from lens arrays are not so perfect; for example, the large binocular telescope uses 2 large telescopes and achieved only about a 40% gain.

That is because the gain is relative to the square root of the number of lenses used. With 2 telescopes, you gain SQRT(2) or ~1.4x...or ~40%. With 50 lenses, the gain would be ~7.1x, or 610%. Even at full retail cost, arraying 50 400mm f/2.8 L II lenses is significantly cheaper than building a gigantic telescope on top of a mountain. With over a seven fold gain in final SNR for the cost of around $500,000, I'd say that's a steal.