#21  
Old 23-06-2015, 04:18 PM
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Atmos (Colin)
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And.... I have to point out,
he waxes lyrical about a new class of low mass galaxy which implicitly is beyond the reach of all other instruments.... then uses images taken through a reflective telescope (Keck) for confirmation... in the form of a measure of its recessional velocity derived from its spectrum no less...
So his smaller cluster of telescopes can detect what a giant telescope cannot BUT can get a spectrum on... Which requires 10-1000x the photon count to do?

Sounds perfectly logical to me
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Old 28-06-2015, 09:47 AM
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Reminds me of the Physicist who needs some dry-cleaning done...& spots a sign in a local shop (oddly, run by a Mathematician). The sign reads
"Dry cleaning done here".

So he walks into the store and plonks some laundry on the counter, and asks
the shop-keep to clean it.

"No!, sorry, we don't do dry-cleaning!" says the shop-keep to the now puzzled Physicist.

"We only make signs!"

My point being, while the F-ratio is an interesting measure of a systems "sensitivity"... fact of the matter is: you still need flux. The ability to capture flux is by-and-large determined by aperture.

The dragonfly 'scope is described by adding extra lenses, as making the system "faster"... but in reality this is being done by increasing the effective aperture (rather than reducing the focal length).....hence I pondering whether this is simply a bit of mathematical sleight of hand.
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Old 28-06-2015, 11:10 AM
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Atmos (Colin)
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Yep, there is a reason why billion dollar telescopes are being developed around the 30-40 metre mark as opposed to a 10m F/0.2.

A faster telescope is all well and good BUT aperture is what captures photons.
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Old 29-06-2015, 08:25 AM
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So, just to be clear, am I to understand from recent comments that Dragonfly has not made any significant advance in imaging to a fainter magnitude than previously possible? If so, it would be more than interesting to have some comments here from the Drangonfly team. I wonder if they would respond if shown this thread?

Peter
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Old 29-06-2015, 01:18 PM
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Shiraz (Ray)
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Originally Posted by PRejto View Post
So, just to be clear, am I to understand from recent comments that Dragonfly has not made any significant advance in imaging to a fainter magnitude than previously possible? If so, it would be more than interesting to have some comments here from the Drangonfly team. I wonder if they would respond if shown this thread?

Peter
looks to me like a very useful tool for doing one specific job - finding dim extended objects - in which role it appears to do very well.

The compromises they made to get there are:
- resolution is nothing like as good as seeing will allow: but they don't care about stars, just the extended objects that lie far beyond.
- no capability for spectra (although bicolour possible): but that's what the huge telescopes are for.
- significant extra processing complexity needed due to multiple focal plane arrays: but that is manageable.

Agree that there seem to be some internal inconsistencies in the presentation, but the overall thrust that large scopes are not necessarily the best approach for really deep low-res imaging looks to be valid. The ability to get down well below F1.0 by adding extra "high end consumer quality" units is particularly appealing and makes this unique as far as I am aware.

Last edited by Shiraz; 29-06-2015 at 03:06 PM.
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Old 29-06-2015, 02:54 PM
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I took the liberty of writing directly to Roberto Abraham of the University of Toronto. He answered me immediately and coincidentally has just arrived in Melbourne for a few days of meetings! He indicated that he was a bit too busy to participate in this thread but he has given me permission to quote from our email exchanges. I hope you find it interesting. I really appreciate that he responded!

"Thanks for your email! I am pretty swamped at the moment so don't have time to participate on the Ice In Space thread, but I did read through the posts quickly and as far as I can tell the posters are making fair comments on the general pros and cons of the Dragonfly setup and I don't have a lot to add. Some of the points of discussion in the thread (focal ratio vs aperture etc) are gone over in our PASP paper describing Dragonfly. Here's a link:


http://adsabs.harvard.edu/abs/2014PASP..126...55A


Some of the issues being discussed in thread are gone over in the footnotes. When writing this we tried to make it accessible to a broad audience (including amateur astronomers) as we figured some of you folks might want to have a go at something similar. Hope we succeeded… if not we'll try to do better if we write a paper describing the upgraded telescope we're working on.

Canon's optical design for the lenses is proprietary but we're pretty sure only one element has the nanostructure coatings… they've clearly done a careful job of modelling ghosts though as remaining ghosts are at a really low level… we quantify it in the PASP paper. I think Canon's standard multi-coatings are really good too.

By the way, check out this nice paper that confirmed our identification of ultra-low surface brightness galaxies in Coma using the Subaru telescope:


http://adsabs.harvard.edu/abs/2015arXiv150601712K


What is really neat is that the authors of this paper (not) only confirmed the existence of the ultra-low surface brightness we detected, they found almost 10x more objects than we did! Subaru is an incredible telescope (the perfect combination of large aperture, amazing optics, wide-field capability and a great site) and its image is far superior to what we're able to do with Dragonfly in terms of resolution and depth on small scales. So why hadn't they identified ultra-diffuse galaxies before? Turns out the ultra-low surface brightness population had been missed in the past because this population was mistaken for compact galaxies (because the low-surface brightness envelopes were missed and they only found the cores) or by flat fielding errors, ghosts etc. Now that Dragonfly has shown that these ultra-diffuse things are real, the bigger telescopes will be able to optimize to detect them and clean up.

Anyway, when it comes to a few niche things like intracluster light and local galaxy envelopes (degree scale structures) I think Dragonfly will do better than Subaru, but in general for most things one wants to tackle in extragalactic astronomy (arcsecond to arcminute scale problems like the Coma objects) bigger telescopes on better sites are a bazillion times better than Dragonfly. Physics is physics! Of course a single night on a large telescope costs about $100K, which is basically what we spent on Dragonfly, so I really have been enjoying seeing how our work with these telephoto lenses has been inspiring this activity with Keck, Subaru, HST etc."

Posted with permission from Roberto Abraham

Peter
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Old 23-02-2016, 05:25 PM
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The Dragonfly Telephoto Array is in the news again, having since been upgraded to a 50 lens system.

Dragonfly telescope shines a light on dark matter

Subwavelength Structure Coating technology featured in the lens design is also explained, in addition to why this system is preferable to having one or more large mirrors.
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