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Old 19-06-2015, 07:49 AM
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The 460mm f/0.89 Refractor

Powered by Canon "EF" series lenses and off-the-shelf components which some will no doubt recognize, this is an an interesting and novel approach to increasing aperture.

If you don't feel like watching the whole presentation, skip ahead to 32:00 for the eyecandy.

Dragonfly 460mm f/0.89 refracting telescope
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Last edited by Eden; 19-06-2015 at 07:57 AM. Reason: Added an image of the telescope for those who can't be bothered faffing around with YouTube.
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Old 19-06-2015, 08:29 AM
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In case anybody is wondering why they went with the Canon EF lens (it looks like either the 800mm or 1200mm EF) in the design of this instrument, the decision was based on the specific optical coatings which are used on these lenses.

The end result was a telescope with 10x less photon scatter than the Burrell-Schmidt telescope at Case Western University, making it ideal for capturing extended galactic regions with ultra-low surface brightness.
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Old 19-06-2015, 09:59 AM
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Interesting, yet funny at the same time.... right at the end I was impressed until I heard "Oh these lenses are amazing, some of them even have a strehl ratio of 0.8!" *facepalm*

Probably of no consequence to what they're using them for, but I'm sure there are far better dedicated astro optics out there at a similar cost? Are Canon coatings that superior to Takahashi, TEC, AP, etc? I guess speed is a big factor here too.
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Old 19-06-2015, 11:06 AM
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My understanding was that the L-series fluorite lenses are made by the same lens manufacturer as Takahashi's.

H
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Old 19-06-2015, 01:41 PM
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thanks Eden - that is a really useful video
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Old 19-06-2015, 02:04 PM
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TEC optimise their lenses at .99 strehl in green with a low of .69 in far blue. So TEC and I am sure AP rate their Strehl at different wavelengths.

The overall average is more like .9 or so. .8 strehl may be the overall average at various wavelengths of the visible spectrum.

Yes I heard Tak got their fluorite lenses from the same company as Canon. I am not sure if they still make fluorite lenses anymore?

Greg.
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Old 19-06-2015, 02:24 PM
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See the attached image -- the Gallium Arsenide coatings shown in the top-left are (according to the video) a very recent advancement in anti-reflective coating technology and are found on the Canon lens featured.

I'm interested to know whether Takahashi objectives have the same coatings, but looking at the structure of said coatings the benefits are fairly obvious.
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Old 19-06-2015, 09:11 PM
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A very interesting seminar. I have a question though - how is using 10 lens at once more advantageous than taking 10 subs with one lens and camera (other than increasing throughput by a factor of 10) ?
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Old 19-06-2015, 10:16 PM
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Quote:
Originally Posted by rmuhlack View Post
A very interesting seminar. I have a question though - how is using 10 lens at once more advantageous than taking 10 subs with one lens and camera (other than increasing throughput by a factor of 10) ?

It's the same result except there would be no other way to
capture 10 times the data in the same amount of time.
The noise would decrease by a factor of the square root of 10
or 3.2 times better.
This means he can go deeper & see low surface brightness.

I always liked the idea of having more than one identical telescope on the same mount.
e.g. Rolf Olsen has shown us all what you can do with long integration times.
He made the best hamburger in town - LOL

cheers
Allan
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Old 20-06-2015, 07:22 AM
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Thanks for posting this! I learned a lot by listening to the whole seminar.

Of course, I immediately compared my puny effort to theirs and was intrigued by the formulas for calculating the apeture and effective fl of multi telescope systems:

A = V n x size

fl = individual fl/ V n

n = number of lenses

V = square root (how do you get this sybol on an iPad?)

I'm wondering if anyone knows the formula for A (aperture) for combining lenses of different size (in my case 140 and 180 mm)?

I took the average of using 2 140 mm + 2 180 mm lenses and got 225mm. Not sure if that would be correct. Since both scopes are f7 I think my system is equivalent to f4.95.

Thanks,

Peter
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Old 20-06-2015, 07:50 AM
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that's kinda what I was referring to with my question.

Is one sub from the 10 lens Dragonfly system equivalent to (from those equations) one sub from a single 460mm f0.89 refractor ? If so, then if 10 subs from a single lens/camera combo is equivalent to one sub from the 10 lens dragonfly, is 10 subs from a single lens/camera combo equal to one sub from a single 460mm f0.89 system...?

If that is true, then doesn't this mean that the problem of systematics is really just about choosing the right type of optical design (refractor vs mirror design) and after that its simply about integration.
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Old 20-06-2015, 08:13 AM
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Quote:
Originally Posted by gregbradley View Post
TEC optimise their lenses at .99 strehl in green with a low of .69 in far blue. So TEC and I am sure AP rate their Strehl at different wavelengths.

The overall average is more like .9 or so. .8 strehl may be the overall average at various wavelengths of the visible spectrum.

Yes I heard Tak got their fluorite lenses from the same company as Canon. I am not sure if they still make fluorite lenses anymore?

Greg.
A few responses: yes, Tak still has fluorite in their range - the new FC-100's are fluorite, the FS-60 is fluorite, the FCT and FET "Buy a house instead" scopes are fluorite.

Canon-Optron make the lenses for Takahashi to order, as they did for Vixen (not sure if they still do - doubt it, as the majority of Vixen stuff now is Synta spec). As far as I am aware, ALL Takahashi optics are made in Japan without compromise.

I believe Tak still uses MgF2 AR coatings predominantly, though this may not be so.
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Old 20-06-2015, 10:43 AM
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Originally Posted by PRejto View Post
Thanks for posting this! I learned a lot by listening to the whole seminar.
Peter
You're welcome, Peter. I found it just as informative and had a feeling that some folks here on the forum would benefit from it too. I wouldn't be so hasty as to write your own efforts off as puny, since any effort made is worth much more than no effort at all.

I feel that the implications of a system like the Dragonfly are enormous and although the concept is nothing new by any means, such a design could be successfully implemented in the amateur field with considerable gains.

Although some might argue that the potential costs of such an endeavor would probably outweigh any benefits, some back-of-the-envelope numbers suggest that there is a sweet spot in the market which could allow for a fast and generously sized instrument to be constructed with reasonably inexpensive and readily available components. I think the primary limitation would be the end-users ability to configure and operate the instrument.
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Old 20-06-2015, 10:52 AM
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Originally Posted by rmuhlack View Post
A very interesting seminar. I have a question though - how is using 10 lens at once more advantageous than taking 10 subs with one lens and camera (other than increasing throughput by a factor of 10) ?
I reckon throughput is the primary motivator for wanting to build something like this. Obviously it depends on your preferred focal length, but for anything outside of long focal length imaging, the performance of such an instrument could easily eclipse the nearest single-OTA alternative (probably a Honders)...particularly appealing to those of us who don't own real estate on the top of a mountain in Chile and are counting clear nights on one hand ;-)
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Old 21-06-2015, 12:56 PM
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Another thing to consider is that 460mm is an 8 inch scope and there are plenty of 20 inch scope options but they all have secondary mirrors typically up to 50% obstruction. So this is 18 inches unobstructed.

Tony Hallas did some images with a specially made AP203 APO. He picked up lots of tiny galaxies and detail not normally seen. So there is a jump when its unobstructed.

Considering there are quite a few good scopes at inch aperture then of these tied together give 16 inches of aperture. It would be a lot of work processing the results and lots of cameras unless you had some optical device that could bring them all together at the back so one camera captured it all. The could be a 16 inch APO for $16K. Imagine 4 x FSQ106ED's linked together. But 4 cameras would really add to the cost.

Focusing would take a while.

Greg.
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Old 22-06-2015, 03:24 PM
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It just goes to show... sometimes it is actually possible to make ground breaking observations underpinned by inductive reasoning.

Frankly I am surprised that no one has pointed out the flaws in the argument.
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Old 22-06-2015, 03:30 PM
LewisM
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Imagine 4 x FSQ106ED's linked together.

Greg.
OllyPenrice in the UK uses 2 x FSQ-106N's in tandem. Double the exposures simultaneously.

One can dream. I mean, you could do RGB on one, and NB the other... make for fun processing
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Old 22-06-2015, 04:25 PM
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OllyPenrice in the UK uses 2 x FSQ-106N's in tandem. Double the exposures simultaneously.

One can dream. I mean, you could do RGB on one, and NB the other... make for fun processing
He probably spends most of his time trying to get them to align on the same target.

Greg.
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Old 22-06-2015, 04:49 PM
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meh.... consider it auto-dithering
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Old 23-06-2015, 04:06 PM
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btw) Thanks for posting the article Brett, it was on the whole very informative and interesting.

I believe the guy in the video could have saved a stack of cash if he had actually applied the scientific method to his assumptions with respect to optics as well.

fwiw) The paper he derives much of his narrative (on optics) can be found here:
http://arxiv.org/pdf/0909.3320.pdf
It is my understanding that he either misquotes or does not understand the conclusions of the people writing the paper.
He infers that the limitation of the (Kitt Peak) 24/36 Schmidt telescope is light scatter implicitly due to it's central obstruction and use of reflective surfaces. The paper actually makes no such assertions, and in fact does not support this conclusion, empirically. The limitations were found to be due to internal reflections off of the surfaces of the refractive elements, ie) filters, Schmidt plate and Dewar window. What is also not stated directly, but there if you read the fine print, is that the read noise level for single exposures is equivalent to 27.6 mag arcsec−2. So the instrumentation (electronic) noise is 10x what he is trying to measure. Interesting that later he derives an expression of the S/N value advantage of his Dragonfly lens system to be a factor of 10x better than the Kitt peak 24/36 Schmidt.....
See page 4 for notes on atmospheric internal scattering.
See page 12 for the statement relating to CCD read noise.
His other reference point by which he determines that 'these new cannon lenses are the best imaging instrument ever devised for faint, extended obects' is by way of comparison to a lens that has 17 elements (without Gallium Arsenide coatings).. does anyone else see a problem here?

Furthermore, if you forward the video to 1:03:45 you will hear him acknowledge that the lens system used in fact has more than one element that does not have the Gallium Arsenide coatings (plus the filters and Dewar window) Therefore, it follows that; it is possible to use an optical prescription that includes refractive elements without these coatings and reach the depth of exposure that this lens system achieves.
ie) He has just proven his base premise to be incorrect by definition.

So much for the scientific method....

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...

Seriously?

Last edited by clive milne; 23-06-2015 at 04:23 PM.
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