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PostPosted: Tue Jul 09, 2019 4:23 pm 
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Joined: Fri Dec 03, 2004 2:41 pm
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Location: 55° 57'N: 03° 08'W
Details of present disposition of Hewitt Camera in Tamworth NSW can be found at


http://www.tamworthastronomy.com.au/aasec.html


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PostPosted: Sun Jul 14, 2019 6:45 am 
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Joined: Sun May 11, 2008 6:11 pm
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Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
David, what does the word 'bokeh' mean? Regards mike.


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PostPosted: Sun Jul 14, 2019 7:44 am 
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Joined: Fri Dec 10, 2004 10:34 pm
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Location: Surrey
Mike,

Wikepedia defines it as 'In photography, bokeh (/ˈboʊkeɪ/ BOH-kay; Japanese: [boke]) is the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens. Bokeh has been defined as "the way the lens renders out-of-focus points of light"'

I wonder if he meant Étendue which is related to how much light a lens gathers ? See https://en.wikipedia.org/wiki/Etendue for a (complicated) explanation. I have seen Étendue used in the sense that a particular telescope and camera system has a greater Étendue and thus is more sensitive to light though this does not seem to agree with the wikipedia page that talks about a single system rather than using it as a comparative measure.

There are some places on the web where people promote high Étendue telescopes. Since high Étendue means high loss you can achive a high Étendue telescope by painting the mirror black or just leaving the telescope cap on !

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PostPosted: Sun Jul 14, 2019 3:48 pm 
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Hi Mike,
Bokeh is a Japanese word as described by John.

I meant bokeh, not Etendue, which I am not familiar with.
I have occasionally left a lens cap on, but the photos aren't much good.
It doesn't happen much with a SLR.

Regards,
David


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PostPosted: Mon Jul 15, 2019 6:13 am 
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Joined: Sun May 11, 2008 6:11 pm
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Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
Thanks for that. Checked the Wikipedia site for the more complicated explanation of the word etendue...complicated indeed, far beyond my level of understanding! Regards maf


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PostPosted: Mon Jul 15, 2019 9:54 pm 
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Joined: Fri Dec 10, 2004 10:34 pm
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Location: Surrey
Mike,

As I understand it Entendu is just a restatement of the law of conservation of energy. Rougly it is Energy at teh focal plane / Energy at the input of the telescope thus seems to be a masure of losses in the telescope. From this it seems that Solar Telescopes are deliberatly designed with a high Entendu so most of the energy is lost and does not fry your eye or other detector.

However this seems to go against the websites that are promoting high Entendu scopes for deep sky work.

As a result I am confused. I will try to have a look at some of my other sources for a definition to clarify whai is really meant.

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