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Diffraction limited astro photography by single optical surface

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18 replies to this topic

#1 Jan_Fremerey

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Posted 19 April 2019 - 07:41 AM

The following close-up view has been extracted from a 4Mpx video taken on April 11, 2019, by a ZWO ASI178MM with the camera chip directly placed at the prime focus of my 10“ f/5 mirror:

 

Moon_211505_lapl4_ap1054_FW-lumi_dSinc15

 

CS, Jan


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#2 Astrojensen

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Posted 19 April 2019 - 09:09 AM

Goodness me, those pixels must be small!

 

 

Clear skies!
Thomas, Denmark



#3 Jan_Fremerey

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Posted 19 April 2019 - 10:49 AM

those pixels must be small!
It's 2.4 µm which in fact allows diffraction limited imaging at f/5. Up to recently I worked with 3,75 µm and 2.2x Barlow magnification at f/11 which only allowed much smaller field of view. Here is the total field of view covered by the single video where the above close-up view was taken from:
 
Moon_211505__g180_linear025.jpg
 
CS, Jan

Edited by Jan_Fremerey, 19 April 2019 - 10:52 AM.

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#4 OldManSky

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Posted 19 April 2019 - 12:09 PM

Outstanding detail!

So...did you rig up a spider that holds the camera, or...?



#5 Jan_Fremerey

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Posted 19 April 2019 - 05:52 PM

 

...did you rig up a spider that holds the camera, or...?

It's just a single arm support, see here: http://www.astro-vr.de/100901_3694.jpg

 


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#6 OldManSky

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Posted 19 April 2019 - 07:26 PM

That is so cool.  Love it :)



#7 Jan_Fremerey

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Posted 12 May 2019 - 11:45 AM

Some 30 min after the above one I took a series of 6 videos, 10 s at 36 fps each, with the same setup in order to prepare a lunar mosaic, see preview below, and original size at 0.39"/px via: http://astro-vr.de/M...1_Mosaik_v2.htm. While the detail resolution is less than before, maybe due to loss of seeing and/or focus, the mosaic anyway demonstrates the extent of wide field imaging with the camera chip placed right at the primary mirror f/5 focus. So it was possible to cover the whole illuminated moon surface by only 6 partial images.

 

Moon_190411_Mosaik_Vorschau.jpg

 

CS, Jan


Edited by Jan_Fremerey, 13 May 2019 - 02:40 AM.

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#8 dustyc

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Posted 12 May 2019 - 09:02 PM

It's just a single arm support, see here: http://www.astro-vr.de/100901_3694.jpg

I thought that scope looked familiar, it's pictured on the Royce mirror site. Nice and simple design



#9 luxo II

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Posted 13 May 2019 - 08:03 AM

Love it - the ultimate in minimalism !

#10 Jan_Fremerey

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Posted 14 May 2019 - 03:21 AM

Love it - the ultimate in minimalism !

It's not just for minimalism. Due to the absence of tubus and/or truss and mirror cell, the system doesn't require readjustment after scope repositioning and even transport, and it also doesn't suffer from tubus seeing and dew.

 

CS, Jan


Edited by Jan_Fremerey, 14 May 2019 - 03:27 AM.


#11 fgraus

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Posted 15 May 2019 - 12:35 PM

I would have thought dew would be more of a problem because the mirror is exposed. Is there a heater on the mirror?



#12 Jan_Fremerey

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Posted 15 May 2019 - 04:05 PM

I would have thought dew would be more of a problem because the mirror is exposed. Is there a heater on the mirror?

The mirror is fully exposed to ambient air, and is shielded against thermal radiation into night sky by its shiny optical surface. So unlike common telescope tubes and front lenses it doesn't cool down below dew point of ambient air. There is no need at all for active heating.



#13 starman876

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Posted 18 May 2019 - 08:27 AM

It's just a single arm support, see here: http://www.astro-vr.de/100901_3694.jpg

I want onebow.gif



#14 Jan_Fremerey

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Posted 19 May 2019 - 04:00 AM

I want onebow.gif

You may have the drawings and do it yourself. - CS, Jan



#15 Jan_Fremerey

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Posted 20 May 2019 - 05:16 AM

On multiple request, here are the drawings: http://www.astro-vr....s_190520_FW.jpg - CS, Jan



#16 Spoonsize

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Posted 20 May 2019 - 07:01 AM

“The mirror is fully exposed to ambient air, and is shielded against thermal radiation into night sky by its shiny optical surface. So unlike common telescope tubes and front lenses it doesn't cool down below dew point of ambient air. There is no need at all for active heating.”

I do not understand. Around here when the ambient air temperature reaches the dew point the dew “falls” out of the air and lands on everything, moisture on everything not heated unless of course it is inside a domed observatory and even then it is only a matter of time.

Edited by Spoonsize, 20 May 2019 - 07:01 AM.


#17 Jan_Fremerey

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Posted 20 May 2019 - 09:39 AM

when the ambient air temperature reaches the dew point the dew “falls” out of the air and lands on everything

When temperature of ambient air falls below its dew point fog will develop right in the air. Otherwise fog, i.e. water droplets (dew) or frost, will only occur on surfaces staying at lower than dew point temperature of ambient air. This in particular occurs due to heat radiation into the night sky which acts as a heat sink at temperatures down to beow minus 50 degrees C. The shiny front surface of a telescope mirror is an effective shield against thermal radiation into sky, so the mirror block, if not hidden inside a tubus-and-mirror-cell assembly can easily accommodate to ambient air temperature by free convection, hence does not fall below dew point temperature of ambient air. On several nights during winter time I enjoyed undisturbed observation with no precipitation on the mirror face while other components of my telescope were fully covered by frost, see here:

 

110130_4075.jpg

 

If you like to discover the effect of radiation shielding by a shiny surface on your own, you may put down two cookie tin lids under clear night sky and humid air conditions on a thermally insulating support, one lid with its shiny, as shown below, and the other one with its painted side up, and see what happens.

 

110130_4080.jpg

 

The lid with its shiny side up will stay shiny, while the one with the painted side up will accumulate frost or dew on both sides.

 

Telescope mirrors mounted inside a tube-an-mirror-cell assembly will usually follow the tubus temperature which in cold and humide nights may easily fall below the dew point of ambient air for the reasons outlined above.

 

CS, Jan



#18 starman876

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Posted 20 May 2019 - 08:47 PM

The things you learn.  Does this also have something to do with the emissivity of the surface?



#19 Jan_Fremerey

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Posted 21 May 2019 - 02:42 AM

Does this also have something to do with the emissivity of the surface?

Exactly. High reflectivity corresponds to low emissivity. You may directly feel thermal reflectivity by holding a cookie box lid with its shiny side close to your cheek.


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