another quick question for SHG users

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highfnum
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another quick question for SHG users

Post by highfnum » Mon Sep 21, 2015 11:20 pm

what does increasing FL of collminator lens do?
make slit image look larger or smaller?
my guess larger

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Merlin66
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Re: another quick question for SHG users

Post by Merlin66 » Tue Sep 22, 2015 1:15 am

For a fixed telescope say 1000mm fl f10, then collimator would be matched (=f10) to give the best fit to the grating size. For a 30mm grating, then a 300mm collimator would be OK , for a 50mm grating you could go up to 500mm fl.
If you leave the imaging lens unchanged then you have a reduction magnification factor (fl imaging lens/ fl collimating lens) this in turn would reduce the effective slit gap as seen by the camera.(which should match the Nyquist/ pixel size = 2-3 pixel width.
Example:
Collimator - 500mm
Imaging lens - 500mm
Entrance slit gap - 20micron
Effective slit gap - 20 micron

Collimator - 500mm
Imaging lens - 300mm
Entrance slit gap - 33micron
Effective slit gap - 20 micron

The collimator fills the grating to give maximum resolution (based on l/mm) and the imaging lens gives the linear dispersion (A/pixel) to allow you to record the best resolution.The final dispersion/ resolution is dependent on the collimator fl and imaging lens fl - longer imaging focal lengths give higher linear dispersion.

The imaging lens fl is always the same as the collimator fl (Littrow) or less.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
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marktownley
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Re: another quick question for SHG users

Post by marktownley » Tue Sep 22, 2015 5:36 am

Excellent answer Ken, thanks :)
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http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!

highfnum
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Re: another quick question for SHG users

Post by highfnum » Tue Sep 22, 2015 9:58 pm

ok
so I got 50X300 objective
50X190 Collimator

I want to switch to 50 X 300 Collimator
that should be OK according to rule above

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Re: another quick question for SHG users

Post by Merlin66 » Tue Sep 22, 2015 11:03 pm

Terminology....
I assume the 50/300 "objective" is the telescope size (?)
The "collimator" is the first lens in the spectrograph, between the slit and the grating.
and the "Imaging lens" is between the grating and the CCD chip.
It's the Imaging lens which affects the linear dispersion (and resolution) in the SHG.
In your post - you want to change the collimator from 50 x 190 to a 50 x 300 (?) this will give a much longer instrument and really would need 50mm gratings (for maximum performance)
The telescope will produce a solar image at the entrance slit of 3mm which should be OK for a small webcam type camera - no reduction magnification required.
This then infers that the imaging lens should be upgraded to match the "new" collimator fl = 300mm. A longer focal length imaging lens will give an improved resolution.
The question is - what are you trying to achieve?? More resolution?
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: another quick question for SHG users

Post by highfnum » Wed Sep 23, 2015 10:19 pm

better(easier) focus on slit
longer FL should be a bit easier to focus no?

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Re: another quick question for SHG users

Post by Merlin66 » Wed Sep 23, 2015 10:58 pm

Hmmm
The critical focus zone (CFZ) - the depth of focus/ focus tolerance to maintain optical accuracy is dependent only on the f ratio of the lens - not the focal length.
Delta = 4* f ratio^2 * wavelength
For green light this gives:
f10 =220micron
f5 = 55micron
f4 = 35micron
Pretty tight requirements with fast systems!
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.yahoo.com/neo/groups/ast ... scopy/info
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer

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Re: another quick question for SHG users

Post by highfnum » Fri Sep 25, 2015 10:25 am

well I tried 50X300 lens for collimator (f/6)
results were not what I expected
slit "seems" smaller and further away
image of sun got smaller
not worth any improvement in focusing which is about same effort anyway
the pain now is I got to re-focus my original 200 lens

Merlin66 now I understand what you said about using longer FL on camera side

here is test shot - no attempt to make nice
if you compare to other shots i took sun is smaller
testshot.PNG
testshot.PNG (129.52 KiB) Viewed 1251 times

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