WHY no side illumination in darkfield? (noob)

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Simon Quinlank
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WHY no side illumination in darkfield? (noob)

#1 Post by Simon Quinlank » Tue Sep 08, 2020 10:14 pm

ok, i recently purchased a second-hand Brunel SP200, its a decent scope that came with a set of plan and a set of phase objectives, the light field stuff is fine but the dark field is not. I'm not getting any side illumination. i have dozens of darkfield stops / masks and regardless of which size i put in the filter tray and the height of the condenser, i just can't get any side illumination, I can get a small sharp circle dead centre on my sample and can see a circle of light around the edges with my 10x...but the sample material is not illuminated against the dark circle.
ok, i'm using the standard condenser lens...the scope also came with a what i believe is a darkfield condenser lens (it has a black circle in the middle and the lens is concave) but i can't attach it to the rest of the condenser assembly, i can install it using the ring that holds the main condenser but, obviously i can't use any filters or stops, in any case everything is still very dark regardless of the height...also i have seen plenty of youtube vids that refer to the stops and some about the darkfield condensers but not both
Any suggestions / pointers would be appreciated, thank you

BramHuntingNematodes
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Re: WHY no side illumination in darkfield? (noob)

#2 Post by BramHuntingNematodes » Tue Sep 08, 2020 11:30 pm

Oh how ho very interesting let's see a picture or two.

A dark field condenser will not need stops but may well need a drop of oil on top and to be touching the bottom of the slide
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

apochronaut
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Re: WHY no side illumination in darkfield? (noob)

#3 Post by apochronaut » Wed Sep 09, 2020 1:11 am

With the standard 1.25 N.A. condenser, you may need to oil the condenser to the slide in order to get the stops to work.

A DF condenser with a concave upper surface is a dry DF condenser . Do not oil.

Nevertheless, it sounds like your problem is in getting the correct working distance established with either condenser. The condenser needs to focus the beam, just so. Also, phase objectives have a built in annular phase plate which can interfere with DF. While low power phase objectives can establish DF with the larger phase annular diaphragms ; in other words they function as DF stops, with proper DF stops their ability to function with phase objectives would be guesswork and very hit and miss. You need to use BF objectives.

MichaelG.
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Re: WHY no side illumination in darkfield? (noob)

#4 Post by MichaelG. » Wed Sep 09, 2020 5:46 am

BramHuntingNematodes wrote:
Tue Sep 08, 2020 11:30 pm
... let's see a picture or two.
Pictures of the actual instrument would be useful ... but here is Brunel’s SP200 page for starters:
http://www.brunelmicroscopes.co.uk/rese ... scope.html
.
and the SP200 accessories:
http://www.brunelmicroscopessecure.co.u ... ories.html

MichaelG.
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Roldorf
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Re: WHY no side illumination in darkfield? (noob)

#5 Post by Roldorf » Wed Sep 09, 2020 8:29 pm

Just a thought, have you fully opened the condenser diaphragm?
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Simon Quinlank
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Re: WHY no side illumination in darkfield? (noob)

#6 Post by Simon Quinlank » Wed Sep 09, 2020 9:33 pm

BramHuntingNematodes wrote:
Tue Sep 08, 2020 11:30 pm
Oh how ho very interesting let's see a picture or two.

A dark field condenser will not need stops but may well need a drop of oil on top and to be touching the bottom of the slide
mine is a dry / concave condenser

Simon Quinlank
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Re: WHY no side illumination in darkfield? (noob)

#7 Post by Simon Quinlank » Wed Sep 09, 2020 9:35 pm

apochronaut wrote:
Wed Sep 09, 2020 1:11 am
With the standard 1.25 N.A. condenser, you may need to oil the condenser to the slide in order to get the stops to work.

A DF condenser with a concave upper surface is a dry DF condenser . Do not oil.

Nevertheless, it sounds like your problem is in getting the correct working distance established with either condenser. The condenser needs to focus the beam, just so. Also, phase objectives have a built in annular phase plate which can interfere with DF. While low power phase objectives can establish DF with the larger phase annular diaphragms ; in other words they function as DF stops, with proper DF stops their ability to function with phase objectives would be guesswork and very hit and miss. You need to use BF objectives.
mine is a concave / dry condenser and im not using my phase objectives, just my LF / plan

Simon Quinlank
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Re: WHY no side illumination in darkfield? (noob)

#8 Post by Simon Quinlank » Wed Sep 09, 2020 9:36 pm

Roldorf wrote:
Wed Sep 09, 2020 8:29 pm
Just a thought, have you fully opened the condenser diaphragm?
yes, it's fully open

apochronaut
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Re: WHY no side illumination in darkfield? (noob)

#9 Post by apochronaut » Wed Sep 09, 2020 9:47 pm

Your field diaphragm is open all the way? Illuminator intensity all the way up. DF starves the field of light, so even at low magnigications the intensity needs to be quite high.

Greg Howald
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Re: WHY no side illumination in darkfield? (noob)

#10 Post by Greg Howald » Mon Oct 26, 2020 4:44 am

Position of condenser is crucial. With my scope if the condenser is lowered by more than one mm everything goes black. I'm usually confused about this for a moment or two before remembering to raise the condenser as far as it will go. This happens because in bf I have to lower the condenser slightly for good focus.
Then I often forget to raise it when I put in the df stop.
Good luck. Greg

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