Purple spot on Field Lens

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MikeBradley
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Purple spot on Field Lens

#1 Post by MikeBradley » Mon May 06, 2019 6:28 am

The field lens on my Olympus BH2 has a bright perfectly circular hot-spot visible at the centre of the field lens. When the field diaphragm is closed to a minimum the hot-spot is only a pinpoint, when fully open it is about 3mm. I have no trouble achieving Kohler illumination over 10/0.30 - 40/0.95. What does it indicate, what would be causing this and how could it be eliminated?
Thanks
Michael
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Field iris open
Field iris open
IMG_6520.jpg (70.91 KiB) Viewed 2965 times
Olympus BH2,
AO110
Carl Zeiss Standard WL
Canon 90D

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75RR
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Re: Purple spot on Field Lens

#2 Post by 75RR » Mon May 06, 2019 6:42 am

Would not have expected to see a hot-spot on the Field Diaphragm dust glass. They are more likely to appear on the camera sensor due to the added camera lens in afocal setups.
It might help to identify the source if you post images of your illumination system
Last edited by 75RR on Mon May 06, 2019 3:13 pm, edited 2 times in total.
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PrecisionInstruments
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Re: Purple spot on Field Lens

#3 Post by PrecisionInstruments » Mon May 06, 2019 12:07 pm

Have you placed a business card directly under the condenser to see if it disappears? It may well be a reflection.
I have a headache, For which the future’s made

MikeBradley
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Re: Purple spot on Field Lens

#4 Post by MikeBradley » Mon May 06, 2019 4:43 pm

I had tried the business card idea, it isn't a reflection from the condenser.
Michael
Olympus BH2,
AO110
Carl Zeiss Standard WL
Canon 90D

MikeBradley
Posts: 94
Joined: Sat Sep 22, 2018 4:29 pm
Location: Sunshine Coast, BC, Canada

Re: Purple spot on Field Lens

#5 Post by MikeBradley » Mon May 06, 2019 5:26 pm

75RR, As far a I know the illumination is standard BH2 (image attached), I've never tried to open it to inspect it because the hotspot doesn't appear to be interfering with the visual observing or DSLR imaging. I have occasionally seen a faint blue area close to the centre of an image when the afocal set up is misaligned but I have fixed that!

I've attached the image from the BH2 manual but I can't understand what it shows about the light path. Other Olympus documents show item A as a simple stop.

Michael
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BH2_lightpathedit.png
BH2_lightpathedit.png (346.23 KiB) Viewed 2901 times
Olympus BH2,
AO110
Carl Zeiss Standard WL
Canon 90D

apochronaut
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Re: Purple spot on Field Lens

#6 Post by apochronaut » Mon May 06, 2019 10:30 pm

The fact that the spot diminishes and increases as you close and open the field diaphragm, tells you where the problem lies. You are seeing a reflection of the field window on the field window.
The light is likely reflecting back down the line and then re-reflecting back to the window. If there is any surface film or dust film( a good possibility in an old scope) on any surfaces in the illumination path, that will reduce transmission and increase the possibility of that film/dust acting as a kind of screen, showing a ghost image of certain components or physical structures, depending on the design of the system.

MikeBradley
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Re: Purple spot on Field Lens

#7 Post by MikeBradley » Tue May 07, 2019 1:21 am

Thanks Apochronaut, I'll start by opening the field lens unit.
Michael
Olympus BH2,
AO110
Carl Zeiss Standard WL
Canon 90D

PrecisionInstruments
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Joined: Mon May 06, 2019 11:44 am
Location: Perth, Western Australia

Re: Purple spot on Field Lens

#8 Post by PrecisionInstruments » Tue May 07, 2019 1:04 pm

I agree with Apochronaut. If you have removed the possibility of reflection from above, then it may well be a hazed screen on the under side of the field lens presenting an image of the field diaphragm. This optic does get dirty on the underside, though this optic assembly can also eventually show lens separation when the optic bonding glue ages, which may also create a similar effect.
The orientation of field optic in the BHS is easily seen as the two optics bonded together are of a different thickness, thinner optic on top from memory. BHT is a much more difficult story if you were to accidental forget orientation.
Rubber suction cup will be your best friend here!!
I have a headache, For which the future’s made

MikeBradley
Posts: 94
Joined: Sat Sep 22, 2018 4:29 pm
Location: Sunshine Coast, BC, Canada

Re: Purple spot on Field Lens

#9 Post by MikeBradley » Thu May 09, 2019 1:55 am

Thanks for the suggestions everyone. I'm off for a months vacation now so I'll start dismantling and cleaning when I return!
Michael
Olympus BH2,
AO110
Carl Zeiss Standard WL
Canon 90D

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