Manual Dowser for Projector

EWCguy

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Nov 17, 2009
Location
Wyoming
I'm working in a high school that has a ceiling-mounted projector in the tech booth that hits a screen that comes down in front of the stage. I'd like to use that for supertitles in Thoroughly Modern Millie, but just access the top-most portion of the screen - and block out all the rest of the projected "black" that will bleed onto the stage below.

So, I'm planning a manual dowser that essentially works like an ellipsoidal shutter to block the lower portion of the projection. Since we won't need all of the projector image in the show, the manual dowser can be set and left throughout the show.

My question is: What is an appropriate distance from the projector lens for the dowser to be in play?

I've never used the DMX dowsers (ie. http://www.citytheatrical.com/products/-projector-dowser), so I have no reference. The manual for the CT instrument just says "as close to the projector lens as required." Any guidance?
 

Eric W.

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Oct 16, 2018
Location
Southern California
Maybe someone with more experience than me will have a more mathematical idea. But, I made a manual dowser before out of black foamcore and I just tested the distance until it looked good. You also may need to sort of box it in as no matter how black you make your dowser, there will be reflection which could flood your booth. I don't know your booth situation, but it would light up our booth like crazy. Just make sure to have plenty of ventilation (for instance, on top,) this will likely create a fair amount of heat.
 

EWCguy

Active Member
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Nov 17, 2009
Location
Wyoming
I made a manual dowser before out of black foamcore and I just tested the distance until it looked good. ...this will likely create a fair amount of heat.
I was going for something aluminum or light steel. Were you concerned about setting fire to the foamcore?
 

dmx

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Apr 10, 2010
Location
Virginia
One point to consider here is the focal point of the lens of the projector. If you place the blocking mask too close to the projector, you will find that the entire image bleeds through and it will just make everything blurry. The traditional dowsers that you linked to, like the one from City Theatrical are designed to dowse the entire image.

The last time I had to do this, our mask was placed ~10ft from the projector.
 

ruinexplorer

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Agreed. The reason that various distances are required is because of varying focal lengths of the projector lens. With a lighting instrument, you have the framing shutters before the final lens assembly. You can't do that with projection optics (though the big projectors do put their internal douser there. I haven't tried (because I don't want to risk scratching the lens), but I have heard that some people use black wrap at about the distance that the lens cap would be from the lens.
 

macsound

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Jun 15, 2018
Location
San Francisco, CA
It's pretty normal to mask a projector, in fact, even movie theatres do it.
I'd reccomend the mask itself starting about the size of a Letter sheet of paper, but stronger, and then see how far away it has to be so you can usefully cut a hole in the paper.
Like said above, focal lengths are different for different projectors and lenses, so that's why I use the piece of paper as a size reference.
 

Ancient Engineer

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Sep 21, 2017
Location
Sandusky, Ohio
Yes, yes, yes... I've used blackwrap cut with a razor and steel straightedge.

Then fudging with the distance till it was good.

THEN a friend told me to use two pieces spaced about 1" apart.

Futz with the first piece till you like it, then add the second one and play with it and you'll have a razor sharp cutoff.

I used metal coathangers and blacktak to attach the blackwrap to clamps...
 

EWCguy

Active Member
Joined
Nov 17, 2009
Location
Wyoming
...THEN a friend told me to use two pieces spaced about 1" apart.

Futz with the first piece till you like it, then add the second one and play with it and you'll have a razor sharp cutoff.
Do you mean to say the second piece was placed closer or further away than the first piece, so the two pieces were in line with the beam?
 

RonHebbard

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Jun 12, 2004
Location
Waterdown, ON, CA
Further away, in line. like: | |


My ASCII graphics not withstanding... I think you get it. The second one kills the diffraction spill from the first edge. (which is part of what makes the edge look soft...)
@EWCguy and @Ancient Engineer I'm making an assumption here but I suspect this is very similar to putting a donut in an ellipoidal's gel holder to sharpen shutter cuts and / or gobos. @Ancient Engineer Is this a valid assumption?
Toodleoo!
Ron Hebbard