Ok, so I've had enough of my OHP with a 15" panel on it and its appetite for bulbs. I've decided to do a proper DIY and I need to start building my case. I've decided to use a 18" Beseler and 550/330 fresnels.

FocalCalc says my distance from the LCD to the centre of the triplet should be 515.5mm. And I'm assuming the 330 fresnel should be right against the back of the LCD and 330mm from the arc of the bulb? For proper focusing and lighting, what do I need to make adjustable and by how much?

Triplet obviously, but by how many mm in each direction?
Do I fix the LCD in place and make the front fresnel adjustable or the other way around?
Does the bulb or rear fresnel need to be adjustable?
The pre-condenser probably has to be adjustable as well?

Design:
Vertical design
FSM on the top OHP style
18" Beseler
550/330 fresnels
15" LCD - Upgrade to 15.4 WUXGA with 1:1 pixel mapping in the future
Some sort of pre-condenser
Reflector from OHP
S51 ballast and a Ushio S400DD bulb

Yes, typically, the rear fresnel should be placed at its focal length from the bulb. But when you add a precondenser, that distance changes and you will need to play with Dazzla's precondenser calculator to get an idea to what degree. Assuming you are going with split (to achieve some keystoning), you leave a ~20mm gap between it and the LCD. Then another 20-30mm for the front fresnel, which will need to be tilted double the angle that the mirror is tilted past 45 degrees (if you end up keystoning to lift the projection).



Most people at least make the lens adjustable, about 2 inches in each direction. Probably don't need quite that much if your measurements are dead on, and your throw will be fixed. Problem with your proposed design (mirror on top of lens) is that you will have a pretty narrow margin of movement away from the mirror before the projection gets too big for your typical OHP FSM, or too close for the Beseller not to obstruct (in fact, the larger diameter of the Beseller may already be too big for a OHP FSM. But perhaps you were planning to get a larger one?) Unless you plan to keep the mirror fixed with the lens, in which case you'll need to be changing the mirror and keystone angle every time you focus.

[EDIT: That last part is assuming a fixed screen. If you're just projecting on a wall and it doesn't really matter how high or low the projection falls, then raising or lowering a fixed lens/mirror, OHP-style, could work just fine. With OHPs, you can tilt the mirror to shift vertical positioning and not worry too much about the pincushion, barrel-rolling effect. With a projector, you need compensate for that with keystoning.]

All of which is to say that if you want to go with your proposed design, then adjusting the LCD for focus may be your best bet. Supraguy has actually suggested that is a better way to get the best balance of focus and brightness, but making an externally adjustable optics sled in a vertical design is definitely a challenge.

The more you make adjustable, the less likely you are to build yourself into a corner, particularly if you are planning for a complex build that involves a precondenser, a fold, and keystoning.

Hope that helps.

Wow, thanks for the lengthly reply. That more than covered my questions.

As for the FSM, I did some calculations and I agree an OHP FSM will be too small. Mine is 6"x3.5". The light cone exiting the Beseler is 2.3" wide but widens out too fast to fit onto a 6" mirror at 45 degrees. I may have to find something larger.

The vertical design I'm using is in a cabinet that is 48" tall. With 8' ceilings in my house, that allows for the maximum screen size without needing any keystone correction - thats why I chose to use a FSM in front of the lens.

Fixing the mirror to the lens is a neat idea - never thought of that.

One other question I never did find a solid answer to is what people are using to hold their optics and screen. I'm going to fix the Beseler to the rack and pinion setup from the OHP for that mount/adjustment. And I was planning a wood frame for the LCD and fresnels but am unsure how to secure them. I'm thinking that because it's a vertical design, I can just router a small groove in the top of the cutout and let gravity hold everything in place. My concern is that heat and lack of a solid support will cause the fresnels to warp.