Lhoumeau Sky-System (Digital Planetarium with Fisheye)

For those who have already built a block with a 50mm condenser, and did not find suitable projector, here is a list of compatible Beamers:

some Compatible Projectors (at mars 2020 ) ,
ultra short zoom factor (no rate limitation )

Benq MH680
Benq W1070
Benq W1400
Benq W1500
Panasonic PT RZ475E / U ( Led )
Viewsonic PJD7820HD

However, only 2 were tested (February 2014)acer P1500 and benq W1070 . My preference is naturally acer, where the light leaks are quite low and consistent with the use of the planetarium (which is not always the case, remember Optoma HD10).

Because of the size of the rear lens of the objective, which is often too small. Only the bright optical lens allows the entire field to be projected.
with a F/D=1.8, the edge of the field is lost and you lose 10° on the horizon.

If you use a mount lenses (e.g. CANON or NIKON) beware of the distance between the Fisheye and the condenser. Depending on the case, it will imply to stick the optical fisheye into the diagonal a little bit more, contrary to what is presented here, with a larger space.

- Peleng 8mm fisheye
- 50 mm objective F/D1.4 with 42 screw mount (here Takumar SMC)
- A 2 inches diagonal (Kepler brand)
- 2 extension tubes 32 mm screw mount M42x1
- 2 adaptor rings: 2 inches outside (50.8 mm 1/24 inch) / 42 mm inside, 1 mm (metric)
- 1 ring inversion M49 / M42x1

Optical assembly

A good surprise!
Wanting to equip the planetarium Bressan with a 2nd projector , I wanted to test some new features. My choice went to the ACER P1500 , a compact Full HD at 3000 lumens !

But here was a big trouble : this projector is INCOMPATIBLE with based Heligon blocks! I did not pay attention to the angle of projection particularly wide ; it is a new generation of devices requiring a wider condensor for getting a good fit. So I went back with my old block 50mm based system.

And then, surprise : the projection angle is perfectly suited ! And the quality is awesome even at horizon. No need to put diaphragm or any added lens. We tested this block with lionez Ruiz under the dome of the planetarium Marseille ( 6m). Nothing to say, the 3000 lumens is perfectly suited for it.

No more need to run after those precious Heligon 75mm anymore ! Just choose the right projector for the right block.

Now you understand, hunting good old 50mm is open! (typically between 50 and 58mm). Especially we can find used and new ones ! For my tests , I used a 50 mm Takumar F/D1.4 (why so open, see page link), a diagonal 2 inches and a few rings .

The lens is held by a bracket that holds the wood bent pinch, easily achievable with few tools and a sense of very rough DIY reference (click the image). The rest is very classic , wedges to tilt the projector, etc. .

Where to find the right elements and the right connectors ?

For this block, we need :

-a condenser consisting of a 50 mm F/D1.4 ( lens size ) , nine Canon USM 50 mm f / d or 1.4 : Used Lens screw ( M42 ) for reasons of rings coupling

-a diagonal ( threaded 2 inch 2 sides , the case of Kepler and Skywatche "basic" , but not necessarily all other models)

-Peleng 8mm fisheye one with his mount M42 screw ( the Samyang is an aspheric and therefore less interesting because it deforms the projection on the horizon)

for adjustments, here are some details ; Warning they are experimental

for mounting a M42 camera lens

for mounting a "Canon" camera lens

Abbreviations used

    M : male F: female
    T2: 42mm diameter 0.75 mm
    M42 : 42mm diameter 1mm
    SCT: diameter 50.8 mm (2 inches) , not 1/24 "
    M48 : 48mm diameter ( internal thread of 2pouces ) not 1/24 "

and the icing on the cake : the list of supplies (also to validate).
I do not have shares in TS, but they have everything you need (fast and cheap) :

Yves LHOUMEAU - mars 2020