Parametric LED Flatbox V3 for Astrophotography — 100–210 mm Telescope Tubes

rototype status — not yet physically tested

This V3 design has not yet been printed, assembled or validated on a telescope. The source geometry, mating interfaces, screw lengths, support-free print orientation, electrical calculations and optical model have been checked digitally. Real light uniformity, thermal behaviour, light sealing, fit and long-term reliability must still be confirmed by physical testing. Treat this release as an engineering prototype.

What this project is

This is a parametric LED flat-field panel for astrophotography. It is intended to provide an evenly illuminated surface for acquiring calibration flats while keeping the camera rotation, focus and optical train unchanged.

V3 uses a compact 225 × 225 mm square enclosure, a removable 60-LED plate, two mechanically retained opal-white acrylic diffusers and a replaceable telescope adapter. The adapter can be configured for telescope tubes from 100 to 210 mm outside diameter. The clear optical aperture is configurable from 100 to 200 mm.

Main design features

• 60 wide-angle 5 mm flat-top white LEDs arranged in four optical rings: 1 + 13 + 10 + 36 LEDs.

• Two 2 mm opal-white acrylic diffusers with 28 mm clear spacing.

• 19.99 mm from the LED lens plane to the lower diffuser.

• Matt-white internal optical surfaces to improve mixing.

• Separate, removable LED plate and electronics bottom.

• Mechanically retained diffusers: no glue and no felt.

• Parametric telescope adapter and matching dust cover.

• All printed parts designed to print without supports.

• Maximum printed footprint: 225 × 225 mm; a 235 × 235 mm or larger bed is recommended.

Included printable parts

• LED plate — holds all 60 LEDs, the fifteen 330 Ω resistors and the wire passages.

• Flatbox body — monolithic light-tight enclosure and lower diffuser supports.

• Electronics bottom — supports the controls, regulator, heatsink and wiring.

• Upper diffuser module — fixed spacer and support for the second diffuser.

• Telescope adapter — published for a 136 mm outside tube diameter and a 136 mm clear aperture.

• Dust cover — published to match the 136 mm adapter.

• Diffuser cutting jig — reusable template for shaping both acrylic sheets.

Recommended print settings

Material: PETG. Nozzle: 0.4 mm. Layer height: 0.30 mm. Infill pattern: Gyroid. Supports: disabled for every part. Brim: disabled. The supplied meshes are already placed in their intended print orientation, centred in X/Y and seated at Z=0; do not auto-orient them again.

Calibrate flow, XY dimensions and holes with a small test piece before committing to the 225 mm parts. Do not repair a fit problem by scaling only one member of the assembly.

Complete mechanical hardware

• 8 × M3 heat-set inserts, 4.0 mm outside diameter × 5.8 mm length.

• 4 × M4 heat-set inserts, 5.6 mm outside diameter × 8.55 mm length.

• 4 × M3×8 ISO 4762 / DIN 912 socket-head screws for the LED plate.

• 4 × M3×10 ISO 4762 / DIN 912 socket-head screws for the electronics bottom.

• 4 × M4×70 DIN 7991 / ISO 10642 countersunk screws for the optical stack.

• 2 × opal-white PMMA sheets, 218 × 218 × 2.0 mm, cut with the supplied jig.

• Matt-white paint for every internal optical surface.

The listed screw lengths are standard commercial sizes. Do not substitute the next longer size: the insert pockets are blind and an overlong screw may bottom out before clamping.

Complete electronics bill of materials

• 60 matching 5 mm flat-top white LEDs, 3.0–3.2 V, 20 mA, stated 160° viewing angle. A 100-piece pack from one production batch is recommended for matching and spares.

• 15 × 330 Ω, 1/4 W resistors — one per four-LED string.

• 1 × 8.2 Ω, 1/2 W resistor.

• 1 × 470 Ω linear panel potentiometer with an approximately 7 mm threaded bushing, wired as a rheostat.

• 1 × LM317T in TO-220 package.

• 1 × TO-220 heatsink rated at 20 K/W or better.

• 1 × GX16-2 panel socket and matching plug; the model uses a 16 mm panel opening.

• 1 × panel switch rated for at least 30 V DC and 1 A; the model uses a 6.5 mm panel opening.

• 1 × inline fuse holder and 0.5–1 A fuse rated for at least 32 V DC.

• 0.5 mm² / AWG20 stranded red and black wire.

• 3 mm heat-shrink tubing and two or three 2.5 mm cable ties.

• 24 V DC source. The design calculations cover a 21–29.2 V battery range; it is not a 12 V circuit.

Electrical arrangement

Battery positive passes through the fuse and switch to LM317 IN. The control path is OUT → 8.2 Ω → 470 Ω rheostat → ADJ / positive LED bus. Tie the potentiometer wiper to the used rheostat terminal so a lost wiper contact cannot force maximum current. Fifteen parallel branches leave the positive bus; each branch contains one 330 Ω resistor and four LEDs in series, then returns to battery negative.

Calculated limits are approximately 152.44 mA total maximum, 10.16 mA maximum per branch and LED, 2.61 mA theoretical total minimum, 0.191 W in the 8.2 Ω resistor, about 0.034 W in each 330 Ω resistor and about 1.92 W worst-case dissipation in the LM317. The calculated minimum useful supply is approximately 20.80 V.

For a TI LM317T viewed from the marked face with the leads pointing down, the TO-220 pinout is 1 = ADJ, 2 = OUT and tab, 3 = IN. Always verify the datasheet for the exact component you buy. Reference: Texas Instruments LM317 datasheet.

Safety: a 24 V LiFePO4 battery can deliver extremely high short-circuit current. Put the fuse in the positive lead close to the battery. Disconnect the battery while wiring, insulate every joint, and perform first power-up with a current-limited bench supply, the panel open and the potentiometer set to maximum resistance.

Changing telescope diameter and dust-cover size in Fusion

1. Measure the telescope tube outside diameter at its mouth in at least three directions and use the largest value.

2. Measure the actual unobstructed optical opening; do not rely only on the catalogue aperture.

3. Open Modify → Change Parameters in Fusion.

4. Set FB_D_TUBO between 100 and 210 mm to the measured outside tube diameter.

5. Set FB_D_OTTICA between 100 and 200 mm to the required clear opening. Keep FB_D_OTTICA ≤ FB_D_TUBO.

6. Start with FB_GIOCO_TUBO = 0.8 mm. If a dimensionally calibrated print is too tight, increase it in 0.2 mm increments.

7. Keep FB_SP_COLLARE = 4 mm, FB_H_COLLARE = 18 mm and FB_SP_ADATTATORE = 4 mm unless you are intentionally redesigning the structure.

8. Regenerate the model. The dust cover follows FB_D_TUBO automatically.

9. Check the adapter, cover and optical opening, save a new clean Fusion version, then export only the updated adapter and dust cover.

Do not uniformly scale the adapter STL. Scaling changes the four M4 screw spacing and breaks compatibility with the 225 mm enclosure. The published setup uses FB_D_TUBO = 136 mm and FB_D_OTTICA = 136 mm.

Assembly overview

1. Deburr and dry-fit all printed parts and both diffusers.

2. Mask all mating faces, LED holes, insert pockets and diffuser ledges, then paint every internal optical surface matt white.

3. Install four upper M3 inserts, four lower M3 inserts and four upper M4 inserts flush and square.

4. Install and test the 60 LEDs as fifteen strings of four. Secure every 330 Ω resistor in its printed cradle and insulate joints with heat-shrink.

5. Lower the LED plate into the body and secure it with four M3×8 screws.

6. Install the potentiometer, GX16-2, switch, LM317 with heatsink and 8.2 Ω resistor in the electronics bottom.

7. Wire and bench-test the circuit before closing the enclosure.

8. Cut two identical 218 × 218 × 2 mm diffusers with the jig.

9. Install the lower diffuser, upper diffuser module, second diffuser and telescope adapter. Cross-tighten the four M4×70 screws gradually.

10. Route wires without pinching them and close the bottom with four M3×10 screws.

Required validation before astronomy use

• Check for shorts and verify GX16 polarity with power disconnected.

• Measure minimum and maximum current using a current-limited supply.

• Verify that all 60 LEDs light evenly.

• Run at maximum brightness for at least 20 minutes while monitoring the regulator, heatsink, switch, wiring and PETG walls.

• Inspect every seam in a dark room for ambient-light leaks.

• Acquire test flats at several brightness settings and analyse radial gradients, bands and local spots across the whole sensor.

The optical Monte Carlo model, assuming matt-white internal walls, predicted radial deviations of 1.132% at 100 mm, 1.656% at 136 mm, 2.434% at 150 mm and 2.989% at 200 mm. These are simulation results, not measured performance. Physical flat-field data always takes precedence.

Use and maintenance

• Keep focus, camera rotation and the complete optical train unchanged between science frames and flats.

• Adjust brightness and exposure so the histogram is roughly one third to one half scale without clipping.

• Use the dust cover during storage; dust and fingerprints on the diffusers become part of the calibration image.

• Clean acrylic only with air and PMMA-safe products.

• Repeat electrical and uniformity validation after changing LEDs, paint, diffusers, wiring or adapter geometry.

• Periodically inspect screw tension, swelling around heat-set inserts and heat marks in the electronics compartment.