Every astrophotography setup eventually runs into dew. Here in Temecula, dew is less of a problem than coastal California, but any session that runs past 1am in the winter has a real chance of fogging the front element — especially when you’re pointed at a low declination target and the scope is angled down toward the wetter air.
The commercial dew shields I looked at were either too short to be effective, made of the kind of foam that disintegrates after two seasons, or absurdly overpriced for what they are. A cylinder with some flocking on the inside is not a precision instrument.
So I printed one.
The Design
The constraints were straightforward: needs to fit the OTA snugly without clamping (I wanted friction fit, not hardware), needs to be long enough to actually block off-axis light and slow dew formation, and needs to be light enough not to throw off the balance.
I modeled it in Fusion 360. The body is a simple cylinder with a 2mm wall — thin enough to keep weight down, thick enough to print reliably without warping. There’s a slight inward taper on the objective end so it self-centers, and I added a small lip on the back end to give the dew heater strip something to sit against.
Total design time: about two hours including the first failed measurement. I measured the OTA twice, transposed two digits, and had to stop the print at layer 12 when I realized it wasn’t going to fit. Classic.
Print Settings (Bambu X1C)
- Material: Bambu matte black PETG
- Layer height: 0.2mm
- Infill: 15% gyroid — this part doesn’t need strength, it needs to not warp
- Walls: 3
- Supports: None — the geometry doesn’t need them
- Print time: 4h 22m per half (I split it into two halves that snap together)
PETG was the right call over PLA. Dew heaters get warm, and PLA creep under heat is real. I’ve had PLA parts deform on the scope before. PETG handles the temperature range without complaining.
Iterations
Rev 1 — Wrong diameter. See above.
Rev 2 — Correct diameter, but the friction fit was too tight. Getting it on and off was a two-hand job, which means doing it in the dark while wearing gloves is not happening. I added 0.4mm to the inner diameter and printed again.
Rev 3 — This one. Fits cleanly, comes off with one hand, light enough to keep the balance point forward of center. I lined the inside with black felt from the craft store and it’s genuinely dark inside — no stray light getting past it.
Result
Three sessions in, no dew issues. The heater strip sits in the groove and runs off a USB power bank I had lying around. Total cost in material was under $4.
The design files are saved on the NAS. I’ll post the Fusion 360 file if anyone wants it — leave a comment below.