Dwarflab Dwarf 3 Version 2 external filter holder and dew heater

This is the second and final version of the original filter holder. The changes were prompted by the realisation that the design risked raising the internal temperature of the telescope and the filter probably didn't need the 2.5° tilt previously introduced.

Going a step further than shown in the teardown, and removing the front plate with the wide and telephoto optical windows it became obvious that it doesn't just get warm but is designed to radiate internally generated heat. The base of the new design has less area in contact with the front plate, permitting better heat dissipation. Whether the difference is of practical significance hasn't been tested but on a matter of principle it seems the better way to go.

Surprisingly, to me at least, the external and internal faces of the plate aren't parallel but it's actually a shallow wedge with a slope of about 2°. Looking at the front of the plate with the telephoto window to the left, the bottom long edge is thicker than the top edge. Although I haven't managed to take the OTA to bits yet it seems on first inspection that the internal face of the plate most likely lies parallel to the optical axis. Perhaps the relative tilt of the external face is to reduce reflections. In any event I've assumed there's no need for the filter to include tilt as there could well be just about the right amount already.

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I also took a few temperature readings to get a feel for whether or not the heating element is actually necessary. In the end I don't know, but it certainly should help if the self generated heat of the device is insufficient. Four sets of readings were taken an hour apart with the Dwarf taking continous 60 second shots at gain 60, having started capturing images an hour before the first readings. The assumption being an hour should have been enough for temperatures to stabilise. Readings were taken with a laser temperature gun pointing at a 4mmx4mm piece of black insulation tape stuck on the surface being measured. Ambient temperature was about 23C.

The table below shows the results where Holder is whether it was mounted or not, Volts as applied to the heater element, Window the telephoto window temperature, Filter the filter temperature and App the temperature reported by the Dwarflab application. Temperatures are all relative to ambient.

 

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For anyone interested, STL files of the parts can be downloaded here Dwarf 3 Filter Holder V2 STL Files.zip. Included are short 11mm and long 38mm dew shields.

It's sized for a ∅46x2mm unmounted filter. The magnets are ∅6x3mm and heater strip 100x12mm.

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