Lightbox, industrial sized

[Horwig]

Those of you with a long memory might recall this: 

My thread on building a large Newtonian Astrograph, well it's still ongoing, the good news is that finally, after a lot of stops and starts, things are getting together, the dome finally works, and the software all sings, one of the last remaining jobs is a lightbox for flats, but like everything else with this scope it's going to be big.

I built a lightbox for my refractor last year, and results on the whole have been superb:

So now to use what I learnt from the small box, and scale it up.

That's a 600mm square board, with 18 segments of LED strip, same as the ones used in the other lightbox.

Then built into a box about 170mm deep, the foil covered ring is 100mm tall, and holds the first opal diffuser

The octagonal frame directs light from the edges of the 600mm square diffuser to the front diffuser, 500mm square

Then a front cover to hold everything together, and here's the output

The Correx sheets are glued together with hot glue, works well. At the moment the box is closed with masking tape so that I can see how much attenuation I need to give a reasonable illumination for flats. With the other box my dimmer would not go low enough, and I resorted to sheets of paper over the first diffuser.

That will be for another day, and also working out a rig to hold the box in front of the scope.

 

Huw

[Horwig]

Question, should I hang this on the front of the telescope, or on the dome, and point the scope at it?

[Horwig]

I've measured the clearance between the scope and dome, the idea was sound, there's sufficient clearance between the end of the OTA, but the guide camera on the OAG sticks out some distance past the end, so it's not going to work. Back to mounting the box on the end of the OTA

Huw

[Horwig]

I've been playing with my dew heater controller to work with a lighbox. My controller, which I built many years ago, is based on this one:

When I originally built it, I'd changed the pots for 220k ones and the 22k resistors for 10k ones. The 3.3uF capacitors became 0.1 uF. The resulting frequency was 50Hz, which might be fine for dew heaters, but might be on the low side for banding from the lightbox on short exposures, and the lowest setting was still way too bright.

So more changes to the circuit, the 10k resistors were changed to 1k, and the 0.1 uF changed to 0.01, so now it gave 500Hz, much safer.

The resistors should have given a mark space ratio of about two hundred to one, but it was closer to 30 to one when measured with a scope, the culprit was the 4.7K  source to gate resistor, it was not forcing the FET to turn off quickly enough, so that was changed for 1k, now everything is as expected.

One more step done.

Huw

 

[Horwig]

And here it is in situ, it's a tight fit between the OTA and dome, but it all works. This is at 100% brightness, I'll be using it around 0.5% brightness for flats, theory is that should be about 5 seconds fro un-binned Luma.

Huw