Light box for flats...

[fwm891]

I've just started building a light box to fit over my 8-inch reflector.

Found these on Amazon when I was looking for something else!

1 of 12v LED Flexible WHITE SMD Strip Light 1 metre / 60 LED's ** IDEAL FOR GARDENS, HOMES, AQUARIUMS, CARS, ETC **

This is just over one wrap around the tube diameter so I decided to bring the remaining tail outside to act as an indicator to confirm I have lights

I made a simple plywood panel with a central hole to go over the tube, a short thin ply sleeve to fit inside the hole. The thin ply sleeve extends about 30mm inside the light box and has the LED's fixed to it.

This then has a reflective box fitted with white interior walls - made from foam board with white card either side of a 4mm foam core giving about 5mm total thickness - very very lightweight.

Photos and some test frames to follow once the glue has dried

[pritc]

Looking forward to the Photo's

I still need to make one for my 200P

Wayne

[yesyes]

I too am looking forward to seeing photos of it... ;-)

[fwm891]

Schematic with dims - photos to follow. Its in use at the mo gathering flats.

Note finding the brightness a bit high for luminosity flats with my f4 system so I'm going to add an extra diffuser just above the LED's to reduce the intensity.

Or use a pale grey card on the end wall...

Also need to add 3 or 4 stops on the end of the plywood collar so it rests on the end of the tube

[yesyes]

Interesting approach with the indirect lighting....

[Merlin]

Excuse my ignorance, Folks, but what exactly does this kind of light box do?

The only "light box" for flats that I know about is of the recess kind, directly behind the flat in the wall of the main tube to reduce around-the-flat glare.

[fwm891]

Hi Merlin,

Used to provide the light source for 'flat-frames' by illuminating a reflective panel as evenly as possible - indirect illumination being best for this.

The unit slips over the front of the telescope and the telescope 'sees' an evenly lit field of view.

[fwm891]

OK - image of the lightbox on the scope. Ignore the yellow colour as that is simply the light going through the foam-board. If you look at the background you will see a more natural colour from the extra externally exposed LED's

[Merlin]

I still don't get it. It looks like self-inflicted light pollution to me.

[yesyes]

that's exactly what it is... ;-)

The light box is used to take flat frames. The box is an evenly illuminated light source. Then you take pictures of that even light source through the telescope. The resulting image will NOT be evenly illuminated but will contain information about imperfections in the light path of your telescope and camera. For example vignetting, reflections of stray light, dust on the mirrors/lenses and more....

These images are then used in the image stacking process as calibration frames in order to remove these imperfections from the final image.

[Gina]

Nice clear and concise explanation Chris

[Merlin]

Clear it is Gina. Thanks Chris.

[fwm891]

Update folks - As it stood it was way too bright and exposures for the 'L' filter were so short the shutter hadn't cleared the chip completely. So I've put a mid grey card as a screen. Used grey to keep things neutral.

May end up using two different 'greys', one for the Ha, OII and SII NB filters as their flats were stretching to 15 secs and a darker grey for the LRGB's.

If anyone is building using these LED's I would cover every other LED with opaque tape to bring levels down.

Already shown me that my 'hot-spot' is off centre so even though collimation looks good - I need to make some adjustments...

Francis

[pritc]

Would this work to dim the LED lights??

12v LED dimmer controller for led flexible strip ribbon | eBay

Wayne

[yesyes]

It doesn't say how it works. If it's pulse based (PWM), then I think it's not a good idea for taking photos of.

[Chris Rowland]

Why would you object to a PWM based dimmer?

Chris R

[yesyes]

Because it flashes the LEDs. If the duration of these flashes is near (but not equal to) the duration of the exposure you're taking of them, you could get weird effects where the field is not evenly illuminated. A bit like pointing a camcorder on a CRT TV.

I might be totally wrong though. I haven't actually tried that.

[fwm891]

I think the problem rather than uneven illumination across the reflective screen would be uneven exposures when they are of short duration as each frame may receive a different number of LED on/off cycles. Longer exposures would probably be ok as the odd flash missed or added would have little effect over say 5 - 10 seconds but when you are exposing for 4-5 hundredths sec then it would.

[yesyes]

That is a good point too...

But what I meant wasn't uneven illumination of the reflective screen but the image sensor. For argument's sake say the exposure time of one flat frame is 300ms but each flash of the LEDs is 200ms with 100ms off time. So the LEDs go off after 2/3rd of the exposure time which would leave the bottom 1/3rd of the image dark.

Having said that, maybe I'm thinking too much of a video sensor that is read in scan lines. Does this work differently on a still image sensor?

Oh, and sorry for going off topic... ;-)