Making a light box for flats

[Horwig]

I've always used a disc of white opal perspex for flats on my FSQ, with a section of cardboard packing tube to hold it in place.

I use this to get flats at dusk, but thought I could do better with a light box, so got an LED strip to play with off ebay. 5 Metre, 300 LEDs for £2.99 delivered from China. Is it me, how can anybody compete with that in Europe!

Anyway, glued sections of the strip to the salvaged plastic end of the packing tube, driven by a PWM circuit...

Lined the section of tube between the LEDs and the opal perspex with white paper, and put a sheet of paper across the tube for luck.

Here's what the resulting illumination looked like, by 53mm off centre, it was down 5%, so not very good

 

V2 of the light box was an array of leds in a box 200 mm square, but it gave the same edge effect as the above with the disc of perspex.

 

V3 was to cover the whole 200mm square of the light box in perspex, and as well, put another sheet of perspex 25mm away.

Then the original piece of cardboard tube was fixed to a cover over the front

 

and it gave this illumination, no drop off whatever across the field

The PWM freq I hope will be high enough to not create any problems with banding, the switch in the circuit is to give two preset levels, one for LRGB, the other for narrow band.

I should now have no excuses for bad backgrounds to my images, but  I'm glad I went through this learning curve before trying to build one for my 16 inch Newtonian!

 

Huw

 

[Horwig]

First results with the new lightbox are very encouraging. I'd always had vignetting before, now all I have to deal with is skyglow and LP

[Horwig]

a quick followup to this, thanks really to a LIKE vote from Alpollo drawing my attention back here.

I'd recently done a measure of illumination using this flat box, and the image second to last above makes it obvious,

I measured the ADU level for a one second exsposure through filters, and found the following

L 20844 ADU

R  6649 ADU

G  8484 ADU

B  6286 ADU

Which explains the green cast to the image. Not in itself a problem, just nice to be able to read figures and see the matching visual result.

The lightbox remains my favourite project of the last few months, it's improved my flats no end.

Huw

[Alpollo]

Yes I am going to build one of these based on your results. I like the even illumination from the oversized box.

You mentioned something about banding, I thought about adding as single pole low pass filter to smooth out the PWM that may cause banding, but I see in application it doesn't seem to be a problem for you, so maybe I won't worry much about it.

Great project.

[Shelster1973]

looks great.

I am looking to change the way I grab my flats as used to use an LED lamp fitting that was just rested on the end of my OTA, but this is not the best.

Do you have a parts list or any wiring / construction diagrams for the project?

Cheers

[Horwig]

Hi Shelster. I'm not organised enough to start with drawings etc, I just tend to have a rummage in my parts boxes, and make it up as I go along. The box is made up of foam art sheets, glued together, and masking tape to strengthen the joins

The PWM circuit is similar to the 555 circuit in this website: https://learn.sparkfun.com/tutorials/led-light-bar-hookup/example-circuits

but I modified it as I went along, for instance, there are two preset lighting levels by means of a dpdt swith selecting between two variable resistors, and both have quite high value resistors in series on the 'bright' side, to give more control, these LEDs are really bright.

The LED strips were similar to these: http://www.ebay.co.uk/itm/NoneWaterproof-5M-3528-Cool-Day-White-300-LED-Strip-Lights-Flexible-Rope-String-/262883817942?hash=item3d3518b9d6:g:73sAAOSwqBJXUVTE&autorefresh=true

Don't know if that is of any help, don't be shy if you need more info

 

Huw

[Shelster1973]

16 hours ago, Horwig said:

Hi Shelster. I'm not organised enough to start with drawings etc, I just tend to have a rummage in my parts boxes, and make it up as I go along. The box is made up of foam art sheets, glued together, and masking tape to strengthen the joins

The PWM circuit is similar to the 555 circuit in this website: https://learn.sparkfun.com/tutorials/led-light-bar-hookup/example-circuits

but I modified it as I went along, for instance, there are two preset lighting levels by means of a dpdt swith selecting between two variable resistors, and both have quite high value resistors in series on the 'bright' side, to give more control, these LEDs are really bright.

The LED strips were similar to these: http://www.ebay.co.uk/itm/NoneWaterproof-5M-3528-Cool-Day-White-300-LED-Strip-Lights-Flexible-Rope-String-/262883817942?hash=item3d3518b9d6:g:73sAAOSwqBJXUVTE&autorefresh=true

Don't know if that is of any help, don't be shy if you need more info

 

Huw

Know what you mean... I tned to do things on the fly and then think 'maybe I should have documented that' at the end of the process.

I ordered a load of plastic sheeting yesterday so I can build it up.  Nealy made a school boy error and ordered it at 200mm x 200mm for the opal sheets and the board for the base.....then realised I have an 8" OTA so need 250mm.  Spotted before I hit buy, so all good.  Don't have the carboard tube for fitting so am planning to use some of the boards to make an octagonal collar that will fit over the end of the OTA. 

Also ordered one of these for controlling the dimming.  Am thinking I can re-purpose the LED strip that is in the light fitting I use currently too. 

[Horwig]

Glad to be of some assistance

I hope that dimmer goes low enough, the output of mine is virtually off, and I still have to dim the box through multiple sheets of A4 writing paper to get a reasonable duration for flats.

 

Huw