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Lightbox for taking Flats


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I've finally got around to making my flats box.

I decided to go for a cylinder rather than the normal square as I thought it would maximize the amount of reflected light and limit any 'dead' areas. I could also use the Celestrons dust cap retaining pins to lock the flats box onto the 'scope.

I purchased some of the craft board that has a thin foam sheet sandwiched between two sheets of thick paper/thin card. In order to bend the card into a cylinder, I creased the board every 20 mm by pressing the edge of a steel ruler into the board. It took two of the sheets to make a cylinder big enough to fit my C9.25, with only a couple of cm trimmed off.

   IMG_20191230_161223.thumb.jpg.007212d56aa8f0a4a3f7f05e033352ec.jpg  IMG_20191230_161129.thumb.jpg.451655123f8cb94e3bcaf5b712e7f9d9.jpg  IMG_20191230_161034.thumb.jpg.db398c709038585141f7c179d9b28831.jpg


I then made a reinforcing ring/defuser holder from two strips of the foam board; this time creasing them at 15mm intervals. I stuck these level to the bottom edge so the joins were 90° to the main cylinder joins. These strips were cut wide enough to ensure that the diffuser cleared the secondary housing.

IMG_20191230_161300.thumb.jpg.165c3b10097043989b438c6b785356d0.jpg  IMG_20191230_225442.thumb.jpg.faec62ac72e41ae12daea6052a1dc5ad.jpg


The cylinder was designed to lock into the C9.25s dust cap retaining pins so next I cut two keyways into the bottom outer side. They looked a little weak so I reinforced them with some Christmas chocolate reindeer plastic packaging!



Although the foamboard is quite shiny, I wasn't happy with all the grooves, so I lined the inside with white A4 paper. The Perspex sheet was cut to shape and hot glued into place onto the ledge.

IMG_20191230_162024.thumb.jpg.0fa4322c767ea3a1f33214bc35616108.jpg  IMG_20191230_161949.thumb.jpg.328762bbf6af4e2017dc5437b98d1e49.jpg


Next, starting at the top, I notched the edge of the cylinder to run the LED string lights cable through and then started to spiral the LEDs around and down the cylinder.

IMG_20191230_230932.thumb.jpg.8a7b97a7d069a61cfb391ab72e0fe23e.jpg  IMG_20191230_161841.thumb.jpg.f6099780cdc354df77cc03a5326cc79a.jpg


The top cap/reflector was made from two discs of foamboard. One to go inside the cylinder and one to sit proud of the edge. They were glued together before being hot glued onto the top of the cylinder. The LED light string that I bought has an integrated on/off button as well as both up and down brightness buttons with a 3M sticky pad on the back, so I stuck this to the top cap.



As I had previously made myself a 'scope mounted power distribution box with aircraft sockets for power, I removed the 3 pin UK plug/ac-dc converter and soldered on an aircraft plug to match my 12 volt DC supply socket.

IMG_20191230_161426.thumb.jpg.806f7c1e4232d01797f271e24af4d385.jpg  IMG_20191230_161737.thumb.jpg.212f3c86489bc522420c596c7cdf6a46.jpg


The lightbox illuminated.



Edited by Stargazer33
Stray picture removed!
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  • 1 year later...

Hi there, thank you for sharing the instructions.

Did you have to trim the LED string lights? Or did you use the whole 5m length? 

Was it just one circular piece of Perspex/acrylic sheet you used? 

Thanks again,


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That’s a really neat idea and nice build Stargazer33 👍 The light must be super-diffused and even with no direct illumination like that. Great :)


oops- didn’t notice the date 😂

Edited by markse68
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On 14/09/2021 at 22:41, tony8gj said:

Hi there, thank you for sharing the instructions.

Did you have to trim the LED string lights? Or did you use the whole 5m length? 

Was it just one circular piece of Perspex/acrylic sheet you used? 

Thanks again,


Yes I  used the whole length, but there are marked lines where you can cut without breaking the circuit.  To be honest I  am thinking about reducing the overall size and therefore the length of the LED string as it is a bit too bright even on the low setting.

Just the one disk cut from the sheet.


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On 22/09/2021 at 13:01, Stargazer33 said:

Yes I  used the whole length, but there are marked lines where you can cut without breaking the circuit.  To be honest I  am thinking about reducing the overall size and therefore the length of the LED string as it is a bit too bright even on the low setting.

Just the one disk cut from the sheet.


I wondered about the brightness being high. Until now I've been using my iPad screen turned down to 1/4 brightness or so with two sheets of diffusion material (Mylar based Kodak product from my horde of darkroom stuff - '70s vintage) to get exposure in the 1-2 second range on my GT-71 refractor.

I am currently working towards an Arduino controlled LED source I can use with Kstars/Ekos/Indi for semi-automated flats exposure. So far I've "borrowed" Arduino code from several sources and can control a test LED with Ekos.  Ekos wasn't handling the flat panel properly (using indi driver FlipMan) but the issue is currently being actively  sorted). The next step is the actual LED panel/scope attachment mechanism design and build.  I am leaning towards a circular/cylindrical design rather than a panel design. (interesting information in this article: https://www.ing.iac.es//astronomy/observing/manuals/ps/tech_notes/tn108.pdf

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I haven't thought much about this, but the normal way to control brightness of a LED or a LED string is using a PWM signal on a MCU. Does anyone know what limitations are there in terms of PWM frequency vs exposure time?

For example, it seems obvious that a PWM frequency of 1kHz would limit the minimum exposure to at least 1/1000sec, but is it as simple as that? Does the duty cycle matter, or is frequency the only factor?

If PWM isn't suitable, does that leave a series resistor as the only option? (not good for adjustable brightness!)

Edited by Padraic M
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4 hours ago, Stargazer33 said:

😁 ahh yes, I remember the 70s well! 

The days of Cibachrome printing and unsharp masking of slides.  I don't really miss that part.  (the youth though... that's another thing)

ps: nice cats

Edited by DaveSw
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