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Building a flat field box

13 Mar, 2019 07:25
Hello, I have some questions about creating flats. I found an article on the internet about building a flat field box. The author described that he rotates the box seated on the telescope after taking some flats for a better result of the masterflat. Do you think this makes sense? The article also says that you have to distribute the LEDs evenly on the surface of the box. Has someone built such a box on its own and can give some advice, e. g. how many LEDs to be installed for a 150mm telescope? I don't want to pay about 150 Euro for such a box that can only give some light. In addition, how do I find out whether I selected the best exposure time? I red an article saying that if the flat is not bright enough in the corners you have to choose a longer exposure time, but not longer than 1 second. Do you have some values for the best exposure time, e. g. for a 150 Newton or Apo? Thank you in advance for your help!
13 Mar, 2019 09:11
Why don’t you get an aurora flatfield panel ? That’s cheaper , often found used , and works perfect , here is a German website , maybe it helps to find it. The price of course depends on the size you need , I got the 8“ , works of course for all smaller sizes as well smile
13 Mar, 2019 12:54
Hi Achim

I have built a flatfield box myself and used electroluminescent foil rather than LEDs underneath a cover of opaque polycarbonate to even out irregularities of the foil. Foil and Polycarbonate are sandwiched between two wood plates, one of which has a dewshield-sized hole for my refractor. Otherwise you only need an inverter and a cigarette lighter plug. I got the electronic stuff here:

and the Polycarbonate (PLEXIGLAS® GS weiss WH73 GT 3mm) and front wooden plate (Multiplexplatte Zuschnitt 12mm) with a correctly sized hole here:

Don't forget to line the inside of the wood with velours foil, otherwise you'll get brighter fringes due to reflection off the wood surface. By my guesstimate, all materials will set you back about 70 €, plus you get the fun of assembling the Box (If you like doing that kind of stuff, that is smile ). It really works a treat, i got rid of the funky pattern background noise and gradients I got using a white t-shirt for flats, and also of dust blobs (I will post a picture where I applied my flatfield box soon). Also I do't need to rotate it as illumination is very even. On the other hand, for 90 € you can get an aurora flatfield panel (160mm) from Gerd Neumann which also uses electroluminescence and might work even better. Mine was cheaper (about 30€) because I only use a small refractor and the foil is the most expensive part.
As for the exposure time of the flats: you want to get as high as possible without leaving your sensor's linear response, before the brightest pixels hit the anti-blooming gate. For DSLR users like me that means the Flat's Histogram peak schould be roughly in the middle of the scale to be on the safe side. For example, on my 73mm f/6 refractor the exposure time is 1/20 s, but that will vary depending on your setup.

Clear skies, Marc
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