If its too bright wire all the LEDs together and run off a variable power supply or controller to dim the lights to the correct intensity needed.
Quote:
Thats a very sensible suggestion Andrew (very different to last night Eh )
Amazing what a little sleep does to a person... Must have come to me lastnight in a dream, my wife said I was mumbling in my sleep so I guess I was working it out
10 out of 10 for effort mate – well done! You may find you have too much power in your set up? I have built a flat field box, but used a shallow washing up bowl. I have 6 flashlight bulbs around the circumference of the rim, pointing inside the bowl so their light bounces off the bottom of the bowl and then onto the back side of a sheet of milky white Perspex. The front side of this Perspex is what my C9.25 then sees.
I made a plastic ring/shield that sits inside the bowl to prevent any light from the flashlight bulbs shining directly onto the back of the opaque Perspex. I also sanded the bottom of the washing up bowl as I read somewhere that this produces an effect called "Lambertian reflection". Apparently the light reflected from a roughened surface is more even than from a shiny surface.
With my ST7, my flat fields are ½ sec using the above apparatus. The flashlight bulbs are wired in parallel and I use a 1.5A power pack that is adjustable from 3V to 12V dc. I usually have it set to 3V dc and that provides sufficient illumination to half fill the charge wells (pixels) on my ST7.
I took a series of flat fields and then rotated the apparatus 90 degrees on the end of the OTA, and then took another series. I then subtracted them and got null, implying I had a true flat field, evenly illuminated.
Gee, this is difficult to describe in words. I'm off to get the flat field box and take some pictures so I can post them later.
Here are the pictures. Whilst taking the apparatus apart, my memory was jogged as follows:
I only use 4 flashlight bulbs, even though I wired it for 8.
I sandwiched one layer of x2 Neutral Density (ND) film with the opaque Perspex to reduce the overall illumination.
The black, circular ring is made from computer binder plastic covers and prevents the bulbs from directly illuminating the Perspex/ND filter sandwich.
I use the power pack at 4.5V dc to provide adequate flat field illumination.
The inside walls and bottom of the bowl have been sanded, to roughen the surface, a la Lambertian Reflectance (the surface luminance is the same regardless of angle of view) which I am hoping will produce a more even illumination.
As stated previously, the illumination is sufficient to half saturate my ccd so I'm thinking yours may be too bright?
Cheers
Dennis
Last edited by Dennis; 26-06-2006 at 10:32 AM.
Reason: 2nd Bullet point changed to read one layer of ND (not two). 5th Bullet point added.
Here are the pictures. Whilst taking the apparatus apart, my memory was jogged as follows:
I only use 4 flashlight bulbs, even though I wired it for 8.
I sandwiched two layers of x2 Neutral Density (ND) film with the opaque Perspex to reduce the overall illumination.
The black, circular ring is made from computer binder plastic covers and prevents the bulbs from directly illuminating the Perspex/ND filter sandwich.
I use the power pack at 4.5V dc to provide adequate flat filed illumination.
As stated previously, the illumination is sufficient to half saturate my ccd so I'm thinking yours may be too bright?
Cheers
Dennis
Mmmm thats way better that what i concocted, might have to rethink and regig afew things, thought the dslr is not as sensitive as the ccd in that respect
Yawn – just woke up for the England/Ecuador soccer match, so I thought I’d post a flat field taken with the above flat field apparatus. This one had the Celestron F6.3 Corrector/Flattener in the optical train on the C9.25.
Whatcha need is something like the material used on the old light tables.
Needs to be somewhat thick (2-3 mm) so that it can be epoxied into the bucket. If you go to Bunnings they sell storage bins for between $5 - $30 which are made of similar material. You could buy a square one and chop the side out? Maybe?
Well well well, this has to be a trick question. Without breaking the laws of physics wouldn't it be easier to empty the contents of the bucket to make it a truly light bucket.
PS even take the handle off!!
Last edited by The Unbeliever; 01-07-2006 at 04:39 PM.
Waht are astronomers up to now?
