My Flat Field Box

[obscura]

Stimulus gratefully acknowledged to Chris and his ideas on flat field boxes

Thought I would try this out rather than expend too much dosh.

Made from 3mm mdf sides with 6mm mdf for base. Sides screwed to base and then taped around sides. This means I can take it apart if necessary. Lined in white foam board.

Light are 2 x led strings from ebay held down with double sided sticky - and it does stick! First opal perspex panel is 2" above the base to eliminate bright spots from leds and the second 1" above that to give a flat even image. Perspex also off ebay.

I have tested it out on the WO80 during daylight varying both drive volts and shutter speed on the camera. Shutter speed seems to be the better alternative with this dry run indoors. However, this does appear to be trial and error on how bright or dim this image should be for flats. Would appreciate any views on this. Camera was driven off Astrojan which makes it so easy to take any images.

Pics give a clue what its about plus one exposure taken at 1/200 sec in Tv mode which at this time appears to be the optimum. But, real field trials are needed to confirm.

[Bizibilder]

I assume you mean set the camera in "Av" mode? If you do then it will automatically give the correct exposure for the box. Looks like a nice simple solution to the problem - well done.

[obscura]

Thanks for that. I take your point - I did use Tv to play with exposure. In the mock up I first used Av and it didn't look right somehow. Now the box is complete I'll try Av again - makes more sense.

[yesyes]

Very nice. And it seems to work well.

I have taken the liberty of playing with that one flat frame you posted. When you do an extreme histogram stretch you can see that the corners are darker than the middle. This is exactly what the flat frames are for.

You can also notice a few dust bunnies... ;-)

When using these flat frames while stacking, both of these will be compensated for in the final image.

[cgarry]

Nice work, the MDF looks nice and robust.

Cheers,

Chris

[obscura]

My pleasure but oo, what mucky optics!

Would say this frame and 20 others like it would be OK using DSS?

I will re-take the flats in Av mode for comparison.

[swag72]

Where have you put the lights to get an even lighting? All around the inside? I would like see an inside pic if possible, showing the lights. Great stuff by the way.

[obscura]

Hi Sara. I initially had some problems in getting the leds to stay down. I was hoping that cable clamps would do the trick - but no. Eventually double sided tape proved to be the answer. This was stuck all over the base (on top of white foam board rather than the base direct.) Because of all this faff, I forgot to photograph them. I left one side of the box off while doing the following.

There are two strings of leds each mounted on the base. The first string runs around the perimeter of the base and approx. 5mm or so from the sides. The second string runs within the first in a zig zag type fashion. The sticky holds them in place during all this.

The key for me was to get the leds evenly spread over the base - as best as one can - no rocket science, just awkward.

From a previous mock up I knew the approximate gaps for the opal perspex. I stuck 50mm wide white foam board around the sides and loose laid the first perspex board on those strips. This placement was to ensure that there were no bright spots or shadows showing through from the leds.

I then laid 25mm strips around the sides resting on that perspex board and then loose laid the second board on that. This placement was to ensure an evenly lit field over the surface. In fact, the second board could have been 25 - 50mm+ to achieve that tho' the image reduced in brightness. Another set of strips hold the second perspex in place as can be seen from the pics.

This box was for a WO 80 and this is about 150w x 200l x 150h. The perspex was A4 and I just cut that in half.

Hope this helps, Sorry no pics tho'

[obscura]

Re Bizibilder and yesyes comments above.

Attached is a shot taken in Av mode - 1/320s @ ISO800.

I think this is the way - looks good to me.

Of course, I have left the mucky optics in place purely for test purposes!!?????

Played with histogram with similar results to yesyes

[yesyes]

Yes, it comes out a bit better in Av mode. The histogram has moved to the lower side a bit. That peak in the histogram should be between 1/3 and 2/3 of the max. With the Av setting it is at about 2/3 which should be fine.

Time to test it on some real imaging data... ;-)

[obscura]

Sounds good - I could reduce volts to leds a nadge to bring the peak down. I'll play with that.

I have nothing more to build and or anymore so I've run out of excuses, 'cept it looks a trifle cloudy and wet out there.

Thanks for your help.

[yesyes]

Hmmm, I think reducing the voltage will not "help". In Av mode the camera will just expose shorter. You could play with white balance and/or exposure compensation. But I think the flats are just fine as they are now.

[obscura]

As soon as I'd posted that reply I realised my brain wasn't in gear - the old 18% grey.

One thing if I can bother you further :

1) from the histogram, how do you determine the 1/3 to 2/3. Is it by position on x-axis?

2) I assume I use the flats as taken by the camera as are?

[yesyes]

One thing if I can bother you further :

1) from the histogram, how do you determine the 1/3 to 2/3. Is it by position on x-axis?

Yes, correct. In the histogram the darkest shades are on the left, the brightest on the right. Ideally flats should have the peak between 1/3 and 2/3. But as long as it is not clipped on either end of the histogram, you should be fine. The flats are about the brightness difference between brighter and darker parts. The absolute values of the brightness does not matter.

2) I assume I use the flats as taken by the camera as are?

Yes, ideally take them as RAW images, not JPEG to avoid artefacts in the images caused by lossy JPEG compression.

Take the flats with the camera set to auto exposure (Av), use the same ISO setting as the light frames. Do NOT change anything to the imaging train between taking light frames and flat frames. Especially, do not change the rotation of the camera on the scope. Otherwise your dust bunnies end up on other places on the flats compared to the light frames.

[obscura]

Ok. Got that. The camera is usually in RAW mode - its not used for any other purpose.

You have been a great help and that's much appreciated. The learning curve doesn't get any less steeper, as yet. But it will.