Another homemade light box

[Lee]

This one is literally a light box. I sat down to make a box today, and then thought of all the cardboard boxes I have stored away..... so one of the observatory building purchases finds a new use....

I went looking for LED's and resistors etc to make a white LED array - but found these LED modules at Jaycar - $19.95 for 10x3 LED's, prewired ready for 12V DC - draws about 150mA. (Jaycar (http://www.jaycar.com.au) cat no ZD0492)

These are double-sided-taped to a white board in the bottom of the box, and the top of the box is cut out for a piece of 3mm opal acrylic. Produces a very nice even light, and very light on the arms to hold up to the objective too. I wired an RCA plug to match my DIY dew heater arrangement.

This was made as a prototype, but I may just cover it in contact to fight dew and see how long it lasts!

[Terry B]

Not trying to be critical but I thought you would get a more even light if the inside of the box was white. This would reflect things around more.

[h0ughy]

how does this one work Lee? is it like a rear projection screen - onto an opal surface?

[Lee]

Exactly - there are 30 LEDs at the bottom, their light is diffuse by the time their beams get to the opal, which diffuses it more. Just hold in front of the objective....



I thought of lining the insides with the same white card from the bottom, but since I was prototyping thought I'd see how it went without - seems fine.

I'll post a flat frame soon.....

[Lee]

Flat frame - there is a slight darkening towards to top of this frame - unsure if this is due to the box or the scope - will do some more tests.....

[h0ughy]

yuk whats that on the right!!! LOL

[iceman]

Looks pretty noisy - what ISO did you shoot it at?

[Lee]

One spec of dust - happy with that - you should have seen it before I cleaned the CCD cover! :eyepop:

It is a bit noisy - I haven't quite worked out the exposure settings yet Mike - too tired last night. Flat shot with DSI Pro - so no ISO.

[iceman]

ah ok. All these DIY lightboxes.. i'm going to have to make my own one day soon.

[Dennis]

Hi Lee

Nice job with the light box, the results look very promising – well done! I used single high intensity LED’s (x6) and mounted the LEDs in the “mouth” of my light box so they reflected back down the bucket I used, to then reflect back towards the OTA aperture where they were further diffused by a sheet of opaque Perspex.

I usually plan to shoot 10 Flats and then do a Median Combine and it makes a significant difference to the appearance of the combined Flat. The graininess almost disappears and the final median Flat looks so much smoother. I think the recommended exposure is that required to fill the well depth of the sensor to around 30%?

I’ve yet to test mine!

Cheers

Dennis

[iceman]

Another one!

[h0ughy]

LOL its a light bucket!! nice take on that Dennis

[Dennis]

What appears promising with Lee’s design is that it is simple and appears quite effective from the results posted so far. I don’t have the smarts to perform any mathematical modelling or light ray tracing, so I just followed the advice on several websites which had the light bouncing off the inner walls and the bottom of the bucket to “flatten” it, before finally striking the main diffuser.

Lee’s design seems to achieve this via direct illumination of the diffuser by a bank of LED’s, which certainly simplifies the design and build process in my book.

Cheers

Dennis

[joshman]

will that bucket fit on my head? i have the urge to wear a lit bucket...

[h0ughy]

Josh your a sad man - I have no desire to take flats of your noggin:whistle:

[gbeal]

Now we are talking, see what you started Huff. That ugly duckling of yours has spawned all this DIY, well done.
I too have one, and will shoot a picture of it. Unfortunately it is redundant, as the scope I built it specifically for (TSA102) is no longer with me, and the replacement is an 8" newt, so it don't fit.
The light bucket is where I am headed, good idea Dennis. I normally aim for an ADU of about 20,000, is this about right?

[Dennis]

Hi Gary

Yep – 20,000 ADU is about the number I aim for too with the ST7E.

Cheers

Dennis

[iceman]

Gary - is it something that can be shipped? :whistle: You know my DIY skills are less than skillful :)

[[1ponders]]

Seeing this is a show and tell. :P
http://www.iceinspace.com.au/forum/attachment.php?attachmentid=25744&d=1176622217 (tidied it up since then and no white showing)
http://www.iceinspace.com.au/forum/attachment.php?attachmentid=25745&d=1176622217 A shot from the front just for the hell of it :P Yes I know it is very yellow but

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=25747&d=1176622481

for the SBIG depending on what colour I want depends on how long I make the exposure. The colour doesn't seem to matter with the DSLR as it is converted to 16bit grey scale in IP for flat fielding anyway.

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=25775&d=1176643216

The result from sitting on my 8" SCT

I measured the output across the face of the lightbox and there is less than 0.2 lux difference between the center and any other spot around the screen.

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=25779&d=1176643216

The final result.

[leon]

Hi Guys, well I'm probably going to ask the dumbest question of all time and should probably know the answer, but, do you use the light box in the day or at night, never used one, or seen one for that matter other than on IIS. :whistle:

Leon :thumbsup:

[Lee]

Whenever you like.

[Dennis]

Hi Leon

'Aint no such thing as a dumb question. I guess the 2 most important factors in taking “Flat Field” images with a light box are:

The field “seen” by the ‘scope/ccd camera must be flat (evenly illuminated) and,
The configuration (rotation, focus) of the sensor and telescope must not change between taking your light frames and the flat field frames.

So, I usually take my flats at the end of an imaging session, usually in twilight so I’m not wasting any imaging time.

If you do say, rotate the CCD camera in the focuser, or re-focus the image after taking your light frames, then any vignetting and dust shadows will not line up between the light frames and flat field frames.

Cheers

Dennis

[leon]

Thank You gentlemen, question resolved, now i might just build one of those things. :whistle:

Leon :thumbsup:

[aerobrake]

Chappies if I wanna take a flat with my 10D how do I know how much to saturate the CCD to have an effective flat?

Cheers
Mike

[gbeal]

Mike,
of course. E mail me, I have tried e mailing you a couple of times, with no reply.
Gary

[[1ponders]]

Mike (aerobrake) it's a bit of hit and miss to start with. What I do is, after switching on the light box, adjusting its intensity (I have a dimmer switch inline) and placing it hard up against the lens or scope I then start taking images adjusting the exposure until it is right. To check the histogram I use the info button on the back of the camera to display the histogram and other data for each shot.

[gbeal]

How do you read the histo Paul? OK, I don't normally use one, as most of my imaging is done with the ST2000, and Maxim gives me an ADU count, but for those that use a DSLR, how??
I suppose if you capture with something like MaxDSLR it will also give the ADU count, correct?

[[1ponders]]

If you turn on the info button when you have an image displayed ti will show the distribution of the image curve. I simply aim for between 1/3 and 1/2 across the graph.

Maxim probably would if it new the gain of the camera.

[aerobrake]

So Paul you arnt to concerned with the "height" of the histogram just as long as the histogram is about 1/3 across the horizontal axis of the graph, is that correct?

Cheers
Mike

[[1ponders]]

No, height isn't an issue at all.

[Dennis]

Hi Paul

This is interesting stuff and is showing me I have a bit of a sketchy grasp on the histogram, especially as displayed by DSLR’s.

I thought that the histogram was a representation of the range of brightness levels (0-255) across the bottom axis (X-axis) and a representation of how many pixels of intensity “n” fell into each brightness level (Y-axis).

So,

A big hump in the middle would indicate lots of pixels (high ADU count?) in the mid-gray range.
A big hump on the right would indicate lots of pixels at the white end.
A big hump on the left would indicate a lot of pixels at the black end.

So, this gave me the understanding that the height of the histogram was a representation of the quantity of pixels which were “held” in each brightness level bucket. If that is the case, the Y-axis would then be a sort of ADU measure?:help:

Cheers

Dennis

[[1ponders]]

Now you've got me thinking. I could be way outside the box here :P



That's about how I understand it as well, the histogram is a measure of the range if brightness levels across the x axis and the y is the number of electrons at each level.

But a large hump in the middle, as I understand it, represents lots of pixels that have their well half full, ie they haven't reached saturation yet. The closer to saturation the further to the right a particular well will appear. So if the aim is to have the chip exposed for a flat so that it reaches between 1/3 and 1/2 full well capacity then a good sharp spike between those ranges would tend to indicate that. The Histogram is giving you a graphical representation of the numeric distribution of the images electrons based on intensity, not physical pixel site.

An example of this would be the more spread out the hump more your vignetting. You can see by the image that there is vignetting around the edge, ie less electrons per pixel. This is shown in the histogram by the tail to the left.

Maybe I'm way out, but it seems to work. :shrug:

[Dennis]

Thanks Paul - more stuff to think about eh!

Cheers

Dennis

[turbo_pascale]

In the Canon Digital Photo Professional (looks like the screen cap above) just run your mouse around the image - you will see three sets of numbers indicating the levels of R, G & B from 0 to 255.
If I have the theory right, you want a number between about 85 and 128 as evenly as you can. Obviously, if your light is not "true white" you're going to get variations across the RGB mix.

I took my first flats last night with one of my 2 year old's size 1 t-shirts he no longer fits in to, elastic banded around the dewshield of the 80ED, and until my prototype lightbox comes to life, I actually used a completely white image on my little 11.5" notebook (only weights 1.2kg), and held it flush against the tshirt and took a whole bunch of automated exposures. Depending on how bright the laptop screen is, you've got to adjust your exposures - mine worked out to be about 1/2 second or so, and the light is decidedly blue-ish.
When you use something like DeepSkyStacker and subtract your bias and darks etc (all automatic), it build a master flat that is grey.

Turbo

[turbo_pascale]

By the way, the blue light is not all that much of a surprise - the laptop screen uses "white" leds for illumination, which are actually a derivative of blue leds. (if you're interested in the detail read here)

http://en.wikipedia.org/wiki/LED#White_LEDs

Turbo

[[1ponders]]

Thanks for that Turbo, I've never noticed it before.