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.
Leon
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.
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.
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?
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.
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?
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?
Now you've got me thinking. I could be way outside the box here
Quote:
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).
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.
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.
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)
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