Digital Imaging - Flats and Darks Explained!

[Doug]

The quality of light used for a flat field certainly seems to affect one shot colour. I took a series of flats today with a blue sky lighting through white cartrige paper and the results are noticably different from flats taken with an overcast sky as the light source.
I think it is time to consider building a light box. Jerry Lodregus seems to think LEDs are not a good idea, but doesn't seem to say why.
I used to service a certain negative film scanner that used arrays of red, green and blue LEDs to illuminate the negs. quite impressive display of colour as it flashed thru the colours, varying time intervals to give a white balanced exposure.
Of course in that application there was no need to try to produce white light, as the beastie just read the three channels and went away and did its homework. Still, I wonder if a Light box could be worked with a set of LEDs tuned to give white light?

[gbeal]

I haven't done anything re lights, but now that the new refractor has arrived (and is likely to be THE imaging scope) I need to.
I'll check this end, but any suggestions on type/brand etc are appreciated.

[netwolf]

Great article Eddie, makes some sense now what the DSO guys are on about.

Can someone however explain what a bias frame is and how it is different from a Dark frame?

Regards.

[[1ponders] (Paul)]

A bias frame records the random noise contributions from the sensor's on-chip amplifier. To take one, you set your camera to the shortest exposure it can take.

A dark noise is a measure of the noise generated by the chip over the timeframe of a light exposure. However when you take a dark frame you are including a bias measurement anyway. Depending on what you are doing with the image then just a dark frame may be all that is necessary without taking a seperate bias frame.

[Doug]

Well yes, a bias frame is only necessary if you want to scale the dark frames.
Because a bias frame is fundamentally part of a dark frame, it becomes attractive to be able to subtract its influence prior to multiplying or dividing the time of a dark frame. In other words if I want to use a 5min dark frame on a 7min light frame, multiplying the dark by 1.4 will not be the same as taking a dark at 7mins because the bias value would also have been multiplied by 1.4. So scaling will sutract a bias frame from the dark, multiply/divide as required and add the bias value back to the resulting scaled dark value. So Bias frames are only required for scaling work.

[Rigel003 (Graeme)]

Great article. Thanks, Eddie.

I'm pretty much a beginner with darks and flats with my DSLR and still have lots of queries. The discussion about the quality of light for flats is interesting. I've been doing T shirt flats with the scope pointed at a wall illuminated by a tungsten bulb. I was worried that these had a pinkish colour cast and wondered if I should change them to grayscale before using them, but Mike Unsold said that it didn't matter when the automatic image processing routine in Images Plus dealt directly with the RAW frames. Does this mean the nature of the light source doesn't matter?

Also, Jerry Lodriguss seems to be saying that if you keep your light source sufficiently bright that the flat exposure can be under 1 second with a low ISO, then it's not necessary to take extra dark frames for the flats. I hope this is so because an extra set of darks for the flats seems like one step too many. Life is too short.

[darrellx (Darrell)]

Hi All

In this article, the statement "The main aim is to get a flat field that has an average pixel value of about 30% of the maximum pixel value that your camera is capable of. For a 16-bit camera, this would be approximately 20,000. Most image processing software will give you the average pixel value in an image so this value is generally easily obtained" has me a bit stumped.

I use CS3 and PaintShop Pro, and I just cannot find anywhere in there a place to find the average pixel value. I must be missing something.

Can someone point me oin the right direction to find this "average pixel value".

Thanks
Darrell

[Doug]

G'day Darrell,
Mostly I suppose people have dedicated asto-imaging software in mind such as images plus, maxdslr etc.
If you are using a DSLR and saving in BMP etc. you might try playing around with the expanded histogram window in photoshop to find the mean pixel value. If you are in a position to save in FIT(s) format, you will most likely find a useful readout using fits liberator, as a photoshop plug in.
What are you imaging with? A lot of capture s/w can also calculate the average...well some programs anyway.
cheers,
Doug

[darrellx (Darrell)]

Doug

I have an EOS400, and usually save my shots as jpeg and raw. With a typical photo, I use the histogram, and for the work I do on the photos I am comfortable with this (at the moment).

So I have been reading about flats and thought I should give it a go - I currently just do the darks. When I take the flats, there isn't anything in the histogram. Its really odd. There is just nothing there. In this article, mention is made of the average pixel value, so I thought I would look at that to see if I am on the right track.

Until the last month or two, I have used PaintShop Pro. But just before Christmas, I got CS3 and am still finding my way around.

Darrell

[EddieT (Eddie)]

Hi Darrell,

Obtaining the value in Photoshop could be considered to be a little "too late".

The idea is that you must achieve this value when capturing the flat field, so you really need to be able to read it from your camera control software after you capture the image.

You would adjust the exposure time to achieve your 30%. For example, if you take a flat and the average pixel value reads 10000, then you would need to roughly double the exposure time and retake the flat.

It all depends on the brightness of the light source being used to take the flat from.

Make sense ?

Eddie Trimarchi
http://astroshed.com

[darrellx (Darrell)]

Eddie

It does. Thanks. I have been taking a set of them; downloading to my laptop; checking them; then redoing it if neccessary.

So if I interpret all this correctly, I should either get the camera control on the laptop working, or check the histogram on the camera after the first shot and make neccessary adjustments.

I might give it another try tomorrow.

Darrell

[EddieT (Eddie)]

Hi again,
What are you capturing the images with ?

If it's Maxdslr (which I have no experience with), and it is the same as MaximDL, then choose View/Information Window (Ctrl-I).

Check the Aperture size (View Area) in the window, it should default to the entire window and it should show you the min, max and average pixel values. If it doesn't try minimising the view to 25% to fit the entire image on the window, then drag the cursor round the entire frame to highlight as much of it as possible. If nothing still shows up in the information window, then you have a problem, most likely with Maxim if you can see that the image actually has data in it, (indicated by varying brightness in the frame).

[darrellx (Darrell)]

Ah, sorry. Well, I don't actually use any software to capture the images. I just use the 400D and take a lot of frames. I attach the camera the way I take the photos - sometimes prime focus, sometimes with EP projection.

I just thought I could analyse the jpegs or raw files after I load them into CS3.

Darrell

[EddieT (Eddie)]

Hi Darrrell,
ah, I see now! Do people do that ?

Sorry mate, I'm so into my own headspace I never realised that you might not be using image capture software.

In Photoshop you should still be able to see the mean, I guess you can see the histogram (Menus: Window/Histogram) but unfortunately Photoshop only shows the screen histogram which is only 8-bit even if you have a 16-bit image loaded. In CS2 anyway, perhaps CS3 is more friendly.

You can still get an approximation of 30% maximum, only the mean will be 30% of 256, rather than your 16-bit value. Try going for a value of 65-70 in Photoshops 8-bit histogram and see how you go.

[darrellx (Darrell)]

Eddie, sorry for the confusion. I am still struggling a bit on the whole image processing thing. Until I started on this last year, I really did believe that all those amazing shots you seen on sites like this and in magazines where taken as a single exposure from an amazingly expensive camera. From that, I suppose the good news is that it need not be that expensive. The bad news is that it is much much more complex than I thought.

You are correct about CS3, the screen histogram is 8 bit. I also have a menu box at the right side of the screen with three tabs - Navigator/Histogram/Info. If I select the Info tab, I have values against R, G, and B, and also for C, M, Y, K. As I move the cursor around, the values change. So I figure that is the value for the pixel the cursor is on.

How do I see a value for the average of the entire image? The "mean". When I look at the Histogram tab, there is only the curve displayed - no values for the overall image, and no values on the x or y axis.

Thanks for your help.
Darrell

[darrellx (Darrell)]

Eddie
Ahah! Got it. In Measurement Log/Record Measurement there they are - all the measurements I could want.

Thanks for all you guidance. Now to put it to good use.

Darrell

[Terry B]

My 40D and I assume the 400D should show you the histogram on the LCD screen on the camera after you have taken the image. Just expose the flat to make the histogram in the middle.

[rogerg (Roger)]

I might be a bit late on this conversation but better late than never:

If you did want to work out the average brightness of an image in photoshop this is how I would do it:

1) perform a gaussian blurr (filter menu -> blur -> gaussian blur) with an extremely high value, if not maximum value.

2) show the "info" toolbox (window -> show info, if not already visible). this shows mouse cursor x, y and RGB colour values as well as a few other things.

3) select a tool such as the eye-dropper or Move Tool.

4) move your mouse over the image, the areas of average brightness (if there is still any variation after the gaussian blur) and watch the RGB values in the info toolbox. They should be quite consistent. That will be your average brightness.

As others have said, they should be in the range of 60-70. If not, you need to increase or decrease your exposure times appropriately.

Roger.

[darrellx (Darrell)]

Guys

Thanks for the pointers. I have had a look at both - the histogram on the camera and the gaussian blur. I think I should be fine now. At least I sort of understand what I am doing.

Now, I might build myself a lightbox.

Darrell

[Quark (Trevor)]

Hi Eddie,

Have read your most excellent article and am in throws of building a light box, as per your basic design.

Can you provide any more info on the bulbs that you use. My local supplier assures me there is no such animal as an ultra white halogen globe, that halogen globes provide warm light ie: a yellowish light.

Any clarification would be greatly appreciated.
The box I am building is for a 16" scope so the diffuser will be quite large.

Regards
Trevor