Digital Imaging - Flats and Darks Explained!

iceman (Mike) · #1 20-06-2005, 08:09 AM

Eddie Trimarchi, one of Australia's best astrophotographers, has kindly written a how-to guide for the use of Flats and Darks in digital imaging.

You can find the article at the IceInSpace How-To page, or by clicking on the following link:

Digital Imaging - Flats and Darks Explained!

Many thanks to Eddie for contributing such a great article, and sharing his knowledge so that others can benefit and produce better images as a result.

You can see Eddie's great astrophotography at his website: The Tin Shed Observatory

[1ponders] (Paul) · #2 20-06-2005, 09:11 AM

Thanks heaps Eddie. At least now I understand what it is I'm trying to do and why. You make it sound relatively simple. Now if I can just understand how to get the imaging programs to do what you've suggested I'll be laughing.

I love the idea of the flat box. I've tried to do a few off light coloured wall but I've not been able to get the illumination even enough. Great article.

One thing I'd like to see as an adjunct to this article (and I'm not trying to put you back to work Eddie, you've done a fantastic job

Anyone who is able and has the knowledge could do this ), is some additions to the "Iceinspace Glossary of Terms" explaining some of the terms you've used, like Normalization, Registration, Optimization, median combining etc. This could also apply to any other article that is posted that uses specialized jargon to explain the topic.

Mike is there anyway the articles could have a link to a "printer friendly version". I'd like to be able to print articles like these for "in field referencing" and currently it's take 8 pages to print.

iceman (Mike) · #3 20-06-2005, 09:17 AM

Paul, there is a "Print" icon at the bottom of the article, which makes it a print-friendly version. It's a long article, I don't understand what you mean by "printer friendly" and trying to fit it in less than 8 pages?

If there's only some sections you want to print (to take into the field), just copy it into word and mangle it how you'd like.

[1ponders] (Paul) · #4 20-06-2005, 09:23 AM

That's what I normally do Mike (ie take it to word). By "printer friendly" I mean when it comes up in print preview, its minus all the frames at the sides and expands the margins so that more info and less webpage layout appears in the printed version.

See here http://www.designplace.org/tutorials.php?page=1&c_id=27

iceman (Mike) · #5 20-06-2005, 09:42 AM

You mean, like this?

Just click the "print" icon at the bottom of the article (not the standard IE print toolbar icon).

iceman (Mike) · #6 20-06-2005, 09:43 AM

oh don't go and delete your post, it looks like i'm talking to myself!

[1ponders] (Paul) · #7 20-06-2005, 09:44 AM

But that's how I get my best conversations

OK I'll move it back if you want

Yes that's what I'm looking for. I guess if I really want to use as much of the page as possible I'll take it into word.

Ok I'll be quiet now and get back on topic.

h0ughy (David) · #8 20-06-2005, 09:47 AM

Quote:

Originally Posted by iceman

oh don't go and delete your post, it looks like i'm talking to myself!

At least Mike you get the answers you want

EddieT (Eddie) · #9 20-06-2005, 01:30 PM

Quote:

Thanks heaps Eddie.

You're very welcome!

Quote:

At least now I understand what it is I'm trying to do and why. You make it sound relatively simple. Now if I can just understand how to get the imaging programs to do what you've suggested I'll be laughing.

Yes indeed. I tried to keep it simple and introductory focusing mainly on the reasons for image calibration (with examples), without specific step-by-step instructions, because these will vary from application-to-applicaton. I think it's better to know what you are trying to acheive and then finding out how to acheive it with your specific spftware.

Quote:

I love the idea of the flat box. I've tried to do a few off light coloured wall but I've not been able to get the illumination even enough. Great article.

Making a good flat field is paramount. It doesn't take much of an inconsistency to create an inaccurate flat, and applying a bad flat can make your images worse than before. Much worse in fact.

Quote:

One thing I'd like to see as an adjunct to this article (and I'm not trying to put you back to work Eddie, you've done a fantastic job Anyone who is able and has the knowledge could do this ), is some additions to the "Iceinspace Glossary of Terms" explaining some of the terms you've used, like Normalization, Registration, Optimization, median combining etc. This could also apply to any other article that is posted that uses specialized jargon to explain the topic.

This was constantly in my mind as I wrote the article. But whenever I tried to concisely explain a statement that needed clarification, the article grew in size exponentially

I had to draw the line somewhere. In doing that, I decided that certain terms would have to be resourced elsewhere and this article would be the catalyst for that "further research". It was that or end up writing an online book!

I am open to questions regarding any part of the article so if anyone needs help to understand something, please ask. This will also give me a better idea of which parts are unclear and also framework for an addendum if needed, or a future, deeper article.

[1ponders] (Paul) · #10 20-06-2005, 04:51 PM

Quote:

Originally Posted by EddieT

I am open to questions regarding any part of the article so if anyone needs help to understand something, please ask. This will also give me a better idea of which parts are unclear and also framework for an addendum if needed, or a future, deeper article.

I'm working on that.

Geoff45 (Geoff) · #11 11-05-2007, 04:41 PM

I'm just finishing off a light box now. One question: does anybody know if the color cast of the lights in the box is important? It seems to me that if you are subtracting a flat from a colour image, the effect may be different for different lamp colours.

EddieT (Eddie) · #12 11-05-2007, 06:12 PM

Hi Geoff,
Going for bulbs as white as possible is safest. The spectral response of your camera/filters will have an effect on what colour the flats end up. Most one-shot cameras aim at a white balance that shows white as white. i.e. even amounts of R,G and B where the colour is white. The interior of the box should also be white as well as the diffuser. The aim being to get an evenly distributed flat white light source to calibrate with.

gbeal · #13 12-05-2007, 08:53 AM

Eddie,
I saw this article when you first floated it, and was impressed. I have mastered darks, but really I struggle with flats, silly I know, but I do.
I have decided to rattle out a light box like you suggest, and may come back for a little more fatherly guidance.
One thing I did wonder, is any specific suggestions for bulbs. I too am on a one shot colour CCD, so this is equally important for me. A suggestion from somewhere like DSE (which we have here as well) would be appreciated.
The flat wall flat isn't really as good as I had hoped.
Regards,
Gary

Doug · #14 12-05-2007, 01:57 PM

I have been using a white cartridge paper over the dew cap with the OT aimed at the zenith. Colour balance on the calibrated image is done using a G2V star. I think this should compensate for any white imbalance in the paper.
Eddie's light box has some attractive advantages over a sky flat. Not sure how white a low level light source is, however a G2V star balancing act should take care of any colour bias introduced by the flat field image in the final calibrated image.

Geoff45 (Geoff) · #15 12-05-2007, 05:07 PM

What happens if you just desaturate the flat and get a grey image?

[1ponders] (Paul) · #16 12-05-2007, 05:24 PM

I've been using standard 20W 12V halogens (couldn't get the ultra whites

) and the box works a treat. What I did have to do though was hook the bulbs up in parallel (as suggested elsewhere) and install a dimmer switch on the 240V side of the transformer. Does a great job.

Doug · #17 12-05-2007, 05:28 PM

You don't convert the flat to colour Geoff; it is a calibration frame. My reasoning, FWIW is that if there exists within the flat frame any bias toward one or two of the primary colours. that will translate as a colour caste to the final colour image. By having a G2V raw and calibrating that, when it is susequently colour converted, and balanced, the same colour values will give a correctly balance image of whatever it is you are imaging. The G2V is used as a reference colour, and adjusting the colour scaling at the time of colour conversion such that the resulting image of the G2V star has equal values of RGB, should therefore result in correct colour in subsequent images calibrated with the same set of flat frames.

Doug · #18 12-05-2007, 05:55 PM

20W

, what sort of exposure times do you use Paul?
And how do you position the box? Sit it atop the OTA, or on a bench at a distance? I have not ventured into the realm of light boxes, one thing that worries me is introducing an artificial gradient. I think I would be happy with a dim enough light source such that the shutter/blind travel time is an insignificant % of the overall exposure time. The other concern;real or imagined is if the light box diffuser surface is not perpendicular to the optical axis, this could lead to an unwanted gradient. Been thinking of trying it out though because it can be a real pain setting up in daytime. With the flat gathering method I use I've found it best to use a mid day(ish) sky rather that twighlight (contrary to the words of the Gurus) because of the rapid change of light levels around dusk or dawn.

Quote:

Originally Posted by [1ponders]

I've been using standard 20W 12V halogens (couldn't get the ultra whites

) and the box works a treat. What I did have to do though was hook the bulbs up in parallel (as suggested elsewhere) and install a dimmer switch on the 240V side of the transformer. Does a great job.

[1ponders] (Paul) · #19 12-05-2007, 07:13 PM

I just followed Eddies lightbox design but with the bulbs in parallel (I had problems with them in series as in the design) and put a dimmer switch inline on the ac side of the transformer.

Yes I just invert the ota and put the box on the top. It is very light and a snug fit so it doesn't fall off. The diffuser screen I use is perspex that is used in the lighting industry for that particular purpose.

Here are a few piccies and associated data.
The first one is a shot of the finished box.

The next three are a shot just using a lense in a dark room with the box at full setting (very bright) with it's colour histogram and the raw camera data.

The next three are of a shot taken through my 8" with the 300D with a colour histogram from PS and the Raw image data.

The last is of the box sitting on the scope. Works just as well on the ED80.

It should be noted that I checked the consistancy of illumination using a light meter and there was less than 1% difference in reading anywhere over the box. Any gradient that may be seen in the image is an artifact or produced by the scope or lens or response from the camera

btw the box has now been fully taped around the edges. When I took the shot I thought I may have needed to still get into the box to make adjustments.

[1ponders] (Paul) · #20 12-05-2007, 07:30 PM

Remember too that the raw Canon images are only 12 bit. When I convert them to 16bit fits files any variation in illumination differences is increased. Plus the accuracy of the light meter is an unknown with the responsiveness centered on the green. It would be interesting to split the colours and just check the green channel.

I did do a conversion (colour to fits without splitting the channels) and using the image of the whole box illumination and checking the mean value at various points there was a less than 6% variation in intensity from the edge to the center.