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SniffTheGlove

Simple Clarification of Dark,Flats,Bias and Dark Flats)

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Hello,

I have take a few steps into shooting with my 350D and then playing around with the image to get an understand of image processing.

I have had trouble with Registax 6 stacking my shots but I have found DSS much better especially for a newbie to understand.

Part of the DSS procedure to is also use Darks, Flats etc to improve the image.

I have read many pages here on SGL and elsewhere but even though they all talk about the same thing they do differ in the implementation or do not describe how to implement (settings exposure etc ) and just give a brief explanation.

So, I am looking to fully understand and here is what I think but would like help to point out my mistakes or when my understand make be wrong.

Dark

This is taken using the same settings (Exposure and ISO) as your main "Light Frames" but with the telescope capped off so not to allow any light to enter the camera sensor. Take at least 25 exposures.

Flat

This is taken using the same exposure as your main "Light Frames" but with the ISO set at least 100 with the telescope pointed to an evenly illuminated light source (Sky at twilight, Electroluminescent Panel, Light Box). Take at least 25 exposures.

Bias

This is taken using the same ISO as your main "Light Frames" but with the shortest exposure time the camera can handle (350D is 1/4000) with the telescope capped off so not to allow any light to enter the camera sensor. Take at least 25 exposures.

Dark Flat

This is taken using the same settings (Exposure and ISO) as your "Flats (as above)" but with the telescope capped off so not to allow any light to enter the camera sensor. Take at least 25 exposures.

Is this correct?

Once I have this sorted I will write it up and place it into my imaging session "Check List"

Thank you.

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Flat

This is taken using the same exposure as your main "Light Frames" but with the ISO set at least 100 with the telescope pointed to an evenly illuminated light source (Sky at twilight, Electroluminescent Panel, Light Box). Take at least 25 exposures.

Hello.....not quite right here.

Your flats aren't the same exposure length as your lights.

You need to expose so you get between 1/3 and 2/3 of the full well depth of your camera.

Not being a DSLR imager, you'll need to ask someone else what this looks like on the histogram, but on a CCD, I go for between 20 and 30,000 ADU.

Flat darks aren't strictly neccesary, as they are usually of such a short duration that they are almost indistinguishable from bias.

Have a look at the 'primers and tutorials' section of my website for a more detailed explanation.

Cheers

Rob

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Rob, your site was one of the one's I looked at from the primers section and where I got some of the details from but I was still in the "dark" about some of the settings.

I also used Astrophotography a, Jim Solomon's Astrophotography Cookbook and a few others gleam the information and try and piece it all together.

So for flats I need to work out an exposure that will give me and ADU (?) of 33-66% of well depth. According to one of the sites above, the 350D which I use has a

The Canon Digital SLRs (350D included) have a 12-bit A/D converter for each sensor, which means that each sensor will produce a digital number in the range 0–4095 when shooting in RAW mode. The mid-point of this range is 2048, and therefore a Median of 2048 is the target value for the Flat Light exposure

So I need to work out how to get an exposure that will give me a the value of a Median of 2048 which will put me into the 33-66% ADU.

OK, and then to do the Dark Flats I just need to replicate the FLAT exposure/ISO with the telescope capped off to avoid light entry to the camera sensor.

I presume I have understood the BIAS and DARKS correctly.

So for everything else I wrote about Darks, Bias and Dark Flats is correct

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I authored the Astrophotography link provided above.

A couple of points.

If you take flats and calibrate them, in theory you need to subtract darks that are the same exposure as the flats. These are called Flat Darks by the whole world, only the author of DSS calls them Dark Flats.

An easier way to do this as the thermal contribution from a short exposure is not much is to just subtract Bias and forget about Flat Darks.

As to the overall exposure for a flat this tends to tax people quite a lot. You hear about many different methods of assessing the 'workability' of your flats but in truth the best possible answer is to suck it and see. Two or three flats, combined, will do for such a test. Depending on how your camera control software normalises the flat (typically it ignores the edges and corners) it might tend to over or under correct. I have found that with my ST10 a flat average of 25,000ADU (from a max of 65,000) is too high. I have been shooting flats at around the 8-9,000 ADU level and they work better.

There is another school that says you need five thousand flats to make it worthwhile. OK, maybe only 100. Others say the total level (the average value x number of flats) should be at least 500,000; maybe a million. The reason for this is to reduce the added noise.

I use twenty to thirty at a relatively low level (flat average) combined with a median output. This is nowhere near the number of flats a lot of people insist on and nowhere near one million ADU total. The thing is, they work. Also the added noise is not a problem. The actual value can be easily calculated (noise adding in quadrature, see in the link above under Calibration - A Primer) and provided the actual, visually perceived noise in the picture can be dealt with by a mild application of Neat Image there is no need to worry.

I would urge you to shoot three sets of flats, three flats per set and with differing ADU values. Say the equivalent of 10, 20 and 30,000 and calibrate the same single light frame with each flat. Stretch each sub frame identically. (set up an action in PS and run it on the other two). Then look at all three simultaneously. Study the corners and pick the best value. If there is no visible difference use somewhere near the highest value to keep the noise down.

I believe you can do all this when it is cloudy or raining.

Dennis

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Hi Sean - As the 350D doesn't have liveview, you'll need to do a little trial and error for the flats the first time, but when you've taken them, at least then you'll have a rough idea.

When taking flats for the DSLR, right at the end of the imaging run, you leave everything in place (basically don't move the focuser or anything in the light path at all). I see you use a 10" dob, so an EL panel might be a tad expensive - I used to use a white t-shirt over the end of the OTA and then set the camera from Manual to AV mode and left the ISO and initially the aperture settings the same. As AV mode will try to expose for an "averaged" 18% grey, it makes a best shot at it and sets its own shutter speed.

If you're attached to a laptop (ie APT?) you can download it, or at least a JPG and look at it via Camera Raw or your photo editing software... if not attached, you'll need to take the chip out of the camera and then look at it in the software. What you need to find is the RGB histogram of the frame in the software, and although the peaks may well be shifted, you're looking for them to more or less about 1/3 the way over from the left hand side...

Here's an example of one of mine (taken at random 40D, MN190, ISO 800) - It's not a brilliant example, but it sort of gives you the idea as you can see some slight darking (vignetting) towards the corners (NOTE: The reason it's blue is that I use an Astronomik EOS clip filter ;) - This was taken at 1/60s, but obviously that will change depending on light conditions)

flat.jpg

Once you have the settings sorted, you then switch back to Manual and set the shutter speed and aperture to the same as the AV setting and take c. 25... and after that, take the T-shirt off, put the scope cover on and take another 25 (Flat Darks) - Job done(fingers crossed!)

EDIT - Ah... I should have known that Dennis ("the callibration guru") would spot this thread - I bow to his far greater knowledge!!!

Edited by AndyUK

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Thanks Andy, a nice explanation and also thanks for that screen grab, really helps to understand.

Yes, I use a 10inDob, and use APT to do the camera control with the Canon350D.

Now, I can practise this as Dennis said when it cloudy or raining (which seems to every day/night for months) and get to grips with it.

I am trying to write myself a Imaging Check List so I can make sure I get everything done in a correct and professional way from placing the 350D into the focuser to applying the final finishing touches in Photoshop.

I am glad there are way more knowledgeable people out here in SGL

Thanks

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