StarTools

Hi!

I have a problem taking flats with my modded DSLR (Canon 1000D): using the white t-shirt method results in flats which have a reddish tone. As a consequence, the peaks of the R, G, and B histograms are at fairly different locations and if I keep the red from saturating, the blue is underexposed. I was planning of using a "blue" t-shirt method, but I am not so convinced: am I doing anything wrong? How would you address this issue?

Thanks!

Nico

Have you tried a white laptop screen with a sheet or two of printer paper as a filter. I find this takes reasonable flats, aiming for 70 - 75% camera histogram with a shutter speed of 1/8 to 1/4 second.

Hi Rowland,

no, I didn't try with a laptop: I have a C8 and my laptop screen is barely enough for covering its aperture, but I could try. Moreover, I could also display a colored background to correct for the enhanced red sensitivity of my modded camera... let's see!

Thanks,

Nico

I think I read somewhere that it does not matter if flats have a color cast. If it did , wouldn't taking sky flats cause terrible trouble? Not sure where I read it. Could be wrong
Che
Found it, from DSS manual:
Flat Frames
The Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in your optical train.

To create good flat frames it is very important to not remove your camera from your telescope before taking them (including not changing the focus).
You can use a lot of different methods (including using a flatbox) but I found that the simplest way is to put a white T shirt in front of your telescope and smooth out the folds. Then shoot something luminous (a flash, a bright white light, the sky at dawn...) and let the camera decide of the exposure time (Av mode),

The flat frames should be created with the ISO speed of the light frames. The temperature is not important.

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master flat frame.

Is it possible to use colored flat frames?
The short answer is yes.
The overall tint of your flat frames is not really important because DeepSkyStacker is processing each channel separately and is applying the flat frames to each channel accordingly.
Of course if your flat frames are fully red only the red channel of your light frames will be properly calibrated but otherwise as long as as the peak in each channel is between 1/3 and 2/3 of the maximum you should have no problem using slightly colored flat frames.

Thanks so much for this post Rowland!

Flat calibration is not done in colour (i.e. pre-deyabering or pre-(L)RGB combining), so colour casts are irrelevant -- except in the sense that if using separate RGB filters, a different flat will be needed for each filter, due to different dust mote patterns different transmissibility.

This has been my experience over 5 years of working with a Canon 1000D, unmodded, modded, uncooled and cooled - right now, cooled and modded. There are times even now when I don't get it right, because it is easy to get it wrong. Trouble, trouble, toil and trouble - if you don't get it right - hours of unproductive processing.

If you have AstroArt, the procedure below is easy to set up and do in one step. If you are using Pixinsight or Nebulosity, several steps - result = same. Recommended reading, Craig Starks work on DSLR non-linearity.

Some programs will allow you to calibrate all three channels - this is a waste of memory and time. It is easier to convert all frames to greyscale and calibrate. When the light frames are debayered, they convert to RGB, which is what you would expect.

If you want consistently good results with a DSLR, dithering is essential - the more the better. Dithering a DSLR is a different technique to dithering a scientific quality CCD. Spread the image broadly over the sensor - move each frame by ~12 - 15 pixels and not by 0.5 or 2.5, as described in some CCD literature.

DSLR data is modified in camera by the manufacturers software/firmware, as such, DSLR data cannot be likened to CCD data, because DSLR data is not linear. Therefore, a different approach is required for calibration, as well. The following frames and the order of application is important for good results;

Lights - of course
Darks to match lights - time and temperature +/-5C. The less the temperature difference the better - that's why I cool my camera.
Bias - lots and lots and lots
Flats - any evenly illuminated light source will do. The exposure should place the liveview luminance histogram at approx 70 - 75%.

Create a master bias and master dark by stacking each without pixel rejection - the camera tidys up the RAW image significantly, and it cannot be considered linear.

Calibrate the flats with the master bias. Stack using the correct parameters for flats - noise evaluation, multiplicative, equalise fluxes and a bit of pixel rejection (PI), or whatever your software does. Note: I prefer not to apply pixel rejection whatsoever to flats; EDIT: Actually, I have recently found pixel rejection to be advisable when integrating flat frames.

Next - Calibrate the lights with the master dark and master flat only - don't touch the lights with the master bias - the bias is in the dark. The bias may/will truncate your data if used on the lights. Debayer the light frames, register/align and stack/integrate.

You should have produced a silky smooth high SNR image, which will process very easily in StarTools using the basic flow.

This is about as detailed as it gets. As long as care is taken with dark frames, time and temperature, and the flats are correctly exposed, and the frames are dithered, the rest is straight forward.

Rowland,

I read somewhere that for DSLR bias frames are already added to every image, so that would mean that lights, darks (as you said) and flats already have bias images added to them, so there is no need to use them separately in stacking. Can you explain why you use a master bias with the flats if they already have the bias from the time of the camera taking the image?

Paul

Having unfortunately just done exhaustive research in this area, the color of the flats don't matter in DSS. If you look at your master flat DSS is converting to greyscale and processing. I did find that my Atik flats had some artifacts when the exposure was under about 1/10th of a second and that if you don't get all the histograms far enough off the left wall you get extensive noise.

asterix2020 wrote:Rowland,

I read somewhere that for DSLR bias frames are already added to every image, so that would mean that lights, darks (as you said) and flats already have bias images added to them, so there is no need to use them separately in stacking. Can you explain why you use a master bias with the flats if they already have the bias from the time of the camera taking the image?

Paul

Hi Paul. Sorry, missed your post.

I think you may be referring to the pedestal that people quote as being added to the RAW data to avoid -ve pixel values. Camera manufacturers add a pedestal to RAW data to keep pixel values +ve, but manufacturers also manipulate the data such that it is not linear and this is a problem.

Bias is noise and better removed...

The master bias is used to remove/subtract bias from the flat frames, otherwise the bias in the flat will add noise to the lights when they are divided by the flats.

To remove the bias from the light frames, do so with the dark frame by leaving the bias-in-the-dark. This is a more reliable process and avoids data truncation, as opposed to subtracting the bias separately from the dark and light frames as you would when subtracting the bias from 16bit linear data, with the intention of scaling the data.

I don't get a lot of traction with this, but recommend DLSR imagers give it a try. A lot of processing difficulties originate from less than optimal preprocessing, truncating the data. If you cant Develop the image in ST it is likely that the image is short of data.

Edit: to avoid introducing terms that amount to splitting hairs, amended to refer to bias.