Flats & Calibration question with stubborn dust mote

[mcbbcn]

Hi,

This is my 2nd newbie question today.

My flats have a dust mote that is so black (in compare with some of the other grayish dust motes) that the calibration process is having a challenge removing it completely during calibration, and it's leaving a light shadow of the dust mote in the subs, which later on become an issue during color balance.  Is there a calibration setting that can help remove completely the dust footprint?

By the way, when I super-impose my master flat with my luminance, the dust shadow from the luminance perfectly aligns with the shadow in the luminance, and all the other dust rings are corrected, it's just this one.

Also, when I create my flats, I use an illuminated panel for consistent lighting.  During the Master Flat creation process, I used Winsorized sigma clipping (as per Pixinsight tutorial when using light boxes) with values of Sigma Low 4 & Sigma High 2.

Thank you so much for your help,

Miquel

[chris.bailey]

I'm no expert but have you tried a slightly more permissive sigma limit for the flats. I use 4 and 3 with flats from an EL panel.

[zvrastil]

Hi Miguel,

I'd also suggest to inspect the pixel values of the dark dust mote on the master flat (or simply use Statistics process to find out master flat minimum value). If they're too low (close to zero), you might need longer exposure for your flats. Flat correction is done by dividing your light frames with your flat frame. Division with very low (dark) values from master flat can lead to lower precision of the correction for these particular pixels.

regards, Zbynek

 

[Lex]

Hi,

I also had this phenomena, the only solving issue if found was applying an other logarithm than WSC...
I found that median worked fine in my case..

Best

Lex

[mcbbcn]

Great information.  I'm going to try tonight & see if I can resolve this issue one way or another.

Thanks a lot for all your help,

Miquel

[mcbbcn]

Hi,

I tried several combinations to create a master flat, but they did not remove the dust mote.

Then - although I'm still learning & I don't know very well what I'm doing - I followed Zbynek's directions and I applied the statistical process to find the minimum pixel value for the black dust mote and it came back with 2.870764e-001 which should be 0.2870764.  I also took the minimum pixel value of the area in the luminance and it came back 5.2537e-002 which should be 0.052537.

I've uploaded my Luminance_Master_Flat to my public drop box folder, so if you would like to take a look to it.  When you look at the flat, imagine a 12 hour clock, and you'll see the stubborn black mote between 7-8pm.  Also, I've uploaded one of my luminance subs, so you can see how the dust aligns perfectly, and it actually, it's able to remove all the dust rings from all other places but this one.   In addition, I've uploaded my Master Bias and Master Dark in case you want to try to calibrate it. 

Here you have all the files:

Luminance sub: http://dl.dropbox.com/u/8971416/Luminance_20110928_150s_M31_-15degC_2138_000000182bck.fit

Master Flat: http://dl.dropbox.com/u/8971416/Median_Master_Flat.fit

Master Bias: http://dl.dropbox.com/u/8971416/QHY9_Master_Bias_0s_minus15c.fit

Master Dark: http://dl.dropbox.com/u/8971416/QHY9_Master_Dark_150s_minus15c.fit

Thanks for your help,

Miquel

[Lex]

Hi Miquel,

I've downloaded your calibration images and also the target one,
I followed the ImageCalibration routine with no special setting with a proper result..
The Calibration took place and removed all "dust traces"...

I additionally ran ABE to check if there where some errors in the image but no...

Here you are

Best

Lex

[chris.bailey]

Lex,

You beat me too it. I got the same result, I wish my flats were as effective all the time.

Chris

[mcbbcn]

Alex and Chris,

Thanks so much for giving it a shot and I'm quite impress.  I'm going to paste a couple of things, so you can tell me what I'm doing differently.

Here are my calibration settings & what I'm seeing on my FIT image after I apply automatic STF.  What I circle in the M31 image is a shade that becomes color imbalance later on.

I'm wondering why your M31jpeg look a lot smoother than mine, but most likely I'm doing something wrong.

Thanks for your help,

Miquel

[zvrastil]

Hi Miguel,

are your master_dark and master_flat pre-calibrated? In other words, have you subtracted at least master_bias from them? If not, then check "Calibrate" option for both Dark and Flat frames in the ImageCalibration. Then, the process will do it for you. Especially for flat, subtracting bias (and dark in case it's long-exposure flat) is crucial for the calibration to be mathematically correct.

regards, Zbynek

[Lex]

Miguel,

It is as Zbynek says, you have to activate all the calibration cases for a propper endresult...
If the Dustparticle is in a region of the targetimage with higher pixelvalues, there can be one ore more calibration cases staying unselected to avoid the negative result of the calibration..

Cheers

Lex

[mcbbcn]

Hi Lex & Zbynek,

Thank you so much.  I'm glad we figured out what I was doing differently.  I can't wait to try it tonight and see the result.

I appreciate everybody's help.

Miquel

[Lex]

Miquel,

That's why this forum is simply great, you always find an appropriate answer 

You're absolutely welcome!

Cheers

Lex

[mcbbcn]

Hi Lex,

You're totally correct.  This forum is very awesome.


...and...YESSSSSSSSSSS! I just calibrated my sub following the instructions, and it looks beautiful.  I'm sooo happy!   

Thank you for everything!!!

Miquel

[Lex]

Hehe,

Cool

Cheers

Lex