Calibrate NB Light Frames has Hot Pixels

[pmarshall]

I am following Inside PixInsight to process LRGB. After Light Frame Calibration, the images are free of Hot Pixels and mots are mostly gone.

But when I process NB (Ha-O3-S2) the same way (same work flow and settings), these Light Frame Calibrations have Hot Pixels.

I have double and triple checked my settings in each of the process steps.
Blink (sunset-flats, lights), Sub-Frame, Master Bias/Dark, Calibrate Flats, Calibrate Lights.

It seems to me that I should be able to use the same settings and processes with both LRGB and NB, but am getting Hot Pixels in  the NB images after Light Frame Calibration.

By the way, the Hot Pixels in the calibrated NB light image matches the Hot Pixels in individual Dark Frames as will has the Master Dark, which is 900 seconds.

This has been happening with other NB projects as will.

I must not be understanding something.
Any insights would be helpful.

Clear Skies to All; Phillip Marshall

[pfile]

what is the duration of the LRGB lights vs. the duration of the NB lights, and what is the duration of the darks?

rob

[pmarshall]

hi Rob;
Exposure for RGB is 900 sec., L is 480 sec and NB is 900 sec. Flats for BB and NB were sunset flats ranging from 1 to 18 seconds. With MaximDL, I take Flat Darks to match the Light Flats. I think PI does not require Flat Darks for calibrating the flats.

I did look at some earlier work with NB and found that most Hot Pixels were removed from the Ha Calibrated Lights.

My present work is with the Horse Head Nebula and Flame Nebula imaged back in Jan. 2016, before knowing about PI.

I know that the CCD ADU is higher for BB than for NB filters.

Since early this year, I am using a light box to generate Flats. This way I do not get star trails and have equal exposures for each filter, wither BB or NB.

Thanks for your toughts....Phillip

[pfile]

whether or not to use flat darks is up to you... with 18s and depending on the camera, you may need the flat darks. but a master bias could be OK.

but my question is, how long are the darks you are using to calibrate the NB images? for the most part, people take darks that are longer or as long as their longest subexposure time and then scale them down, not up.

rob

[pmarshall]

oh, sorry;
Forgot your original question.
Light exposures for this particular project were 900 seconds.
Also the Darks were 900 seconds.

Flats ranged from 1 to 18 seconds were sunset flats for this Jan 2016 project. Now I am using a light box, so my exposures are fixed for each filter.

I do use Bias frames to make Master Bias and have used Super Bias, though I am not sure of the benefit.

My setup for this particular project is;
Tak TOA-130 NS, 5 inch refractor configured as f/5.8 for a nearly two degree FOV.
QSI 583WSG-8 ccd camera and auto guide using an SBIG St-I
Have backyard roll off roof observatory. About 12 miles east of San Diego, CA

I did not understand your statement about "then scale them down". How do I do this?

Thanks; Phil

[pfile]

by default, ImageCalibration is set up to scale darks. but in order for this to work, the master dark has to be bias subtracted. i was just curious about what the dark duration was like, because if it was shorter than the lights then it would be scaled up, which generally people don't do.

the way IC scales darks is to iterate over different scaling factors and subtract the dark from the light, then measure the noise in the resultant calibration. when it finds the scaling factor that results in the lowest noise, it then scales the dark by that factor and subtracts it from the light. this factor is almost always < 1.0. this is equivalent to using a dark that's of less duration than the lights. however, this can lead to undercorrection of hot pixels because hot pixels don't scale with exposure time.

one thing you can try is to turn off "optimize" in the dark section of ImageCalibration. then PI will do a straight subtraction. the hot pixels may disappear from the result, but the calibrated frame may have more noise than it otherwise would.

generally speaking, hot pixels in calibrated frames are usually inconsequential. you can clean them up with CosmeticCorrection, but most commonly (as long as you have dithered during aquisition) the hot pixels can be easily rejected during ImageIntegration of the lights.

rob

[pmarshall]

Hi Rob;
You have explained it clearly
Some of which I did not know.

I will take it from here.

Clear Skies;  Phil Marshall