Question about my Flats approach

[STEVE333]

I have a Canon DSLR (modified) and take all images with camera RAW (CR2).

• After taking many Flat frames I load them into the ImageCalibration process (see first attachment) and calibrate them.
• I load the calibrated frames into the ImageIntegration process (see second attachment) and integrate them into my MasterFlat.

The problem is that if I DeBayer the MasterFlat it doesn't look anything like the original Flat frames. In fact it looks almost black unless I use the STF to expand it.

Am I doing something(s) wrong?

PS: See my second Post for the second attachment. Sorry.

Thanks in advance for any help.

Steve

[STEVE333]

OOPS!!

Below is the second attachment.

Steve

[pfile]

i merged your two topics...

[STEVE333]

Thanks

[STEVE333]

I found a tutorial on creating Flats for a DSLR in another post here on the forum (see link to tutorial below). The tutorial helped me to see that I was way underexposing my Flats. I was using the Histogram display in BYEOS to estimate my exposures. That turned out to be a bad idea. That Histogram is quite inaccurate and greatly overestimates the exposure levels.

http://www.myastroscience.com/proper-flats-with-dslr

Following the tutorial allowed the actual counts of the flat frame to be determined. My Flats now have at least three times higher ADU counts than in the past thus reducing the noise in the Master Flat frame. The underexposure of my previous Flats accounts for their appearing so dim.

As an aside, I am now using the new Triad filter. It has a 3nm bandwidth at the Ha wavelength and an 18nm bandwidth at the Oiii wavelength. Thus, the G and B channels of the DSLR have much higher signal than the R channel when using an LED Drawing/Tracing tablet as the light source. To help correct this I purchased some "light pink" cellophane, folded it in half to doubled the effect, and placed it between the telescope and the LED tablet. This reduced the G and B channels and made them only about twice as bright as the R channel.
I took a test frame then DeBayered it and looked at the Mean counts in the R/G/B channels to make sure none were saturated. The exposure was then adjusted to raise the G/B channels while making sure they weren't saturated. The Mean R/G/B channel counts are now 3555/9608/8333. I may increase the exposure time a bit more to boost the R channel even more (along with the G and B channels of course). This has been a good learning experience for me.

Hope this is helpful for someone else as well.

Still looking forward to my first chance to use the Triad filter (shen rain and clouds leave).

Steve

[pfile]

it's just that the histogram in BYE mimics what a canon DSLR would show back-of-camera, that is, it's a histogram of the image with default stretch / gamma curve. this may have been a conscious choice on guylain's part, since there's a rule of thumb in DSLR astrophotography that the back-of-camera histogram should be "well-detached" from the left side of the histogram graph axis when shooting lights. since people rely on that it makes sense for BYE to display that instead of the histogram of the linear image (which is what's important for flats.)

the pink cellophane is a good idea. it's not important that the flat be grey but it is important that no one channel is terribly underexposed.

rob

[STEVE333]

it's just that the histogram in BYE mimics what a canon DSLR would show back-of-camera, that is, it's a histogram of the image with default stretch / gamma curve. this may have been a conscious choice on guylain's part, since there's a rule of thumb in DSLR astrophotography that the back-of-camera histogram should be "well-detached" from the left side of the histogram graph axis when shooting lights. since people rely on that it makes sense for BYE to display that instead of the histogram of the linear image (which is what's important for flats.)

the pink cellophane is a good idea. it's not important that the flat be grey but it is important that no one channel is terribly underexposed.

rob

Thanks for the clear explanation Rob - Makes sense, but, definitely messed up my old Flats.

Steve