Use only flat correction: impossible??

[ippiu]

While i was experimenting out the effectiveness of dark and flat, i found that using only flat frame caused an error.
If i want to apply just flat frame correction (and not dark frame correction), it isn't possible.
I show you my 3 final stacked image examples

Dark, no flat. Yes

dark_noflat.jpg (317.86 KiB) Viewed 1789 times

Dark, flat. Yes

dark_flat.jpg (373.4 KiB) Viewed 1789 times

No dark, flat. No

nodark_flat.jpg (87.14 KiB) Viewed 1789 times

A screenshot of live stacking session
https://drive.google.com/open?id=14k1p0 ... vdvYKFBrX-

So, why isn't possible to use just flat frame correction (without dark frame too)?

[admin]

Hi,

Flat correction without a dark frame is really limited to solar/lunar/planetary imaging where you are removing the effects of any dust on the sensor and you are not applying a strong image stretch.

If you tried to image with just the flat frame when applying a strong nonlinear stretch for deep sky imaging, you tend to get sort of results you have seen since you are not correcting for the black point offset in the image frame which means that the flat correction tends to make rather a mess of the background.

If your exposures aren't too long then using the subtract bias option in the capture flat dialogue will help as it will ensure that the black point offset is corrected for, but it's still not as good as subtracting a proper dark frame which will also remove the majority of the effect of any thermal noise, hot pixels, et cetera.

Cheers, Robin

[ippiu]

Thanks for exhaustive explanation.

So is that sharpcap effect normal if i use only flat correction? I didn't do any sort of stretching histogram, just use flat correction, as you can see in the live stacking screenshot here

nodark_flat_screenshot.jpg (101.84 KiB) Viewed 1767 times

So from your explanation, it isn't useful to use just flat frame correction and if i try to use just flat without dark, sharpcap behaves like you can see in the above screenshot: have i understood correctly?

So would the answer to my topic title "Use only flat correction: impossible??" be yes?? (for deep sky imaging)

[admin]

Hi,

You certainly have a bit of a stretch set up in the life stacking histogram which is visible in your screenshot. Basically flats only work correctly if the image has no offset (dark pixels have a value of zero) or if corrections can be applied to allow the calculations to be performed as if the dark pixels had a value of zero.

Applying flats to solar/lunar/planetary is a bit of a special case because the brightness of the image tends to be close to the brightness of the flat frame and errors due to black level offset tend to cancel out enough not to be noticeable.

So the answer is that applying only flat frame correction without dark subtraction is inadvisable for deep sky imaging except under very special circumstances.

Cheers, Robin

[ippiu]

Thanks robin.

Tonight flats correction applied correctly without dark, so is possible to use just flats without darks.
Basically it isn't correct as you explained, but tonight sharpczp accepted only flats correction without dark. No strange behaviour like in my image posted above.
But tonight i use darks as usual and flats weren't so useful as other nights even if i took them in the same way as i usually do (darks are the same because i have a library): tonight flats didn't correct perfectly the final stacked image.... i can't explain why...

Here 2 examples

m87_markarian's chain_22X660s=660s_gain300.jpg (142.11 KiB) Viewed 1740 times

Anonimo.jpg (159 KiB) Viewed 1740 times