SharpCap Forums

Hi,

I would like to hear your suggestions on the apply Darks and Flats during imaging.
The manual states, "When applying a flat to light frames, a dark should also be applied.
Applying only the flat without the dark can result in overcorrection."
So, what happens if we apply Hot & Cold Pixel Removal feature instead of the master dark, and simultaneously apply the master flat?

I actually tried this, and it seemed that vignetting was resolved and there was no overcorrection. ”At least to my eyes...”
I would like to hear the opinions of those with more experience. What differences might there be between these two approaches?
1. Master Dark & Master Flat
2. Hot/Cold Pixel Removal & Master Flat

Background of the Question:

・I aim to balance EAA and imaging, so spending time capturing darks feels wasteful.
・I frequently adjust exposure and gain, making it difficult to prepare master darks in advance.
・The noise level of my camera (533) is not significant, and the exposure time per frame is at most 120 seconds.
➡ I want to operate by applying only the master flat.

Thanks in advance,
Takuya

Hi Takuya,

flat correction is almost never 100% perfect - if you stretch the resulting image enough you will usually find some hint of either under or overcorrection in the corners or where there are dust shadows. Using a dark as well as a flat frame is listed as best practice because it gives you the best chance of getting high quality flat correction and therefore being able to apply a fairly strong stretch without noticing any remaining problems.

If you do decide not to use a dark frame then SharpCap does its best to cope by subtracting the bias level out of the light frames before applying flat correction (the bias level is calculated as part of the flat capture process and is stored in the filename of the captured flat frame). This will often give you decent results - certainly it should be fine for solar system imaging where you are trying to deal with dust spots and are not likely to stretch the images. It will also work to some degree for deep sky imaging, but - on average - this approach will start to show imperfections at smaller amounts of image stretching than using dark frames.

cheers,

Robin

Hi Robin,

Thank you for your prompt response(^^)
I see. I'll plan to closely examine the effects of flat correction by comparing images with applied dark frames and those with Hot & Cold Pixel Removal.
・Compare the details by zooming in.
・Compare how well they withstand stretching.

…Here is a theoretical question:
Since dark subtraction only subtracts what is captured in the dark frame (hot noise) from the light frame, I believe the negative impact on flat correction from not applying dark frames is caused by the hot noise remaining in the light frame.
The reason for overcorrection in flat correction is due to the influence of extremely bright pixels (hot noise), which increases the brightness difference between the bright parts and the surrounding dark parts of the image.
If this understanding is correct, then if Hot & Cold Pixel Removal can sufficiently eliminate the noise, it should also be beneficial for proper flat correction.
What do you think about this?

Thanks in advance,
Takuya

Hi,

there are three components that need to be subtracted from light frames for flat correction to work correctly - in approximate order of importance:

* The bias level of the camera - that is the signal level that is produced for zero light input with a short exposure. This is almost never zero, since trying to set this to zero introduces other problems with some pixels being clipped to zero value when they should really have negative values due to random fluctuations.

* Thermal noise of the camera accumulated over the length of the light frame exposure (this can be more significant for uncooled cameras and/or CCD cameras which tended to need longer exposures and have higher thermal noise rates)

* Hot/warm pixels - these are least important as they only affect a very small percentage of pixels so don't tend to give a problem with noticeable over/under correction over a larger area of the image.

cheers,

Robin

Hi Robin,

Thank you for your response.
I understand well now. Based on this reasoning, it seems that using Hot & Cold Pixel Removal along with applying the master flat would also have a reasonable effect. I'll continue experimenting further.

Let me confirm just one more thing. If I don’t use the master dark, is it correct to understand that, when creating a master flat, it is better to choose the option to subtract the bias rather than the option to subtract the darkflat?

Thanks in advance,
Takuya

Hi,

the choice between those options really depends on the exposure time you are using for flat frames - if you are taking short exposure flat frames (less than 0.5s maybe) then bias subtraction should be sufficient. In some circumstances (narrow band filters) you need to take longer exposure flats of several seconds, and then the 'dark flat' option may be better.

Hope this helps,

Robin

Hi,

My understanding of flats has deepened.
Always very grateful for your prompt support.
-& I apologize for asking so many basic questions.

Thanks a lot,
Takuya