flats and Bias frames

[sinbad40]

Hi all,

My dark frames are the same temp, time and gain as what is used to capture the image, so those are fine.

I have tried a few different timings for light/dark flat files, but they always seem to make the image worse when stacking.  I have tried doing 1/10th second up to 4 seconds (about to do a new set due to dust pixies)  What times  do other people tend to use?  This is with with a ONC 533.  I take the light flat frames with the same filter that was used to capture the image file, and the same gain.

I find that when i use them in stacking, i am getting heavy light edges on the final image, when i don't use them, the image is a lot better, but i want to get them right.  I know i can fix them later in editing (still getting to grips with that as well) but i think if i can get the calibration files right, i would have less work to do to get it looking right.

I am using a light screen to gather them, so not sure if that could be the cause

 

Thanks for any pointers.

[tomato]

I would aim for around a 3-5 second exposure adjusting the brightness of the light panel to achieve the histogram peak around 2/3 along the horizontal axis.

You will likely get some edge effects when stacking large numbers of subs, but these should only affect the edges, not encroach very far into the image.

[alacant]

35 minutes ago, sinbad40 said:

i am getting heavy light edges on the final image

Hi

You don't mention how you deal with any offset, but your description is consistent with incorrect or absent bias subtraction.

The easiest way is to deal with this is to simply subtract a constant value corresponding to the offset you used. The more laborious, applying dark flat frames.

Cheers and HTH

Edited December 10, 2022 by alacant

[ONIKKINEN]

If you have overcorrection (bright corners) something is wrong. Either offset is different or offset is incorrectly removed from your lights and flats. The second happens when you have light leaks, so maybe look into that. Your darks and darkflats should be within a couple of ADUs of a bias frame taken in complete darkness.

Short flats are fine with the new CMOS cameras. I have used 50ms->205ms flats and never had bad calibration because of that. If your panel gives you 1/10s flats, just go with that and dont bother adding extra dimming to the mix.

[sinbad40]

1 hour ago, alacant said:

Hi

You don't mention how you deal with any offset, but your description is consistent with incorrect or absent bias subtraction.

The easiest way is to deal with this is to simply subtract a constant value corresponding to the offset you used. The more laborious, applying dark flat frames.

Cheers and HTH

Looking at how i did the last set of light flats, I didn't have the capture and subtract bias frames ticked in sharpcap, will give that a try next.

One more thing, the dark flats should have the same settings as the light flats?

Thanks all for the info.

Edited December 10, 2022 by sinbad40

[scotty38]

Are you using bias and flat darks? If so then that is most likely the cause of your over correction

[sinbad40]

9 minutes ago, scotty38 said:

Are you using bias and flat darks? If so then that is most likely the cause of your over correction

No, I don't use bias, i was using Darks, flat darks and flat lights.  One more thing i am thinking of is,  if PI, even though i set the flat darks in as bias frames when doing a stack, they may not be marked as that, as i need to use add custom and tell it what they are.

[iantaylor2uk]

I know some people say flat frames should be a few seconds exposure but I have found with my ZWO 071 camera they are usually only around 50 milliseconds or so with a UV/IR filter, and around 300 milliseconds with an L-Enhance filter. This is with a flat light pad over the top of the telescope. I tend to use bias frames rather than flat darks, and these are usually taken at the shortest exposure your camera has, with a cover over the telescope. You can re- use the bias frames from one session to another.

[scotty38]

49 minutes ago, sinbad40 said:

No, I don't use bias, i was using Darks, flat darks and flat lights.  One more thing i am thinking of is,  if PI, even though i set the flat darks in as bias frames when doing a stack, they may not be marked as that, as i need to use add custom and tell it what they are.

They need to go in the Darks section

[Chefgage]

18 hours ago, iantaylor2uk said:

I know some people say flat frames should be a few seconds exposure but I have found with my ZWO 071 camera they are usually only around 50 milliseconds or so with a UV/IR filter, and around 300 milliseconds with an L-Enhance filter. This is with a flat light pad over the top of the telescope. I tend to use bias frames rather than flat darks, and these are usually taken at the shortest exposure your camera has, with a cover over the telescope. You can re- use the bias frames from one session to another.

I use a zwo asi294mc pro camera and agree with what you say. I know some people say aim for 3 seconds for the flat darks as that will give you the correct flat dark. But I always use 0.4 seconds with the flat led panel on a low setting.

[AMcD]

I have been having an infuriating time with flats overcorrection on my Ha frames.  Having spent the weekend trawling the forums for solutions I set out below what I have learnt, in case it is any use.  I am still working my way through the suggested fixes.  

Others more than experienced than I may have views on the accuracy of some of this, but appeared to be a pretty consistent view across SGL, CN and Astrobin regarding the problem and potential solutions:

Potential Causes:

• A failure to bias subtract or flat-dark subtract the flat frames.  
• Differential temperature and / or gain between the frames.
• Differential offsets between the frames.
• Using bias frames with a CMOS camera rather than flat darks.
• Variability in temperature of CMOS chip when taking very short flat frames.
• Differential vignetting profile in the flats and the lights.
• Brightness / distance of flat light source.
• ADU of flats not in linear range of camera.
• Light leak resulting in light in the flats that does not come from objective lens of the telescope.
• Reflecting parts (e.g. extension tubes) in the optical train.

Solutions (root cause):

• Ensure flat frames are bias or flat dark subtracted.
• Do not use bias frames for CMOS cameras, just dark flat frames.
• Use frames all at same temperature, gain and offset.  
• Increase / decrease ADU to ensure it is in linear range of camera.
• For CMOS cameras ‘flush’ the chip by adding a delay of 15s between flat exposures, especially where flat exposures are very short.
• Do not change system elements / orientation between lights and flat frame acquisition.
• Adjust brightness / distance of flat light source.
• For CMOS take flats of at least 3-5 seconds to ensure linear data.
• check for light leaks.
• Check for internal reflections and light leaks.

Solutions (mathematical):

• Add a pedestal in PixInsight WBPP (for example 10 or use Auto).
• Add a pedestal to the master flat using PixelMath with the formula $T+0.xx with a constant value, for example 0.43.
• Subtract a fixed value from flats to apply an offset to the flats.
• For flats with a very high dynamic range, measure the contrasts by taking values manually on the flat, at the center and at one of the darkest corner and the same areas in a Light. Derive the contrast ratio in both images and computed the ratio between them .  Then, with Pixelmath, tweak the contrast using (MasterFlat - mean(MasterFlat))/(2*1.7) + mean(MasterFlat).

[iantaylor2uk]

I think some of the above "solutions" depend on the type of CMOS camera used. 

I use a ZWO 071 camera and have no problem with much shorter flats than 3-5 seconds, and simply use bias frames rather than dark flats, and don't have the problems you seem to be having.