Avoiding flat overcorrection

[anirban]

I want to share with you the steps that I am using to avoid flat field overcorrection. This may be useful for people who are using LED panels or T-shirt method and observing overcorrected images.

Here is an integrated image using WBPP where the flat field correction was not proper. I had taken 20 flat images using a LED panel laying on top of the dew shield. The overcorrection is evident on the sides.

When I analyzed the master flat image with a regular sub, I observed that the light fall-off on the flat image is much higher than that in the light frame. As seen below, the illumination drops by 25% (~0.5 to ~0.4) to the edges in the master flat compared to about 10% (~0.064 to ~0.058) drop in the light frame.

A good tool to normalize the master flat in this situation is the FourierTransform tool. I created the magnitude and phase components from the master flat using FourierTransform.

The objective here is to reduce the amplitude difference, so that we get a illumination drop-off comparable to the light frames. I used the HistogramTransformation process to pull down the midtones in the Amplitude component. The phase components shouldn't be touched, as it can alter the image features.

Now a new master flat image can be reconstructed using InverseFourierTransform process using the modified Amplitude and the original Phase components.

With few iterations of trial and error (of fixing the amplitude midtone), a flat field representing appropriate illumination characteristics can be obtained. A test calibration can be quickly tested by the following PixelMath operation.

($T-master_dark)*mean(new_master_flat)/(new_master_flat)

This is a quick comparison between a previously overcorrected calibrated image and the new calibrated image.

Hope you find this useful.

Thanks,
Anirban

 

[pproulx]

In the reconstruction using InverseFourierTransform does is the modified amplitude image the First DFT component and the original Phase component the Second DFT component? Does the order matter? Is 'Truncate' chosen? The images in that step are pixelated. Thanks - Pete

 

[anirban]

In the reconstruction using InverseFourierTransform does is the modified amplitude image the First DFT component and the original Phase component the Second DFT component? Does the order matter? Is 'Truncate' chosen? The images in that step are pixelated. Thanks - Pete

The order for InverseFourierTransform expects the first component to be Magnitude. Appropriate FITS keywords are added to the images during the FourierTransform process. I don't think 'Truncate' will matter here (since flats are generally targeted around middle of the histogram).

My apologies for the screenshots,.

 

[pfile]

while it is possible your flats just don't match right, the most common source of flat overcorrection is improper calibration of the flats or the lights. what did you use to calibrate your flats?

rob

 

[anirban]

while it is possible your flats just don't match right, the most common source of flat overcorrection is improper calibration of the flats or the lights. what did you use to calibrate your flats?

rob

Hi Rob,

I just used Flats and corresponding FlatDarks for calibrating the flats using WBPP.

Thanks,
Anirban

 

[pfile]

between the flats and flat darks, were there different capture software versions? different driver versions? was the gain set the same?

how about the light calibration?

your method is certainly novel and quite clever, it's just that such heroic measures really shouldn't be necessary. usually there is some problem with calibration that when resolved, fixes the overcorrection.

rob

 

[anirban]

Everything was proper as I can recall. I suspect the LED panel was not evenly illuminated for a Celestron Edge HD11.

Thanks,
Anirban

 

[pfile]

that is possible - maybe it's just not big enough for an 11" aperture?

rob

 

[ngc1535]

Anirban,

Although you show, I think, a calibrated single frame...small errors will still add up when integrating many frames. Did you integrate many images and find the result still satisfactory for this example?

As Rob noted, this seems to be a processing solution for a acquisition problem. As you are probably aware (I made a video on this), this kind of over/undercorrection is typically due to a constant/pedestal value in the mix somewhere. For example, if the incorrect bias level is used to calibrate the flat image this is exactly what the result looks like.

-adam

 

[anirban]

Hi Adam,

I agree to both you and Rob that is definitely an acquisition problem. I have seen your video. I will try out the suggestions from there as well.

The integrated image had little amount of vignetting, which was easily removed with DBE.

As I read your note again, I think you are right that the bias might have some contribution as well. For this scope (Edge HD 11), I was using a regular LED panel (which was pretty bright) and the flat exposures were sub-seconds (~100ms). The camera's (ASI294MM) bias response for small exposures might be different than the longer exposures.

Thanks,
Anirban

 

[Abdur]

Hi Adam,

I agree to both you and Rob that is definitely an acquisition problem. I have seen your video. I will try out the suggestions from there as well.

The integrated image had little amount of vignetting, which was easily removed with DBE.

As I read your note again, I think you are right that the bias might have some contribution as well. For this scope (Edge HD 11), I was using a regular LED panel (which was pretty bright) and the flat exposures were sub-seconds (~100ms). The camera's (ASI294MM) bias response for small exposures might be different than the longer exposures.

Thanks,
Anirban

Hi Anirban,
My flats have the exact same issue with the C11 EdgeHD in hyperstar using an ASI1600MM and a cheap LED panel. Did you ever manage to resolve the issue? I use darkflats that are several seconds long as required with the ASI1600MM but still get overcorrection.