Trouble with texture from flats in processed images

[rweed]

I am having an issue where after I calibrate, align, and stack all of my images, I end up with a lot of texturing left over from the flats. Is this an issue with my calibration? How do I prevent this from happening/remove the texture from my final image? I am attaching a calibrated image and my final stacked image. Thanks!

[pfile]

what was the calibration flow for the flats? matching darks? scaled darks? just bias?

rob

[jkmorse]

What do your flats standing alone look like?  Also how are you taking your flats?  If possible, post a flat sub and your fully calibrated flat as examples.

Best,

Jim

[rweed]

I make a master bias and dark, calibrate my flats, then stack to make a master flat, then I calibrate the images using my master calibration frames. I take my flats on a blank sky at dusk. I am attaching an image of an uncalibrated flat and then my master flat (the uncalibrated is obviously the one that has stars). Thanks!

[jkmorse]

Is that cross hash pattern on all your images or just the flats?

[pfile]

if the duration of the flats is reasonably short you might try calibrating them with only bias frames to see what happens.

that pattern is quite strange, what kind of sensor is this and do you know if it suffers from residual bulk image?

rob

[jkmorse]

I am also concerned about the uneven illumination in those flats.  I fully understand the dark edges due to vignetting, but really struggle with the bright ring around the edge and the bring center but darker regions in between.  That and the pattern are, as Rob says, really strange.  Where are you pointing the scope when you shoot your sky flats?  it should be very near the zenith, just east of center.  In the end, if this is just happening with your flats it is probably best to process your images without them unless we can find a solution.

Jim   

[chris.bailey]

Is that scope an unshrouded truss tube RC by any chance? Stray light reflecting off the primary can cause havoc with dusk/dawn flats.

Chris

[rweed]

So I ended up figuring out that this is most likely an issue with how I'm processing my flats. When I created a master flat using a different script, it still had the texturing, but when I calibrated the images with the new flat, they didn't. Which I guess indicates that the texture is there in the images as well. I think the problem is that my master flat has an average value of 0.3 instead of 1. Does anyone have any idea what I might be doing wrong to make this the case? If it helps, the flats have that value after calibration (before I stack them). Thanks!

[pfile]

if you open a flat sub, what's the mean value? if you want to see the value in ADUs you can switch the readout cursor to 16-bit unsigned integer mode (click the > triangle down in the bottom toolbar to get the menu).

depending on how the flat master was produced, the mean value can be all over the place. if the first flat sub in ImageIntegration (the integration reference frame) happens to be dim and you have normalization turned on, then the value in the master will be similar to that dim flat. if normalization is turned off, then you're getting the straight average of all the flats.

rob

[rweed]

my mean value is about 23000. i integrate my master flat with multiplicative normalization and no weights. could those settings be part of the problem? thanks!

[jkmorse]

No, those aren't the issue.  Here are the settings I use, building from only 5 flat subs, to create beautiful master flats:

Flat Frames:

Step I - Calibration:

Open Image Calibration Tool
Load Raw Flat Frames
Load Appropriate Master Bias or Superbias
[Load Appropriate Master Dark]
[for low noise CCDs, it is best to just use the master bias since that creates a cleaner Master Flat in PI]
Global Apply

Step II – Integration:

Open Image Integration Tool
Load Calibrated Flat Frames
Settings:

Combination: Average
Normalization: Multiplicative
Weights: Don’t Care (all weights = 1)
Scale estimator: Iterative k-sigma / biweight midvariance (IKSS)
Pixel Rejection (1):
   Rejection Algorithm: Winsorized Sigma Clipping
   Normalization: Equalize Fluxes
   Check all boxes
Pixel Rejection (2):   
        Sigma Low 4.0
   Sigma High 3.0
   Range low 0.0
   Range High 0.98


I don't think you have ever stated whether this pattern is exclusive to your flats or whether its in all your lights as well.  If its only the flats, then you have a glitch in your method for shooting your sky flats.

Jim

[pfile]

my mean value is about 23000. i integrate my master flat with multiplicative normalization and no weights. could those settings be part of the problem? thanks!

assuming your camera outputs 16-bit fits files, then 23000/65535 = 0.35. so 0.3 is not an unreasonable mean for the master flat.

if you are wondering why the brightest part of the flat is not 1.0, that's because it does not need to be - ImageCalibration itself does the rescaling of the flat while calibrating the lights. ImageIntegration does not really know that it's integrating a flat or anything else for that matter. of course the settings form a heuristic that points to the targets being a flat but there's really no certainty... so ImageIntegration does what it says on the tin... it integrates images.

rob

[rweed]

Ok so I think I mostly fixed the flat issue (I think it was a problem with the binning of my flats) but now I have a new texture in my final image... Possibly also related to my flats but I'm not sure. I'm attaching my final image, has anyone seen this texture before? Thanks

[jkmorse]

That's getting better, at least the pattern is gone.  Question is whether that image has had DBE applied to it yet.  If not, that should help.  If so, then that may be the issue.  Still, flats should cure that level of disparity.  From the looks of it, you still have a ring issue with the flats where there is a brighter outer edge and center than the spaces in between when in fact that should be much more uniform, typically being darkest in the vignette portions and brightest in the center. 

Post the flat master you used for that latest image.

Jim