Apparent Image grid

[glatiak]

Being a novice with PI, and only a couple of images massaged so far, I have noticed that after drizzle integration my stacked images have a distinct grid as though seen through a screen. I assume I am doing something wrong someplace and have made a few unsuccessful attempts to remove it but to no avail. The source is a Mallincam DS16c -- debayered in PI. Sensor is 4656x3518 pixels, bin 1. The attached image is a crop -- roughly 1/4th the total frame. Any thoughts or suggestions as to how to not have the window screen effect?

Side comment -- colour was almost non-existant, assume I underexposed the thing...

Thanks,

greg latiak
avalon observatory

[cliff_n]

This is a "Me too" post.  I'm getting the same type of grid pattern during image registration (Star Alignment).  This is the same image taken at different steps.  Other images in the group (not shown here) also have a grid pattern but sometimes it is larger and sometimes it is smaller.

[dhb2206]

Are you debayering before integrating? Looks like a debayer pattern to me. Though I notice you mention debayer - is it perhaps drizzling from CFA or not as the case may be?

[wadeh237]

I see this (with my non Bayer camera) if I stretch the background too far.

I believe that it's an artifact of imperfect calibration, combined with stacking dithered frames.  In my images, it looks like some very faint residual of the fixed pattern noise (as seen in a stretched bias frame).  When you align the images, I think it's like an interference pattern (or actually an interference pattern).

In my case, I have to stretch beyond all reasonable measures before it's noticeable, so it is not an issue at all.

Note that I don't think that has anything to do with the color filter array because the scale is way too large.

[glatiak]

Don't know about other folks, but I found my grid was coming out of drizzle-integration. So by reducing the scale and drop shrink I could make it completely disappear. The later appears to be the driver of the grid. Another 'user error on device' problem -- or perhaps 'de fault is always de user'.

greg

[dhb2206]

Just had a thought. You have set your raw up for Pure Raw on the Format Explorer?

[ApolloLaunchControl]

And I'm getting one on a much larger scale. 3 nights of mono NB images with the ASI1600MM, drizzled, DBE, etc. and integrated with ChannelCombination. I didn't notice the pattern until I did a StarMask, and then it really popped out.

[pfile]

if this happens with mono images then it is likely a calibration problem - the sky background is so weak in a narrowband image, that dark subtraction sometimes leaves negative values (which get clamped to 0) in the calibrated frames.

you can try adding a pedestal of like 100DN in ImageCalibration and the problem should resolve itself.

rob

[mmirot]

I see this in my narrow band images with registration on auto/Lacnos3. I changed to Mitchell and it goes away. I would like know the cause and how to fix it before registration.

Max

[mmirot]

I will try to add a pedestal. What is the proper method to do this ?

Max

[pfile]

add a pedestal to the calibrated frames with the Output Pedestal section of ImageCalibration. try maybe 100 or 200. this will prevent the pixels from getting clamped at 0.

rob

[mmirot]

 Do I need to do this to make the master bias and dark too?

Thanks

Max

[bulrichl]

No. (I am assuming that you prepare the MasterDark by integration of the dark frames, i.e. NO calibration of the dark frames, NO calibration of the integrated dark frames).

Bernd
 

[mmirot]

To be clear,  integrations of the masters without adding 100 adu etc?

Max

[pfile]

only do the output pedestal when you are calibrating lights. adding pedestals to darks/bias will just make this problem worse...

rob