Issue with interlacing on FITS images taken in Nebulosity 4 with QHY6 camera on CCDSPEC spectrometer

I am experiencing an issue with interlacing on images taken with the QHY6 camera on my CCDSPEC spectrometer.

See also two other posts:

Image interlacing with QHY6 camera attached to CCDSPEC spectrometer: EZCAP vs Nebulosity 4 software for image acquisition

Effect of altering exposure on QHY6 camera attached to CCDSPEC spectrometer on spectra of daylight and compact flourescent bulb 1/8/2018

Photograph of the setup is below:

The issue is that sometimes I appear to get an interlacing problem in the images – alternating black lines between the image lines. This is intermittent and sometimes goes if I change gain. It has been particularly prevalent when I took spectra of compact fluorescent bulbs at home.

This is what should be seen:

But instead, I am sometimes getting this:

Here is the same FITS file from Nebulosity 4 opened in FITS Liberator 3.0:

When the image above is saved as BMP file rather than FITS from Nebulosity the interlacing still exists:

However, a set of trial solar spectrum images in Nebulosity 4 today with the QHY6 have demonstrated that this is clearly linked to exposure time (below):

Exposure 0.002 seconds:

Exposure 0.05 seconds:

Exposure 0.5 seconds:

Exposure 3.0 seconds – now clearly showing the effect:

Now if I reduce the exposure back to 0.5 seconds, the effect disappears again:

In the following screenshot, I have used FITS Liberator to highlight white and black clipping in the 3.0 second image:

The problem originally appeared as I took spectra of compact fluorescent bulbs in my study for calibration purposes. I wonder if the interlacing reflects over-exposure there? I have taken a new set of images this afternoon of one of these bulbs with difference exposures (images below) and they do indeed appear to suggest this is the case. I can bring on the effect by increasing exposure time and remove it by reducing exposure time. The 0.8 second exposure time image below is particularly interesting here as it shows the effect only in the highly exposed areas.

This time I went from long exposure to short to prove that the effect could be removed by reducing exposure.

Exposure time 5.0 seconds – very prominent interlacing effect:

Exposure time 3.0 seconds – interlacing effect still prominent but reducing as exposure shortens, until 0.8 seconds exposure:

Exposure time 1.5 seconds:

Exposure time 1.0 seconds:

Exposure time 0.8 seconds – the interlacing effect is only present in the over-exposed areas:

Exposure time 0.4 seconds – the interlacing effect is now completely gone:


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