ASI294MC - Master Dark and RGB Light not ‘aligned'.

[Hughsie]

I have never experienced this before and hope that I have just missed something in the calibration process.

Below is a rather poor unprocessed but calibrated image of NGC4236. Below it is the Master Dark taken later the same day. Both images have the same exposure time, gain, offset, binning and sensor temperature. As you can see the RGB image has both a positive (light) and negative (dark) ‘glow’ on the right hand side. However, when you look at the Master Dark image it is almost like it is horizontally inverted to the RGB image.

In calibrating the lights I did try inverting the dark frame about the horizontal so the pattern matched but PixInsight kicked out an error saying the images did not match. Has anyone seen this before and what was the outcome.

Separately the flats have not corrected, and I see there is copious material on the forum regarding that so I will dive in.

 

 

[michael8554]

Lights taken before and after Meridian flip ?

Michael

[scotty38]

1 hour ago, michael8554 said:

Lights taken before and after Meridian flip ?

Michael

Calibration is done before registration so that shouldn't be an issue but I cannot work out what process has taken place really.....

[david_taurus83]

The master dark looks OK as the 294 amp glow is top right like other examples I've seen. I would guess that the lights have been aligned prior to dark subtraction for some reason. Load up all your raw lights in Blink and see where the amp glow is. It should be top right in them all.

[Hughsie]

Thank you to everyones reply and apologies for the delay in responding. Having got to the point where the master dark was not being applied properly I decided to ‘park it’ and save it for a rainy day.

Many cloudy and rainy days later I have now resolved the problem. But first, here are the answers to some of the points raised so far;

• The image session was short and no meridian flip took place.
• The light frames were registered in PixInsight AFTER calibration.
• David’s response was much closer to the mark......

With PixInsight I had come to the conclusion that every solution has to be complex. I was looking for a difficult solution when a simple one was staring me in the face.

First, I recaptured my dark frames then subtracted a dark from a light frame using pixel math....the issue still remained. Then I recalled David’s comment about the location of the ‘amp glow’ being in the top right hand corner of the dark frames (as shown above). This was in fact true. Then I blinked the light frames and the amp glow was in the bottom right hand corner!?

So, having integrated the dark frames I then flipped the master dark about the vertical and then recalibrated the lights and hey presto it worked.

The next question is why did the orientation of the lights and darks not match...but that is for another day.

[sharkmelley]

Did you take your darks with the same software as your lights?  FITS files can be stored in a "top-down" or "bottom-up" row order and different software might store them differently.

Mark

[ollypenrice]

On 24/10/2022 at 12:40, sharkmelley said:

Did you take your darks with the same software as your lights?  FITS files can be stored in a "top-down" or "bottom-up" row order and different software might store them differently.

Mark

Yes and, even with the same software, there is sometimes an option in the capture part to save the data in a choice of orientations. If this is inadvertently changed it will disorientate the calibration files. I've had this in Artemis Capture.

Oly