Chasing down a Flat issue or something else ?

[mgirdwood]

Hi,

Not sure if this is in the correct forum area so please move if not.

 

I have been having an issue with an area of my images using my Newt (GSO 150/750 ) and would now like to try to get to the bottom of it.  ASI1600MM.  Processing is with PI

Here is an overstretched view of a Master flat taken with the Red Filter.  All the filters show the exact same issue at the same angle and position on the respective Flat. The area starts to show itself in the integrated image once I start to increase saturation and stretching beyond the standard initial processing steps. 

These Flats were taken at night using NINA auto flats. I am using a panel to try to get consistency.  I checked back over some flats that were taken in March and the "lighter patch" appears in the same area of the Flat at the same angle, even though I have rotated the camera between those 2 nights.  I use Dark, Dark Flats and Flats in the PI WBPP 

I do have the base of the Newt covered with a sleeve to try to exclude light from around the Primary mirror ( and there were gaps you could drive a truck through).  Where the telescope is located is on a pier with no obvious external bright light sources.

Given the apparent shape of the "blotch" could this be some form of phantom reflected Secondary mirror ? Focuser light bleed ?

Any suggestions to track further would be very much appreciated as I would really like to both Me/this scope working so I can use it to its best.

In a stacked image the areas where the "patch" is appears to cause colour aberrations once it gets past a certain amount of processing.   Perhaps this is normal however i don't recall having the same issue when working of data taken with the refractors.

 Below is a a very rough quick view on the latest image, simply integrated, DBE, background neutralisation, and then ramped up the saturation to show the difference in the pixel colour that seems to correlate to the same area that is showing through with the Flat.  

I have just checked an ASI1600MM Mater Flat taken with the Quad earlier this year and it shows a clean "flat" image with the usual dust motes plus i have a vignette, nothing like  the Blotch that i am seeing with the Newt - hence my thought that this is a specific issue with this telescope or with Newts in general.

 

Thanks for any suggestions.

Mark