Satellite streak: reject image or treat later?

[niccoc1603]

I am very new to PI

I have a few subs with satellite streaks across the galaxy (M101).
I am aware that streaks can be eliminated later in the integration process, also with he new Large Scale Pixel Rejection

But in a case like this where it goes right across the imaged object, is it better to completely reject the sub?



Thanks

[Geoff]

There is no need to reject the sub. The satellite trail should go when you select the appropriate data rejection method in image integration
Geoff

[STEVE333]

I often have a jet airliner trail across an image, sometimes right through the target. The jets usually leave three or four parallel trails with bright red dots from the blinking red lights. They have always been completely and invisibly removed by the PI Integration processes.

I do use dithering with my guiding, so, that does help. However, even without dithering if you have enough frames to combine it should be OK.

So, I agree with Geoff and would keep the frame.

Steve

[ngc1535]

Just to dot the i's on this thread. The most important consideration is the sample size (number of measurements, exposures). Often conversations around what is the best exposure time entirely neglect the statistical requisites for removal of outliers. I think it is safe to say that for any given exposure time you need at least 7-10 images in order to really take advantage of rejection algorithms and hope to take care of things we can clearly see with our eyes. I usually acquire 15-25 measurements if I can.

Concerning the OP's original question- there is an implicit concern that statistical rejection of something is on a per image basis. I believe for this conversation this isn't how it works- and we are talking about not including values (or in the case of Winsorized rejection, substituting some values) in the calculation of the mean. So it needs to be stressed that the fact the satellite "went through the galaxy" has little impact on the effectiveness of the rejection. I highlight this because it is so common to unnecessarily throw out frames- especially when the statistical requirements are met.

-adam

[gamempire]

Hi Nicco!

With a faint satellite streaks such as the one in your image, I've not had the best luck with the streak being completely rejected when integrating a stack of images where I may only have 5 or 6 frames per filter.

Rick and Juan posted a great PixelMath expression to draw a solid white antialiased line wherever you have a satellite streak here: https://pixinsight.com/forum/index.php?topic=8342.msg54473#msg54473 . It makes the rejection algorithm's job much easier.

d = d2seg(x1, y1, x2, y2 );
iif( d <= r, $T*d/r + ~(d/r), $T )

with declared symbols:

d, r = 2
x1,y1 are the pixel locations where the streak starts, and x2,y2 are the pixel location where the streak ends. Sometimes you'll need to increase the width of the line, so adjust R accordingly.

Now the one issue I ran into with this method is if I drizzled my data afterwards, I suddenly get the satellite streak back. When drizzling, the drizzle data file refers to your cosmetically corrected images, not the alignment images. So what I've had to do is draw the line on the cosmetic correction image, and then go through my normal alignment -> integration -> drizzle process. It will reject out during the integration, and then the drizzle data file will reject it when you drizzle.

The other option may cause a bit of degradation to SNR in the area of the streak, but sometimes it works when the other process wont. First, align two images before you start your workflow, make a mask with the line of the satellite trail that masks everything but the trail. Then apply mask to image with the satellite trail. Type the name of the image without the satellite trail in pixelmath, make sure replace image is selected, and apply it to the image with the trail. It will then take data from the image without the trail and only apply it to the area shown by the mask.