Orientation of light frames in BatchPreprocess

[Jules]

Hi

I don't know if this a dumb question but here goes:

How does the Batchpreprocess script know how your images are orientated with respect to a before and after a pier flip prior to the application of darks, flats, bias etc?

Thanks

Julian

[Geoff]

It doesn't know and it doesn't matter. Flats, darks and biases depend only on the properties of your camera and optical system and are independent of the images you take.
Geoff

[Jules]

Hi Geoff

OK, say you have a hot pixel or a dust mote at an XY location on a dark/flat. Then when you go through a pier flip the software would need to know that the position of the hot pixel or dust mote as your whole image train had flipped?

Or perhaps I am not getting this?

Julian

[oldwexi]

Hi Julian!
Simply if your chip has a defect hot Pixel on Location x,y  1,1
The hot Pixel will be on all Images, dark, bias, light(!!!!) on position x,y 1,1 no matter how you turn your camera around.
Calibration therefore will correct all these pixels

For the lights the sky will turn 180 degree after flip mount, but this turn on the image is recognized by the Star Alignement Prozess and rotated back correctly,
BUT AFTER the calibration (with bias, dark,flat)!!

So your hot Pixel at 1,1 will be removed by Calibration and afterwards StarAlign rotates the calibratet lights so they fit together.
Gerald

[Jules]

Gerald

Thank you for getting back to me.

I am still having problems with this. I have a hot pixel position in XY position 1,1. Batch preprocess applies the dark frame to said image and the hot pixel is gone.

Later on, I apply the same dark frame with a correction for the hot pixel at position 1,1 to some light frames that have been meridian flipped.

How does Batch preprocess know that the light frames have been meridian flipped and so the hot pixel will be in a different position?

Thanks

Julian

[pfile]

the hot pixel will not be in a different position. if you take your camera, put it upside down in your closet and take an image, the hot pixel is still at 1,1. if you take your camera to the moon, the hot pixel is still at 1,1.

maybe you're hung up on the fact that StarAlignment has rotated the image for you? calibration happens *before* star alignment runs. it's actually impossible to calibrate a frame that's been translated and rotated (without translating and rotating the dark, which would be silly), so SA runs after calibration is complete.

the only relevant problem with calibrating images and meridian flip is when you have a rotator, and the rotator undoes the image flip for you. then you (may) need a different set of flats for that post-meridian flip position angle.

rob

[Jules]

OK Rob

I have got it now thank you very much. It was a case of finger up backside and brain in neutral.

Thanks

Julian

[pfile]

believe me this hobby is full of counterintuitive stuff. how about trying to change the orientation of secondary vane diffraction spikes by rotating the camera!? for a while i thought that would work. 8)

[Geoff]

believe me this hobby is full of counterintuitive stuff. how about trying to change the orientation of secondary vane diffraction spikes by rotating the camera!? for a while i thought that would work. 8)

Just the other day I was wondering how to be sure the diffraction spikes would line up when I came back to an image a year later. I had previously known they would, but somehow managed to forget it!  Back on track again.
Geoff

[Jules]

Geoff, Rob and Gerald

Gentlemen, thanks very much for taking the time to get back.

Julian