Hi Everyone,
I have been troubleshooting and trying to fix issues with my sensor tilt and focuser squareness for a while now. Ultimately it came down to field flattener being loose in the focuser barrel. I wanted to share the method I have used to find the problem and also how to help adjust for sensor tilt related issues during the day and not to have to waste the rare good imaging nights to do so.
I have printed an A4 sheet with a grid of black dots and hung it on the wall around 5-6 meters away from the telescope. Then added enough spacers between my flattener and the camera to be able to achieve focus at that distance.
I took a test shot, inverted the image in Photoshop, adjusted levels so our dots became bright stars on a dark background.
Then imported the image into ASTAP and measured both aberration and tilt.
This method will work for both DSLRs and dedicated astro cameras.
A bit of further explanation:
Because we are moving a focusing distance closer to us, we are also reducing our depth of field.
Also because we are moving the sensor further away from telescope lens, we are exaggerating any sensor tilt or off-axis errors we have in our imaging train.
Both exaggerated tilt and narrower depth of field allow us to perform a much more precise adjustment to ensure that our imaging train is as close to square as possible.
The principle behind it is similar to doing macro photography with a regular DSLR lens and extension tubes.
Just to answer any questions about how perpendicular should telescope imaging axis be to the paper, here is my setup and calculation:
80mm refractor @480mm F6, QHY163M 4/3 camera and distance to the paper is ~5m. That gives me a working focus depth of field of ~2cm.
I hope that helps someone with their struggles
Clear Skies and Kind Regards,
Michal
Images
Test page (Test Page.pdf):
Inverted and levels adjusted:
Out of square imaging train results:
Focuser tube squared with imaging train:
I have been troubleshooting and trying to fix issues with my sensor tilt and focuser squareness for a while now. Ultimately it came down to field flattener being loose in the focuser barrel. I wanted to share the method I have used to find the problem and also how to help adjust for sensor tilt related issues during the day and not to have to waste the rare good imaging nights to do so.
I have printed an A4 sheet with a grid of black dots and hung it on the wall around 5-6 meters away from the telescope. Then added enough spacers between my flattener and the camera to be able to achieve focus at that distance.
I took a test shot, inverted the image in Photoshop, adjusted levels so our dots became bright stars on a dark background.
Then imported the image into ASTAP and measured both aberration and tilt.
This method will work for both DSLRs and dedicated astro cameras.
A bit of further explanation:
Because we are moving a focusing distance closer to us, we are also reducing our depth of field.
Also because we are moving the sensor further away from telescope lens, we are exaggerating any sensor tilt or off-axis errors we have in our imaging train.
Both exaggerated tilt and narrower depth of field allow us to perform a much more precise adjustment to ensure that our imaging train is as close to square as possible.
The principle behind it is similar to doing macro photography with a regular DSLR lens and extension tubes.
Just to answer any questions about how perpendicular should telescope imaging axis be to the paper, here is my setup and calculation:
80mm refractor @480mm F6, QHY163M 4/3 camera and distance to the paper is ~5m. That gives me a working focus depth of field of ~2cm.
I hope that helps someone with their struggles
Clear Skies and Kind Regards,
Michal
Images
Test page (Test Page.pdf):
Inverted and levels adjusted:
Out of square imaging train results:
Focuser tube squared with imaging train:
