Image alignment issues

markuspaul

New member
Aug 4, 2020
3
0
Hello.
I have taken a set of M27 images with an 11" inch telescope (2800mm length) consistently get error messages when I try to use the star alignment process. When I use the same process on a set of images taken with a 400mm Nikon lense it works just fine. The 27 images were done at high gain with lots of noise and the stars are rather large. I have tried to play around with the parameters (RANSAC tolerance etc.) but consistently had errors.

The images are posted in the below directory with the reference file named as such. If someone is able to tell me either how I can change the parameter to get PI to align the images or if there is something fundamentally wrong with the images I would be very grateful.

 

Attachments

bulrichl

PTeam Member
Nov 2, 2016
831
58
La Palma, Canary Islands
The histogram shows that the data are heavily clipped in the low range, see appending histogram of frame "Capture_00004_a_d.xisf". A fraction of 18 % of the pixels is clipped in the red, 10 % in the green and 16 % in the blue channel. This is the fundamental error.

It is not clear to me what was done prior to Debayering. What does the "_a" mean in the filename? Also, the data seem to be stretched already, and I'm wondering why this is the case. So I conclude that you applied an improper workflow.

For raw data of OSC cameras, the appropriate workflow is:
- ImageCalibration (dark frames, flat frames and either bias frames or flat darks are necessary)
- if necessary, CosmeticCorrection
- Debayer
- StarAlignment
- ImageIntegration


If you previously applied image calibration, it failed completely. If these data are uncalibrated, either the offset setting of the camera is absolutely wrong (too low) or an aggressive black point setting was applied to the data.

The model properties in the Process console output of the StarAlignment process:

Root mean square error:
delta_RMS : 0.000 px
RMS error deviation:
sigma_RMS : 0.000 px
Peak errors:
delta_x_max : 0.000 px
delta_y_max : 0.000 px

show that alignment was performed on hot pixels and not on stars.

You will have to follow the workflow outlined above. Please also see my guide For beginners: Guide to PI's ImageCalibration .


Bernd
Clipping.JPG
 

markuspaul

New member
Aug 4, 2020
3
0
The histogram shows that the data are heavily clipped in the low range, see appending histogram of frame "Capture_00004_a_d.xisf". A fraction of 18 % of the pixels is clipped in the red, 10 % in the green and 16 % in the blue channel. This is the fundamental error.

It is not clear to me what was done prior to Debayering. What does the "_a" mean in the filename? Also, the data seem to be stretched already, and I'm wondering why this is the case. So I conclude that you applied an improper workflow.

For raw data of OSC cameras, the appropriate workflow is:
- ImageCalibration (dark frames, flat frames and either bias frames or flat darks are necessary)
- if necessary, CosmeticCorrection
- Debayer
- StarAlignment
- ImageIntegration


If you previously applied image calibration, it failed completely. If these data are uncalibrated, either the offset setting of the camera is absolutely wrong (too low) or an aggressive black point setting was applied to the data.

The model properties in the Process console output of the StarAlignment process:

Root mean square error:
delta_RMS : 0.000 px
RMS error deviation:
sigma_RMS : 0.000 px
Peak errors:
delta_x_max : 0.000 px
delta_y_max : 0.000 px

show that alignment was performed on hot pixels and not on stars.

You will have to follow the workflow outlined above. Please also see my guide For beginners: Guide to PI's ImageCalibration .


Bernd
View attachment 8874
Thank you Berndt. In fact, there were no darks, flats and bias frames. I have added the original fits for reference into the folder. The "a" suffix just stands for approved after the blink module and subframe selection which should have no impact on the image. The only subsequent operation performed was debayering. I have also added a set of fits of NGC 6992 where there were no problems with alignment after having followed the same process. Is it possible that having set the camera gain to its maximum for M27 of 700 is the cause of the issue. The NGC 6992 pictures were taken with a gain of around 630.
 

bulrichl

PTeam Member
Nov 2, 2016
831
58
La Palma, Canary Islands
The NGC 6992 frame Capture_00001_a is clipped as well, albeit not so severe (about 2.6 % of all pixels), see appended screen section.

I recommend to try a gain setting of 100 and the default offset setting of 50 with longer exposure duration (1 - 5 min).

The clipped frames are not usable.

Bernd
Clipping_2.JPG
 

markuspaul

New member
Aug 4, 2020
3
0
Thank you Bernd. I am learning my way with the new camera and this is very helpful. Best regards, Markus