How reliable is the tilt analysis function?
Stacking program, astrometric (plate) solver, fits file viewer
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han59
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I have a set of subs that have a tilt that can be spotted when looking closely at the image. I used the ASTAP tilt fonction and it reports there is no tilt. How reliable is that function in ASATP? Is there something I am missing?
For tilt, ASTAP measures the half flux diameter of the stars and use the median of 9 areas to measure the tilt. If the four corners are symmetrical it will report no tilt. See
http://www.hnsky.org/astap.htm#inspector
I doubt it will fail as long there is no bright nebula in the image. Can you attach one of your images?
Han
Thank you for the reply Han. Here is a jpeg file. I could not upload a fits file as the site refused it. I hope that will work.
Best Regards
Very good resolution. There is no tilt but the stars are oval due to loss of tracking. They point between 8 and 2 o'clock. Ovality is about 1.4 everywhere in the image. So everything is symmetrical. The length/width of the stars is about 1.4. I assume the guiding was not working correctly.
Han
The ovality is in RA so east/west which is typically. Look to the red north/east arrow pointer and compare it with the stars. The arrow is pointing north. The small part of the arrow is pointing east.
Thank you for this.
That's puzzling as the guiding was 0.7-0.8 RMS (Phd2) all night with an mage scale of 1.3.
Do you have any idea or suggestion as to what could be going on here? Are the stars jumping ahead towards the west or east according to what you see on the image?
Difficult to say. First thing to do is to analyse the image series. You can load all images in the ASTAP light tab (cntrl+A) and press analyse. Look to the column HFD (half flux diameter). Sort on column HFD. Most likely most images are okay and only a few are poor which should be indicated by a higher HFD value. It is not abnormal to have a few poor images. Inspect visually the poor and good images.
You could also look to the log of PHD2 and load it in the PHD2 log viewer and look if there are some bad moments.
Han
If in all images the stars are similar oval , one thing to check if the RA control is not oscillating by a to high gain.