Why Hour Glass comet coma

[Pierre]

I keep getting an hourglass shape coma for my Lovejoy using CometAlign. Any ideas? Is it a dithering issue? Following Harry's video as usual.
Pierre

[Don]

Hi Pierre,

What does the coma look like in a single sub?  If it's reasonably circular, then the hourglass shape could be a dithering issue.  If you dither, the CometAlignment process can't exactly register the comet.  In my case it came close, but I used the DynamicAlignment process to tweak the subs after CometAlignment (one sub at a time).

You can use the Blink tool to look at the subs after CometAlignment and see if the coma is well registered or not.

Don

[Pierre]

The subs were only 30 seconds each and the coma are round in each one. I have 121 of them. I guess that there should be an advisory to NOT use dithering if comet imaging.

[pfile]

what if you just registered them all first using StarAlignment? the only problem would be if the acquisition time FITS header keyword is not preserved by StarAlignment. assuming that works you can then run CometAlignment on the images coming out of SA.

rob

[Pierre]

I will give that a shot.

Still playing, but I ran the Sigma High under the Pixel Rejection 2 tab, down to under .600 and the hour glass has disappeared.
Pierre