Rainbow Stars in Mono LRGB image

[Martin Dowd]

Ive noticed my broadband LRBG images have some rainbow stars ( mostly the smaller fainter stars ) when you zoom way in ( x 800 ) or pixel peep across all areas of the image. They are not noticeable at normal frame size .
Could this be a result of alignment issues between each of the channels after they are Composed in Startools Version 1.9 ?
The image in question was M22 Globular Cluster.
My data set weightings were as follows -
Lum 148 x 20 sec
Red 149 x 20 sec
Green 145 x 20 sec
Blue 55 x 20 sec

Full Calibration suite ....Flats , Flat Darks and Darks

Telescope 8" f5 Carbon Newt
Camera 2600MM Gain 0 , temp -10C
Bortle 8 Skies

https://www.astrobin.com/6c0tkp/

Thanks

Clear Skies
Martin

[han.k]

Most faint stars have the same orientation and pattern green (left top), yellow, red (right bottom). That indicates the alignment was not perfect. The image is very star rich. That could result in alignment problems. You could try in tab alignnment to increase the setting "Max number of stars" from 500 to 2000 or reduce the quad tolerance to 0.002 or 0.003.

The blue halo around the bright stars is a problem I'm also facing. Refractors often have this problem, the focus for blue is not so good as for red and green.

I could not see it in the jpeg image of astrobin. A colour smooth could help to hide the problem.

Cheers, Han

[Martin Dowd]

Han,
Firstly , thanks for your timely reply
Much appreciated
I restacked the data sets using the latest version of ASTAP. ( my version was a year old ) Then in Alignment increased the number of stars to 2000 followed by reducing the quad tolerance to 0.003 as recommended.
Performed a Calibration and Alignment for each set. Loaded the individual aligned sets into Startools Compose and commenced processing.
I did not use the Star colour smooth as Ivo recommends the data should be "virgin " as possible when loaded into Startools. No stretching , colour calibration or noise rduction and the like.
Finally arrived at the color module and set my parameters , zoomed way in and was pleasantly surprised with a significant improvement ( see attached )
The rainbow stars were almost completely gone . The blue halos on the medium to larger star don't bother me too much as I can deal with them in another module of Startools.
Your recommendation was spot on
Thankyou

Question: When I image my next target which is the Helix ( very few stars ) should I return to default settings in Alignment ( No. of Stars and quad tolerance) ?

Thanks for your help

Martin

[han.k]

Good it is solved.

I assume this problem will not occur so often. It happens only in areas with a huge amount of stars with a very similar magnitude resulting in some false quad detection's which then result in a small error. Having selected 2000 stars will reduce the stacking speed. I would put it back to the typical value like 500 stars. Leaving the tolerance at 0.003 does not result in a speed penalty. Only the 2000 stars does, so you could consider leaving the 0.003. It is then less likely it will pick up the wrong quad (4 stars figure). As long it find enough quads the 0.003 quad tolerance is fine.

Han

[Martin Dowd]

Thanks Han
At least now I can adjust settings as required for each project
Clear Skies
Martin