Carina with Easter Stars

[oska (John)]

I've been experimenting with a Startravel 120/600 for narrowband, for fun.
Ordinarily one might be disappointed by cruciform stars (top left is worst) but hey it's good Friday

Just combination, DBE, SPCC, STF stretch, starless, removed most of the green from the stars, quick histogram tweak to bring down the background a touch (100px clipped), rescreen.

BX really highlights the astigmatism (2nd).
The top left crop shows it better (3rd)

I would love some pointers on how to fix the stars?

About 5hrs of 300s & 600s in SHO

SW 120/600 + orion flattener
ASI160MM + Antlia 3nm SHO
CEM40EC (0.21" total )

NIINA + PI

Peace Love & Mung beans.

[Bodon (Steve)]

John
I have the same problem with stars. You can use the image inspector in ASTAP to inspect a file. Over my head. My stars are a shocker when using a more sensitive camera. Fairly certain curvature may be at play. Slight tilt could be corrected by pulling the train apart and putting back together if it exists. My SW72 is not the best thats why I crop....poor optics; I have what I see in my reflector....coma. They are great with eyepieces but a camera will show flaws.
Two weeks ago I sold one of my kidneys and bought a ZWO 2600 Pro and filters. I manged to get out the other night and took 8x240sec shots with Sii Ha Oiii, 24 pics all up. Flying seagulls in the corners. If I was using my ZWO 183 they would be less pronounced.

[JA]

Originally Posted by oska

BX really highlights the astigmatism (2nd).
The top left crop shows it better (3rd)

I would love some pointers on how to fix the stars?

About 5hrs of 300s & 600s in SHO

SW 120/600 + orion flattener
ASI160MM + Antlia 3nm SHO
CEM40EC (0.21" total )

NIINA + PI

Peace Love & Mung beans.

Hi John,

When the image is looked at in its entirety the aberrations are not that obvious and still a fine image, but if you want to chase things further you could:

1. Critically compare all 4 corners to see if there is any difference in the aberration. If you see a significant difference in corner to corner aberrations then you could try some form of tilt adjustment to even this out / reduce it before....

2. Reducing the aperture of the optic. Rather than crop the image as some do to reduce such artifacts, try using an aperture mask up in front of the optic. Try reducing the OTA from its native 120mm to say 100mm, by using a black paper or similar annulus cut to size. This will prevent light entering the more extreme edges of the optic where the light is bent more to get to the focal plane and thereby reduce some of the aberrations, chromatic aberrations in particular, but also astigmatism and coma. Of course doing this has the disadvantage of "losing light" and your 5 hours of imaging, in light gathering terms is now more like 4 hours 10 minutes, but with better stars and potentially the full field of view available to your camera, without the cropping that would otherwise have been necessary to remove the aberrations somewhat, at the expense of a lessened field of view.

Maybe aperture masking to 100mm is/is not enough, so experiment to taste.


Best
JA

[oska (John)]

Originally Posted by Bodon

John
I have the same problem with stars. You can use the image inspector in ASTAP to inspect a file. Over my head. My stars are a shocker when using a more sensitive camera. Fairly certain curvature may be at play. Slight tilt could be corrected by pulling the train apart and putting back together if it exists. My SW72 is not the best thats why I crop....poor optics; I have what I see in my reflector....coma. They are great with eyepieces but a camera will show flaws.
Two weeks ago I sold one of my kidneys and bought a ZWO 2600 Pro and filters. I manged to get out the other night and took 8x240sec shots with Sii Ha Oiii, 24 pics all up. Flying seagulls in the corners. If I was using my ZWO 183 they would be less pronounced.

Thanks Steve. I've got a better focuser for it but I have to machine an adapter yet. Congrats on the 2600! I need one.

Originally Posted by JA

Hi John,

When the image is looked at in its entirety the aberrations are not that obvious and still a fine image, but if you want to chase things further you could:

1. Critically compare all 4 corners to see if there is any difference in the aberration. If you see a significant difference in corner to corner aberrations then you could try some form of tilt adjustment to even this out / reduce it before....

2. Reducing the aperture of the optic. Rather than crop the image as some do to reduce such artifacts, try using an aperture mask up in front of the optic. Try reducing the OTA from its native 120mm to say 100mm, by using a black paper or similar annulus cut to size. This will prevent light entering the more extreme edges of the optic where the light is bent more to get to the focal plane and thereby reduce some of the aberrations, chromatic aberrations in particular, but also astigmatism and coma. Of course doing this has the disadvantage of "losing light" and your 5 hours of imaging, in light gathering terms is now more like 4 hours 10 minutes, but with better stars and potentially the full field of view available to your camera, without the cropping that would otherwise have been necessary to remove the aberrations somewhat, at the expense of a lessened field of view.

Maybe aperture masking to 100mm is/is not enough, so experiment to taste.


Best
JA

Cheers JA!
Very much appreciated. I'll get onto both and report back.

[xelasnave]

Nice work and you will find your stars will improve when you get set up permanent in an observatory
Great effort I only wish I could come close.
Alex

[oska (John)]

Thanks Alex, I'm pretty sure a 2600mm will help in the interim.