Help with the lens module

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almcl
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Location: Shropshire. UK

Help with the lens module

Post by almcl »

Would some kind soul point me in the direction of info on how the lens module can best be used to remove coma?

The help file suggests it can be used to correct coma but the default settings (curvatures red, blue, linked all at 100%, shifts and centres all at 0) don't make appear to make any difference.

Perhaps I'm not using it correctly, almost certainly, in fact, but have a look at the stars bottom left and top right in the attached image, that's coma, isn't it? What settings in the lens module would help?
Attachments
coma example
coma example
Coma.jpg (215.17 KiB) Viewed 7275 times
Skywatcher 190MN, ASI 2600 or astro modded Canon 700d, guided by OAG, ASI120, PHD2
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Re: Help with the lens module

Post by admin »

Hi,

Would you be able to post the full image stack (before cropping?) - the distortion correction needs the full frame.
With your permission (and full credits to you for the image), I'd love to do a proper tutorial for the Lens module's manual entry (and maybe see if we can fix up your colours a little).

In a nutshell, you tweak the 'Curvature Linked' parameter (typically upwards of 100%) to counter the curvature. You can use Center X and Center Y to define the center of the distortion.
The one downside of using the Lens module is that your diffraction spikes no longer look straight unfortunately.

Result with the following parameters;
Parameter [Center X] set to [43 pixels]
Parameter [Center Y] set to [-81 pixels]
Parameter [Curvature Linked] set to [134.61 %]
Coma_fixed.jpg
Coma_fixed.jpg (124.34 KiB) Viewed 7268 times
It's a little bit trial and error and YMMV, but using the full, uncropped frame will help.
Ivo Jager
StarTools creator and astronomy enthusiast
almcl
Posts: 263
Joined: Wed Jan 21, 2015 7:15 pm
Location: Shropshire. UK

Re: Help with the lens module

Post by almcl »

Thank you, Ivo. Wonderful response as always!

You have a PM with a link to three files that may be of use.

I'll definitely have a play using the setting above and see what I can achieve.
Skywatcher 190MN, ASI 2600 or astro modded Canon 700d, guided by OAG, ASI120, PHD2
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Re: Help with the lens module

Post by admin »

Hi Al,

Many thanks for the data. I'm going to have a look and see if it is indeed a good example for the Lens module. The one thing the Lens module struggles with a little is diagonal tilt of which there seems to be a fair bit in the data. It's tricky data - there is a severe gradient in there as well. Do you apply flats?

At the very least though, I wanted to understand where you're having problems with coloring. Have a read here for more information on how to color calibrate in StarTools.
Did you apply the Color module as one of the last steps before switching Tracking off?
M31(1).jpg
M31(1).jpg (217.67 KiB) Viewed 7242 times
Ivo Jager
StarTools creator and astronomy enthusiast
almcl
Posts: 263
Joined: Wed Jan 21, 2015 7:15 pm
Location: Shropshire. UK

Re: Help with the lens module

Post by almcl »

Ivo

Thanks so much for taking a look.

That particular data was from my 'pre-flat' days. Also from before the light pollution filter, so there would be a lot of sodium street light scatter.

Could you amplify/explain the 'diagonal tilt' comment a bit more? Not clear what this is - a problem with the optical train?

Thanks for the link to the colour module info, pretty certain this had the module applied but probably only with the auto settings. Unfortunately a change of hard drives and moving to the 64 bit operating system means I can't go back and check the log file.

It also would have been done with version 1.3 rather than 1.4
Skywatcher 190MN, ASI 2600 or astro modded Canon 700d, guided by OAG, ASI120, PHD2
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Re: Help with the lens module

Post by admin »

almcl wrote:Ivo

Thanks so much for taking a look.

That particular data was from my 'pre-flat' days. Also from before the light pollution filter, so there would be a lot of sodium street light scatter.

Could you amplify/explain the 'diagonal tilt' comment a bit more? Not clear what this is - a problem with the optical train?
I merely meant that the imaging plane seems to have been ever so slightly tilted towards one corner across a diagonal axis. It introduces another type of distortion that Lens doesn't correct for that well.
Ivo Jager
StarTools creator and astronomy enthusiast
almcl
Posts: 263
Joined: Wed Jan 21, 2015 7:15 pm
Location: Shropshire. UK

Re: Help with the lens module

Post by almcl »

Ah! Got you. I was making all sorts of mistakes at that stage and failing to fully secure camera and adapters was likely one of them.
Skywatcher 190MN, ASI 2600 or astro modded Canon 700d, guided by OAG, ASI120, PHD2
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