Author Topic: What Type of Gradient is it?  (Read 2661 times)

Offline tdgm

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What Type of Gradient is it?
« on: 2016 January 15 14:55:19 »
Hi Everyone

Attached is a link to the image. I was just wondering what type of gradient is in this image. It is 36 5 minute images stacked and pre-processed in PixInsight. My imaging system is bothered by vignetting due to inherent properties of the Takahashi FS 78. Even if I do not see a gradient I know that the stars at the corners are elongated in different directions from coma. if the image is bothered by a multiplicative as well as a additive WHICH Gradient is removed first?

https://www.dropbox.com/s/pj2rrpcnur40ir1/integration.xisf?dl=0


Thanks for any suggestions all you guru's have since this will help me removed gradients that effect all my images.


Thanks again

Offline cfranks

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Re: What Type of Gradient is it?
« Reply #1 on: 2016 January 16 15:03:50 »
I might have this completely wrong of course.  My image of this object is about the same size and, using my arcsec per pixel of 1.5, CCDInspector reports you have a Field Curvature of 96% which will give very confusing results.

Offline tdgm

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Re: What Type of Gradient is it?
« Reply #2 on: 2016 January 16 22:32:40 »
Yes I am bothered by a significant vignetting in all my images Possible due to optical alignment of camera and tube and also that the refractor does not give a flat field. Need an astrograph that gives a flat field. I am just interested when I do DBE do I do a division first or after a subtraction grtadient?



Steve Herrman

Offline tdgm

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Re: What Type of Gradient is it?
« Reply #3 on: 2016 January 17 09:41:04 »
Hello cfranks

After checking in CCDInspector even the original images are bother by field curvature. This I think is due to the telescope Takahashi FS 78 F 8.1. Might just need a Takahashi FSQ 85mm to flatten field. I do use a flat field in image calibration there does not seem to be anything I can do about thje field curvature because that is a hardware abnormality so a different scope is what is needed.


What do you think?

Steve

Offline cfranks

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Re: What Type of Gradient is it?
« Reply #4 on: 2016 January 17 15:02:39 »
Hi Steve,

Takahashi 'scopes being the high quality they are, I'm sure there will be (hardware) Field Flatteners for their specific models.  Flat frames for ImageCalibration will have no effect on field curvature.  William Optics have a variable FF that has done wonders for my FLT132.

Charles

Offline tdgm

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Re: What Type of Gradient is it?
« Reply #5 on: 2016 January 17 15:27:30 »
Hi Charles

Have Focal reducer / field Flattener for FS 78 and actually adds some color fringing and really does not decrease curvature. Still thinking about getting few 85mm


Steve

Offline tdgm

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Re: What Type of Gradient is it?
« Reply #6 on: 2016 January 17 18:38:02 »
Hi Charles

Here is my final processing of above mention and downloaded image:eve


Steve


Offline cfranks

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Re: What Type of Gradient is it?
« Reply #7 on: 2016 January 18 13:47:22 »
Hi Steve,

That is coming along nicely but until you can reduce that curvature, you will be struggling a bit.   I would almost guarantee that your elongated stars in the corners are due to the curvature.  If the 85mm is an Astrograph, you should improve your subs considerably.  What the heck, a hobby like ours is just an enjoyable way of spending money!  :moneyinmouth:

Charles

Offline tdgm

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Re: What Type of Gradient is it?
« Reply #8 on: 2016 January 19 14:15:26 »
Hi Charles

Takahashi 85ED on the way. Yes it is an Astrograph so am hoping for a lot better subs than the ones I have now with The Takahashi FS78 (know field curvature Problem for imaging).


Steve

Offline Alejandro Tombolini

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Re: What Type of Gradient is it?
« Reply #9 on: 2016 February 13 16:24:57 »
I was just wondering what type of gradient is in this image.

Hi Steve, I would consider as additive and use Subtraction, however in this case DBE solve perfectly well the gradient using either Subtraction or Division.

Saludos, Alejandro.