Annoying gradients

[Dave_D]

I'm using PixInsight to postprocess my images but there's one thing that i'm having trouble with in that a lot of my images are being left with either a halo gradient or a bright 'bar' along the bottom of the image (as can be seen in this pic of M97 and M108.

Anyone know what is causing  these gradients to be left after using either the PI dynamic or automatic background removal. Is it because i'm not using flat frames? but the strip along the bottom doesn't seem to be in the original fits.

[Guest]

Sorry can't see any bright bar on that picture, looks good to me!!!

MM

[swag72]

Are you using flats?

[Dave_D]

Are you using flats?

no just darks and bias. haven't got the equipment to do flats properly yet and i can nowhere near afford an A3 EL panel at the moment. tried with the 'white shirt over the aperture' but that didn't work too well. just couldn't get an even illumination.

[lensman57]

I'm using PixInsight to postprocess my images but there's one thing that i'm having trouble with in that a lot of my images are being left with either a halo gradient or a bright 'bar' along the bottom of the image (as can be seen in this pic of M97 and M108.

Anyone know what is causing  these gradients to be left after using either the PI dynamic or automatic background removal. Is it because i'm not using flat frames? but the strip along the bottom doesn't seem to be in the original fits.

Hi Dave,

I use flats and I think that they are good but I too end up having exactly the same thick bright bands  along the bottom of the image. So I too am interested in finding out why?

Regards,

A.G

[Dave_D]

ok, if you can't see what i'm refering to, here's a deliberately overstretched version.

[Langy]

Not knowing too much about this, but would the bottom of the image be where the horizon is? If so it could be that it's LP from the horizon.

I suspect that the software takes an average of the background to get it mostly black otherwise you lose details in the remaining image.

[Dave_D]

Not knowing too much about this, but would the bottom of the image be where the horizon is? If so it could be that it's LP from the horizon.

I suspect that the software takes an average of the background to get it mostly black otherwise you lose details in the remaining image.

it's possible, but to get the objects in frame like they are, the bottom of the frame as seen by the camera wasn't the closest to the horizon. unless i'm mistaken, it would have been the top left corner.

[Langy]

OK just thought I'd give a different view. It sounds more like an issue with the software then where it processes the bottom part of the image.

It might not be an easy test but if you could get images into the software with the bottom edge on a different side to see if it's coming from the software or the recording.

[Dave_D]

interesting... i've just looked at one of the individual RAW files and this is the info shown:

File Name IMG_3947.CR2

Camera Model Canon EOS REBEL T3

Firmware Firmware Version 1.0.5

Shooting Date/Time 23/01/2014 01:29:20

Owner's Name Dave Dennett

Shooting Mode Manual Exposure

Tv( Shutter Speed ) 60

Av( Aperture Value ) 0.0

Metering Mode Evaluative Metering

ISO Speed 800

Auto ISO Speed OFF

Image Size 4272x2848

Image Quality RAW

Flash Off

FE lock OFF

White Balance Mode Custom

AF Mode Manual focusing

Picture Style Standard

Sharpness 3

Contrast 0

Saturation 0

Color tone 0

Color Space sRGB

Long exposure noise reduction 0:Off

High ISO speed noise reduction 0:Standard

Highlight tone priority 0:Disable

Auto Lighting Optimizer Low

Peripheral illumination correction Enable

Dust Delete Data No

File Size 15300KB

Drive Mode Single shooting

Live View Shooting OFF

Camera Body No. xxxxxxxxxxx

Comment

just been reading this:

"What is Peripheral Illumination Correction all about?

Virtually all camera lenses are designed so that the volume of light transmitted to the image sensor tends to decrease somewhat from the center of the image to the corners. Many different terms are used to describe this phenomenon, such as “vignetting,” “light fall-off,” “unevenness of illumination,” etc.  Most experienced photographers are well aware of this common lens performance characteristic, and some take advantage of it for creative effects. Vignetting, for example, has been a popular artistic technique for centuries. It draws attention to a well-lit main subject by darkening the areas surrounding it. Intentional vignetting can be effective in a wide variety of photographic applications, including landscapes, portraiture, and advertising photography to name a few.

However, there are many other shooting situations where uneven peripheral illumination can be very distracting and undesirable. Examples include aerial photography, sports photography, seascapes, and any other kind of composition where consistent, even illumination across the frame is preferred. For these situations, the less falloff there is in an image, the better. Generally speaking, uneven peripheral illumination or light falloff is at its worst at the maximum aperture of the lens, whatever that happens to be. It could be f/1.2 with a fast prime lens, f/2.8 with a professional zoom lens, or even f/5.6 with a consumer-grade zoom lens. In most cases, peripheral illumination is also affected by the distance setting – it gets worse at infinity because the entire coverage of the lens is being used, but it gets better at closer distances because the lens is projecting a larger image towards the image sensor and as a result the sensor is effectively seeing a cropped view. In almost all cases, uneven peripheral illumination quickly diminishes as the aperture of the lens is stopped down. For most high-quality lenses today, uneven peripheral illumination is no longer a concern once the lens is stopped down by a couple of f/stops or more."

wondering if this could be anything to do with it??? guess i'll have to wait for the clouds to depart to find out...

[Langy]

Hopefully worth a shot. Yes the dreaded clouds , bring on the clear nights.

[lensman57]

interesting... i've just looked at one of the individual RAW files and this is the info shown:

File Name IMG_3947.CR2

Camera Model Canon EOS REBEL T3

Firmware Firmware Version 1.0.5

Shooting Date/Time 23/01/2014 01:29:20

Owner's Name Dave Dennett

Shooting Mode Manual Exposure

Tv( Shutter Speed ) 60

Av( Aperture Value ) 0.0

Metering Mode Evaluative Metering

ISO Speed 800

Auto ISO Speed OFF

Image Size 4272x2848

Image Quality RAW

Flash Off

FE lock OFF

White Balance Mode Custom

AF Mode Manual focusing

Picture Style Standard

Sharpness 3

Contrast 0

Saturation 0

Color tone 0

Color Space sRGB

Long exposure noise reduction 0:Off

High ISO speed noise reduction 0:Standard

Highlight tone priority 0:Disable

Auto Lighting Optimizer Low

Peripheral illumination correction Enable

Dust Delete Data No

File Size 15300KB

Drive Mode Single shooting

Live View Shooting OFF

Camera Body No. xxxxxxxxxxx

Comment

just been reading this:

"What is Peripheral Illumination Correction all about?

Virtually all camera lenses are designed so that the volume of light transmitted to the image sensor tends to decrease somewhat from the center of the image to the corners. Many different terms are used to describe this phenomenon, such as “vignetting,” “light fall-off,” “unevenness of illumination,” etc.  Most experienced photographers are well aware of this common lens performance characteristic, and some take advantage of it for creative effects. Vignetting, for example, has been a popular artistic technique for centuries. It draws attention to a well-lit main subject by darkening the areas surrounding it. Intentional vignetting can be effective in a wide variety of photographic applications, including landscapes, portraiture, and advertising photography to name a few.

However, there are many other shooting situations where uneven peripheral illumination can be very distracting and undesirable. Examples include aerial photography, sports photography, seascapes, and any other kind of composition where consistent, even illumination across the frame is preferred. For these situations, the less falloff there is in an image, the better. Generally speaking, uneven peripheral illumination or light falloff is at its worst at the maximum aperture of the lens, whatever that happens to be. It could be f/1.2 with a fast prime lens, f/2.8 with a professional zoom lens, or even f/5.6 with a consumer-grade zoom lens. In most cases, peripheral illumination is also affected by the distance setting – it gets worse at infinity because the entire coverage of the lens is being used, but it gets better at closer distances because the lens is projecting a larger image towards the image sensor and as a result the sensor is effectively seeing a cropped view. In almost all cases, uneven peripheral illumination quickly diminishes as the aperture of the lens is stopped down. For most high-quality lenses today, uneven peripheral illumination is no longer a concern once the lens is stopped down by a couple of f/stops or more."

wondering if this could be anything to do with it??? guess i'll have to wait for the clouds to depart to find out...

Could it be the stacking software? I use DSS.

A.G

[Alien 13]

I can only suggest what has already been said lack of flats and the Peripheral illumination correction setting, it could be worth running a few subs through DSS to see its effect.

Alan

[harry page]

Hi

Could you let me have a look at you fits stack and I will have a look and see what the problem is

Harry

[Dave_D]

Hi

Could you let me have a look at you fits stack and I will have a look and see what the problem is

Harry

sent you a pm with the link

just watching your PI tutorials btw... very helpful 

[Dave_D]

Hi Dave,

I use flats and I think that they are good but I too end up having exactly the same thick bright bands  along the bottom of the image. So I too am interested in finding out why?

Regards,

A.G

And the answer shall appear as if by magic...

Apparently, it's noise from the camera electronics http://lightvortexastronomy.blogspot.co.uk/2013/12/tutorial-post-processing-pixinsight.html 

just above 'STEP 4. Extracting a Luminance image from the RGB image'

i read somewhere that the internal battery generates heat and affects the image. anyone know anything about this?

[lensman57]

And the answer shall appear as if by magic...

Apparently, it's noise from the camera electronics http://lightvortexastronomy.blogspot.co.uk/2013/12/tutorial-post-processing-pixinsight.html

just above 'STEP 4. Extracting a Luminance image from the RGB image'

i read somewhere that the internal battery generates heat and affects the image. anyone know anything about this?

Hi Dave,

Many thanks for the link but I am  not only using DSLR but the problem is there even when I use my 428EX, no chance of the battery heating things up with the Atik @ -20C .

Regards,

A.G