Mild glow around edges

M

mventi1

Active member
So as I mature in this software I discover more and more as I process. This is just a stacked image in PI format. Does anyone know what is the mild gray glow around the edges? I get the same result when I stack in Siril so I am guessing i am doing something wrong in the imaging process. Link below to images xisf from PI and FITS from SIRIL

PixInSight - Google Drive

drive.google.com drive.google.com

Mick
 
This looks like a problem with flat correction. What flat source did you use? Could you upload samples single of dark, flat and light frames.
 
fredvanner said:
This looks like a problem with flat correction. What flat source did you use? Could you upload samples single of dark, flat and light frames.
Click to expand...
Hey Fred,
I just uploaded 5 of each of the subs. Also how I take the flats: I put a T-shirt over my scope and use a light board over that. I use the ASIAIR and the Autorun feature. I set it to 30 exposures and the exposure time to auto. I then turn the light board off and do the same for the flat_darks using the same exposure time as the flats. I do this while it is still dark out as well.
Thanks,
Mike
 
Last edited:
Mike,

The reason your Flats are not correcting as they should is that your Dark frame is showing a significant light leak. The actual Dark frame looks more like a Flat frame when you examine the illumination profile using the FlatContourPlot script. Below is a screen shot of your Dark frame's light patterns.

Dark_60_0s_Bin1_533MC_gain101_20240430_183752_10_1C_0001_contourPlot.jpg


The effect this will have is to make the Flat fail to correct properly in ImageCalibration. The Flat could still be a cause but until the Darks are correct, you will not be able to tell for sure how well the Flats really work.

Very often such light leaks are seen when using a Newtonian reflector for imaging. If your imaging scope is some form of Newtonian, you will need to track down the source of the light leaks. The usual culprits light entering from the back of the tube, light getting in around the focuser-to-tube attachment point, or incomplete sealing of the front aperture while taking Darks. You may do better taking Dark frames with the camera off the telescope and a metal cap on the camera. If the camera came with a plastic cap rather than metal, you should line the plastic camera cap with a single layer of aluminum foil to ensure that no infrared light can pass through the plastic.

John
 
jdupton said:
Mike,

The reason your Flats are not correcting as they should is that your Dark frame is showing a significant light leak. The actual Dark frame looks more like a Flat frame when you examine the illumination profile using the FlatContourPlot script. Below is a screen shot of your Dark frame's light patterns.

View attachment 23059

The effect this will have is to make the Flat fail to correct properly in ImageCalibration. The Flat could still be a cause but until the Darks are correct, you will not be able to tell for sure how well the Flats really work.

Very often such light leaks are seen when using a Newtonian reflector for imaging. If your imaging scope is some form of Newtonian, you will need to track down the source of the light leaks. The usual culprits light entering from the back of the tube, light getting in around the focuser-to-tube attachment point, or incomplete sealing of the front aperture while taking Darks. You may do better taking Dark frames with the camera off the telescope and a metal cap on the camera. If the camera came with a plastic cap rather than metal, you should line the plastic camera cap with a single layer of aluminum foil to ensure that no infrared light can pass through the plastic.

John
Click to expand...
Hi John,
Thank you for the effort and reply. I uploaded a pic of my rig to the same drive you got the pics at. For my darks I take the camera and put the plastic cap over it for the 60 sec darks. I do this in the dark of night as well. They seem ok to me. For the flats i use a pillow case with a light board over the pillow case. I make sure the exposure is in the middle of the histogram. I take those when it is still dark outside. Then the flat_darks or biases, I really have no idea why they are called different names, I take with the cover over the scope and the same exposure time as the flats. I really don't see where I can be getting light from.
Standing by,
Mike
 
mventi1 said:
Hi John,
Thank you for the effort and reply. I uploaded a pic of my rig to the same drive you got the pics at. For my darks I take the camera and put the plastic cap over it for the 60 sec darks. I do this in the dark of night as well. They seem ok to me. For the flats i use a pillow case with a light board over the pillow case. I make sure the exposure is in the middle of the histogram. I take those when it is still dark outside. Then the flat_darks or biases, I really have no idea why they are called different names, I take with the cover over the scope and the same exposure time as the flats. I really don't see where I can be getting light from.
Standing by,
Mike
Click to expand...
Mike is correct that the dark should not be showing this vignetting pattern. There may be another explanation than a light leak, but that is a reasonable one. Note that black plastic lens caps are often rather translucent in near IR, so you should not assume they will actually block all the ambient light (and even in the typical dark there is usually still enough light to expose pixels in these sensitive cameras). So using aluminum foil or some other truly opaque material is a good suggestion.

The term "flat dark" is deprecated. Flats are calibrated with either darks or with biases. Biases are essentially just darks with virtually zero exposure time. Your sensor should not require that you create darks for your flats. Bias frames should work just fine, and are easier to take and to manage.

Your short exposure darks here don't show the same pattern, presumably because they simply aren't exposed long enough for any extraneous light to make it through.
 
cloudbait said:
Mike is correct that the dark should not be showing this vignetting pattern. There may be another explanation than a light leak, but that is a reasonable one. Note that black plastic lens caps are often rather translucent in near IR, so you should not assume they will actually block all the ambient light (and even in the typical dark there is usually still enough light to expose pixels in these sensitive cameras). So using aluminum foil or some other truly opaque material is a good suggestion.

The term "flat dark" is deprecated. Flats are calibrated with either darks or with biases. Biases are essentially just darks with virtually zero exposure time. Your sensor should not require that you create darks for your flats. Bias frames should work just fine, and are easier to take and to manage.

Your short exposure darks here don't show the same pattern, presumably because they simply aren't exposed long enough for any extraneous light to make it through.
Click to expand...
CB,
Good to hear from you again. My dark_flats or biases are just a fast exposure the same as the flats. Should I do them with the camera removed and an opaque cover like I do my darks? If you remember from earlier chat when Fred told me my darks were a bit old so I redid them. When I did the red glow went away and the pics were fine. Shortly after that this phenomena started. I agree i have something wrong with my process due to as mentioned I see the same thing in Siril as PI. Just not sure where to go look for the leak. Also, the pic in the folder with the link up top shows my rig. If there is an obvious area to check I am open ears.
Thanks,
Mike
 
Mike,

Since you are using an SCT and taking the camera off the telescope for your Darks, then the light leak issue is most likely IR light passing through the plastic camera cap. This is a reasonably common issue. Just take a single layer of aluminum foil and place it loosely over the camera nose-piece, then put the plastic cap on over that. It will allow you to take Darks frames of almost any length with no light (even IR) getting to the sensor.

There is one more place that light might leak into the camera when taking Darks but is more rarely seen. If your ASI533MC camera has a built in tilt plate like the ASI2600Mx cameras I am familiar with, a few users have reported that light can leak into the gap between the main camera body and the attached tilt plate. If using aluminum foil in the camera cap does not eliminate all the light leakage, then try wrapping a single layer of black (plastic-type) electrical tape around the area where the tilt plate and the camera's main body meet. That will prevent any light from entering that area.

Because your short exposure Darks (Flat-Darks) for the Flats are so short, you need not change what you are doing. The short Darks look good and are not exposed long enough for any leak of similar amount to be a problem.

John
 
jdupton said:
Mike,

Since you are using an SCT and taking the camera off the telescope for your Darks, then the light leak issue is most likely IR light passing through the plastic camera cap. This is a reasonably common issue. Just take a single layer of aluminum foil and place it loosely over the camera nose-piece, then put the plastic cap on over that. It will allow you to take Darks frames of almost any length with no light (even IR) getting to the sensor.

There is one more place that light might leak into the camera when taking Darks but is more rarely seen. If your ASI533MC camera has a built in tilt plate like the ASI2600Mx cameras I am familiar with, a few users have reported that light can leak into the gap between the main camera body and the attached tilt plate. If using aluminum foil in the camera cap does not eliminate all the light leakage, then try wrapping a single layer of black (plastic-type) electrical tape around the area where the tilt plate and the camera's main body meet. That will prevent any light from entering that area.

Because your short exposure Darks (Flat-Darks) for the Flats are so short, you need not change what you are doing. The short Darks look good and are not exposed long enough for any leak of similar amount to be a problem.

John
Click to expand...
John,
I am a bit confused and apologize. I take the short darks that same way i take the flats. In the dark of night i put the scope cap on the front and take them directly after I take the flats. Both are done on the full rig. Flats with white pillow case and light board and the dark shorts with the scope cap on. From what I am understanding is my flats have a lighting issue and I am not sure where to look further. The only place I can see light my enter is the Edge 8HD has cooling vents on the tube after of the primary mirror. Thoughts?
Mike
 
Mike,

I am sorry if I was not clear. Let me try again.
  • The 60 sec Darks are the main problem here. They need to be taken again. Match all the parameters you used before. Take the new Dark frames with the camera off the telescope and capped with an aluminum foil layer lining the camera cap.

  • The Flat frames are OK to take just like you described. Put the pillow case and light source over the telescope aperture to shoot the Flats.

  • The short Darks to match the exposure of the Flats are short enough to just put on the telescope cover as you describe. For the very short exposures, the short Dark frames are not showing any effects for leaking light. If you start shooting longer Flat exposures, you may need to take the camera off the telescope and shoot Bias frames just as the Darks need to be shot (#1 above).
Until the Dark are cleared of any light leaks, don't worry about something being wrong with the Flats. In fact, they looked good when I looked at your uploaded frames. The over-correction of the vignetting went away when I substituted a synthetic 60 sec Dark frame in Image Calibration using your other frames. That makes me think your Flats are probably good and it is just the bad 60 sec Dark frames that are causing the flat fielding issue due to leaking light.

John
 
jdupton said:
Mike,

I am sorry if I was not clear. Let me try again.
  • The 60 sec Darks are the main problem here. They need to be taken again. Match all the parameters you used before. Take the new Dark frames with the camera off the telescope and capped with an aluminum foil layer lining the camera cap.

  • The Flat frames are OK to take just like you described. Put the pillow case and light source over the telescope aperture to shoot the Flats.

  • The short Darks to match the exposure of the Flats are short enough to just put on the telescope cover as you describe. For the very short exposures, the short Dark frames are not showing any effects for leaking light. If you start shooting longer Flat exposures, you may need to take the camera off the telescope and shoot Bias frames just as the Darks need to be shot (#1 above).
Until the Dark are cleared of any light leaks, don't worry about something being wrong with the Flats. In fact, they looked good when I looked at your uploaded frames. The over-correction of the vignetting went away when I substituted a synthetic 60 sec Dark frame in Image Calibration using your other frames. That makes me think your Flats are probably good and it is just the bad 60 sec Dark frames that are causing the flat fielding issue due to leaking light.

John
Click to expand...
John,
Thanks for the detail. Currently working new darks now and will update. Have camera off of scope, plastic cap on, camera face down on dark wooden table with cooler on.
Mike
 
mventi1 said:
John,
I am a bit confused and apologize. I take the short darks that same way i take the flats. In the dark of night i put the scope cap on the front and take them directly after I take the flats. Both are done on the full rig. Flats with white pillow case and light board and the dark shorts with the scope cap on. From what I am understanding is my flats have a lighting issue and I am not sure where to look further. The only place I can see light my enter is the Edge 8HD has cooling vents on the tube after of the primary mirror. Thoughts?
Mike
Click to expand...
Your flats don't seem to be the problem. It's your darks that don't look right, and the longer the exposure, the worse the problem. Which does suggest a light leak of some kind.
 
mventi1 said:
John,
Thanks for the detail. Currently working new darks now and will update. Have camera off of scope, plastic cap on, camera face down on dark wooden table with cooler on.
cloudbait said:
Your flats don't seem to be the problem. It's your darks that don't look right, and the longer the exposure, the worse the problem. Which does suggest a light leak of some kind.
Click to expand...

Mike
Click to expand...
OK I just uploaded a screenshot with on the left the old dark and on the right the new dark. I do see a difference in the contours. Going to process an image now.
 
Update: I just uploaded 5 of the new darks I just took. I processed mine as well and no more gradient. Thanks for the help. I look forward to hear what you found in the new darks.
Mike
 
Mike,

With the new Dark frames, the data looks very good. I see no hint of residual vignetting after calibrating with the new Darks. The Flats (and corresponding short Darks) you had were all good and the only issue was the light leak ruining the initial set of Dark frames. Following calibration, I see some large scale mottling in the five mage integration but the only orderly gradient I can see is a small linear light pollution gradient in the stacked image.

John
 
You must log in or register to reply here.
Back
Top