15 replies to this topic

[MjNorthstar]

Hey everybody,  I've now have 3 nights of imaging with my new setup and after getting a decent amount of data I'm noticing a dark vertical band in the lower left hand side of the image when processing.

 

This is an overstretched starless image of 5.5 hrs of data over 3 nights using the flats from night 1 (the image train was not touched between nights).

I used asiair auto mode for flats using an ipad as a light panel with no diffuser.  I tried stacking each night separately and this dark band shows up on all of them.  This photo was stacked in Siril, but I get the same result using DSS.

 

I'm wondering if I somehow messed up taking flats or could this be something completely different?

I would have rotated the camera and taken frames to compare, but it's been very cloudy lately, so I thought I'd ask and see if there is anything obvious I'm missing.

Thanks for any advice.

 

 

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[WadeH237]

It doesn't look like a flat calibration error to me.  Flat calibration problems are typically over or under correction, or circular gradients.

 

If I were troubleshooting this, I would do these things, in this order:

 

Register and stack the raw files, with no calibration.  This would tell you immediately if the problem has anything to do with calibration, or if it's in the light data.  If the problem shows up in the lights, then you are looking at some kind of camera problem.  That said, it doesn't look like a camera problem to me.

 

Assuming that the problem doesn't exist in the lights, the next thing to do is calibrate with just darks and stack them.  If the problem shows up here, then you know that the problem is in your dark frames.  I see this as a possibility.  For example, if there was a light leak in some or all of the dark frames, it would over correct in that spot, resulting in a dark area in the calibrated files.  The fix for this would be to retake your darks and ensure total darkness.

 

Note that some people calibrate modern cameras with bias instead of darks.  If that describes your work flow, just do the same test as the above, but with bias instead of darks.

 

You should have a much better idea of where the problem lies once you have done this.

[fmendes]

I usually take flats for each night, as dust may accumulate or move from one night to the other.

 

About flats, I recently had a big problem, that turned out to be due to internal reflexes and light reaching the camera without passing by the mirrors (on a Newtonian). 

 

A few things to ckeck:

- See if any frame is defective and messing with the whole stack. On light frames, look for airplane or satellite trails.

- With Pixinsight, I sometimes apply ABE (Automatic Background Extraction) on my flats to visualize underlying hard gradients. Tried to do something similar with Siril, but didn't work quite as good. A good substitute may be Graxpert with smoothing=1. That defect will likely show with that strategy.

- Try sky flats instead of panel flats. Use a paper sheet (white shirt, etc)in front of the telescope and point to the zenith. I don't like LCD screens because some of them flicker (I heard iPads do not flicker, but may be worth to try not using it).

[WadeH237]

About flats, I recently had a big problem, that turned out to be due to internal reflexes and light reaching the camera without passing by the mirrors (on a Newtonian). 

Sure, but the problem here almost certainly has nothing to do with flats.

 

This could be completely prove or disprove whether flats are involved by processing the stack twice, once with flats and once without, and comparing the results.  This should certainly be done before making any changes to either the system or how flats are acquired.

[MjNorthstar]

Thank you for the reply on the process to figure this out.

 

I got my lights stacked and it seems everything is ok. I don't see anything out of the ordinary, just some vignetting.  As I have time, I'll work on the darks and see how those turn out.

I have a quick question though.  How many darks should I be taking?  I've heard with the 2600mc I could get away without using any, but with Siril I need at least 4 in the folder to run the stacking scripts.

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[WadeH237]

That looks pretty good.  Is it the same data from the result you first posted?

 

If so, then we can rule out the camera as the source of the problem.  The only risk here is that I am reaching my conclusions based on an assumption that you are changing one and only one thing each time you process.

[MjNorthstar]

I found it!  There is definitely something going on with my flats.

 

I suppose my next question is what is the best method for taking flats?  Do I keep the Ipad and use the paper/tshirt method?  Do I use daytime sky flats? Or is there a specific flat panel that's popular within the AP community?

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[fmendes]

I found it!  There is definitely something going on with my flats.

 

I suppose my next question is what is the best method for taking flats?  Do I keep the Ipad and use the paper/tshirt method?  Do I use daytime sky flats? Or is there a specific flat panel that's popular within the AP community?

 

The problem is with the flats, that I was almost sure. The underlying problem may be a different issue. I had problems with shiny surfaces on the coma corrector (the equivalent to a field flattener): 

Before:

https://www.cloudyni...2#entry14169483

 

After:

https://www.cloudyni...2#entry14169598

 

After I got it fixed, I can do flats with whatever.

 

You can start cheap with sky flats+tshirt or paper. Try what I suggested: open your flat with Graxpert background extraction, smoothness=1, subtraction. That will likely make evident any defects. You may have a light leak, or an internal reflex. In my opinion, that's the culprit.

 

As for flat panel, I'm using the Gemini flat panel. It works fine, but you need to put it on top of the telescope every time: https://www.aliexpre...8564537169.html

[Phil Sherman]

Your problem is should not be your flats. I ran an analysis of the flat you posted and there's no evidence of the bright stripe in the flat that is necessary to cause the dark area in your images.

 

If your naming convention of the images in your prior post is accurate, then the naming convention indicates a probable processing error. Flats can not be directly applied to lights. Your processing needs to include, at a minimum, lights, bias, and flats. DSS should apply the bias to both the lights and flats during its processing of the stacks. You can get away with not using darks if your light+bias output meets your requirement for a reasonable number of hot pixels.

[WadeH237]

Your problem is should not be your flats. I ran an analysis of the flat you posted and there's no evidence of the bright stripe in the flat that is necessary to cause the dark area in your images.

Agreed.

 

In the sample image posted, the flats are strongly over correcting.  This is probably revealing, but not causing, the stripe.

 

The flat over-correction is almost certainly user error in doing the calibration.  I would love to help with specifics, but I've never used Siril, so I could suggest what needs to be done, but not how to do it in Siril.
 

[fmendes]

I found it!  There is definitely something going on with my flats.

 

I suppose my next question is what is the best method for taking flats?  Do I keep the Ipad and use the paper/tshirt method?  Do I use daytime sky flats? Or is there a specific flat panel that's popular within the AP community?

 

Do you happen to have the stacked files, both the complete with darks, bias and flats, and the master flat? Can you share them?

 

Again, I use Pixinsight. The problem I was having (documented above) could be seen not on the flats. They looked pretty normal. But when I applied background neutralization, the problem appeared. So I applied bachground neutralization (I did Pixinsight's ABE, but GraxPert with smoothness=1 yields similar results) to the flats to see what would happen. This was the result:

 

On the left, my flat frame. On the right, the same frame with GraxPert/smoothness=1

 

And this is a link to my raw stacked image if anyone wants to take a look: https://drive.google...iew?usp=sharing . A similar gradient to what's seen on the image on the right can be seen on the stacked image.

 

The problem was not exactly on my flats, but on how light behaved entering my telescope. Light emmited from far will reach my telescope at a certain angle. Light from close (such as the flat panel) will reach the telescope differently. 

 

I still suspect the issue that you're having is that the light of the device you're using for flats is reaching places where it shouldn't. So while the issue, while not being strictly speaking "with the flats", can be mitigated with "how flats are taken", or fixing the underlying problem, that is some light leak or obstruction to the light path.

[MjNorthstar]

With Siril, you can do manual stacking or there is a script you can run that will do it all for you. I haven't learned how to manually stack there yet so I've been using scripts.  When stacking, it creates a folder where it puts the master biases, darks, and flats.  It then gives the combined stacked image.

I ran a second script that stacked the lights only.

These are large files, so I'm linking them separately.

 

Stacked image  https://drive.google...iew?usp=sharing

Master Light  https://drive.google...iew?usp=sharing

Master Dark  https://drive.google...iew?usp=sharing

Master Bias  https://drive.google...iew?usp=sharing

Master Flat  https://drive.google...iew?usp=sharing

 

[MjNorthstar]

I still suspect the issue that you're having is that the light of the device you're using for flats is reaching places where it shouldn't. So while the issue, while not being strictly speaking "with the flats", can be mitigated with "how flats are taken", or fixing the underlying problem, that is some light leak or obstruction to the light path.

 

I was able to do some imaging the other night and decided to take sky flats the next morning.  I used the t-shirt method and in order to get the correct histogram, I had to double up the fabric so it wouldn't overexpose.  After processing the data from that night everything looks good so I think I'll keep with this method for the time being.
 

[fmendes]

For one thing, your flats were too bright:

 

Ideally you want to keep your flats' peak between 20 and 50% of the range. Yours is peaking around 90%. That will capture every uneveness of your system. 

 

Looking at the master flat after background extraction, yes, the annoying smudge was there. But so it was on your lights without flats, although weaker:

 

 

Still I managed to get a decent image from your data. Needed to darken the background more than I like, but didn't even clip much:

 

 

Colors look funny because I didn't bother testing what sensor/filter you used. But I'm sure if your flats didn't peak so high, you'd have no problems processing this image. If you want to further improve your imaging, chase all light leaks and internal reflexes.

Edited by fmendes, 20 June 2025 - 09:02 PM.

[MjNorthstar]

Thanks for taking the time to go through and process this.

 

 

For one thing, your flats were too bright:

It surprised me to find the flats were so bright since I do watch the histogram, but after thinking about it there is a setting on the AIR that allows autostretch on the preview and you can zoom in on the histogram.  I'm guessing one or both of these were turned on which visually shifted the histogram left which made me think it was darker than it actually was.  I'll be paying more attention to that in the future.

 

If you want to further improve your imaging, chase all light leaks and internal reflexes.

I don't know if it's common on telescopes, but the inside surface of my dew shield is covered in some sort of microfiber/felt material to prevent reflections.  Pretty neat.
 

[fmendes]

I don't know if it's common on telescopes, but the inside surface of my dew shield is covered in some sort of microfiber/felt material to prevent reflections.  Pretty neat.

 

 

Refractors are mostly fine in that regard. Sometimes the focuser leaks a bit of light, but it's only a problem if you take flats during the day. Newtonians leak a lot of light everywhere.