Flat troubles with dark corners - Forums

Topic starter

Nov 9, 2020

I'm having trouble with using flats to get flat images with a full frame sensor and 2 inch filters. Due to the large size of the of the sensor and the size of the image circle, the corners on my shots are significantly less illuminated than the center of the image. This seems to be causing trouble when using flats to get a flat image. I am having the most trouble with broadband filters and OIII specifically. HA and SII seems to come out flat regardless.

I use SGP to take my flats. In the past, I've used the flat wizard to take flats and get a centered mean ADU value. This works great if your sensor is not blocked in any way and the imaging circle illuminates all of the sensor. This method does not work if your sensor is not fully illuminated and the edges are significantly darker. I have found lower mean ADUs works great for HA, SII (~14k ADU). I have had a lot of difficulty with broadband filters.

I have gone through the process of taking flats with my filters at differing mean ADU values in increments of 1000. Depending on the exposure length of my light frames and how bright the target is, I have to select the best matching flat set. This is very tedious and I do not know of a method to calculate the best set of flats I will need.

For my testing, I've been using the batch pre-processing script in PixInsight to generate my stacked images. I am hoping there is some methodology or setting that can help me with the process of getting a flat image. Here are some images to illustrate my struggles:

Flat (B) ~10k ADU:

Light (B):

Light (HA):

HA always seems to come out looking flat after stacking using a 14K mean ADU on the flats.

Helpful

andrea tasselli

Frequent contributor

Nov 10, 2020

I'd suggest to shoot flats with 2/3 of the histogram covered, given that you have scant illumination at the corners (something due to your optical train so nothing you can do about it). You need enough signal at the corners to have any hope of a decent flat and then it will still be hard I'm afraid. Myself I've never bothered with anything less than 1/2 of the histogram covered in my standard practice. Frankly I don't even understand why bother with scaled flats at all.

Helpful

Iñigo Gamarra

Nov 10, 2020

I had the same problem in the past, especially with broadband, some friends from Astrobin recommended me to raise the explosion time of the planes to 5 seconds while maintaining a medium range, it worked very well for me.Hope it can help.

CS Iñigo.

Eric Beckinger

Topic starter

Nov 11, 2020

Iñigo Gamarra:
I had the same problem in the past, especially with broadband, some friends from Astrobin recommended me to raise the explosion time of the planes to 5 seconds while maintaining a medium range, it worked very well for me.Hope it can help.

CS Iñigo.

.Thanks for the tip. How did you go about accomplishing this? You would need a light source but very faint. Any recommendations?

mjgood

Nov 11, 2020

I have the same issue using a full frame camera as well... I found this post on how to adjust the flat using pixelmath. I was able to correct the dark corners out of my images..

https://www.cloudynights.com/topic/685094-tuning-flats-in-pixinsight-using-pixelmath-process/

hope this might help

Iñigo Gamarra

Nov 11, 2020

Indeed, at the moment I use a very low-priced light sheet, it has three intensities but to attenuate it more I use the potentiometer on the heating tape. It is not perfect but it works, in the next few days I will receive the Artesky 250mm flat field generator, it is not cheap, but it calculates and memorizes the brightness of each filter.

Hannah

Dec 9, 2020

Hi my first post on the site so forgive me if I have missed anything.
I am relatively new to astro imaging and use a SW200p with a full frame Canon 6D, and getting images similar to yours.
I have just replaced the M42 mount with a 2" to M48 unfortunately I have not been able to take any deep sky images since doing that but I do have this, which shows the improvement.

Related discussions

Trouble with Deep Sky Stacker applying flats correctly

I'm having trouble with stacking photos lately and not sure what changed. I'm seeing dust mote artifacts in stacked & calibrated images that shouldn't be there. Below is an image straight out of DSS. No further editing, STF applied. Y...

Dec 9, 2024

Both posts describe technical problems with astrophotography image processing where calibration frames (flats) are not properly correcting optical issues like vignetting and dust artifacts.

extreme offset vignetting in flats and lights

This issue has persisted for a while now through at least 3 set ups and I honestly cannot for the life of me figure out what is causing this. Here is an image of one of my flats which displays the extreme offset in vignetting in my imaging train whic...

17 days ago

Both posts discuss problems with vignetting and uneven illumination affecting astrophotography flat field correction.

Focuser trouble when NINA starts

Hello, I’m new to NINA (was using ASIAIR before), I was able to connect and set everything and to perform two nights of imaging. Here is my issue, the last two times I turned all my equipment on, my focus was lost (I mean stars wereway out of focus, ...

Sep 3, 2025

Both posts discuss technical challenges with astrophotography equipment setup and image acquisition processes.