Banding issue when taking flats with light box

JoelRed (Joel) · #1 08-06-2022, 01:07 PM

Hi all,

Hoping someone can help me with an issue I've been having while using a recently purchased light box.

When attempting to take flat frames using the newly purchased light box, I get a horizontal banding issue on all my frames. This only occurs when using the light box, not when using the old t-shirt and sky method. Nor does this banding show up in frames of any other description (light, dark, bias etc.)

The light box used LEDs and is powered using a 5.5x2.1mm 12V DC connector with a variable resistance dimmer in-line. I have tried powering it directly from my ASIair and also independently from a 12V DC Li-Ion battery pack.

I have varied the intensity using the included dimmer all the way from full bright to full dim with exposure times varying from 110ms to 5sec, the banding persists throughout.

I have tried rotated the light box 90 degrees and the banding remains horizontal.

I have also tried taking the dimmer out and using the inbuilt PWM function of the ASIair to no avail.

I have attached some photos to help demonstrate what I am talking about. The banding is not a consistent pattern and it changes with every frame. Please forgive the poor image quality, its hard to get them below 200kb!

The relevant equipment I am using is as follows:

Esprit 120ED
ZWO ASI2600MM Pro
ZWO ASIair Pro
12V/10A AC to DC power adapter
Powertech 12V DC Li-Ion portable power centre

If anybody has any ideas of whats going on here, it would be greatly appreciated!

Cheers,

multiweb (Marc) · #2 08-06-2022, 01:16 PM

My guess would be a USB cable running close to a power cable with a pulse. Like a dew heater. Some EL panels flicker as well when you lower the voltage to dim them and that can cause banding in the same fashion.

JA · #3 08-06-2022, 01:57 PM

I think it may have something to do with LED flicker being picked up by the imaging system. It may be that pulse width modulation is used in the dimming circuit of the lamp or there is AC in its power supply. To the human eye with its relatively slow response the light may appear continuous and appears as always ON, but if recorded with video or a suitably short exposure you may, if I'm correct see the truth of a changing illumination pattern, some rows on, others off then changing....possibly. Why others may not see this phenomenon would depend on the nature of the LED source and camera type and exposure. Try increasing the exposure duration in steps say: your initial setting, then 2x, then 4x, then 8x.... what differences do you see? If you see a more integrated or whole pattern as you increase the exposure duration that is a sure sign of what I suggested. You could also use an incandescent light source with white diffuser or t-shirt to verify that its the LED light source which is the issue due to its inherent flicker/fast response compared with the pedestrian speed of an incandescent source. If that shows no change (lines still present) then it's a camera issue or interaction

It could also be an interaction / sort of interference effect between the led row pattern and the camera pixel pattern and associated frame rate/exposure duration and refresh rate.
Best
JA

AdamJL · #4 08-06-2022, 02:28 PM

Subscribing. I'm having the same issue with a Pegasus Flatmaster.

Evans replied to me with the following (I haven't yet replied, but will)

Quote:

Hi Adam,
What is the exposure that you use for the calibration flat frames?
Also which is your camera and what is the gain/offset you have set?
This has to do with the LED sync frequency that matches your camera frame rate.
Regards,
Evans

I've tested out USB cables and tried running the Pegasus off a different power source with no change. It still bands.

JoelRed (Joel) · #5 08-06-2022, 02:56 PM

Quote:

Originally Posted by multiweb

My guess would be a USB cable running close to a power cable with a pulse. Like a dew heater. Some EL panels flicker as well when you lower the voltage to dim them and that can cause banding in the same fashion.

Are you suggesting the USB cable is affecting the power cable running to the light box and making it flicker, or the power cable affecting the USB from the camera, creating the banding?

Quote:

Originally Posted by JA

I think it may have something to do with LED flicker being picked up by the imaging system. It may be that pulse width modulation is used in the dimming circuit of the lamp or there is AC in its power supply. To the human eye with its relatively slow response the light may appear continuous and appears as always ON, but if recorded with video or a suitably short exposure you may, if I'm correct see the truth of a changing illumination pattern, some rows on, others off then changing....possibly. Why others may not see this phenomenon would depend on the nature of the LED source and camera type and exposure. Try increasing the exposure duration in steps say: your initial setting, then 2x, then 4x, then 8x.... what differences do you see? If you see a more integrated or whole pattern as you increase the exposure duration that is a sure sign of what I suggested. You could also use an incandescent light source with white diffuser or t-shirt to verify that its the LED light source which is the issue due to its inherent flicker/fast response compared with the pedestrian speed of an incandescent source. If that shows no change (lines still present) then it's a camera issue or interaction

Best
JA

I feel like this is the most likely culprit.

I have tried two different ways of dimming the box, one being the inbuilt function of the ASIair (which I believe is PWM) and also the dimmer that came with the light box. The included dimmer is of the variable resistance variety, I am no electrical engineer, but I don't believe that it's the same as PWM, correct?

I was using the automatic exposure function for flats on the ASIair and was varying the exposure time using the dimmer. I will experiment today with more precise exposure differences and see what happens.

Flat frames taken with the sky and t-shirt as well as using an white screen on an iPad do not replicate the banding.

multiweb (Marc) · #6 08-06-2022, 03:47 PM

Quote:

Originally Posted by JoelRed

Are you suggesting the USB cable is affecting the power cable running to the light box and making it flicker, or the power cable affecting the USB from the camera, creating the banding?

I mean the USB cable that's connected from the camera to your computer. Is it running close or bunched up to a power cable nearby? The power cable interfere with the data that comes through the USB cable. You can use ferrite blocks on your USB cables to minimise this. Also try to have the camera and the EL panel on separate power circuits see if that helps.

JoelRed (Joel) · #7 08-06-2022, 03:56 PM

Quote:

Originally Posted by multiweb

I mean the USB cable that's connected from the camera to your computer. Is it running close or bunched up to a power cable nearby? The power cable interfere with the data that comes through the USB cable. You can use ferrite blocks on your USB cables to minimise this. Also try to have the camera and the EL panel on separate power circuits see if that helps.

The USB cable from the camera to the ASIair runs right next to the 12v DC cable that powers the cameras cooler.

However, I don't believe this is the issue as it has been this way since the beginning and I've never had this issue before. The only thing that has changed is the addition of a light box. The issue persists when the light box is powered completely independently also.

Startrek (Martin) · #8 08-06-2022, 04:41 PM

You mentioned using the white T shirt for sky flats , are you using a white T shirt for the light box ??

I use a few different sized Huion dimmable led light panels and a white T shirt is a must to eliminate low brightness micro flicker and provide a even flat field.
Most of these LED panels and light boxes have a constant voltage ( not constant current) power supply which can cause issues at low illumination levels.

Just a thought !!

JoelRed (Joel) · #9 08-06-2022, 05:40 PM

Ok, been experimenting today and I have a little bit of an update.

Started off by trying the light box plugged directly into the battery with no dimmer.... No banding!

Next I plugged the light box directly into the ASIair port (running at 100%), without the dimmer attached.... No banding also!

Next I set the ASIair power output to 50%.... Banding returns. This was expected as the ASIair uses PWM to vary the power output to light boxes and dew heaters.

Following that I plugged the light box back into the battery and reconnected the included dimmer.... Banding was back again.

After a quick google search of the dimmer switch, I discovered that it also uses PWM to dim.

Obviously I am happy that I have discovered what is causing the banding issue, and I can indeed use the light box with t-shirts or pillowcases to dim it.... but the whole point of getting the light box was to make the task of taking flats simple.

So now I have another questions, does anyone know another way to dim LEDs without using PWM?

bojan · #10 08-06-2022, 06:03 PM

I am using clear blue sky before dark for flats...

Exfso (Peter) · #11 08-06-2022, 08:55 PM

Joel another way to make the image dimmer is to add paper sheets over the diffuser, this is what one guy did when he had a too bright diffuser image, as I said in our emails, I have never encountered this before, interesting to say the least

The light you see coming through the diffuser in my light boxes is not direct led illumination, as their output is aimed at the white rear interior and you see this. it is like looking at a projected screen through the diffuser.