asi1600mm cool - flats horizontal bands

[Andyb90]

Hi Everyone,

I've got myself a light panel and had a first go at producing some flats for my asi1600mm cool camera.
 
I've used the flats calibration wizard in SGPro and ran it for my Baader Ha filter. Here is the result from the wizard with an average of around 22,000 ADU, which seems about right:

But the resulting flat frame with exposure 0.55 sec, gain 300 has horizontal bands on it. Here is a screenshot from within SGPro:

The light frames and dark frames that I've created so far with the camera don't show these bands on the flat frames. I thought it might be the light panel so I changed the orientiation of it but the lines are in the same position so it can't be the panel. I also get this banding with my other filters including my Baader Lum.

Has anyone seen this with their asi1600mm cool? Is there something I need to do to remove the banding?

Andy.

[Filroden]

I noticed it for the first time yesterday and it seemed to occur on shorter exposures of less than about 2 seconds. I also assumed it was my light source and didn't have time to test. 

[Andyb90]

This is information I recieved from CN:

"Is the flat panel running off LED? If so, LEDs are usually controlled by PWM, which results in the light oscillating on and off at a high rate. Some cheaper PWM only operate in the 100s of hertz range, which can interfere with the rolling shutter rate of these modern CMOS cameras at higher frame rates. The best solution would be to get or build a flat panel that uses a PWM operating in the KHz to Mhz range, or diffuse the panel and/or scope more so you can use flats that are a few seconds long to average out the oscillations."

I've covered the light panel with some white paper to diffuse the light and tried a longer exposure of 4.3 seconds. I can't make out the banding on the resulting image any longer.

Although is putting white paper over the light source a legitimate way of diffusing the light for flats?

I think as a longer term solution I'll look for a light panel with a higher frequency.

Andy.

[hughgilhespie]

Hi Andy,

I am almost certainly getting this wrong, but doesn't an exposure of 22,000 adu mean that you have exceeded the well depth (4096 ?) of the chip in the ASA1600mm?

Just a (wrong ?) thought.

Regards, Hugh

[Filroden]

2 minutes ago, hughgilhespie said:

Hi Andy,

I am almost certainly getting this wrong, but doesn't an exposure of 22,000 adu mean that you have exceeded the well depth (4096 ?) of the chip in the ASA1600mm?

Just a (wrong ?) thought.

Regards, Hugh

It's shown in SGPro multiplied by 16 to make it a 16bit readout. So it's exposed to 22k/65k.

[Filroden]

6 hours ago, Andyb90 said:

Although is putting white paper over the light source a legitimate way of diffusing the light for flats?

Yes, paper or a white tshirt. I use about 5 sheets and folded the top 20% of the sheets so they stay flat on the scope under their own weight. I used to wiggle them to prevent their own grain affecting the image but so long as you have enough sheets it seems to take care of that itself.