17 replies to this topic

[Wjdrijfhout]

It started as a fairly innocent question on the need of matching gain between Flat and Light frames. But it ended in a series of experiments/comparisons on what factors contribute to taking the good Flats. Today I published the second blog in a series of three, describing the outcome of those experiments. In this second blog, four flat panels are compared and their pros and cons are investigated. You can find the blog here: https://www.astrowor...panels-compared

The third (and last) blog will look into the topic of Flat-Darks vs Bias.

The first blog that focused on the need of gain-matching flats and lights led to a follow up discussion here on Cloudy Nights: https://www.cloudyni...r-light-frames/

[james7ca]

Nice work. Thanks for making this available.

[gatsbyiv]

Did you test the Aurora for banding or assume that it didn't cause it because it is an EL panel?  I get banding with mine, and that's why I switched to a different panel (Geoptik).

[Midnight Dan]

An interesting comparison.  I'm surprised that you didn't use a sky flat as a reference to subtract from the others.  That would have given you a true flat reference point with no chance of variable illumination that you get from the flat panels.

 

Another test I've seen done for flat panels is to take flats, rotate the panel 90°, then take flats again.  Optical path defects will remain in the sam position, but panel non-uniformities will not.  So if you subtract them from each other, you will see the non-uniformities without any confusion from two different panels interacting with each other.

 

The color balance is also interesting in that it's very different from what I've seen.  I have two flat panels, a generic electroluminescent panel, and an LED tracing panel.  Comparing the two side by side, the EL panel shows a distinct pinkish color, while the LED appears pretty neutral.  In addition, the EL panel needs time to warm up.  It is stored outside in an observatory, and when first turned on in cold weather it is dimmer and bluer, and changes color and brightness over time.

 

One more thing to point out about LED panels. There are different types, with different ways of adjusting brightness, which can affect the uniformity.  Some units have the power control circuitry built into the frame.  People have reported that these show a glow from the area near that circuitry, possibly due to heat and associated near-IR, which the camera can record.  

 

In addition, as you noted, some use PWM which can produce banding at short exposures.  But some (not many) use a straight 12V dc supply and can be varied in brightness simply by adjusting the input voltage.  I chose a panel that used DC voltage for control and did not have any power circuitry in the frame.  It does not produce banding at any exposure length.  

 

-Dan

[nwcs]

I agree about the review of the flatmaster, which is what I have. I have to use a lot of diffusion to get the exposure time to 6 or more seconds to smooth out the banding. That works fine but wasn’t what I was expecting. Would have been nice if they had made this very clear in their documentation as a side effect.

[Wjdrijfhout]

Did you test the Aurora for banding or assume that it didn't cause it because it is an EL panel?  I get banding with mine, and that's why I switched to a different panel (Geoptik).

I tested it down to 0.1s and could not see any banding. Anything shorter than that is not applicable to my setup. Are you using the 12V converter? There is a little intensity knob on there that I always put on maximum, could that make the difference?

[Wjdrijfhout]

An interesting comparison.  I'm surprised that you didn't use a sky flat as a reference to subtract from the others.  That would have given you a true flat reference point with no chance of variable illumination that you get from the flat panels.

 

Another test I've seen done for flat panels is to take flats, rotate the panel 90°, then take flats again.  Optical path defects will remain in the sam position, but panel non-uniformities will not.  So if you subtract them from each other, you will see the non-uniformities without any confusion from two different panels interacting with each other.

 

The color balance is also interesting in that it's very different from what I've seen.  I have two flat panels, a generic electroluminescent panel, and an LED tracing panel.  Comparing the two side by side, the EL panel shows a distinct pinkish color, while the LED appears pretty neutral.  In addition, the EL panel needs time to warm up.  It is stored outside in an observatory, and when first turned on in cold weather it is dimmer and bluer, and changes color and brightness over time.

 

One more thing to point out about LED panels. There are different types, with different ways of adjusting brightness, which can affect the uniformity.  Some units have the power control circuitry built into the frame.  People have reported that these show a glow from the area near that circuitry, possibly due to heat and associated near-IR, which the camera can record.  

 

In addition, as you noted, some use PWM which can produce banding at short exposures.  But some (not many) use a straight 12V dc supply and can be varied in brightness simply by adjusting the input voltage.  I chose a panel that used DC voltage for control and did not have any power circuitry in the frame.  It does not produce banding at any exposure length.  

 

-Dan

Those are good suggestions Dan on experimental design. I sort of rolled into this whole topic by accident. It was not my intent from the start to do a flat panel comparison. But as I was diving deeper into the subject of taking flats, I noticed some differences in different panels and that got me to this side-by-side comparison. But as you say, there are definitely other (and probably better) ways to investigate the non-uniformity. 
The voltage-dependent LED panels sound interesting. Do you have a link to those?

[Midnight Dan]

The voltage-dependent LED panels sound interesting. Do you have a link to those?

Unfortunately no.  It was a no-name brand from Amazon and those brands come and go.  The link in my Amazon orders listing brings up a page-not-found.  

 

There are a ton to look through on Amazon.  You first have to skip any that have the controls built into the frame.  Look for one with the controls built into the cable.  Then you have to search for specs to see what the actual input voltage to the panel is, NOT the input voltage to the cable.  That's rarely called out.  So if you think you might have a suspect, you need to ask questions and wait for the manufacturer to get back to you.

 

It's a pain to find them, and I'm not even sure if they're made any more.  They may have all moved to USB powered, which is probably PWM.  But that's how I'd go about looking for one.

 

-Dan

[Fegato]

Nice comparison. I use the Aurora and it works really well for me, so can only agree with your conclusion. 

[Zambiadarkskies]

Did you test the Aurora for banding or assume that it didn't cause it because it is an EL panel? I get banding with mine, and that's why I switched to a different panel (Geoptik).

That's a good write up. Interesting. I have an aurora (small as powered one) and doing short exposures with a mirrorless camera I ran into banding. I was able to sort it with ND filters, but it is there. I haven't ever had an issue with my zwo 533.

[Midnight Dan]

That's a good write up. Interesting. I have an aurora (small as powered one) and doing short exposures with a mirrorless camera I ran into banding. I was able to sort it with ND filters, but it is there. I haven't ever had an issue with my zwo 533.

The banding issue isn't strictly a problem from the flat panel.  It's an interaction of the sensor and the flat panel.  

 

As mentioned above, using PWM (pulse width modulation) means the panel is flashing off and on extremely quickly.  But it is NOT doing so in bands.  The entire surface is flashing off and on, which will not cause banding by itself.

 

Most sensors use what's known as a "rolling shutter" for shorter exposures.  Instead of activating all the pixels at the same time, it activates a band of pixels that rolls down the sensor surface.  That band becomes narrower for shorter exposures and wider for longer exposures.  But as that band moves down the sensor, the flat panel is flashing off and on, so the exposure records that as bands where the sensor was active while the panel is on.

 

Some sensors do not use a rolling shutter, and will not show banding.  

 

-Dan

[Zambiadarkskies]

The banding issue isn't strictly a problem from the flat panel. It's an interaction of the sensor and the flat panel.

As mentioned above, using PWM (pulse width modulation) means the panel is flashing off and on extremely quickly. But it is NOT doing so in bands. The entire surface is flashing off and on, which will not cause banding by itself.

Most sensors use what's known as a "rolling shutter" for shorter exposures. Instead of activating all the pixels at the same time, it activates a band of pixels that rolls down the sensor surface. That band becomes narrower for shorter exposures and wider for longer exposures. But as that band moves down the sensor, the flat panel is flashing off and on, so the exposure records that as bands where the sensor was active while the panel is on.

Some sensors do not use a rolling shutter, and will not show banding.

-Dan

Thanks. I was aware of rolling shutter artifacts when using electronic shutter with my Canon R6 - e.g. fast moving action. As well as pwm (with lower quality lights and AC lighting) interference in video, hadn't thought about it much in astro.

Edited by Zambiadarkskies, 14 February 2023 - 10:33 AM.

[FiveByEagle]

Can attest to the FlatMaster. 3s seems to get rid of banding for most filters! I have yet to solve Luminance flats with my FlatMaster though.

[nwcs]

Can attest to the FlatMaster. 3s seems to get rid of banding for most filters! I have yet to solve Luminance flats with my FlatMaster though.

I use a folded up white pillowcase as a diffuser (since I already used up some plastic balanced diffusion material elsewhere). That generally gets me about 7s flats which always fixes the flatmaster problem for me.

[oatmeal]

My cheapo LED tracer panel seems to work fine for my mono setup (with either my persnickety ASI294MM or the much more accommodating ASI2600MM); though I usually need a couple layers of fabric and/or semi-opaque acrylic as a diffuser since it's so bright. 

 

However, when I briefly owned an ASI294MC, I could not get it to work for flats with that camera. The histogram of the flats using that combo was always tri-modal, and I got blotches galore when attempting to use them to calibrate.

[T~Stew]

There are a ton to look through on Amazon.  You first have to skip any that have the controls built into the frame.  Look for one with the controls built into the cable...

My cheapo amazon tracing pad has the controls built into the frame. Are you saying they all have PWM? I can't detect any PWM effects and have no problems take very short flats with it, but admit I have not tried to reverse engineer the circuitry or anything. 

https://www.amazon.c...01?ie=UTF8&th=1

[FiveByEagle]

My tracing panel was weirdly inconsistent - and I am thrilled to see I wasn't alone. Re-reading the entire  blog post shows this really well of what I struggled with for a while.

[Midnight Dan]

My cheapo amazon tracing pad has the controls built into the frame. Are you saying they all have PWM? I can't detect any PWM effects and have no problems take very short flats with it, but admit I have not tried to reverse engineer the circuitry or anything. 

https://www.amazon.c...01?ie=UTF8&th=1

No, the controls in the frame are not necessarily an indication of PWM.  There are two concerns with controls in the frame.

 

First is the potential glow from that side.  I've seen people report that the panel shows up brighter near the controls, similar to amp glow in a a sensor.  This may not be the case in all panels though.

 

Second is the ability to control the brightness with an external adjustable 12V supply.  If the controls are built into the frame, the you need to provide a constant voltage to the frame and must manually adjust the brightness using the controls.  I was looking to control the brightness remotely, or as part of an automated sequence.

 

With the controls in the cable, it means that the voltage supplied to the actual panel is being controlled externally, from the cable controls.  Which means you can cut the cable between the controls and the panel, and control that voltage from something else like a Pegasus Powerbox Adjustable output.  This gives you full remote control to turn the panel off or on, or adjust the brightness.

 

-Dan