Flat frame light panel brightness

[Allan3026 (Allan)]

Hi All

I don't know if I'm missing something here but I can't seem to figure this out. These are the things I've read.

1. When taking flat frames, the general consensus for exposure time is around 1 second (correct me if I'm wrong).
2. Camera settings and optics should be the same as light frames.
3. I should aim for around 1/3 along the histogram.

The light panels advertise super bright LED light output.

When I try taking a 1 second exposure with a light panel with ANY light output, my image is pure white and massively overexposed.
Why are light panels so bright? What settings are used to get to 1/3 histogram. I can't seem to achieve this, without dropping my exposure to a tiny fraction of 1 second.

Any help on this one please?

[ronson]

Hi Allan,
If you are using drawing/tracer LED panels (like this one: https://www.amazon.com.au/Ultra-Thin...s%2C233&sr=8-6), they are not really meant for astronomy purposes, hence they are a bit brighter as you have found. One way to reduce brightness is to use something semi-transparent between the panel and your scope. Some people use white t-shirts or sheets of paper to effectively reduce the brightness.

[Retrograde (Pete)]

Originally Posted by Allan3026

Hi All

I don't know if I'm missing something here but I can't seem to figure this out. These are the things I've read.

1. When taking flat frames, the general consensus for exposure time is around 1 second (correct me if I'm wrong).
2. Camera settings and optics should be the same as light frames.
3. I should aim for around 1/3 along the histogram.

The light panels advertise super bright LED light output.

When I try taking a 1 second exposure with a light panel with ANY light output, my image is pure white and massively overexposed.
Why are light panels so bright? What settings are used to get to 1/3 histogram. I can't seem to achieve this, without dropping my exposure to a tiny fraction of 1 second.

Any help on this one please?

Hi Allan,

I have purchased a couple of semi-transparent, acrylic sheets to cover the LED panel, which is still too bright even at its lowest setting. A single sheet was fine for narrowband, but still not dark enough for broadband.

[Startrek (Martin)]

If using an LED tracing panel ( I use both an A3 for my 8” Carbon Newt and an A2 for my 10” Carbon Newt ) for taking Flats then it’s a must to have a panel with dimming control adjustment.
I use the Huion brand which have a touch control dimmer and at low brightness levels mine don’t suffer from colour shift like some of the cheaper brands.
Exposure time for all Flats are not necessarily around 1 sec. Your exposure time for Flats is dependent upon my factors including the type and characteristics of your camera and what filters you are using. With my Broadband filters I have Flats exposure times of between 0.1sec and 0.5sec and with Narrowband filters have Flats exposure times between 2sec and 5sec.
My cameras are the ZWO2600MC and ZWO2600MM cooled to -10C
I aim for a Target Range of around 8,000 to 32,000 or half way along my histogram and adjust both my Target ADU and brightness setting on my LED tracing pad to achieve this.
I have added a layer of 4.5mm opal acrylic sheet to my tracing pad as my lowest setting on the pad was still slightly too bright. Some folk use a white T shirt but I used an acrylic sheet as it’s just gaffer taped to my panel so permanent.

I’ve been taking Flats using the “Flats Aid” in APT this way for years with no issues at all.

Martin

[Allan3026 (Allan)]

Thanks for the replies.
I've just been using a dimmable whiteboard app on a tablet on my esprit 100. I'm looking to purchase a light panel that will also suit my 9.25 Edge HD. My camera is the 2600MC Duo.
It looks like the acrylic sheets are required and a panel with very low brightness. I will look into the Huion panel but any other suggestions?

[Max Vondel (Peter)]

Just drop your exposure length so your histogram is at about 1/3 peak from 0

[Bodon (Steve)]

Im far from a guru but I get a pleasing result.
I use a flat panel which has 4 touch settings, piece of frosted white perspex and a white bin bag liner. Ie White plastic bin liner over scope with elastic or velcro and get it fairly tight (dont worry about crinkles as you wont see them) then perspex I then adjust the light panel from low to high to get the desired ADU. Turn the light panel around if still too bright. I like around 24000ish for LRGB SHO. My flats (dependent on filter) are from 3.0s and up. I do not like any flats them lower than 3.0s. If still bright throw some A3 A4 between the perspex and flat panel. I use a ZWO 2600MM.

[Addos (Adam)]

The 1/3 peak histogram levelling is guidance for DSLRs.

As others have said, for dedicated astro cameras you want around 50% available. IMX571's have 65k max well depth at unity gain so ~30-32000. Advice that flats should be 1 second in length is bunk - you use whatever exposure time gets you to that adu level (special cases like imx294's notwithstanding).

Flat panel / light source brightness is then nothing more than a variable in that calculation. Dimmable drawing/led panels usually have 3 or 4 brightness settings, so between the different filters you should be able to find something that works. use nina's flat wizard/dynamic flat mode and it will do it all for you, hunt down the optimum exposure time to reach set adu range. If it (or you manually) cant hit a short enough sub length to hit target adu (common with the lum filter), damp the light source with some plain white tshirts / other fine woven cloth.

Gain/offset settings and frame orientation if changed may impact the quality of the flat subtraction, so try to keep that consistent. not super critical however there are post procedures that can reasonably mitigate alot of flat correction related issues, albeit time consuming and fiddly. you also dont necessarily need new flat sets each time you shoot, although with a newt you'd have to do it far more often than I do with a refractor (larger, open optical train gets dirtier easier and more often, collimation shifts).
Best of luck!

[The_bluester (Paul)]

I generally shoot sky flats, but with my cameras which are both ASI2600's, I configure the software for anything between 0.1 to 30 seconds at low gain (I actually use gain -20 with my pair of cams for flats and broadband subs) A master bias at the same gain and temp calibrates over that exposure range nicely.

I would not get too hung up on aiming for 1/3 on the histogram, my approach is to aim for around the 50% mark (so ADU of about 32K) but so long as the flat does not have pixels at zero or saturated after calibration then it goes in the pot to create the master flat.