flat frames with light panel banding

[alacant]

Hi. I'm trying a Huion lcd light panel to make flat frames. Some appear with a dark band across, always in a different place; really annoying. The faster the shutter, the worse it gets. Example attached at 1/6s. Anyone? TIA.

[Art Gecko]

No expert, but I'd say the refresh rate of the LCD screen is somewhere near the shutter speed... have you tried moving it further away and slowing the shutter speed so you don't pick up the 'flicker' of the screen?

[bobro]

Do you mean a 'Huion led light panel'? Assuming this is powered from the mains, the internal electronics may be a bridge rectifier producing a 'choppy' DC that powers the leds in the panel. Although the eye will not see the resulting fast changing led brightness a camera will, especially with a focal plane type shutter.

Taking flat frames needs a constant (and even) source of light as you are no doubt aware. Looks like another source is the answer....

[frugal]

Or put something light absorbing between the panel and the scope, such as a piece of paper or two. That will force a longer exposure which should get rid of the banding.

Also, having a quick look on Amazon, a lot of the Huion lightboxes have adjustable brightness. Try setting the brightness to the lowest setting which will also force a longer exposure.

[alacant]

27 minutes ago, bobro said:

Huion led light panel

Hi. Not sure. It looks like the one mentioned in this post. Unfortunately, no exposure details or conclusions emerged...

21 minutes ago, frugal said:

a lot of the Huion lightboxes have adjustable brightness

Yes, this one has. Any guidelines? TIA.

[frugal]

Just now, alacant said:

Hi. Not sure. It looks like the one mentioned in this post. Unfortunately, no exposure details or conclusions emerged...

Yes, this one has. Any guidelines? TIA.

I have not used one specifically. However I would set it to the dimmest setting you can. That will mean you need to expose for a longer time; which should in turn mean that any fluctuations due to AC/DC conversion are evened out.

[bobro]

Recalls taking photos of previous generation televisions with cathode ray tubes and scanned images - there was inevitably banding. To ensure banding isn't a problem the focal plane shutter needs to be fully open for a significant period - perhaps for a second or two but can be reduced with good light even spreading IMHO. The illumination of the light panel may not be even, especially with reduced brightness, so best to ensure at least a few inches of distance between the panel and attenuating material to help ensure an evenly distributed illumination. Sounds a bit over the top, but it defeats the object if the source doesn't produce even light - especially if it is not constant.

 

[alacant]

Hi and thanks so much everyone for your time and pointers. Getting a bit further, but research time is running out as the panel has to be returned. I found out how to dim it. This makes the banding worse. It seems that dimming works by lowering the refresh rate. The only way to lose the banding is to use a t-shirt between panel and the telescope giving 0.5s. Anything shorter and the banding appears.

So, dunno. Is it worth the €40 or so? Do these things deteriorate or go wrong I wonder? Is it gonna be better than my current hit and miss t-shirt-sky flat frames? Is anyone using one for this purpose? TIA.

[Uranium235]

I used to get the same issue with using monitors, I found the best solution the refresh rate issue was dimming/diffusing the light source with copier paper or a white t-shirt and simply take longer flats. Youre looking into the order of at least a few seconds, perhaps 4-6s for (for my CCD its 10s in luminance). Whether you need dark flats after doing that is debateable, but I took some just in case (dark flat = a dark sub that is as long as your flat sub).... it never hurts to have the full set, just in case.

[Guest Tuomo]

I use Huion at brightest setting. Eventually I will use some sort of fabric/plexi-glass in front of panel, but at the moment I use white double folded T-shirt in front of dew shield. No banding at 30cm-50cm distance. I also figured that dimming actually made everything worse. 

[alacant]

55 minutes ago, Tuomo said:

use Huion at brightest

OK, so this can't be correct. A snap of my dimmed 0.5s setup attached...

1 hour ago, Uranium235 said:

a dark sub that is as long as your flat sub

What is the meaning of dark/flat sub? Do I have it/them? TIA.

[bobro]

As light from the panel is not focussed, if the cloth is left on the scope and the panel moved away from the scope (say 12 inches), the banding should reduce if not disappear.

[Guest Tuomo]

Thats what I did. It worked.

[Uranium235]

9 hours ago, alacant said:

OK, so this can't be correct. A snap of my dimmed 0.5s setup attached...

What is the meanif dark/flat sub? Do I have it/them? TIA.

A dark flat is a calibration frame for flats that are are over a certain length - to remove any added noise that may have been introduced by such a comparitively long flat frame exposure time (ive done flats 25s long in narrowband).

I see you have the panel flush to the tube... I learnt that with a newt it is better to place the light source at least 6-12 inches away from the telescope apeture. The reduces the risk of light bypassing the primary/secondary mirrors and going stright up the drawtube.