Flat frames problem

[yusbot]

Hi Guys,

I read about typical flat frames should be a bright ball with darkened corners or edges because of the vignetting. But in my case, it is so weird that the dark edges is only comes in the bottom to the right of the image, and it's not evenly circular like a vignette.

Do you know what cause this? When the flats is applied to light frames, the image is getting worst than before it was flat fielded :(

And I notice a horizontal darker lines on the very top of the image. Is there something problem with the sensor?

[billdan]

Hi Yusfi,

Are the flats taken with a flat panel, if so the panel may not have been square to the scope. If they were Twilight flats with a T shirt across the front of the scope make sure the T-shirt is stretched and held tight with a rubber band and also check there are no wrinkles in the shirt.

As you say it does not look right, if nothing has changed with your telescope you may be able to take another set of flats.

You can also make synthetic flats with Photoshop or other Astro imaging software from your light frames.

Cheers Bill

[multiweb]

Does your camera use a mechanical shutter? If your flat duration is too short it might be the shutter shadow.

[yusbot]

Ah I see, I suspect it is. yea I took it below 1 sec of exposure to get average pixels around 20,000. The camera is QHY9.

So, maybe I should dim the light a little bit. Thank you!

[multiweb]

Hehe... I have a QHY9 too, a mono, and I recognised the flat shading. Nothing under 2 or 3 seconds or you'll get that ghosting. :)

[Manav]

Hello Marc - How exactly are you taking your flats when the exposure is 2-3 sec? Curious

[multiweb]

I lower the voltage on my EL panel to dim the light.

[Atmos]

I personally didn't go below 5s with a QHY9, used a Light Box with variable intensity.

[yusbot]

I did it with a computer screen as a flat light...so, I dimmed the light from the monitor brightness. If it's not enough, I change the white screen to grey or darker until the exposure is around 5-7 secs. I use "Paint", the native windows software to create flat light and put the tip of my telescope onto it.

Thanks folks, all of my flats are good now!