Flats...is there a level of brightness rule?

[astroimpulse]

Just wondering (as I plan my DIY flat panel) is there a rule for light intensity when it comes to flats?

Dim, bright etc?

My panel will be very even but I'm unsure on the strictness of brightness needed

[Photosbykev]

with my DSLR I just monitored the histogram to ensure the peak was well to the right without being burnt out. In AV mode +1 or 2 stops compensation seems to be about right.

[astroimpulse]

Ah, of course!

Good point well made *thumbs up*

Knew I could rely on you

Post 1000 coming up too

[Photosbykev]

damn I am a sad muppet, I've turned into a sub dwarf lmao

[brianb]

well to the right without being burnt out

You do want the light level high (to minimise quantization noise) but not so high that you run into the non linear response near to saturation. (This is assuming that your sensor has anti-blooming ... without the anti-blooming gate the response should be linear until the buckets actually overflow, leaving horrid stripes across the picture) About half full well is reasonable for flats with anti-blooming sensors. If you go too high any vignetting in the dark grey background will be undercorrected.

[astroimpulse]

I have no idea if I have an anti-blooming sensor *blush*

I'm of the understanding that the flats are taken at the same settings as the lights (like darks). But presumably I'd have to change the exposure (or close the aperture if AV) otherwise I'd just have a flattened/over exposed pure white image?

[Photosbykev]

You need to use the same aperture for the lights and flats so you adjust the shutter speed to get the correct exposure for the flat. using AV mode allows you to select the same aperture and lets the autoexposure work out the correct shutter speed.

However the metering will try to expose the flat as an 18% grey image so you need to compensate to increase the exposure which will push the histogram peak further to the right. You can either use exposure compensation or just switch to manual and set the same aperture and manually adjust the shutter speed for the optimium exposure. Personally I use the manual mode which will ensure all the flats are taken with the same settings.

[astroimpulse]

Aye, I'll go the manual way to That'll keep everything spot on and I'm more comfortable in the manual mode (not that AV is complicated or anything lol)

EDIT: Dunno why I've even considered camera aperture...it's irrelevant as I'm using the scope! *slaps head*

[brianb]

I have no idea if I have an anti-blooming sensor *blush*

I'm of the understanding that the flats are taken at the same settings as the lights (like darks). But presumably I'd have to change the exposure (or close the aperture if AV) otherwise I'd just have a flattened/over exposed pure white image?

They nearly all are. If you want a sensor capable of professional photometry, you specifiy a sensor without anti-blooming gate.

To set the exposure for you flats: use manual exposure mode: if using a camera lens, the aperture must be set to the same value as it was for the "light" frames you wish to calibrate. Change the exposure time / shutter speed until the histogram peak (which should be quite sharp) is well into the upper half but not near the right hand edge - about 3/4 is ideal. Go under rather than over if unsure. Take a reasonable number of flats and median combine them to make a "master flat".