Lights, darks, flats, bias, lions, tigers and bears. Oh my!

[Kirscovitch]

So I understand light and dark frames. Lights are the image you want and darks are taken with the same settings and around the same temp but with the lens cover on.

I am still a little sketchy on the flats and bias frames. I've been doing some reading but I just get more confused. This is my understanding of the 2.

Flats are taken to remove the effects of dust or other particles from the image. Putting a white t shirt stretched over the lens and taking frames with the same settings as the lights. 

Bias frames are taken with the same ISO of the light frames but with the fastest exposure the camera will allow, lens covered.

So do the flats and bias frames need to be the same temp as the lights? Also with the flats I read that exposure is critical so setting the camera to AV. But with the camera attached to the scope I don't think I can take the flats with BYEOS. If I remember correctly if the camera is not in manual mode BYEOS scolds you. So will I have to take the camera off the scope and put a lens on to do it? Or is there another way to get around this? 

[Ant]

Flats and bias don't really need to be at the same temperature - although I normally take them as I am packing away.

Bias, as you say are as quick as possible exposures and are used to elimatinate read noise I beleive.

Flats will help correct for a non flat field, vignetting and as you say dust bunnies. I normally take flats to about a 30% saturation. On my camera the well depth is 64000 ADU, so I adjust the exposure to achieve around 20-25000. With stardard LRGB filters they can be pretty quick, but with the 7nmHa filter they can be 5,6,7 seconds long...

All depends on your camera and filters used.

Someone who knows what they are talking about will be along soon

Cheers

Ant

[dph1nm]

So will I have to take the camera off the scope and put a lens on to do it?

Definitely not. Flats need be taken with the camera and telescope in the same arrangement  as for the lights, or they won't work.

NigelM

[Thalestris24]

Hi

I find the best way of taking flats is with a monitor screen a few feet away and with Notepad open. This seems to give an even white light. With a dslr APT allows you to setup a flat frame plan (I usually take about 20 flat frames) which enables AV mode on the camera. I'm not familiar with byeos but I'd be surprised if it doesn't do the same. With a ccd camera it's a bit more difficult to get the exposure right. The flat image needs to have the vignetting gradient so it can get subtracted from the light frames.

Hth

Louise

[Kirscovitch]

Hi

I find the best way of taking flats is with a monitor screen a few feet away and with Notepad open. This seems to give an even white light. With a dslr APT allows you to setup a flat frame plan (I usually take about 20 flat frames) which enables AV mode on the camera. I'm not familiar with byeos but I'd be surprised if it doesn't do the same. With a ccd camera it's a bit more difficult to get the exposure right. The flat image needs to have the vignetting gradient so it can get subtracted from the light frames.

Hth

Louise

There is an imaging plan in BYEOS where you can select between light, darks, flats, etc. I have never messed with flats so I'm not sure if it will enable AV mode. I'll look into it next time out.

[Alien 13]

I am on the fence regarding flats, they are useful if taken at the same time during an imaging session but i have noticed that with a DSLR you do get movement of the dust bunnies sometimes especially if the camera has an inbuilt ultrasonic sensor cleaner.

The result is that flat will correct for vignetting etc but then add other artifacts to the image.

If you think about it all the data required to make a flat is in your final image anyway so by using various processing an artificial flat can be produced, this method works best for me.

Alan

[Christopher Davenport]

I am on the fence regarding flats, they are useful if taken at the same time during an imaging session but i have noticed that with a DSLR you do get movement of the dust bunnies sometimes especially if the camera has an inbuilt ultrasonic sensor cleaner.

The result is that flat will correct for vignetting etc but then add other artifacts to the image.

If you think about it all the data required to make a flat is in your final image anyway so by using various processing an artificial flat can be produced, this method works best for me.

Alan

I must say, I am in the same camp on artificial flats. Sometimes flats add to my problems, ie during stacking star counts are affected, so I avoid if possible.

I do flats with a white laptop screen or early morning misty sky.

I only take flats when I see a big bad dust bunny.

Fridge Dark library per iso used

Bias 64x temp bucket (5c steps)

Darks 64x temp bucket (5c steps) per exposure length.

Hope this adds a bit of perceptive.

[Kirscovitch]

I like the idea of using a laptop screen. Do you still cover the lens with a white cloth or t-shirt? Also what would be a good ratio of lights to darks and bias? 2:1? And should the bias, flats and darks be of equal amount?