Problems with flats

[Mr niall]

Hello there

I could do with some advice on taking flats if at all possible? At the moment I'm using a Canon 18-55mm standard kit lens. Trying to take flats with this lens is quite tough because the focus is changed by rotating the whole element at the front of the unit (which moves it back and forth at the same time). Its quite delicate - so stretching a T-shirt or anything else over the front is a no-go as it only takes the slightest pressure to move it off-focus.

I've therefore tried pointing it at a laptop scree but I'm not sure I'm doing it quite right; as the vignette across the frames is visibly very different depending on how perfectly perpendicular the screen is to the camera. So I end up sort of wiggling the camera around to try and get the vignette evenly distributed into the corners but I don't think that is really the point is it!?

Do flats absolutely have to be taken at the same time as darks, or could I get away with making a stash of them for various focal lengths (as it is becoming a bit of a pain!). Many thanks

 

[MarkAR]

I had decent results using some 40% translucent acrylic sheet 3mm thick. I taped some foam blocks to it so it will hang from the ew shield.

Laptop screen set to white will work, get as square on as possible, you can take flats anytime. 

Do not change the focal length, keep it the same as your images.

[Chefgage]

1 hour ago, Mr niall said:

Hello there

I could do with some advice on taking flats if at all possible? At the moment I'm using a Canon 18-55mm standard kit lens. Trying to take flats with this lens is quite tough because the focus is changed by rotating the whole element at the front of the unit (which moves it back and forth at the same time). Its quite delicate - so stretching a T-shirt or anything else over the front is a no-go as it only takes the slightest pressure to move it off-focus.

I've therefore tried pointing it at a laptop scree but I'm not sure I'm doing it quite right; as the vignette across the frames is visibly very different depending on how perfectly perpendicular the screen is to the camera. So I end up sort of wiggling the camera around to try and get the vignette evenly distributed into the corners but I don't think that is really the point is it!?

Do flats absolutely have to be taken at the same time as darks, or could I get away with making a stash of them for various focal lengths (as it is becoming a bit of a pain!). Many thanks

 

I think the problem with taking a stash of flats would be that any dust on the sensor would not then be corrected for as you would have moved the camera on your next imaging session. Also additional dust may have accumulated since you last took the flats. That's my understanding as to why the flats need to be taken before moving/removing the camera.

[alacant]

1 hour ago, Mr niall said:

flats absolutely have to be taken at the same time as darks

Hi

No, but take them as soon after (or before) your imaging session to cater for the -albeit rare- occasion where dust settles on the sensor.

1 hour ago, Mr niall said:

vignette across the frames is visibly very different

Camera on tripod. Screen in fixed position (a table? Wall?) at a fixed distance. Set the camera to Av and press the shutter to get the exposure. Adjust the distance or screen brightness to get a minimum of 1s. Switch to M and enter the same exposure. Don't change the aperture or focus. Fire 20 or so frames.

DSLR: dark frames will usually introduce more noise. Use bias frames, flat frames and dither between each light frame.

HTH

Edited April 15, 2020 by alacant

[Mr niall]

31 minutes ago, alacant said:

Hi

No, but take them as soon after (or before) your imaging session to cater for the -albeit rare- occasion where dust settles on the sensor.

Camera on tripod. Screen in fixed position (a table? Wall?) at a fixed distance. Set the camera to Av and press the shutter to get the exposure. Adjust the distance or screen brightness to get a minimum of 1s. Switch to M and enter the same exposure. Don't change the aperture or focus. Fire 20 or so frames.

DSLR: dark frames will usually introduce more noise. Use bias frames, flat frames and dither between each light frame.

HTH

Ok that may help - I've been doing that apart from the "fixed" bit. I may need to work out some way of getting the laptop and camera fixed in a relative position. I notice nobody has mentioned anything about how important getting it perfectly perpendicular is. I suppose my point is that if you compare the two images below - both were an attempt to get as perpendicular as possible but you can see if you can compare them that they produce ever so slightly a different pattern as the camera lens was obviously pointing fractionally in a different direction.

I was worried that I wasn't getting a proper flat as I cant work out a way to tell whether I am 100% completely square to my screen, I'm always going to be off by a degree or two no matter how hard I try - but as long as they're all from the exact same position it doesn't matter too much? Everything else was the same in both pictures.

Thanks again

 

[alacant]

1 hour ago, Mr niall said:

100% completely square to my screen

The frames look ok. If you really want to nail it, a bubble level for the camera and a set-square for the laptop screen, but hey...

Edited April 15, 2020 by alacant