DSLR Flats

[Ronclarke]

What exposure are folk using for DSLR (Canon 1100d) Flats. I did some 0.025s Flats that looked good, ie 50% on the APT Histogram but don't seemed to have worked on the finished, stacked image??

[Seelive]

13 minutes ago, Ronclarke said:

What exposure are folk using for DSLR (Canon 1100d) Flats. I did some 0.025s Flats that looked good, ie 50% on the APT Histogram but don't seemed to have worked on the finished, stacked image??

The flats for my Canon 200d are usually around 1/1000s (0.001s). I guess it could depend upon your light source and whether it has a particular refresh rate that may cause a problem.

[ollypenrice]

Exposure depends on your light source but, if the histogram peaks about half way, the flats ought to work. What makes you doubt them?

Olly

[david_taurus83]

I aim for around 2 seconds with my 6D. The problem with the on camera histogram and APT one is they both measure the 8 bit jpeg yet we work with raw files. I use NINA and expose to around 8000 ADU, half of 14 bit Canon 6D max ADU value. On the camera, this is why over on the right edge of the histogram but they work well.

[Ronclarke]

Hi Olly, some of the dust bunnies still show along with some lines..

(I called in to see you a couple of years ago, looking for Vultures!)

 

Ron

Edited January 23, 2022 by Ronclarke

[happy-kat]

where the flats taken at the same time as the lights with nothing moved?

[Ronclarke]

Not the same time but the same set up..

[Elp]

It's generally good practice to take the flats at the same time as the lights as your focus position, any rotation in the camera sensor in relation to the optical train (telescope lenses, filters, flatteners etc) has to be exactly the same as the lights. Even dust anywhere on the train can be different if you don't take them at the same time as the lights.

Using an led tracer panel for even light on top of the telescope lens I found 100ms gives flats with the peak around a third away across from the left at iso 1600. My personal experience with my dslr however (Canon 600d) I still get tiny black streaks all across the image, the last time they were in a circular pattern like how stars rotate around Polaris in a time lapse.

[michael8554]

I've heard that too-fast exposures (0.025s, 0.001s) with mechanical shutters, as in DSLRs, may show shutter artefacts.

Dim the light source until AV exposures come out at about 1 second.

AV gives a 50% exposure.

Michael

Edited January 24, 2022 by michael8554

[ONIKKINEN]

I had shutter shadow visible in 30s exposures on my lights, so with my 550D i dont think there is a point trying to increase flats exposure time since it will be an issue anyway. In theory though if i somehow managed to take 30s flats it would probably go away, but way too much effort.

Mine were apparently 1/25s or 0.025s and looked like below. Histogram stretched monochrome and a false colour rendering to show the whole flat dynamic range better. The dark patch on the bottom would be the shutter shadow.

 

14 hours ago, Ronclarke said:

Not the same time but the same set up..

Was the camera removed?

If the camera is removed or something else is changed in the optical system, its almost certain you dont get perfect flats afterwards. If you image with a mirrored system its also possible something moves that shouldnt when the scope is just left doing nothing and flats would also not work. The remedy would be to take flats immediately after imaging, or even before imaging if you intend to leave the scope running overnight while you sleep. If the flats still dont work there are some mechanical issues that need tending because your optics change during the session.

[michael8554]

3 hours ago, ONIKKINEN said:

Mine were apparently 1/25s or 0.025s and looked like below.

That's not shutter shadow.

1/25s = 0.04s

Michael

[ONIKKINEN]

1 minute ago, michael8554 said:

That's not shutter shadow.

1/25s = 0.04s

Michael

Hmm, that went away as soon as i changed to a shutterless camera, so what was it then?

[michael8554]

Well I suppose if the vignetting at the bottom of the Flat isn't apparent on the Lights, you may be right.

But I'd say its vignetting by the bottom of the shutter box, or the step between the circular lens mount and the sensor.

Michael

[alacant]

23 minutes ago, ONIKKINEN said:

what was it

The shadow of the mirror. 

HTH

[ONIKKINEN]

19 minutes ago, michael8554 said:

Well I suppose if the vignetting at the bottom of the Flat isn't apparent on the Lights, you may be right.

But I'd say its vignetting by the bottom of the shutter box, or the step between the circular lens mount and the sensor.

Michael

 

3 minutes ago, alacant said:

The shadow of the mirror. 

HTH

Could have been any of these actually. Went away with a change of camera into one that has none of the above.

[Ronclarke]

The camera was not moved and the set up etc was identical.

I'm going to do a range of Flats and spend time stacking to see which is best.

Many thanks

[Albastars]

On 23/01/2022 at 19:31, david_taurus83 said:

I aim for around 2 seconds with my 6D. The problem with the on camera histogram and APT one is they both measure the 8 bit jpeg yet we work with raw files. I use NINA and expose to around 8000 ADU, half of 14 bit Canon 6D max ADU value. On the camera, this is why over on the right edge of the histogram but they work well.

Similar for me with a modded Canon 800D.  I came across this article a while back and found it useful - https://www.myastroscience.com/proper-flats-with-dslr

[Elp]

@Albastars Thanks that link was great.

There was the highlighted point "Very important to remember. When you do your flats at different ISO than your Lights, you have to calibrate your Flats with matched ISO dark-flats or BIAS frames !!! Keep that always in mind."

Haven't done this myself, can it be done in DSS?

[Rustang]

22 hours ago, Albastars said:

Similar for me with a modded Canon 800D.  I came across this article a while back and found it useful - https://www.myastroscience.com/proper-flats-with-dslr

Thanks for this, really handy and probably why my flats arnt working with my DSLR 👍

[ollypenrice]

On 23/01/2022 at 21:12, Ronclarke said:

Hi Olly, some of the dust bunnies still show along with some lines..

(I called in to see you a couple of years ago, looking for Vultures!)

 

Ron

I remember that, Ron. They were overhead today, in fact. We had a squadron of them last year after a wolf-kill of some sheep...  I now have a better telephoto so will try harder! 

 

Olly

[Ronclarke]

Wow great photo's Olly!

I did get to see some that day at Remuzat..

 

Ron

Edited January 27, 2022 by Ronclarke

[Albastars]

On 26/01/2022 at 14:11, Elp said:

@Albastars Thanks that link was great.

There was the highlighted point "Very important to remember. When you do your flats at different ISO than your Lights, you have to calibrate your Flats with matched ISO dark-flats or BIAS frames !!! Keep that always in mind."

Haven't done this myself, can it be done in DSS?

Sorry @Elp I don't use DSS.