Can mechanical CCD shutter affect Flats?

[michael8554]

I have a CCD camera with a mechanical shutter.

Will the shutter affect the flats at some shutter speeds?

I can vary the brightness of my flat panel to give 30K ADU with exposures up to a maximum of 4 seconds.

Michael

[bluedandelion]

Yes. On my Aitk 386+ mono flats below 200 ms will have artifacts. Atik has recommended not going faster than this.

Ajay 

[RayD]

Yes same here with the Atik 16200.  Definitely have to consider the shortest time possible for your particular camera.  I think mine is 250ms after which it is fine.

[drjolo]

I had Atik383+ and measured flats at different exposures during twilight. For my camera flats needed to be exposed over 5 seconds (!) to get rid of visible shutter shadow  

[steppenwolf]

On my QSI I have to use exposures in excess of 5 seconds to avoid the affects of shutter swipe so yes, it is a big potential issue.

[Tim]

As it is important for flat frames to be evenly illuminated, you will likely have to expose for longer than usual to get rid of the shadow of the shutter.

To take acceptable flats with the Atik 16200 I couldn't use my flat panel, just had to dimly illuminate a white board and take longer exposures.

[michael8554]

A fair range of speeds:

Slower than 200ms

Faster than 250ms

Over 5secs

Over 5 secs

Longer exposures

What shoild I look for?

Does the shutter shadow have a sharp edge?

Is there an obvious shaded area?

Michael 

 

[drjolo]

It does not have sharp edge. In theory uneven shutter illumination can be ideally removed only with flats with exposure time equal to light frames - but it is absurd of course You just need to do some trials and check when this effect is not affecting the image. Longer will be better, and I think 5 secs or more will be enough. I have not heard about worse example of uneven shutter than in Atik 383, however it may exist of course

[RayD]

1 hour ago, michael8554 said:

A fair range of speeds:

Slower than 200ms

Faster than 250ms

Over 5secs

Over 5 secs

Longer exposures

What shoild I look for?

Does the shutter shadow have a sharp edge?

Is there an obvious shaded area?

Michael 

 

I think it depends on the type and speed of the shutter also. The 16200 has a multi blade type which I think is pretty quick to open. 

As mentioned above you will probably need to experiment with your particular camera as they do vary. 

[michael8554]

What should I look out for?

Does the shutter shadow have a sharp edge?

Is there an obvious shaded area?

Michael

[carastro]

It will be obvious if you have shutter shadow, it will be a blackened area with a soft edge.  Take a short flat and you will see it, then extend the length until you stop seeing it.  Of course you'll have to dim the light source as the flat gets longer. 

When I had a Atik383 with a shutter Atik told me to take about 3secs and dim the light source.  T Shirt, double T shirt, layers of typing paper, white pillow case etc. 

Carole 

[ollypenrice]

Not so sure that it will be obvious as you move towards the 'nearly long enough' exposure time. If 5 seconds is good enough for Steve Richards it would be good enough for me! (Though I've never had a shutter camera.) Why take chances on adding a gradient? You can dim a panel right down with typing paper.

Olly

[carastro]

Of course it will vary with whichever filter you are using, but find the point where you get no shadow and then add a bit more on as a safety margin, then in future never do a flat shorter than that.  

Carole 

[michael8554]

Thanks everyone, I shall start fast and look for the shadow and then work up to shadowless.

Michael 

[wxsatuser]

I find 4secs is ok with my Atik 383L.
Have an Artesky variable panel which makes life easy.

Here is a 4sec flat at 22000

[michael8554]

Thanks for the image, could you please  post one with shutter shadow?

Michael 

[wxsatuser]

Will see if I can do one tomorrow.

[michael8554]

Took some of my own today. They've been stretched to look roughly similar.

First is at 1/1000 sec, does his have shutter shadow?

 

I increased exposure until I got an even frame at 0.5 sec, and at 1 sec I got 30K ADU:

 

 

Michael

[ollypenrice]

4 hours ago, michael8554 said:

Took some of my own today. They've been stretched to look roughly similar.

First is at 1/1000 sec, does his have shutter shadow?

 

I increased exposure until I got an even frame at 0.5 sec, and at 1 sec I got 30K ADU:

 

 

Michael

The first image certainly has the shutter gradient. The second might not, but I would still dim the source and slow down the exposure.

Olly

[SteveA]

Sorry if this is a stupid question and I've often wondered...

Is there an advantage to having a mechanical shutter?

Steve

[drjolo]

Electronic shutter speed is limited by sensor read out time (unless it is global electronic shutter, but I don't think it is present in astrocameras). While CMOS sensors are pretty fast, then electronic shutter can be implemented. For older/larger/slower/CCD sensors read out time can be quite long.

Another thing that comes to my mind is that you can do dark frames during/after session without covering your telescope. But it requires lightproof shutter, and for Atik383 I found the shutter is not 100% lightproof and some amount of light passed when scope was not covered.

 

[SteveA]

1 hour ago, drjolo said:

Electronic shutter speed is limited by sensor read out time (unless it is global electronic shutter, but I don't think it is present in astrocameras). While CMOS sensors are pretty fast, then electronic shutter can be implemented. For older/larger/slower/CCD sensors read out time can be quite long.

Another thing that comes to my mind is that you can do dark frames during/after session without covering your telescope. But it requires lightproof shutter, and for Atik383 I found the shutter is not 100% lightproof and some amount of light passed when scope was not covered.

 

An interesting topic...and having just read up on it I can definitely say I've learnt something new today...so thanks

Steve