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Shortest exposure to minimize "iris" effect?

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ldj's picture
Shortest exposure to minimize "iris" effect?

Hi, I am commissioning a new Apogee Aspen CG16M Camera. I'm used to the SBIG type rotating shutter and not used to the Iris type shutter that the Apogee's have. I noticed that in short exposures its easy to see the opening/closing iris effect which I'm not used to seeing.

So, as a rull of thumb, how long to do you have to expose, both science images and flat fields, to reduce the effect so that it does not unduly affect your photometry?

--- Dave LDJ

HQA's picture
shortest exposure

Hi Dave,

It depends on your definition of "unduly". :)

Say that the shutter takes 0.1sec to fully open, and another 0.1sec to fully close.  For a 1-second exposure, then, the center sees 1.1 seconds (start of open command until end of close command), and the corner sees 0.9 seconds (end of open command until start of close command).  For a 10-second exposure, it is 10.1 seconds and 9.9 seconds; for a 100-second exposure, it is 100.1 seconds and 99.9 seconds.  The longer the exposure, the smaller the percentage difference.  However, it is always present.

As long as the shutter has a reproducable open/close time, you can calibrate this out.  Take a 1-second flatfield; take a 100-second flatfield.  Divide the two.  That gives you the vignetting pattern due to the shutter.  Scale according to the exposure time and divide into the image after flattening.

Note that this affects flats, too.  You should use relatively long exposure times for your flats; I typically use 10-30seconds.  With a photometric shutter like SBIG, I'll often take 1-second flats, so you can get more of them taken during a twilight session for example than when you have a bladed shutter.


WGR's picture
Exposures with iris shutters

Hello Dave

The question you have to ask Apogee, is the opening time and the closing time of the shutter.  

The answer is --it depends on what you are doing.

My camera has an opening time of 8ms (.008 secs) and a closing time of 8 ms (.008 secs).  So this means that the center of an exposure will be .016 secs longer than the edges.  So a 1 second flat would be 1.6% brighter at the center.  A 2 second flat will be 0.8% brighter.  So if you want to do 1 millimag photometry, you want the center to be exposed no longer than 0.1%.  This implies a 20 second flat, minimum.  My scope will have tons of stars with such a twilight exposure.  I use 2 seconds for the flats--so my error is 0.8% brighter in the center than the edges.  I cannot do any better than this without putting a diffuser or attenuator at the scope exit pupil, which changes the optical system, because the light is not entering the scope as it does under the sky.--another source of error, difficult to quantify.  

If your science frames are 2 seconds, then this error drops out.  If you science is 60-300 seconds, then not so much.  Rarely in VS do we use 2 sec exposures.  

Using this scheme, I am coming close to the .01 mag agreement that Arne is looking for from CCD observers.  

As for darks, and bias', the shutter does not come into play, unless you have light leaks, which must be fixed for other reasons.



ldj's picture
Thanks Gary and Arne, I

Thanks Gary and Arne,

I asked Apogee and they sent me the spec sheet on the shutter. It has an actuation time of 30ms (typical). The opening is 58mm and the chip diagonal is 52 mm so the effect may be a bit less than 30ms. So with the in and out, 10 second exposures would vary about one part in 166. My FOV is greater than 30 arc-minutes square so most of the time if the stars and comp stars were nearer the middle of the chip,  the effect will be smaller. 

Arne, I will have to think some more about your suggestion of the secondary correction. Sounds interesting. 

--- Dave LDJ

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