Camera calibration / Flats using EL display Panel.

[ribuck]

Hi all,

My shiny new earlsman EL display panel arrived yesterday, so I went about trying to calibrate my Kaf8300 based camera and i'm left feeling a little confused.

Process I follow

1) take bias and adjust offset value until average background adu value is around 1000.

2) Place the EL Display panel on the end of my scope and take a 2 second flat and keep adjusting gain until the ccd chip can reach maximum saturation.

Now this is the bit i dont understand. Becuase my camera has a mechanical shutter i have to use a min of 1-2 seconds for Flats.

I've read that background adu values for flats should be around 20,000 - 25,000 or (40% of max saturation value)

So seeing that i had to use 2 secs exposure to calibrate my camera to fully saturate the ccd, how can i do a flat of (1-2 secs - needed due to mechanical shutter) without fully saturating it using the EL Panel.

Would i need to find some way of dimming the panel to get the required background adu value (20,000 - 25,000) for flats ?

Regards,

Richard.

[steppenwolf]

Place several sheets of good quality paper between the telescope and the EL panel to cut down the light intensity - works a treat!

[ribuck]

So this is a normal procedure then. Great glad to see i'm on the right track.

Thanks guys.

[ribuck]

Steppenwolf, just so i'm 100% clear, I would use the EL Panel without any paper when calibrating the camera to get max saturation of ccd, then use it with a few sheets of paper to lower the brightness for doing flats.

is that right ?

[MartinB]

Yes, that will work. You can use flats with much less than 40% saturation but just need to collect more of them. I think 40% is an absolute maximum for an anti blooming chip. A good rule for really good flats is to collect 1 000 000 ADUs worth. So, if your flats were 10 000 ADUs brightness you would need 100 compared with 50 of 20 000ADUs.

My QSI has a mechanical shutter and I've never had a problem with shorter exposures for flats - 0.4 secs are fine. It would be worth finding out at what point the shutter becomes an issue.

[steppenwolf]

The use of the pieces of paper is for the Flat only to allow you to achieve the < 40% ADU count. The other calibration that you are talking about (the BIAS) does not require a light source at all - what you are aiming for here is to just record the 'readout' noise so an exposure at the very lowest exposure value (ideally zero exposure time) you can achieve with the telescope/lens capped off to exclude all light is what you should be aiming for.

[Beyond_Vision]

The minimum shutter time on the Atik is 200ms

[Beyond_Vision]

This is a handy device for adjusting the power.

It also is useful for running from a 24V source to provide a 12V/3A regulated supply. It will also shutdown when the battery is low before it gets fully discharged

therefore saving battery life

3A Digital Car Power Adaptor : In-Car Power Adaptors : Maplin

[ribuck]

Hi Kevin,

Thanks for that. The only thing i was thinking is that when you lower the voltage you increase the current. Does the increased current not damage the display ?

Thank'sfor lettingme know about the shutter speed, i'll try mine around 200ms and see what background value i get.

Thanks for everyones help on this, it has had me stumped.

Rich.

[Beyond_Vision]

The EL Panel power supply output does not stay constant regardless of the input supply. If the input is lower the output voltage is lower and so will the current hence it dims down

[ribuck]

ok thanks kevin, just thought i remembered from my electronics classes at college that if you reduce voltage the current willalways increaseand if you increase current voltage will always drop.

Guess my memory isn't as good as it used to be....

I'll try with a few sheets of paper first and see how i get on.

Rich.

[Beyond_Vision]

Only if the device is a switched mode supply with a regulated output. If the output voltage is constant and the load is static the input current will rise if the input voltage falls.

[barkis]

I always thought Current was directly proportional to voltage, and inversely proportonal to Resistance.

Ron.

[Beyond_Vision]

Good old Ohms law

[ribuck]

Hi Ron,

That was my believe too.

So the bottom line that i am ok to get a variable voltage power supply for the input (4-12v) and then just change the voltage selector down to 4v when i need it without causing any damage to the display.

I'll get a variable power adapter then and then plug it into the power block that came with the display and test the current levels withmy dgital multimeter and see what i get, as that will give me peace of mind.

Rich.