Different exposure lengths for LRGB flats

[alcol620]

Hi folks

Just been practicing taking flats with a filter wheel containing LRGB and a Ha filters. Am I right in saying that the exposure length has to be adjusted to keep the ADU within the range required?

[steppenwolf]

Yes it does. Ha for example will require a much longer exposure time than the others to keep the ADU value within your required range.

[alcol620]

Hi Steve

Would you expect the RGB filters to require different settings? It seems that my R channel requires 8s and the B and G 3s. The Ha filter requires 60s to get the ADU in the 30000's.  I have a IDAS P2 filter ahead of the reducer in the optics train. Using small LEDs as the light source behind perspex panels.

Thanks

Alec

[alcol620]

See my earlier post re difference between the red filter flat and the green and blue filter flats. Any reason for this?

[swag72]

My RGB filters are very different - For me, as long as they are within the ADU figure that I want, then I don't care what the exposure time is

[alcol620]

Sara

Many thanks. New to this monocamera and filters learning curve and was worried I had run into another problem with the kit. Why would the red filter require twice the exposure, any idea?

Regards

Alec

[swag72]

Probably something to do with light wavelengths and physics - Neither of which I understand

[alcol620]

Thanks Sara - Happy New Year from the UK

[steppenwolf]

Would you expect the RGB filters to require different settings?

Yes I would expect differences - this isn't an issue though as the actual ADU figure is not super-critical. A test set will give you the exposure that you will require for subsequent sessions.

[alcol620]

Thanks Steve - Happy New Year.

Regards

Alec

[Whirlwind]

The reason you need different exposures for your flats for different filters is due to the varying sensitivity of your system with wavelength.  Just considering your CCD for the time being, the sensitivity of an 383 camera is similar to the attached image. 

If, arbitrarily, your blue filter passed through wavelengths of 400-500nm, green = 500-600nm, red = 600-700nm (assuming 100% pass through in the stated ranges and 0% in anything else).  In each of these 'pass bands' your detector has a different quantum efficiency.  As such in the 383 case if a blue/red photon hits the sensor then the probability it will be detected is less than if a green photon hits the sensor.  As you have filters in the way a blue filter will stop any green/red photons etc. As such compared to a green filter to detect the same number of photons in the red/blue you have to expose longer because the detector is less likely to detect them.   

However on top of this your filters will have different pass band widths.  My example above assumes that each pass band is 100nm in width.  The actual flux reaching the sensor is integrated from all the photons arriving in the filter pass band.  For example if we take one filter we can split the pass band into bins of say 1nm.  If in a second 1 photon arrives from the source in each of these bins then if you have a 100nm filter pass band your integrated flux is then 100 photons per second (1 photon per second per bin x the number of bins, which in this case is 100).  If however one of your filters only has a pass band of 10nm (e.g. the Ha), then your integrated flux is only 10 photons per second (in this example, note these are not 'real' numbers).  i.e. the narrower your filter the less photons are passed through and hence to obtain a certain signal you need to expose longer (in this case 10x, excluding other factors).

Finally the flat field source you are measuring may not produce an equal number of photons in each of the pass bands.  So in your blue pass band it may only be producing half the number of photons than it does in the green pass band.  Again this will affect how many photons per filter you detect and hence how long you need to expose obtain the same signal.

These are likely to be the three significant factors affecting the time it takes to take different filtered flats, but there may be other factors too.  For example the coatings on your telescope may preferentially transmit/reflect certain pass band photons etc.

I hope this helps

Ian

[alcol620]

Ian

That is great. Thanks for taking time out to explain the reason for requiring different exposure times. I'm much happier now.

Just goes to show what a superb site Stargazers Lounge is and what great people are around to take time out to provide knowledge and reassurance to those of us with questions to get answered.

Happy New Year

Alec