Biassec incorreectly specified on mosaic images
reynolds wrote on Jun 30, 2008
I'm trying to reduce spectral data takenwith an E2V 2K x 4K CCD read out from 2 amplifiers, using mscred. I noticed after the data was acquired that the BIASSEC on some of the images was specified as [1:2048,1:4102], the same as the DATACSEC, rather than something like [2049:2068,1:4102] as I have used in the past. The data look normal, and so I'm wondering whether there's a way to deal with the incorrect BIASSEC? Can I still do an overscan correction somehow? I've read that the overscan correction can be evaluated by looking at the mean of a bias(zero) frame as reported using imstat. Apparently a mean different from 0 indicates the overscan correction has been over- or under-estimated. Can I use this fact to correct my images?
Also, if I try to ignore the overscan region by specifying a BIASSEC of "" when using mscred/ccdproc to merge the two halves of an image, I get an error saying "no BIASSEC specified....". Is there a correct way to ignore the overscan region?
Thank you.
Also, if I try to ignore the overscan region by specifying a BIASSEC of "" when using mscred/ccdproc to merge the two halves of an image, I get an error saying "no BIASSEC specified....". Is there a correct way to ignore the overscan region?
Thank you.
Francisco Valdes wrote on Jun 30, 2008
The BIASSEC you noted is definitely wrong. You can either change it with hedit or put it in the biassec parameter. However, I am not clear about your data. Is the data from each amplifier in a different extension? If this is data in the "flat" format that pastes the amplifiers into a single image then you would need to use a different ccdproc or else convert the "flat" format to the MEF format. With MEF format you can't use the biassec parameter to give an explict bias section unless all extensions have the bias region in the same place.
I suspect your data has a bias region even if the headers are wrong. I would implot some line cuts and if there is any significant sky in the exposure you should see the jump down to bias on the left or right of the image. Blowing up the region should show you the width of the region. Then you hedit the biassec keyword.
To ignore bias/overscan you set the overscan to no. This then does not use the biassec parameter. Simply setting it to "" is not the way to turn off bias subtraction.
I hope this helps but if you need more information maybe including a header listing would help me understand the E2V format being used.
Yours,
Frank Valdes
P.S. I'll be on vacation next week.
I suspect your data has a bias region even if the headers are wrong. I would implot some line cuts and if there is any significant sky in the exposure you should see the jump down to bias on the left or right of the image. Blowing up the region should show you the width of the region. Then you hedit the biassec keyword.
To ignore bias/overscan you set the overscan to no. This then does not use the biassec parameter. Simply setting it to "" is not the way to turn off bias subtraction.
I hope this helps but if you need more information maybe including a header listing would help me understand the E2V format being used.
Yours,
Frank Valdes
P.S. I'll be on vacation next week.
Last post on Jun 30, 2008