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I never trunkate to zero internally !

It is a big error to truncate zero (MaximDL ?) because the noise distribution statistic is modified
(the mean value of the signal is under estimated in the final result).

It is your responsibility to make the master images with high signal-to-noise and correct thermal conditions during obseervations.

ISIS performs all calculations in floating point numbers and considers the negative numbers (remember, a negative number can have statistical significance).

Remember also the order of the operations:

Processeced image = Raw image - Offset master image - K x Dak master image

(with K the ratio of actual exposure time and the exposure time during master acquision - and I recommand k << 1)

You can also write

Processeced image = Raw image - (Offset master image + K x Dak master image)

The term (Offset image + K x Dak image) is probaly always positive.

It is forbidden to add a constant to a flat-field. If the spectrum appears noisy is that you are at the limit of detection. This is not an unusual situation !

A typical problem with the flat-field is the very low flux produced by tungsten lamp in blue part of the spectrum. It is not easy to solve.
I do not have a good economic solutions at the moment. Personally, I realize most 20 to 30 flat-field individuals images for generate final flat-field master and obtain
an approximately correct signal in blue.

For dark master I take typically 15 to 20 elementary frame, exposured each 600 sec, for produce my master dark (during cloudy night or by placing the camera in my refrigerator for reproduce cold night conditions) (600 seconds because it is in general my maximal exposure time in my light polluted observatory).

Note that if you believe that your master dark image seems too noisy (which brings more problems than soution), you can leave
the name (blank) in the corresponding ISIS interface field (but this is not a normal situation!!!). You can also try to erase the flat-field
name for test (only) and compare.

Christian

Thanks for taking the time to reply Christian.

I have just discovered something which I think explains what I have seen. If I open my master dark in ISIS it reports BZERO = 32768 in the FITS header but for the same master dark MaxIm DL and The Sky X report BZERO = 0 ! For a raw image captured using The Sky X Camera add on all three report BZERO = 32768.

Looking at the Master Dark FITS file in Word Pad shows that BZERO = 0 so it seem ISIS knows how to manage the file while other packages can only go on the FITS header data.

The moral is to stick to one package at a time if possible and all will be well.

I apologise for wasting your time.

Regards Andrew

Hello Christian,

I have another question. During processing, there is produced a log in the Process window. This contains the normalization value and some additional data:
...
Normalization spectral range : [5450 - 5600]
Normalization value (final profile) : 31691.4
Normalization value (individulal profile #1) : 31691.4
----------------------------------------------------------------------------
Spectrum #1 ( b ) - ( v ) intensity = ( 0.642 ) - ( 0.457 )
( b ) - ( v ) mean intensity = ( 0.642 ) - ( 0.457 )
( b ) - ( v ) median intensity = ( 0.642 ) - ( 0.457 )
...

I could not find a documentation on the line (b) - (v) intensity.
From looking at the values they seem to be (b/norm) and (r/norm), with b equal normalization in the range around 4210 A, r equal normalization in the range around 6720A and norm the normalization as above (in this case 5450 - 5600). Is this correct?
Is it possible to use these values for "pseudo-BVR" measurements as you used them in your evaluation of RR Lyr lightcurve?

Regards, Martin

Hello Martin,

You've perfectly found the (b) - (v) intensities mechanism ! I added this information to show the effect of chromatism problem (instrumental and atmospheric origin) during the actual current observation. If the relative values ​​(b) - (v) are highly variable from one observation to the next this imply that you have a potential error on the shape of the stellar continuum i.e. blue/red instrumental slant (for example, by using a wide slit, the value (b) and (v) are very stable in general, it is an indication of spectrophometric quality).

Values ​​(b) - (v) are only indicative (the B and V bands are not standard, for example). I do not really recommend using this information for scientific analysis.

Christian