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Hi Christian,

Thanks for the explanation about the problem and how to solve it.

By the way, I use MaxImDL for the calibration with dark frames and do not have a problem in applying them on my cropped images. MaxImDL expects from me full frame images but is able to apply them on my cropped images.

I do not shoot in full frame directly out of the camera, but do shoot already a cropped area because so I have a faster download and the images are much smaller as if I would use full frame.

A full frame image (1600x1200pixel @ 16bit) is ~12x bigger (3'758kb bytes) then my cropped image (1600x100pixel @ 16bit) = 321kb.

I will do some tests starting 15th of December with images higher then 116pixels.

Again, thanks for the explanation.

Acquisition of a good master dark image is a long job : for example mean of 10 x 600 s exposures (6000 s) for a serious master at a given CCD temperature (in darkness during a cloudy night!).

If your cropping heigth is of 120 pixels, you can take star image specrtra, OK (but you have the obligation to position the star in the same place along the slit - not so easy in function of available star for guiding in the field for very faint object spectrocopy - the observed star is not the guiding star).

But now imagine observation of a large nebula (M42, ...) or comet. The object image is now projected along the majority heigth of the slit. For capture max info, idealy, the cropping zone during acquistion occupy the totality height of slit image. But... your master image correspond to a smaller region! you can not process your nebula spectrum

Anticipate! At minimum, the cropping zone correspond to the total slit height image (with marges), for master images (dark, offset, flat) and for science image. An important recommendation.

(Personnaly, because I always lost cropping coordinate I have adopted the full frame acquisition method)

Christian B

But now imagine observation of a large nebula (M42, ...) or comet. The object image is now projected along the majority heigth of the slit. For capture max info, idealy, the cropping zone during acquistion occupy the totality height of slit image. But... your master image correspond to a smaller region! you can not process your nebula spectrum

Hi Christian,

Thank You very much for this information. This is the first time somebody explains me this and so far I did not know that

I will take that into consideration in the future and do some spectra in that way in order to learn more about this behaviour.

I have made spectra of nebula and the lines do not ocuppy the full height of the crop. Why is that so ? Is that due to not long enough exposure time and if yes I guess it would change when I use the 35µ slit. I think next week I will change the slit in my LISA from 23µ to 35µ and do some comparing imaging

here an example of M57

But... your master image correspond to a smaller region!

No, this is not the case. All my Darks, Flats and Bias are full frame images as explained in my prior message and maybe I did not write that understandable enough MaxImDL needs full frame images as calibration images even if I calibrate just a crop of an image, but OK (Max)mDL does choose the correct crop in this case)

Anyhow I will spent some hours or days and do some testing with my spectrum images in order to see how it comes out.

I make my darks unattended during the morning hours (after an imaging session) in the dark and closed Observatory room. I normally make 20 dark images as well as the same amount of BIAS. I am not applying Flat images so far on my spectrum images.

I see there is still a very long way to go ....................