Matching the brightness/ offset value of lights to darks

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timh
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Matching the brightness/ offset value of lights to darks

Post by timh »

Just a quick note about the importance of using lights and darks with the same offset value that would apply to folk who operate in the same way that I do (i.e. doesn't use bias frames and instead builds a dark library and selects a master dark to to exactly match lights during calibration) ... i.e.

I routinely use Sharpcap to prepare master grayscale Flats and Darks. I also use the Sharpcap facility to automatically save and assign master darks into the various subfolders of the Dark library that it assigns for you (according to camera, pixel dimensions, binning, exposure, gain, temp etc.)

BUT - caveat emptor - while the SC subfolders sort out nearly all of the important parameters for you there is one important parameter that is missing. Just as for all of the other parameters, Darks and Lights also need to be closely matched in terms of the offset or (ZWO cameras) image brightness value. It is an easy one to miss because SC doesn't actually separate this one out into the subfolder tree it creates -- but it is easy enough to sort further subfolders manually of course

I tripped up on this particular one because I had a library of darks - all at brightness offset value 10 - for use at home but then went to a dark sky site where I found that I needed to increase the offset in order to move the left hand of the histogram away from vertical axis. I then forgot that I had done this and went on to calibrate the lights using my original set of (offset 10) darks.

The results were quite nasty -- looks similar to vignetting (originally thought that I had messed up the flats) - until I realised what the problem was and produced an appropriate set of darks at the correct offset. Problem then solved

So ..maybe of use to someone ...

Tim
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Menno555
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Re: Matching the brightness/ offset value of lights to darks

Post by Menno555 »

Hi Tim

Actually, you can add it. If you use the parameter {BlackLevel} in the Manual Filename Templates, this will be added to the name.
It's kinda confusing cause BlackLevel, Offset and Brightness are the same things here.

Bit indeed it's a lesson I learned here too: forget 1 thing and the calibration files can be off.
To solve that, I made a rule for myself: everything but the exposure time must be the same as the Lights. So when making flats and darkflats, all I change is the exposure time. I even use the same temperature (usually -10 degrees C) as the Lights.
This can mean with the flats and darkflats that the exposure time is longer. But overall that's always better then shorter exposure times. If I use the Optolong L-eXtreme dual narrowband filter, my flats and darkflats can have up to 10 sec exposure. Works fine plus it minimizes the changes of vignetting immensely.

Menno
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turfpit
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Re: Matching the brightness/ offset value of lights to darks

Post by turfpit »

everything but the exposure time must be the same as the Lights
I always have worked like this and never had any issues with calibration.

BlackLevel (Altair), Offset (QHY) and Brightness (ZWO)are the same things - manufacturers having 3 different names for the same camera control has been a constant source of confusion during my time on the forum.

Dave
timh
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Re: Matching the brightness/ offset value of lights to darks

Post by timh »

Excellent comments you two. Thanks.

Making use of the Manual Filename Template is a good call. I haven't used dark flats up to now but will try building that into my process because, especially with HA filters etc, the exposure times for flats do start to become appreciable.

Tim
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admin
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Re: Matching the brightness/ offset value of lights to darks

Post by admin »

Hi,

just as a warning, I have seen one case recently where it was clear from detailed analysis of the dark frames that they were taken at brightness 16 (ZWO camera), but the FITS headers and capture settings files made it clear that the control was set to brightness 15. This led to darks that were too bright (brighter than the light frame background) and a failure in processing. I don't understand how the problem came about, but as a precaution I have added code to re-send the brightness value to ZWO cameras just before taking long (>2s) exposures in the latest versions of SC 4.0.

cheers,

Robin
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