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

#1

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

#2

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

#3

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

#4

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

#5

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
ChrisR Oz
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Re: Matching the brightness/ offset value of lights to darks

#6

Post by ChrisR Oz »

Hi Tim,

Just a bit puzzled about your offset being too low in the Lights at a dark site. Surely, your Darks at that offset were already fine (or you would have used a higher offset). The Darks should always be lower than any Lights, even at a dark site. :?:

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
timh
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Re: Matching the brightness/ offset value of lights to darks

#7

Post by timh »

Good question Chris

Yes you are right it doesn't make sense. My observation was that under dark skies -sky brightness 6 or 7 fold less than my usual Bortle 6 - I needed to increase the brightness setting of the light frames in order to move the livestacking histogram clear of the left hand axis (where live stack successively adds frames, subtracts the master dark and divides each by the flat) -- and see an unclipped live image - whereas from my home sky this was never necessary. I think it just reflected the fact that at home the sky brightness is habitually about the same or greater than the 1% offset signal whereas under dark conditions the offset becomes the biggest part of the total signal ?

Anyway it was that experience that made me finally twig that the brightness setting did matter. Sometimes before then - not thinking that it would matter I had randomly changed it to a different value when collecting lights- and then seen odd vignetting effects after calibrating with my normal brightness 10 dark masters that I couldn't understand. So just a step in my learning curve -- I just thought that others with ZWO cameras might also stumble over the same thing because 'image brightness' isn't very descriptive of what the control does - and while SC organises within dark libraries all the other parameters of darks and lights that have to match eacxhother the brightness parameter is missing.

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

#8

Post by ChrisR Oz »

Hi Tim,

I have done the same thing. “Brightness” did not gel as offset/black level/pedestal at first until it was explained to me. I then set about working out which I should use. Settled on 4 for a gain of 120 at -10C on 294MC pro. But I did that by closely examining Dark frames and keeping the distribution away from zero. Probably need to investigate how that should change with gain. But at least for the 294MC, 120 is pretty optimum most of the time, outside of planet season and some fast plate solving.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
MarMax
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Re: Matching the brightness/ offset value of lights to darks

#9

Post by MarMax »

timh wrote: Wed Apr 20, 2022 9:59 pm Good question Chris

Yes you are right it doesn't make sense. My observation was that under dark skies -sky brightness 6 or 7 fold less than my usual Bortle 6 - I needed to increase the brightness setting of the light frames in order to move the livestacking histogram clear of the left hand axis (where live stack successively adds frames, subtracts the master dark and divides each by the flat) -- and see an unclipped live image - whereas from my home sky this was never necessary. I think it just reflected the fact that at home the sky brightness is habitually about the same or greater than the 1% offset signal whereas under dark conditions the offset becomes the biggest part of the total signal ?

Anyway it was that experience that made me finally twig that the brightness setting did matter. Sometimes before then - not thinking that it would matter I had randomly changed it to a different value when collecting lights- and then seen odd vignetting effects after calibrating with my normal brightness 10 dark masters that I couldn't understand. So just a step in my learning curve -- I just thought that others with ZWO cameras might also stumble over the same thing because 'image brightness' isn't very descriptive of what the control does - and while SC organises within dark libraries all the other parameters of darks and lights that have to match eacxhother the brightness parameter is missing.

Tim
I have experienced the same thing at a dark site (SQM-L 21.85) versus my home site (SQM-L 18.30). With the ASI533MC-Pro I run a brightness of 10 at home and had to bump this up to 30 at the dark site. At the time I did not re-take the flats and darks but I should have. Thanks for the Brightness/Offset/Black Level tip.
ChrisR Oz
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Re: Matching the brightness/ offset value of lights to darks

#10

Post by ChrisR Oz »

Hi Tim, Marmax,

I am beginning to think I should use the Pedastal option in PixInsight more often to deal with dark site data. If I increase offset in both Lights and Darks, then the calibrated light (av_flat*(light-dark)/flat) will tend to cancel that change and still have a scatter that blends into zero in some pixels, if the background sky glow is low. However, adding a constant Pedastal early in processing, and keeping that until later, would help avoid any issues of zeroes during stacking and processing.

Cheers, Chris.
Celestron EdgeHD 8, reducer 0.7x, Star Sense, CGX-L mount, Focuser, CPWI; Starlight Xpress AO, OAG and Filter Wheel; ZWO 294MC/294MM Pro and 174MM mini; SharpCap Pro, PHD2, Powermate 2x, Baader Neodymium, Astronomik CLS-CCD, ZWO UV/IR, Duoband filters.
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