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QHY268c Gain and offset settings for broadband

CMOS Broadband
QHY Users Group •7 replies•556 views
AstroShed avatar
AstroShed avatar
Hello all,
I just got one of these cameras and wanted a bit of advice on some good starting gain and offset settings for broadband targets in a bottle 6 area using a `Takahashi FSQ85 as native f5.3, 450mm FL…
I have moved from CCD imaging for years, so this CMOS is all new to me…
Thanks
Stewart
Michael Timm avatar
Michael Timm avatar
Hi Stewart,

there is a wonderful article available herefrom @Jarrett Trezzo . This will give you a good overview. I am using the photographic mode with gain 26 and offset 30 at the moment with no problems.

Good luck and CS
Michael
astrodawg avatar
astrodawg avatar
Stewart:
Hello all,
I just got one of these cameras and wanted a bit of advice on some good starting gain and offset settings for broadband targets in a bottle 6 area using a `Takahashi FSQ85 as native f5.3, 450mm FL…
I have moved from CCD imaging for years, so this CMOS is all new to me…
Thanks
Stewart

For broadband work I used my QHY268C at gain 0 and an offset of 30. This works quite well. You get the full well depth & full dynamic range at gain 0 and the camera is at unity gain already at zero gain. I use different settings for narrowband. The above is what I use for full broadband.
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Harry Karamitsos avatar
Harry Karamitsos avatar
Hi, I just received my QHY2600 mono and I was wondering what settings you use for your mono setup.
Well Written
Alan Brunelle avatar
Alan Brunelle avatar
For broadband work I used my QHY268C at gain 0 and an offset of 30. This works quite well. You get the full well depth & full dynamic range at gain 0 and the camera is at unity gain already at zero gain. I use different settings for narrowband. The above is what I use for full broadband.


astrodawg, you should probably also state that you are using readout Mode #1, which is the High Gain Readout mode.  Yes, when set to gain 0 it is at unity,   But its well depth is significantly below other modes, if you should work at below unity gain.  Only 60k compared to ~90k for the others.  Mode #1 at unity gain looks like it has a deeper full well, but I'm likely splitting hairs there.  The others at unity gain are more significantly deeper wells.  Go to zero gain with the other three modes is what gets you the 90k deep wells.  Can  be useful for some situations.
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Alan Brunelle avatar
Alan Brunelle avatar
When I got my 268c I spent a fair amount of time figuring out what the different readout modes could offer me in differing situations.  I basically decided that I would choose the modes depending on whether the object presented itself with a high dynamic range, or low dynamic range.  I.e. a high dynamic range object has both bright and faint or dim subjects.  Low dynamic range objects are those that don't have a huge signal difference between regions of the frame.  Then other considerations are if I want the fullest well depth or if that doesn't matter.  For example, I want to shoot next to a very bright star, but I don't want to fully saturate the star, yet still get most out of the areas of interest.  The readout modes and gain settings can have a big impact on readout noise.  But I do not fret that.  

I think just settling at 30 for offset is fine.  But I am more stingy than that.  I was also not happy with the very wide range for offset found on the internet for this camera.  Clearly there are a significant number of people who choose these settings without knowing what they are doing.  I don't like giving away dynamic range for nothing.  So I did test exposures (darks) before using it to image under each of the 4 mode/gain choices I use to shoot.  With each, I figured out the signals I got.  Looking at the average signal to define a low, but healthy basement number.  Also, keeping an eye on the lowest pixel values and being sure that I am not getting pixels with zero values.  My offsets range from 8 to 12.  So not far from 30.  But the difference between 8 and 30 is still significant.  And as I stated, I am stingy.   While raising the basement values more than necessary may not impact a low signal image, with the dang bright stars or occasionally bright areas that drive values to max, the last thing I want to do is drive significant areas of the image to be clipped at the high end.  This can limit the colors you can get out of the stars, especially brighter ones. 

Use the graphs provided by QHY for each of the 4 modes and mark where unity gain is for each mode, low readout noise, or full well and dynamic range, etc.  Then consider if any particular mode, with settings, will offer an advantage considering the nature of the object.  I use Mode #1 for both very low dynamic range objects (gain 60) and moderately high DR objects (gain 0).  I also use modes #0 and #3.
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astrodawg avatar
astrodawg avatar
Alan Brunelle:
For broadband work I used my QHY268C at gain 0 and an offset of 30. This works quite well. You get the full well depth & full dynamic range at gain 0 and the camera is at unity gain already at zero gain. I use different settings for narrowband. The above is what I use for full broadband.


astrodawg, you should probably also state that you are using readout Mode #1, which is the High Gain Readout mode.  Yes, when set to gain 0 it is at unity,   But its well depth is significantly below other modes, if you should work at below unity gain.  Only 60k compared to ~90k for the others.  Mode #1 at unity gain looks like it has a deeper full well, but I'm likely splitting hairs there.  The others at unity gain are more significantly deeper wells.  Go to zero gain with the other three modes is what gets you the 90k deep wells.  Can  be useful for some situations.

You are incorrect in your assumption. I am not using high gain mode & zero of my statements implied that. I am using standard photographic mode. That should be clear enough. And my original statement holds true.
Alan Brunelle avatar
Alan Brunelle avatar
Alan Brunelle:
For broadband work I used my QHY268C at gain 0 and an offset of 30. This works quite well. You get the full well depth & full dynamic range at gain 0 and the camera is at unity gain already at zero gain. I use different settings for narrowband. The above is what I use for full broadband.


astrodawg, you should probably also state that you are using readout Mode #1, which is the High Gain Readout mode.  Yes, when set to gain 0 it is at unity,   But its well depth is significantly below other modes, if you should work at below unity gain.  Only 60k compared to ~90k for the others.  Mode #1 at unity gain looks like it has a deeper full well, but I'm likely splitting hairs there.  The others at unity gain are more significantly deeper wells.  Go to zero gain with the other three modes is what gets you the 90k deep wells.  Can  be useful for some situations.

You are incorrect in your assumption. I am not using high gain mode & zero of my statements implied that. I am using standard photographic mode. That should be clear enough. And my original statement holds true.

Your first post makes no mention of which mode you use.  So I had no option but infer.

There are 4 modes.  Only one mode gives you unity gain when the gain is set to 0.  That is Mode 1.  It's referred to by the QHY literature I am looking at this moment as High Gain mode.  That would make sense given a unity gain set at zero.  So no, you were not clear.  Clarity would be stating upfront which mode you use.

Photo DSO is Mode 0.  By the QHY graphs I am looking at, at this moment, Mode 0 is at unity gain at a setting of roughly 15.  At unity, Photo Mode offers roughly 65k well depth.  Dynamic range of 13 stops.  So unless I am using outdated information, I don't see where I am incorrect.  Please let me know if my info is wrong.

Mode 1, High Gain (edited out 'Photo Mode', damn this gets confusing!), offers a particularly nice combination of low readout noise throughout the setting range, but particularly when set to 60.  And at 60, the highest dynamic range of all the modes.  It does sacrifice well depth though.  I find this to work great for images of IFN or dark nebulae.  Well depth not such a big deal for that type of object.  Not a lot of photons.
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