CCD Camera gain setting help please

[DougM43]

Hi all, after dipping my toes into imaging with a modified canon 40D I have recently bought a dedicated ccd camera,(Starlight Xpress SXVR-M25C) I have read the manual from cover to cover and also searched the internet but cannot find any info on what gain setting would be a good starting point to use with the camera. APT has a gain setting box so it seems logical “ to me anyway “ that I need to put something in it. 

I suspect this is the first of many questions on my journey from very mediocre dslr imaging to using a dedicated Astro ccd camera.

many thanks, Doug.

 

[vlaiv]

As far as I know - most if not all astronomical CCD cameras have a fixed gain (unlike CMOS sensors).

CCD pixels have certain electron well depth and usually 16 bit ADC. Gain or e/ADU value is set as to optimize ratio of the two.

If you look at the specs here:

https://www.sxccd.com/trius-sx25c

You will see following specs listed:

Full-well capacity: Greater than 25,000 e- (unbinned)
System gain:  0.4 electrons per ADU

Max value that can be written using 16 bit is 65535 so 25,000 / 65535 = ~0.3815 or roughly = specified 0.4 e/ADU gain (gain does not need to be exactly ratio of full well and 16 bit max value - but it is good to have it close as this optimizes value range)

 

[DougM43]

Ah right I see, thank you very much so I assume in APT I just leave the gain blank as it’s already set by the camera?

thanks.

[symmetal]

1 hour ago, DougM43 said:

Ah right I see, thank you very much so I assume in APT I just leave the gain blank as it’s already set by the camera?

thanks.

I would expect the gain setting in APT to be greyed out when you connect the CCD gamera as the camera driver will not have an option to set the gain. If it's not greyed out just ignore it as it won't have any effect anyway. 

CMOS cameras  which have gain/offset settings are only 12 bit so only have 4096 discrete levels as opposed to the 65536 levels in a 16 bit CCD camera. Trying to juggle the well depth of say 20000 -e into 4096 levels you either throw away electrons (low gain settings) or throw away even more bit depth to only 11 or 10 bits (high gain settings), or leave it at unity gain (12 bits) and accept a useable well depth of 4096. To be honest a few more clipped stars at a well depth of 4096 is not as bad as it sounds. This is all balanced with very different noise characteristics at different gain settings as well so different people have different opinions as to what the 'best' gain setting is. 

Vlaiv convinced me to leave my ASI1600 CMOS at unity gain as there is less faffing about. 

As Vlaiv said, with your CCD camera having 65536 discrete levels which is greater than the well depth there is little point in having variable gain.

Here's a useful article by Craig Stark explaing gain, offset and bit depth in more detail. 

Alan

[ollypenrice]

The gain setting, in my understanding, is controlled by a potentiometer inside the camera and is adjustable as a factory setting but not as a user one. 

Olly

[steppenwolf]

I can confirm that that the SX SXVR-M25C is also 0.4 electrons per ADU. Here is the full spec:-

Specifications:

• CCD type: Sony ICX453AQ SuperHAD CCD with ultra low dark current, Bayer RGB matrix and vertical anti-blooming.
• CCD Full resolution Pixel data: Pixel size: 7.8uM x 7.8uM, Image format: 3024 x 2016 pixels
• CCD Image area: 23.4mm (Horizontal) x 15.6mm (Vertical).
• CCD quality: Grade 1 or better - No bad columns, no dead pixels, no more than 50 'hot' pixels (saturated in <10 seconds).
• Spectral Response: QE max at 540nM (~60%), 50% roll-off at 400nM and 650nM. 
• Readout Noise: Less than 12 electrons RMS - typically ONLY 7 electrons! 
• Full-well capacity: Greater than 25,000 e- (unbinned)
• Anti-blooming: Overload margin greater than 800x.
• Dark current: Dark frame saturation time greater than 100 hours. Less than 0.02 electrons/second @ + 10C ambient.
• Data format: 16 bits.
• System gain:  0.4 electrons per ADU
• Computer Interface: Built-in USB 2.0 compatible interface. Also works with USB 1.1.
• Image download time: Typically 3.5 seconds full resolution using USB 2.0, approximately 8 seconds with USB 1.1.
• Power requirements: 115VAC / 240VAC @ 12VA, or 12VDC @ 750mA max.
• Cooling system: Regulated constant current cooling supply with single stage thermoelectric cooler to give a CCD temperature of approximately -30C below ambient.
• Size: 75 x 100mm black anodised aluminium barrel with 42mm 'T2' thread at the CCD window end & input/output plugs at rear. CCD alignment screws are provided for setting the chip parallel to the focal plane.
• Weight: approx. 400g.