Altair Hypercam 294C HCG Mode Switching Point

[PeterC65]

SharpCap switches my Altair GPCAM2 327C into HCG mode at a gain of 200, at which the read noise is a very low 1.1e(rms). My understanding is that there is no point in operating the camera at lower gains because the read noise would be significantly greater and so gains below 200 are essentially unusable.

I would like a wider field of view (without changing the optics) so I’m looking at the Altair Hypercam 294C.

SharpCap switches the Hypercam 294C into HCG mode at a gain of 900, at which the read noise is 1.5e(rms). This seems like quite a high gain value, meaning that much of the gain range is unusable and the full well capacity is down to 7300 from a possible 64000.

The ASI294MC uses the same sensor and switches (automatically I believe) into HCG mode at a gain of 120, at which the read noise is 1.9e(rms). So at a much lower gain value when the full well capacity is about 15000 and the read noise only a little higher.

Why is there this difference when the sensor in both cases is the same?

[admin]

Hi,

the 294 sensor has a limitation meaning that it cannot produce a fully saturated output in HCG mode if the total gain is less than 9x (4.5x from LCG/HCG switch and 2x from gain control). This is where the 900 minimum in SharpCap comes from. I can change SharpCap to allow it to enter HCG mode from gain 450 upwards, but when you do that you end up in a situation where the maximum pixel brightness is about 65% (at 450 gain, working upwards as the gain rises). Nasty things happen if images are taken in this range, for instance what look like perfectly exposed flats can actually be overexposed and not work properly.

ZWO tell me that they have a workaround for this problem that allows the camera to be used from gain 120 (~4.5x gain), but I have seen threads recently with people getting issues on the ZWO cameras at 120 gain that are resolved if they switch to higher gains, so I am not sure if it is a complete workaround.

You can of course use the camera in LCG mode - higher read noise doesn't always cause an issue providing that you can take long enough exposures.

cheers,

Robin

[PeterC65]

Thanks for the quick reply Robin. That all makes sense. I assumed there would be a sensible rationale.

I'm using my cameras for EEVA and want to keep exposure times small and therefore read noise low, so from what you say the Hypercam 294C might not be the best choice. Would you agree?

I've also been considering the Hypercam 269C. From what I've read, initially SharpCap also switched this camera to HCG mode at a relatively high gain, but you have recently changed this so that it now switches at a gain of 200. Can you confirm that is the case? The 269C seems to have inherently more read noise than the 294C so it may still be unsuitable for EEVA.

Peter

[admin]

Hi Peter,

yes, earlier versions of SharpCap switched the 269 camera into HCG mode as late as possible, since there were some image issues in HCG mode. These have now been addressed by Altair SDK updates, so SharpCap now switches to HCG earlier in the gain range.

How about the 533 camera? It's a newer generation sensor that produces less background pattern noise than the 294 or 269, and also has a nice low read noise once it switches into HCG mode.

cheers,

Robin

[PeterC65]

Hi Robin,

Unfortunately the 533 camera is just a bit too expensive for me, even the Altair version.

My current thinking is that the 269 read noise is too high for EEVA and that the 294 pixel size is too big for my 432mm F6 APO (and it is likely to have too much amp glow).

My current favourite is the ASI585MC. Not quite as much field of view but I may be able to overcome that with a suitable (x0.6) reducer. There doesn't seem to be any issues with the 585, but it is new, and ZWO have very recently changed their spec for this camera (the HGC mode switching point has gone from 150 to 252).

Thanks again for your help.

Peter