SharpCap doesn't fully cool my camera

procyon12
Posts: 253
Joined: Tue Jan 14, 2020 11:32 am

Re: SharpCap doesn't fully cool my camera

#11

Post by procyon12 »

Hi,

Maybe this doesn't help here - with QHY174GPS, SC 3.3.7093.0, QHY driver from Feb. 2020: no problems with cooling.

Cheers, Christian
tc.astro
Posts: 22
Joined: Wed Jun 26, 2019 11:02 am

Re: SharpCap doesn't fully cool my camera

#12

Post by tc.astro »

Fine night last night but pretty warm at 20 degrees. SGP cooled the camera down to -20 (40 below ambient - better than the spec of 35) using 100% cooler power. I did a focus run then kicked off a sequence.

But SGP warned about the temperature which had warmed to -13, still using 100% cooler power. I left the camera idle but it didn't cool down. Seems to be a bug. Maybe my cooling problem is a bug in the firmware of the camera. Something isn't idling when it should.

I eventually kicked off the sequence and the camera over 15 minutes cooled to -18 where it stayed all night. I suspect that the window heater stopped it getting down to -20 as it had in the beginning of the session.
tc.astro
Posts: 22
Joined: Wed Jun 26, 2019 11:02 am

Re: SharpCap doesn't fully cool my camera

#13

Post by tc.astro »

I see that it is coming up to nearly two years since my initial post. Guess what - it took me that long to realise what the problem is.

Turns out that (I suspect) light falling on the sensor warms up the sensor. That's why when I cool the camera to -20 then turn on the light source for flats the camera warms up even when the ambient temperature is less than 35 degrees above the desired temperature. I have verified this by cooling the camera to -20 then turning on the flat panel then watching the temperature climb.

In summary: 3 things (at least) that warm up a sensor

1) the camera window dew heater
2) imaging using the camera
3) light falling on the sensor

Cheers
Tony
User avatar
admin
Site Admin
Posts: 13179
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: SharpCap doesn't fully cool my camera

#14

Post by admin »

Hi Tony,

that's an unexpected result :)

I suspect that it isn't the energy of the light itself (which will be really small), but perhaps the extra work done by the electronics on the camera when it has some significant signal to report rather than a nearly black frame.

cheers,

Robin
tc.astro
Posts: 22
Joined: Wed Jun 26, 2019 11:02 am

Re: SharpCap doesn't fully cool my camera

#15

Post by tc.astro »

Just finished some more flats today using an iPad screen as a light source. I've always done flats at 1.35s exposure (to match darks at that exposure). Today with the room temperature at 13 degrees the camera at 100% cooling settled at -17 degrees. I decreased the screen brightness so that the required exposure was 8s. It made a massive difference to the cooling - I obtained -20 degrees with only 80% cooling.

So from now on it's 8s flats for me.

One day I'll do a plot of settled temperature vs screen brightness to see what the curve looks like.

Cheers
Tony
User avatar
oopfan
Posts: 1321
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: SharpCap doesn't fully cool my camera

#16

Post by oopfan »

Hi Tony,

You said something that raised an eyebrow: "I've always done flats at 1.35s exposure (to match darks at that exposure)." I am probably misinterpreting what you mean, but here are the "rules":

Lights: The exposure should be determined by the Smart Histogram/Brain. For example, 30 seconds.
Darks: The exposure is the same as Lights but with the camera covered. Also, 30 seconds.
Flats: The exposure is adjusted so that the mean pixel value is between 25,000 and 30,000 ADU. For example, 2 seconds.
Dark Flats: The exposure is the same as Flats but with the camera covered. Also, 2 seconds.

Brian
tc.astro
Posts: 22
Joined: Wed Jun 26, 2019 11:02 am

Re: SharpCap doesn't fully cool my camera

#17

Post by tc.astro »

Hi Brian

Where did the eyebrow raise come from? My process is almost exactly the same as yours but where you say 2 seconds I used to use 1.35s and now I use 8. A slight difference is that I aim for a mean pixel value between 20,000 and 25,000 ADU because nonlinearity starts to kick in higher than that.

Cheers
Tony
User avatar
oopfan
Posts: 1321
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: SharpCap doesn't fully cool my camera

#18

Post by oopfan »

No worries. Just checking.
Post Reply