How long to let a cooled camera cool down before starting operation (vintage QHY8)

[uhb1966]

Okay, so i have a vintage QHY8 camera that is cooled, but does not report the temperature.

How long should i let the cam cool down before operation? I'm asking because it seems like the camera gets its best performance after at least 2 hours of cooling... and the difference is enormous ! Ambient temp was relatively mild (15° centigrade)

 

[Stuart1971]

It should not take anywhere near that long to cool, a few mins max…I would leave it about 5 mins to be sure

That camera should have a control box that you can set the cooler power on….?

[uhb1966]

Hi Stuart, thanks for your input-and no, there's only a power support box

[Oddsocks]

On 04/10/2023 at 20:11, uhb1966 said:

Okay, so i have a vintage QHY8 camera that is cooled, but does not report the temperature.

How long should i let the cam cool down before operation? I'm asking because it seems like the camera gets its best performance after at least 2 hours of cooling... and the difference is enormous ! Ambient temp was relatively mild (15° centigrade)

 

The original QHY8 (square camera with unsealed sensor chamber and dedicated desiccant storage/drying case) had an unregulated two-stage TEC, on paper capable of -40 delta .C although I doubt many actually achieved that in the real world.

Back in the early days of cooled astro CCD cameras, unregulated TEC cooling was not uncommon and there were two ways of dealing with the resulting changing dark current as the session progressed and the sensor reaches lower temperatures.

Either, take light-dark, light-dark, light-dark repeating pairs during the session and calibrate each light with its matching dark frame before combining the calibrated frames, or, use a master bias frame and master dark frame, taken at the end of the session when the camera has reached minimum temperature, and use the Dark Frame Optimisation routine found in many post-processing applications to automatically vary the ratio of dark frame subtraction from the light frame according to a measurement of residual noise in each calibrated frame, and then combine the results.

For your early QHY8 camera with an unregulated TEC the sensor should reach a reasonably stable temperature after around thirty minutes and cooling can begin at twilight while the OTA is still acclimatising, and for greatest efficiency take master dark frames and master bias frames at the session end and use Dark Frame Optimisation during calibration in preference to matching light-dark, light-dark, light-dark repeating pairs.

On the camera hardware side, if your QHY8 camera has a heatsink fan (or fans) make sure it/they are still working, the electronics was rather crude on cameras of this era and a broken fan might not be monitored, which would result in the camera cooling much more slowly than normal and fail to reach maximum delta .C. Also, make sure that the heatsink fins are clean and not choked with dust, the end result is just the same as with a broken fan, poor cooling.

TEC coolers do have a finite life and if a camera has a two-stage TEC it’s possible for one of the pair to fail, again, its likely that the electronics of this era would not detect a failed TEC and the result would be a greatly reduced delta .C capability, which would fit with your observation of a slowly improving image after many hours of use.

Final hardware tip, the TEC cooler stack will be bonded together, and also bonded to the back of the sensor, with either a thermal transfer pad, thermal grease or thermal paste. The latter two will dry out over extended time and lead to reduced cooling efficiency. If your camera has been in use for many years and it uses thermal grease or compound it’s possible that the old thermal grease/compound needs cleaning away and renewing.

If you feel that camera cooling performance has changed and is not as efficient as it once was then the above tips may point to a possible cause.

Lastly, improving image quality as the session progresses might not all be attributed to a lower sensor temperature, target altitude (rising target), local light pollution levels (auto-dimming street-lights, neighbours turning out their house lights) and daytime atmospheric dust gradually falling back to the ground leaving a clearer sky can all contribute to improved image quality the longer that the session continues.

William.

[uhb1966]

@Oddsocks thank you !