Preventing ice on sensor when using active cooling

[Jannis]

As I'm on a tight budget for the moment, I'm using the Polemaster for imaging. I've noticed how much the sensor temp affects this sensor, and so I made a cooler for it. It cools well, however, there's always this slight issue with things that get cold fast...

The ice appears to be only on the outside, and  when scraping some ice of the filter, the sensor looks clean.

However, this filter is only there temporary attached while i have it disconnected from my filter wheel. So my worry is that when i remove the filter, and attach the camera to my filter wheel, will the front of the sensor get as icy as the front of this filter?
I'm using an EFW, but due to the 1.25" NP adapter there will not be an airtight seal. If an airtight seal would help significantly I could use a different adapter though.

Any thoughts/ideas on how to keep the sensor frost free?

[wimvb]

That's why cooled cameras have dissicant tablets ad an airtight seal. The tablets absorb moisture before it can freeze. To get rid of ice on the lens/filter, you can make a dew shield and heater (NiCr wire). While a small hair dryer could help with keeping frost off when you switch between filter and filter wheel, I wouldn't recommend this as it also blows dust around. I would try to avoid handling the camera at all, if possible.

[davew]

I've seen dew straps wound round the front of the camera to prevent ice. In your case I suppose it would go between camera and filter wheel. I doubt you'd get too much heating to the chip if you only used something like an eyepiece strap.

Another step could be to cool the camera down slowly. Take, say, 5 minutes or so by switching the cooler on and off a few times,

Dave.

[Jannis]

Thanks for the replies.

Naturally the Polemaster was never designed for cooling, and i guess the main issue is that instead of cooling just the sensor i'm cooling the entire camera house. I can make a heater in a few minutes, but how will this work with an exposed sensor? Is the idea that the heating will keep the moisture away? Or did you mean to leave the 1.25" filter on and heat up just the filter?

[davew]

From what I've seen the heater would be around the tube where the filter is and not too near the chip. The filter would be removed when you fit the wheel ?

Some commercial cameras have a small heater built into the chamber in front of the chip for this very purpose.

Dave.

[Jannis]

Yes, the UV/IR filter is removed once attached to the wheel. I have an L filter in the wheel already, and I'm also worried about internal reflections if i leave two sets of filters in place.

I put together a low power heating strap now, and placed it on the outside between the wheel and camera. I'm guessing this should mostly heat up the wheel and air in front of the camera, rather then the camera itself. Turned it on now, and we'll see how it looks in 30 min or so - if i can still see anything, or if things are all covered in ice or dew.

Now if it all works, i just need to work a bit on my cables haha...

[davew]

Ingenious ! Crazy but Ingenious

I hope it works. 

I'm not even mentioning the wiring ! Oh damn, I just did LOL

Dave. 

[Jannis]

The heater helped stopping the buildup of ice in the front, and when i looked trough the telescope i could see the sensor.

However, once i started capturing i noticed it wasn't going to work. I could only see fog, and water droplets, so i removed the camera to have a closer look. Unfortunately not a promising sight..

So not sure what to try next. Lower power on the peltier, higher power on heater, slower cooldown, simply attach the filter again and see if that works, - or accept it without cooling until i can afford a QHY163M... Haven't quite given up yet though, but any ideas and thoughts are welcome

[Jannis]

Giving it another go now after yet another sensor cleaning. Running cooler at 5V now for a less aggressive and longer cool-down time, and with filter in place.
Hopefully the trapped air, slower cooling, and heating around the front will be enough. Just hope i'm not getting any reflections...

Edit: just checked, sensor is clean, but filter is fogging up. Still need to work more on this...

It's just 2c outside, but the difference is still noticeable with only 5V on the cooler. Here are 10 sec darks with gain 100, before cooling, after 10 minutes, and after 30 minutes.

[Stub Mandrel]

When I converted my DSLR I had to fit a ring of nichrome wire around the sensor cover glass to create a heated window. It took some experimentation to get the temperature right.

 

[Jannis]

I can't see any protective glass on this sensor, and so i don't think that will be a safe modification in my case, but how did you attach it, and how much power did you find out you needed?

[Stub Mandrel]

1 hour ago, Jannis said:

I can't see any protective glass on this sensor, and so i don't think that will be a safe modification in my case, but how did you attach it, and how much power did you find out you needed?

I fitted it around the sides of the cover glass, there was a groove there.

I think I'm giving it a watt or so. It's running now I will have to check tomorrow what current it takes.

[michael8554]

Are you regulating the temperature you are cooling to in any way?

https://www.ebay.co.uk/itm/1PCS-Controler-Sensor-Digital-W1209-thermostat-50-110-C-12V-NEW-Temperature/162914939745?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Michael

[Stub Mandrel]

12 hours ago, Stub Mandrel said:

I fitted it around the sides of the cover glass, there was a groove there.

I think I'm giving it a watt or so. It's running now I will have to check tomorrow what current it takes.

I have about 30 ohms resistance in series with ~10 ohms of wire. That gives about 40 ohms, so about 1/3 amp. That means about 1 watt through the heater and wasting 3 watts through the resistors.

[Jannis]

Thanks for the info on the sensor heater power usage. It doesn't look possible to install this on my camera, but I'm sure it will come in handy in the future. May i ask why the 30 ohm resistor instead of stepping down the voltage? 

Michael, i have several of those and they are great for heating, but this type is not very good for peltier cooled sensors as it turns off the peltier completely for too long time and gives a lot of temperature swings even when set to the quickest mode. When i turn off the peltier, it only takes a few seconds before the heat from the hot side transfers and defrosts the camera. What i would need instead was a regulator that instead adjusted the volt/current gradually (not PWM), but i don't have this at hand. 

[michael8554]

10 hours ago, Jannis said:

Michael, i have several of those and they are great for heating, but this type is not very good for peltier cooled sensors as it turns off the peltier completely for too long time and gives a lot of temperature swings even when set to the quickest mode. When i turn off the peltier, it only takes a few seconds before the heat from the hot side transfers and defrosts the camera. What i would need instead was a regulator that instead adjusted the volt/current gradually (not PWM), but i don't have this at hand. 

Oh dear, I have one of those arriving for a cooled dslr project.

What wattage is your Peltier please?

Does your fan run all the time?

Michael 

[Stub Mandrel]

You can fork out for a PID controller which will learn your device's behaviour and change it's switching pattern to minimise over and undershoot

http://www.ebay.co.uk/bhp/pid-controller

 

[Jannis]

20 hours ago, michael8554 said:

Oh dear, I have one of those arriving for a cooled dslr project.

What wattage is your Peltier please?

Does your fan run all the time?

Michael 

I run a rather high power TEC as i don't just cool down the sensor, it's around 60-70W at full power. For just the sensor you don't need more then a few watts, and so it might not be a problem for you. I recommend to leave the fan running at all time, yes.

[michael8554]

Thanks Jannis

Michael 

[michael8554]

On 26/03/2018 at 13:53, Jannis said:

gives a lot of temperature swings even when set to the quickest mode.

Was that with the hysteresis P1 setting changed from the 2 degree default?

Michael 

[Allinthehead]

Hi Jannis. You could try to leave the ir filter in place and see if you get reflections. Pop a few dessicant pills in to soak up any moisture. Then you could place something like this over the filter to prevent fogging. https://astronomy-imaging-camera.com/products/accessories/anti-dew-heater/

 

[Jannis]

Michael, sorry for the late response. Yes this is with the most aggressive hysteresis setting, not the default 2 degrees. The problem is that the temperature keeps dropping for a while after peltier is turned off, causing it to wait for a while before turning on the peltier again. This again causes the temp to raise for a while after peltier is turned on. So even though the controller tries to keep it within +/- 1 degree, it swings with +/- 3-5 degrees at best.

Allinthehead, I tried with the filter in place and didn't seem to get any reflection issues. However, I wasn't able to test much as even with a heater on the filter fogged up. I haven't quite given up yet though, and it does appear the best option is to leave the filter in place and have a bit more heating on the filter itself. A heater strip like that won't really work though as it would heat up the camera body and not the front filter, but I'll see if i can make something that will fit somehow.  

[michael8554]

Thanks Jannis

Michael 

[Adam J]

You have to get some desiccant in there with the sensor somehow and at least semi seal the chamber, you might want to also tone down the cooling.