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CMOS camera dewing?


centroid
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I have seen some discussions where people are having problems with CMOS based cameras dewing up.

Having recently bought a CMOS based camera (294C), I am intrigued as to why this should be, as back in the day, I imaged for 12 years, with a variety of Starlight Xpress CCD cameras, and never experienced any dewing problems.

Is it perhaps that CMOS cameras are of a different physical construction?

Edited by centroid
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I've had my new QHY 268M dew up, but also an older QHY9M CCD and an Atik 383L+ do the same thing.  It humid in Ireland, and my humidity reader is 99% in my dome if I dont have my dehumidifier running.

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11 minutes ago, tooth_dr said:

I've had my new QHY 268M dew up, but also an older QHY9M CCD and an Atik 383L+ do the same thing.  It humid in Ireland, and my humidity reader is 99% in my dome if I dont have my dehumidifier running.

Interesting, perhaps its because I live in the dryest area of the UK, that not one of the five different SX cameras ever displayed any dew problems. Having only just returned to the hobby, after an 8 year break, and only a mile from my previous location, it will be interesting to see how this camera (294c CMOS), copes.

 

Edited by centroid
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3 hours ago, centroid said:

I have seen some discussions where people are having problems with CMOS based cameras dewing up.

Having recently bought a CMOS based camera (294C), I am intrigued as to why this should be, as back in the day, I imaged for 12 years, with a variety of Starlight Xpress CCD cameras, and never experienced any dewing problems.

Is it perhaps that CMOS cameras are of a different physical construction?

Every few years you have to replace the dessicant or it will freeze or dew. Takes about 30mins. The SX stuff is argon filled, as its dry and being a very large atom will not seep past the seals so easy. But it adds expense and replacing the dessicant is not such a hard thing to do. 

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51 minutes ago, Adam J said:

Every few years you have to replace the dessicant or it will freeze or dew. Takes about 30mins. The SX stuff is argon filled, as its dry and being a very large atom will not seep past the seals so easy. But it adds expense and replacing the dessicant is not such a hard thing to do. 

That sounds like a  logical reason as to why I never experienced dewing problems during many years of using SX  cameras. A case of "you get what you pay for I guess". The Altair Hypercam 294C Pro TEC  camera that I recently bought, apparently has a heated window, and a guarantee of 2 years frost free operation, time will tell I guess. It also comes with a spare re-chargeable desiccant cartridge, with a note to say do not replace unless the need is confirmed by the dealer. It will be interesting to see what direction SX head in, now that Sony are ceasing the manufacture CCDs.

Edited by centroid
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6 hours ago, Adam J said:

Every few years you have to replace the dessicant or it will freeze or dew. Takes about 30mins. The SX stuff is argon filled, as its dry and being a very large atom will not seep past the seals so easy. But it adds expense and replacing the dessicant is not such a hard thing to do. 

Actually I will modify that statement I think the reason is the weight means it's harder for air to displace it as opposed to the size of the atom. 

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It’s condensation & then freezing with the shock. My QSI did it first time I used it the cooling power is so aggressive & having never experienced it with my Atik 314 I was taken aback. But it’s just a case of letting it warm up for a bit & then gradually cooling down to target temp. As said my ASI,s have built in dew heaters so I never seen it with them so far.

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I had 5 different Starlight Xpress CCD cameras, over a 12 year period, and never once had any dewing problems. Now I see reports of CMOS cameras, one of which I have just bought, having problems with dewing. An educated guess as to why this never happened with my CCD cameras is because they were of a different physical construction to CMOS cameras. SX CCD cameras use(d) the body of the camera as a heatsink, to dissipate the heat generated by the Peltier Cooler and other components. The body of the camera would get warm, as a result. Maybe this acted like a Dew Heater band, surrounding the optical window, hence never any dewing. As I said, just an educated guess. With CMOS cameras, like the one I have recently bought, and others, they have vents in the body, and a fan to expel the heat, so the body never gets warm. The body of my SXVR H16 got so noticeably warm, that I bought a small 12V fan, fabricated a bracket, and attached it to the camera (see attached photo). This pulled the warm air away from the camera. I posted this theory on another group, which shall remain nameless, where a discussion of dewing problems with their particular CMOS cameras was taking place, and apparently the ADMIN didn't like it, because they deleted it. 

I also discover that people are taking  multiple darks, to create a Master Dark to calibrate out amp glow/starburst out from the light frames, in one case 50 dark frames. 50 dark frames of 300 secs equates to 250 minutes. Add that to the light frames, and flats, and I would be at it until breakfast time 🙄.  A handful a calibration frames for the CCD cameras, was more than enough, and although dark frames were beneficial, they weren't essential.

I am beginning to wonder whether I should have gone back down the CCD route, while they are still available. 🤔

SXVR-H16_with cooler.jpg

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12 minutes ago, rnobleeddy said:

Never had any issues with my CMOS. I did have issues with an old ATIK CCD. 

I don't imagine the sensor has any bearing, it's just the camera + cooling design.

Indeed, it is the physical design of the camera, and not the senor. Albeit the 'starburst' problem is CMOS sensor related.

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18 hours ago, Sp@ce_d said:

It’s condensation & then freezing with the shock. My QSI did it first time I used it the cooling power is so aggressive & having never experienced it with my Atik 314 I was taken aback. But it’s just a case of letting it warm up for a bit & then gradually cooling down to target temp. As said my ASI,s have built in dew heaters so I never seen it with them so far.

Perhaps I have been spoilt by using SX CCD cameras. Back in the day (pre 2014) my scopes were kept permanently mounted in my then observatory, but the cameras were kept indoors. They would be taken out to obsy when needed, attached to scope, and quickly electronically cooled down to the required temperature, without any dewing problems. The opposite of what I am advised to do with CMOS cameras, where the advice is to cool slowly.  Of course, at the moment, this is all hypothetical for me, as I have yet to use the 294c. 

I have been busy acquiring all the gear that I need. I had forgotten just how much kit I used to have. 😅

Edited by centroid
typo
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On 14/10/2021 at 11:58, centroid said:

I also discover that people are taking  multiple darks, to create a Master Dark to calibrate out amp glow/starburst out from the light frames, in one case 50 dark frames. 50 dark frames of 300 secs equates to 250 minutes. Add that to the light frames, and flats, and I would be at it until breakfast time.

I take it you have the cooled version of the camera? You do this once per gain / offset setting and reuse the master dark for six months to a year.

I'm a recent convert from Atik CCDs to CMOS, I'm still getting used to my new camera (QHY 268M) but the results so far are blowing me away. Where I was doing 15 - 30 minute narrowband subs on the Atik I can get better results with 3 - 5 minute subs, albeit over the same total imaging time.  Point being I lose fewer subs due to odd events like planes, wind gusts etc.

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16 minutes ago, Starflyer said:

I take it you have the cooled version of the camera? You do this once per gain / offset setting and reuse the master dark for six months to a year.

I'm a recent convert from Atik CCDs to CMOS, I'm still getting used to my new camera (QHY 268M) but the results so far are blowing me away. Where I was doing 15 - 30 minute narrowband subs on the Atik I can get better results with 3 - 5 minute subs, albeit over the same total imaging time.  Point being I lose fewer subs due to odd events like planes, wind gusts etc.

Yes, I do have the TEC version of the camera, and have started creating a library of darks, taken at various exposure lengths, and the same temperature setting (-10).

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Few observations from my side:

Many of CMOS cameras do have window heater built in (like ZWO , QHY) , for some, it can be switched on/off in camera (ASCOM) setting.

If heater is not built in , ZWO and some other manufacturers do offer camera window heaters. Combined with regular application of desiccant tablets, dewing should not be real issue.

Amp glow is not that big deal anymore on recent cameras and dithering combined with good stacking takes good care about hot/cold pixels, so dark frames are not that essential. Good to have them , though.  Reasons for taking flats( or not) are the same for both CMOS and CCDs 

 

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On 14/10/2021 at 19:48, centroid said:

Perhaps I have been spoilt by using SX CCD cameras. Back in the day (pre 2014) my scopes were kept permanently mounted in my then observatory, but the cameras were kept indoors. They would be taken out to obsy when needed, attached to scope, and quickly electronically cooled down to the required temperature, without any dewing problems. The opposite of what I am advised to do with CMOS cameras, where the advice is to cool slowly.  Of course, at the moment, this is all hypothetical for me, as I have yet to use the 294c. 

I have been busy acquiring all the gear that I need. I had forgotten just how much kit I used to have. 😅

Any cooled  camera cmos or ccd benefits from being cooled or brought back to near ambient temp slowly.. to stop thermal shock

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