What has caused primary mirror on sct to fog up?

[lee g]

So my sct lives outside in a well ventilated shed. Last night I set up 2 hours before sunset when I went out to observe i turned on my dew strip heater on the secondary mirror, but within half an hour for the first time ever the primary mirror had fogged over. This ended the session there and then. How is this possible since the mirror sits in a sealed tube and how can I prevent it in the future.

 

[michael8554]

Hi Lee

My 8" SCT has a Dew Shield, and I rarely need to have the Corrector dew heater on.

But you may have damp air inside the OTA.

So you may need to employ one of these SCT Coolers:

Michael

[LandyJon]

56 minutes ago, michael8554 said:

But you may have damp air inside the OTA.

I agree it could only be damp inside the tube, but I'm not sure how a cooler would help, surely that would just speed up the foging.

I'm not sure a dew band round the base of the ota would get enough heat on to the primary to prevent it either, although they work on newtonians so it could be worth a try.

The ideal solution would be to remove the moisture from inside the tube.  To do that you need to get it indoors warmed up with some desicant in there.  Obviously carefully, hopefully its an SCT without a lens at the back and you could stand it up near a radiator with a net bag filled with silica gel packets tied to string and dropped down from the visual back.

It could take a while, perhaps blowing a hairdryer in occasionally to speed things up, warm the air a little more and stir it up.  You also want a fairly dry atmosphere in the room, if you have a room dehumidifier run that too so its in a dry atmosphere when you seal it back up.

The other thing, it doesn't have any vents on the side like the EdgeHD does ? I'm not sure these SCTs are quite the 'sealed unit' like a camera lens is when left attached to the body. The vents aid cooling but would also allow humidity to enter. Maybe worth considering storing the OTA indoors and then a cooler could be employed to speed up use each time.

Hope you get it solved.

[michael8554]

It's described as a cooler, as it swaps the "warm" air inside the OTA with cool air from outside, so really it's ventilating the inside of the OTA.

Michael

Edited December 2, 2023 by michael8554

[Captain Scarlet]

As to what actually happened to fog up your primary: when, after an observing session say, you bring a cold scope or mirror into the warm house, the glass immediately fogs up of course. Here it’s the same but in different circumstance. By heating the secondary, you’ve added heat to the air inside your tube. So you’ve effectively brought warm somewhat humid air to a cold mirror and in just the same way the water vapor condenses onto your cold mirror.

Cheers, Magnus

[Jules Tohpipi]

21 hours ago, lee g said:

So my sct lives outside in a well ventilated shed. Last night I set up 2 hours before sunset when I went out to observe i turned on my dew strip heater on the secondary mirror, but within half an hour for the first time ever the primary mirror had fogged over. This ended the session there and then. How is this possible since the mirror sits in a sealed tube and how can I prevent it in the future.

 

Last couple of nights in East of England the relative humidity has been at 100% (unless you're on the coast). This is somewhat unusual and probably the key factor. Caused by a combination of rain earlier in the day, followed by freezing cold temperatures in the evening (air less able to hold water vapour at lower temps).  At 100% humidity and below-freezing temperatures, the air is fully saturated, no longer able to hold any further water,  and will dump water vapour onto whatever cold surfaces it encounters. 

Conclusion is that the sealed tube might not be so sealed. Alternatively, there's a very small amount of outside air getting in.

The only solution in such extreme 100% relative humidity conditions is to get the relevant surfaces heated up.

 

Edited December 2, 2023 by Jules Tohpipi

[lee g]

So after reading all these helpful replies I think I’ve figured out what went wrong.

just before the primary mirror fogged up I had used a hairdryer to clear the corrector plate and I probably inadvertently warmed the inside of the tube up thus resulting in warmer air inside. Does this sound feasible in your opinions.

[Martthebass]

To echo a comment above, humidity was really high last night. I had frost growing off all surfaces of my rig at 6:30pm and had to pack up due to freezing fog at about 7:30

[Jules Tohpipi]

1 hour ago, lee g said:

So after reading all these helpful replies I think I’ve figured out what went wrong.

just before the primary mirror fogged up I had used a hairdryer to clear the corrector plate and I probably inadvertently warmed the inside of the tube up thus resulting in warmer air inside. Does this sound feasible in your opinions.

If the tube is truly sealed then no that's not the reason. Same physical principles as why sealed double-glazing doesn't form dew/fog in the air gap [actually usually some inert gas but..] of the two panes when it's cold outside but warm inside. But does fog internally if the seal is broken.

If outside air can get into your OTA, then the act of heating your corrector plate would mean the heated air inside becomes capable of holding more moisture - and that extra moisture is supplied courtesy of the cold (but very humid) air entering from outside - on that particularly humid night. This hotter air then has to dump part of that extra moisture again when it drops below the dew point hitting your very cold primary mirror. 

 

Edited December 2, 2023 by Jules Tohpipi

[lee g]

57 minutes ago, Jules Tohpipi said:

If the tube is truly sealed then no that's not the reason. Same physical principles as why sealed double-glazing doesn't form dew/fog in the air gap [actually usually some inert gas but..] of the two panes when it's cold outside but warm inside. But does fog internally if the seal is broken.

If outside air can get into your OTA, then the act of heating your corrector plate would mean the heated air inside becomes capable of holding more moisture - and that extra moisture is supplied courtesy of the cold (but very humid) air entering from outside - on that particularly humid night. This hotter air then has to dump part of that extra moisture again when it drops below the dew point hitting your very cold primary mirror. 

 

When I say sealed tube I meant that in a loose sense. I mean air must have entered the tube whilst I attached the diagonal thus letting in some cold ambient air. So when I dried the corrector plate using the hair dryer I may have also heated the air behind it inside the tube which in turn caused the primary mirror to dew over 

[Neil H]

Hi I use this when I store my MAK 180 pro never had a problem of dew inside and the MAK is out in a cold shed

https://www.firstlightoptics.com/dew-prevention/astro-essentials-dual-fit-desiccant-cap-red.html

[lee g]

5 minutes ago, Neil H said:

Hi I use this when I store my MAK 180 pro never had a problem of dew inside and the MAK is out in a cold shed

https://www.firstlightoptics.com/dew-prevention/astro-essentials-dual-fit-desiccant-cap-red.html

Thanks for your suggestion I actually ordered one yesterday 

[Neil H]

I use them on my MAK and 127 refractor , when I remove it from the OTA I put a plastic cap over the end with the holes in so it can't draw any air in till I refit it at the end of the night and I pack up 

https://www.firstlightoptics.com/astronomy-optics-cleaning-protection/astro-essentials-125-flexible-nosepiece-cap.html

Edited December 2, 2023 by Neil H