Celestron 925 - Don't leave outside. . .

[TerryW]

This whole thing is very odd, as others have said. My SCT has been out in the damp, on and off, for ten years and never had anything like this. Not remotely. Hope it is sorted.

Olly

That is odd. The history goes something like this . . . . My OTA and mount lived in the conservatory for about 3 months or so. No problems. Then it lived out on its mount in the garden, under its cover, just outside the conservatory for about a month whilst I experimented with webcam photography of Jupiter. I got nothing but 'fuzzy-melon' shots no matter what I did. I also notice a huge amount of flaring around bright objects. Out came the Kendrick dew heaters and shield. Worked ok to clear the corrector lens but I still had the "fuzzy melon" in my avi files.

I brought the OTA in out of the dark for close examination and noticed that when I shone a strong torch down the OTA I could see what looked like a film over the mirror and lots of condensation on the inside of the corrector lens. This was in a large arc surrounding fastar attachment in the middle of the lens.

I dried it all out using my patent drying table (fan heater and computer fan to extract air). This worked well. Moisture gone but mirror still very cloudy.

I then got curious as to whether the condensation formed when the scope was in the conservatory or under its cover in the garden.

I put it back under the cover in the garden with the corrector lens cover on and an eyepiece in-situ and checked it the next day. That is when I discovered it had really bad water ingress (the shots are attached to earlier emails).

Martin (FLO) reliably tells me that the OTA is not sealed. It is classified as "open", It is therefore subject to ingress of air which may be moist. Fair enough this isn't Saudi after all. I knew that already but was basically taking the risk to save time.

It dried out a second time and I ripped it apart for considerable cleaning activity. All done.

What is the solution to water ingress?

I am wondering whether I can introduce a weatherproof seal at the corrector lens end. Currently, as it came from the manufacturer, there are two semi-circular paper/fibre gaskets between the corrector and the retaining ring. They do not cover the whole of the circle and will not form an air tight seal in any way. I am not really sure why they are there. They may even be off a smaller scope. I guess they would complete the circle if they were off the 8" version. Maybe that is a clue.

It looks easy enough to replace the half-hearted seal that is there already and use a Farpoint condensation device at the eyepiece. This has a sealing ring. This "closed" design might well yield a more moisture resistant OTA.

As to why my OTA insists on sucking moisture from the air and yours does not, I have not a clue. I would say make the most of it.

I did notice yesterday morning that the mount was covered in droplets of water under its "green Witch" cover. It certainly isn't dry and ventilated under there so I'll investigate this. I don't want water getting at the tracks and components on the mount's circuit board. That won't be so cheap to fix.

The mystery continues . . .

Terry

[TerryW]

Hi All,

I said I'd report back when I have tested the collimation following corrector removal, cleaning and replacement. The collimation didn't change at all. It remained spot on.

Terry

[mdstuart]

Sounds like you deserve a DRY back garden observatory!

Mark

[Earl]

I think it needs to be dried out for a while to get rid of any moisture inside the OTA, the thingys I linked earlier will probably solve this if you leave them in all the time when not in use.

[jonathan]

I dare say a desiccant cap could easily be manufactured, I have plenty of desiccant pouches lying around (several in my eyepiece cases). Might have a go at making one for myself.

[TerryW]

Should be really easy to identify a 2" plastic tube that can have end-caps fitted. One removable and one fixed.

I think that the desiccant that changes colour would be needed as otherwise you don't know when it has stopped working.

Good luck.

I splashed out and ordered one. No time to build now. I am back at work after a few months off to do DIY.

Terry

[jonathan]

I didn't know desiccant stopped working, I thought it just absorbed and then released moisture indefinitely.

[TerryW]

I didn't know desiccant stopped working, I thought it just absorbed and then released moisture indefinitely.

Well, the blue stuff turns red when you are supposed to change it. It has saturated. Now, whether it will turn back to blue is another matter.

Here is what is says on What Is Desiccant Silica Gel?

================================

Silica gel is an amorphous form of silicon dioxide, which is synthetically produced in the form of hard

irregular granules (having the appearance of crystals) or hard irregular beads. A microporous structure

of interlocking cavities gives a very high surface area (800 square meters per gram). It is this structure that

makes silica gel a high capacity desiccant. Water molecules adhere to the gels surface because it exhibits

a lower vapour pressure than the surrounding air. When an equilibrium of equal pressure is reached,

no more adsorption occurs. Thus the higher the humidity of the surrounding air, the greater the amount of

water that is adsorbed before equilibrium is reached. It is in these higher humidity conditions (above 50%

Relative Humidity) that stored or in-transit items are susceptible to damage.

The beauty of silica gel is the physical adsorption of water vapour into its internal pores. There is no

chemical reaction, no by products or side effects. Even when saturated with water vapour, silica gel

still has the appearance of a dry product, its shape unchanged.

Standard white silica gel is referred to as being non-indicating. As it adsorbs moisture it remains

physically unchanged.

======================================

It is the cobalt in the cobalt chloride indicator that is responsible for the toxic labelling

and disposal restrictions on this product.

It is still though considered as non-hazardous for transportation purposes.

Changing from blue (dry) to pink in a near-exhausetd state, it is supplied in 2 - 5 mm beaded

or 3 - 6 mm granular forms.

=======================================

It can be regenerated and reused if required. Gently heating silica gel will drive off the adsorbed

moisture and leave it ready for reuse.

- It is a very inert material, it will not normally attack or corrode other materials and with the

exception of strong alkalis and hydrofluoric acid is itself resistant to attack.

=======================================

So, it is re-usable. Yippee. I have a boat full of it. I'm going to warm the bags and re-use them now.

Terry

[TROJAN]

I have left my C11 out for hours and the outside has been dripping wet when I have brought it back in - no troubles so far, but I will definately keep a close eye on things...

[proflight2000]

Get some dessicant packs and tie them around the scope, they will draw water out. if you are lucky enough to see a BT engineer anywhere, ask them if they have any dessicant packs you could have, even old ones, they are excellent moisture removers.