19th May Jupiter and Europa transit

[Lester]

Hi all,

got this image in 6/10 seeing with slight jetstream and a bit of light cloud.

Unfortunately the primary dewed up again, and no doubt this would be effecting the contrast in my image. I will have to remove the corrector and thoroughly dry out the OTA.

[iceman]

nice image Lester, good colour and detail. Europa looks 3D. Nice work!

[Dennis]

Although the seeing has just taken that slight edge off the image, it is a mighty fine image for the conditions Lester and as usual, your whopping image scale is impressive, especially with Europa perched on the limb!

Cheers

Dennis

PS - How on earth did the primary dew up on a closed tube SCT???

[Lester]

Thanks Mike and Dennis for your comments.

Dennis, I first had the primary dew up over 12 months ago when I cooled the OTA too much, and continually it would dew up very easily; there must have been damp air that got into the OTA. I opened the OTA and cleaned the primary and corrector, then the problem went away till last month.

Last month I cooled the OTA and it got to 5 degrees below ambient and it dewed up again. Seems to me when the internal gets below the dew point that moisture is sucked into the OTA, (I am only guessing here). The OTA is open where I have 2 fans mounted on the rear of the OTA, so I may have to cover these.

[Dennis]

Ahh – thanks Lester, it had slipped my mind that you use active cooling in your set up!

Hmm, not sure how the dewing can be avoided, unless you can somehow ensure only dry air exists inside the OTA by using an easily replaced desiccant cartridge that you can then recharge prior to each session?

Cheers

Dennis

[Lester]

Hi Dennis,

Martin made me a dessicant canister that I can put into the 2" focuser. I don't know how much of this is necessary to dry out a 14" SCT internally, or how long it will take.

[Quark]

Another nice image Lester,
Some very fine detail in this image, especially considering your problems.

What sort of control do you have for your cooling system.

Even at the Keck's if the air is to close to dew point they wont open the slot of the dome.

Even if the dew point is a reasonable level below ambient, if you cool your primary to much moisture will condense onto you mirror.

If there was some way of sealing your tube then a little dry Nitrogen would do the trick for you, it wouldn't take much.

Many high end telescopic sights for rifles have "O" ring seals that enable the tube to be filled with dry nitrogen, this stops any internal dewing.

Regards
Trevor

[Lester]

Thanks Trevor for your comments.

I have just reassembled the 14" after washing the primary and Corrector, took me 4 hours.

I will seal the OTA where I have got 2 fans installed on it and then the moisture shouldn't get in. These are only 40mm diameter fans and I have not used them since summer. I use them after a hot day to expell the heat build up in the OTA very quickly.

If the OTA is completely sealed and I cool the primary to below the dew point, I cannot see how the primary can dew up.

The control for my cooling system is by how many ice blocks I put into the water in the esky. The time the primary dewed up, we had had some cold nights and then this night wasn't as cold, so the primary was well below ambient, by 5 degrees.

I need to monitor the forcast minimum temperature more closely.

[Chippy]

Nice image. Well done!

[cookie8]

Good one Lester
Europa looks really bright because you have picked the right moment i.e. when it transit the darker limb. Well done!!

[Lester]

Thanks Nick and Vincent for your comments.

All the best.