Covering the primary lens.

[Suzy]

Hi all, this is a fantastic forum. I am new to this hobby. After months of watching, I'm posting for the first time. I have a 6 inch Dobsonian, which i bought due to the recommendations in your forums for best beginner scope. I read concerns with dew being a problem on some scopes. Are the dobs effected? Also, i heard that covering the scope with anything other than cotton, when not in use can actually trap the dew and eventually do damage. My local dealer advised me to use a plastic shower cap on the primary mirror to keep the dust off when not in use, which i have been doing, but am now confused. so, in brief, to cover or not? If so, Cloth or plastic? Your input will be appreciated.

[[1ponders] (Paul)]

Suzy and to IIS. Glad to see you've decided to become a more active member. Any scope can collect dew and it can cause problems. The idea of a shower cap is a good one, a few people use that and it wont cause problems with keeping dew in. The air can still get around the base of the mirror.

I like to throw an old sheet over mine or towel and they are usually cotton. If I leave a scope outside overnight I'll cover it with a more waterproof cover like a garbage bag but I'll take it off in the morning to allow the scope and mount to air/sun dry

[jjjnettie (Jeanette)]

Hi Suzy!
Welcome to IIS and congratulations on the new scope.
The plastic shower cap is good insurance against little critters taking up residence inside your scope..
I've had spider in my dob, and frogs more than once. Frog poop and wee, when it gets on your mirror, and it will, will start to eat into the coatings.
If your scope get dewy during your observing session, wipe down with an old cotton tshirt before storing.

[Blue Skies (Jacquie)]

Hi Suzy,

I can see where the confusion is coming from. The standard procedure to make sure the scope is dry before you put the shower caps on. This could include wiping the scope down but also leaving the scope uncapped in a warm room overnight before capping it again for storage. If you have to chuck it in the car wet, as an example, make sure you uncap it for a while at home for it to dry, then cap it to keep the dust and spiders out.

Also worth noting is that condensation can sometimes form on the primary mirror once you bring it inside in to a relatively warmer environment, so it pays to let it warm up to ambient temperature before capping. You don't need to get the hairdryer out or put it near a fire, just let it dry naturally.

[Wavytone]

The main thing is to make sure dust cannot settle on the mirror. Lying the tube on its side helps. Covering the mirror with something is better. A shower cap is one possibility; another is a cotton pillowcase (which breathes), 5 minutes on a sewing machine and you can add a drawstring.

And do the same for the secondary mirror.

[AstroGuy]

Hi all,

I love Dob's, I want one (eventually)...

When buying my first scope, the decider that pushed me into buying a Mak was the drawback of the Dob's open-ended-architecture design. I just can't bare to think of the mirror being exposed to the elements like that. Makes me wanna cry...

I've done a lot of thinking since then, and figured that a nice Bk7 optical window on the open end would help a lot. This is not as expensive as it sounds. Polished mirror blanks can do the trick. Or a custom 1/3" thick optical window and the nightmares will all go away.

Regarding the mildew/fungus thing, some moisture absorbing silica gel to take care of mildew (possibly sewn into the shower cap?). You know, Silica gel, that stuff you find in those little packets inside vitamin bottles. If used in cloth bags, make sure that you quadruble bag it (or more) cause it can form a fine dust eventually if handled a lot. Ideally a few layers of superfine porous material would be the best to use for a bag (microfibre cloth maybe?). I found that they sell that stuff now as kitty litter. Well I don't know about using it for that but....I find it's great to keep it in a container or a large satchel and placing it in wardrobes, anywhere where you want to keep moisture out, and it's way cheap, about $5 for a huge 2 kilo bag!. They sell it here in Sydney at a store called "The Reject Shop".

If you can't find it, the next best thing is just gelatine powder (not jelly crystals, the sweet flavoured stuff) but the plain, make it yourself stuff. Mix it dry right out of the satchel with a little natural herbal antibacterial/anti-fungus/anti-pest plant matter (lavendar possibly?), and there you go! mildew, bugs and fungus problems gone!

On a different note. has anyone ever tried steam cleaning the mirror when overhauling their scope? Those hand-held portable steam cleaners, like the ones on eBay for $20 sound like they would clean and get rid of any fungus problems real good. I would only use distilled water in it though.