Winter observing with a dob?

[ericc]

Hi all,my first winter of observing with the dob is just round the corner, and I was hoping for a little advice?
I was wondering if dew/fogging etc becomes more of an issue using a dob in the cold months(I assume it does)?

Any other thoughts(so I can prepare for winter observing)would be really appreciated.
:thanx:
Cheers--Eric

[stephenb]

Eric, I have observed with 8 and 12 inch tube dobs in Winter here in Melbourne over the last few years and with a cooling fan at the rear, I can only recall 2 or 3 times which dew only started to form. I think the tube design did contribute to protecting the mirror solmewhat. I had more problems with dew on the eyepieces. I had more issues (naturally) with SCT's on the corrector plate (but that's to be expected).

Other may provide better evidence, but I think it has a lot to do with the environment around you. i.e. out in an open paddock in the country, or a suburban block.


regards,

Stephen

[barx1963]

I am in Colac and the only things that doesn't fog up is the primary on my 12". Finder, Telrad, eyepieces and secondary all get fog.
I have found that investing in a basic dew heating system is a fantastic investment. I have straps on everyting including the secondary.
If that is not possible, keep the eyepieces capped when not in use, keep a cap on the eyepiece of your finder, a stubby holder with the bottom taken out and held in place with an elastic band protects the objective of your finder. Also keep eyepieces in a box when not in use. A hairdryer can be used to remove dew, just don't get it too close to the glass of eyepeices.

[ericc]

Thanks for the tips people!

[Paddy]

HI Eric,

I'm not in a very dewy place, but my 12" GSO did get a bit of dew on the secondary occasionally before I did the mods described in

http://www.iceinspace.com.au/63-400-0-0-1-0.html

In fact, I only did part of the modification. I put the baffle inside the tube about 1-2 cm above the primary (using thick self adhesive door sealing foam tape) to direct airflow across the front of the primary and I put the neoprene gusset on the back of the tube to stop air from the fan bouncing off the back of the primary. I left the existing fan in place as vibration was not an issue. Never had dewy optics again. It also dramatically reduced the length of time for the mirror to cool sufficiently for good seeing. I'm not very skilled with my hands and it was a very easy mod for me.

Finderscope? Just keep putting the caps back on it when you're not using it in the observing session. Dew covers can be bought for Telrads or put a small plastic bag over the business end when you're not looking through it. This all sounds a bit cumbersome, but it becomes second nature very quickly. I don't even know I'm doing it these days.

[GrahamL]

I have one of these with a 60 amp battery in it to run my little fan also does me for camping as well with a couple of cheap flouros for the tent and oustside.
http://www.supercheapauto.com.au/online-store/products/Calibre-Power-Battery-Box-12V.aspx?pid=12358#details


My Secondary and telrad dew up now and then to which a 12 v hair
dryer plugs into a spare socket hanging off the battery box, only takes a minute to warm things up a little .

http://www.whitworths.com.au/main_itemdetail.asp?cat=123&item=40000&intAbsolutePage=1

[mozzie]

on my 8" dob ive straps for the eyepieces,telrad,and a strap around the primary seems to work well for me

[astro_nutt]

Hi Eric
Try a hot water bottle wrapped in a tea towel, place this in a cardboard box to keep your EP's warm.
I have used an old showerproof jacket, minus the sleeves, and drap this over the finderscope and focuser, like the old time photographers used to.
Cheers!

[ericc]

Good ideas people--and I really like the hot water bottle :)
I am determined to try out the alternatives before commiting to the dew heaters(although I do suspect I may need help from Mr Kendrick/etc before winters end)??
I hope I really only need eyepiece(box?) and secondary heating as my normal backyard observing sessions start with the 2 star alignment of the Argo Navis(using a telrad) and everythings left to the A.N. and observation guides!
I dont use the Finder.
Can anyone tell me how noisy is a 12v hair Dryer,compared to the 240v version???
Thanks again,
Cheers-Eric

[DavidTrap]

A 12V hair dryer is reasonably quite (much less than a 240V one - although you probably hear a 240V one in a bathroom rather than in the open air).

Mine draws about 10amps - so about 120W of heating. I don't know how anyone could actually dry their hair with one though - the heat output is pretty pathetic. However, it's more than enough to sort out the dew on the corrector plate of my SCT. Remember you don't really want to cook the optics, just warm them above the dew point. Too much heat too quickly could cause something to go bang!

A 240V hair dryer is probably 500-1000W.

David T

[Wavytone]

Unlikely. Years ago I found the ultimate solution to stop a C8 dewing over, and bear in mind I had a plastic dewcap* on the front that stuck out maybe 50 cm.... the same would work well on a dob.

Point the scope down.
Set up a camping gasring beneath.
Put the billy on top and add water.
Light the gas.
The rising steam and hot air will make the whole OTA quit warm.
Bring the water to the boil, turn off the gas, add tea and make a cuppa, including friends.

By the time you've drunk the cuppa, 20 minutes later the tube currents inside will have settled down enough for a look through the scope and it will stay that way until it's time to repeat the process :)

A lot quieter than hairdryers, it won't flatten your car battery (so you can still drive home afterwards) and best of all you're warmed up and hydrated. The blue gas flame is pretty benign, it shouldn't bother the photogs.

*if the dewcap melts, time to turn the gas off.

[DavidTrap]

I stand corrected!
DT

[ericc]

And I thought I was starting another boring thread-