Re: Dew control for Dobsonian


Bill, I should have mentioned that my reluctance for using the hand-warmer technique stems from a combination of clutziness on my part (i.e., afraid of dropping, knocking secondary out of alignment, etc) and perhaps an bit of over-protectiveness of my scope (likely due to previous reasons).

In any event, secondary fogging seems like a fairly common problem with a surprising lack of solutions for purchase...

--- In, William McLean <preciousmyprecious@...> wrote:

I have had excellent secondary anti dew results by rubber banding a hand warmer to the stalk. Have even de- dewed by doing this and covering the scope for 15 minutes. I have done this many times and it works. Given a set of solutions the simplest tends to be the best- you know who's shaving apparatus.
Carpe Noctem
Bill McLean

From: Paul <paultar@...>
Sent: Friday, December 30, 2011 10:27 AM
Subject: [backbayastro] Re: Dew control for Dobsonian



I have an Orion XT8 and have the fan that was referenced. IMO, it is well worth it as it is one thing I use pretty much all year round to shorten my cool-down time. Also, I have read that it can keep the primary from fogging up. Primary fogging up doesn't happen often in a closed tube dob, but it does happen (3x for me before getting the fan). Mostly I use the fan for 20-30 min at the start of an observing session to bring the mirror to ambient temp. When dew is a concern, I let it run continuously. Vibrations are not a problem.

You raise an excellent question about dew and our type of scopes. I think the dew heaters would be fine for finderscopes, telrads and eyepieces. However, these types of dew control devices don't seem to offer anything for secondary mirrors. Heating the stalk may work, but seems problematic to me: long time to actually heat the mirror, hard to attach without obstructing view, danger of things falling on primary, etc.

As of late, I have been using my hand to warm the back of my secondary when it starts to dew over. It takes awhile (10-15 min) and is boring, but eventually it works. It always seems to give me 30-60 min of observing time before I have to do it again. I don't like attaching hand warmers to the stalk for the reasons I mentioned above (plus the chance of misaligning my secondary in the field).

One idea that may work (I'm planning doing this over the Jan full moon time) was making a homemade tube extension for my OTA (i.e., a dew shield). Perhaps this would add a bit more shielding, just enough to keep the secondary from fogging up on most nights.

I'd be interested in hearing anything you try in the future.

- Paul

--- In, "boba_fett_fan" <nickiwi2@> wrote:

Is there some sort of heating assembly that I can use for my 8-inch reflector? I've had my fair share of observing nights ruined by dew or frost coating the primary mirror. I didn't find anything that Orion sold to use for an 8-inch. Is it even practical to get something for an aperture so small?

Regardless I'm spending about $200-250 of Christmas money on some much needed accessories for my telescope (especially since I have no filters). Here's my list so far:

Deluxe 1.25" Eyepiece filter set:
- 4 planetary filters (#15, 25, 58, 80A)
- SkyGlow Light Pollution filter
- Variable Polarizing Moon filter
4-Eyepiece rack
2x Shorty 1.25" Barlow lens
1.25" O-III filter
I may or may not add the 1.25" UltraBlock NarrowBand filter depending if I want to get the free shipping. The barlow will increase my top magnification to 320x (with 7.5mm). Any opinions are welcome.

-Nick Anderson

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