Secondary dew heaters

[neil phillips]

Just wondering about peoples thoughts on these, I don't want to run it all night, just put it on to clear bad dewing. Then take off. Not sure which one is better for a 300p Newt ?

Large

http://www.ebay.co.uk/itm/W-W-Astro-secondary-mirror-dew-heater-L-/191811586777

Small

http://www.ebay.co.uk/itm/W-W-Astro-secondary-mirror-dew-heater-S-/191811584860

 

 

[brantuk]

Neither really - what you need is a split ring dew heater for the 300P like these:

http://www.kendrickastro.com/newtonian.html

See "Split Secondary Mirror Heater" section on the sheet for precise measurements. You'll also want a dew controller to plug it into.

Hth

[Chris Murray]

Hi Neil, i just use a 1.25 eye piece heater band, leave it on the scope most of the time, i got stuck one night and this saved the day, err i mean night.

chris

[neil phillips]

Hi Guys

I want ease to just quickly put on and off, I don't need a controller, once dew is gone I will remove, and carry on with the imaging of planets. Even though it will likely be back on in a hour.  Are you saying this wont clear the dew ? This looks similar to the eyepiece heater band. but will fit the shape of the flat more.

What size is best here guys ? from the links

[brantuk]

Well I could be wrong but the ones you link to look like stalk heaters - I prefer the ones that sit flat on the back of the mirror. If that's the only one you require then there's nothing wrong with a "direct to battery" plug for sure. Dunno how you're gonna handle routing the cable - usually it's fixed along the spider with as low a profile as possible. So "on/off" all night would be a question of just unplugging it.

If I had to choose I'd o for the 2" stalk size - my 300P secondary was quite large as I remember it.

[theastrodragon]

A stalk heater works fine on most Newtonians - you don't need it to go all the way around, you just want some warmth, and it should conduct through to the secondary.

The problem with a split heater is having room on the secondary to place it.

[Oddsocks]

When I am imaging away from home using a DSLR with minimum kit I use a 12v portable hair dryer, it is faster (and less expensive) than using a standard dew strip once dew has formed, a two minute blast with the dryer will clear the dew and provide enough warmth to keep the optics clear for half an hour or so. Several of the members of my local astro club use them with dobs and sct's.

The one I use is more than ten years old and no longer available but a typical model is linked below:

http://www.ebay.co.uk/itm/12V-In-Car-Hair-Dryer-Folding-Handle-Compact-Size-SWHD-Camping-Festivals-/191747688638?hash=item2ca50d88be:g:hF0AAOSwRLZULKI0

They are pretty poor at actually drying hair as the power output is quite low, and rather noisy,  but adequate for astro use.

William.

[neil phillips]

Cheers guys I think I may go for the 2" one,

A hair dryer on a closed tube newt can fog the primary, it happened to me before,

[neil phillips]

4 hours ago, brantuk said:

Well I could be wrong but the ones you link to look like stalk heaters - I prefer the ones that sit flat on the back of the mirror. If that's the only one you require then there's nothing wrong with a "direct to battery" plug for sure. Dunno how you're gonna handle routing the cable - usually it's fixed along the spider with as low a profile as possible. So "on/off" all night would be a question of just unplugging it.

If I had to choose I'd o for the 2" stalk size - my 300P secondary was quite large as I remember it.

Hi Brantuk

I have never seen one of these stalk heaters fitted on, I thought part of it, sits on the secondary. Is that not the case ? how does it actually fit on, do you know at all

Just found it, Cheers guys

https://www.youtube.com/watch?v=eGAGtA-kG5s

[brantuk]

They just wrap round the stalk and heat transfers to the mirror - particualrly on smaller secondaries - there is a little bit of contact with the back of the mirror. With the split ring type they sit flat on the back of the mirror - I've only used the split ring type on my old 12" and more recently my 16" dob. But those secondaries are quite a bit bigger than the 1.25" ones - just heating the stalk isn't as efficient. But I'm not saying it won't work at all with a smaller one - I just prefer a steady heat on mine for more consistent results.

[Macavity]

Interesting topic! I keep intending to buy something.

I do rather like the idea of the ones that makes FULL
contact with the glass, but my 2nd'ary is firmly stuck 
(in common with many of such ilk) in plastic holder.

I believe they can be extracted from the (probable)
"glue" etc. Maybe one day I will have the courage!

P.S. I too found a 12V hair dryer works well enough.
Do remember DUST can build up inside these. I know
to my cost when it covered all the scope innards!  

[Starflyer]

I have no factual information to back this up but my view is that by heating something in the middle of a newt tube you will generate tube currents and this will reduce resolution. I used to suffer from secondary dewing regularly but since adding a small (6 inch long) camping mat dew shield I haven't had a dew problem, and I've imaged when everything's been dripping with dew.

[brantuk]

Yep - I agree - the idea isn't to heat it up - but rather to keep the mirror surface at ambient temperature so dew can't form. I think there's sometimes confusion cos it's called a dew heater - but very little heat is needed at all.

[astronymonkey]

I removed the secondary of my 16inch lightbridge and fitted a kendrick heater in the holder before refitting the mirror. It moved the mirror about 4mm down the tube so this needed to be adjusted for but it works like a dream and probably one of the best mods ive done.

Where the stalk is bonded however , rather than using a holder, I would use a split heater over a stalk heater as  I dont think enough heat would conduct through to the mirror as the silicone bonding agent would act as an insulator. It also  means you dont run your battery down as fast either as low power is enough to heat the mirror. Cheers

 

 

 

 

 

 

 

 

[neil phillips]

Hi Guys I already use a dew shield, though I do need to figure the correct length, not too short or long. if anyone knows how to estimate that, I thought it was case of looking down the focuser at the end of the tube to see if any overhang occurs is this correct ?

Not sure this heater ive ordered will fit or work, but if not I may go the heater on mirror way

[theastrodragon]

As long as it doesn't sag, I've found a foot is a good length for a big scope (10-12" diameter). You usually need a hard one , though, the flexible ones tend to sag.

[Chris Murray]

i think there is one of those rules that cover these things, one third the focal lenth or lenth of the ota, but if you dont buy a factory made one  and make your own just use the dimentions of the production models, if there not perfect they cant be far off

chris  

[neil phillips]

52 minutes ago, Chris Murray said:

i think there is one of those rules that cover these things, one third the focal lenth or lenth of the ota, but if you dont buy a factory made one  and make your own just use the dimentions of the production models, if there not perfect they cant be far off

chris  

Ok Cheers Chris

[astronymonkey]

I tend to go for 1.5x the aperture for a dew sheild, and this is possible for most scopes if using a £5 camping mat for the material.

cheers

 

[theastrodragon]

Ah, the old black camping mats...been a while since I made one of those, but in some respects they were better than the commercial shields, especially if you lined one with black flock.  I found the thick foam sat on the tube far more securely than one of the thin commercial ones.