DIY dew shield: aperture / length ratio?

[Planetesimal]

I was wondering if there is a rule of thumb about the length of the dew shield compared with the aperture of the scope? I'm sure I came across somewhere that said there is a recommended length: aperture ratio, however I'm a bit confused; here's why:

- My dewshield is 1.33x as long as the diameter of the 'scope, if you measure the length of the shield from the end of the OTA (217mm) and divide by the primary aperture (150mm) - I used A4 cell foam to make the shield.

- An alternative method would be to compare with the actual diameter of the tube (175mm), giving a ratio of only 1.24:1.

- Or I could measure the distance from the end of the shield down to the secondary (250mm) against the tube diameter to give 1.43:1.

What am getting at is, could I get significantly more dew-free time by adding an extra 100mm or so length on the dew shield, or am I better off just being done with it and getting a hair dryer?!

Can you tell I'm getting cloud fever?

[DrRobin]

I make mine about 1.5x the aperture, or whatever size of material I have lying around. I find that on my SCT the dew shield significantly increases the time before the dew collects on the surface and for my Newt, it is less of a problem. When it does dew up I simply remove it with a hair drier.

I am not sure how much extra time you will get by making the dew shield longer, it probably depends where in the sky you are pointing. Straight up a dew shield is not going to make much difference regardless of length unless it is heated.

[midnight_sky_watcher]

Hi Grant here,

Dew shiels are a nightmare, there is a lot of trial and error with them. However dew shields in some cases dont stop "dewing up" i also have heater bands fitted to my scopes and that normally cures the problem. But be careful dont use a normal hair dryer on your scope optics,it can crack your lenses, there are various types of demisters on the market, and most reputable astro stores do them.

Hope this helps.

Grant....

[Moonshane]

I think it depends what scope you have. I use newts and the distance between the end of the tube and the secondary is the key thing and that's bigger in some of my scopes than others. personally, for a newt I'd do it about 1.5x aperture but neasuring from the secondary - in my case 12" for my 16" and 6" for my 6" scope are both fine. in the case of a SCT / Mak there's almost no dewshield (I think?) so personally given their tendency to dew more quickly I'd go to 2x aperture from the end of the scope.

[Planetesimal]

I make mine about 1.5x the aperture, or whatever size of material I have lying around. I find that on my SCT the dew shield significantly increases the time before the dew collects on the surface and for my Newt, it is less of a problem. When it does dew up I simply remove it with a hair drier.

I am not sure how much extra time you will get by making the dew shield longer, it probably depends where in the sky you are pointing. Straight up a dew shield is not going to make much difference regardless of length unless it is heated.

Mines a newt, so at 1.42x it's probably close to as good as it's going to get... I find that the time for cooling (30 mins) overlaps with the beginning of misting up. So the quality of view is already beginning to be compromised by the time the 'scope is cooled down. It then takes another 1 hr tops before I have to pack it in.

Do you know why the position in the sky would make a difference? I usually move around a fair bit in an observing session because my time is so limited before the dew sends me inside.

Also, I've asked around (including retailers) and apparently you can't buy dew heaters for newt secondaries...

[Planetesimal]

Hi Grant here,

Dew shiels are a nightmare, there is a lot of trial and error with them. However dew shields in some cases dont stop "dewing up" i also have heater bands fitted to my scopes and that normally cures the problem. But be careful dont use a normal hair dryer on your scope optics,it can crack your lenses, there are various types of demisters on the market, and most reputable astro stores do them.

Hope this helps.

Grant....

Hi Grant - not sure I can get a dew heater for the secondary. I did find a 12V "dew remover" on 365astronomy, but I don't know if people use these. I was planning to try a hair dryer on "cool" for a few seconds on the secondary (by blowing down the focuser draw-tube) and see if it makes a difference... I wouldn't have thought that would be a risk to the mirror, would it?

[Planetesimal]

I think it depends what scope you have. I use newts and the distance between the end of the tube and the secondary is the key thing and that's bigger in some of my scopes than others. personally, for a newt I'd do it about 1.5x aperture but neasuring from the secondary - in my case 12" for my 16" and 6" for my 6" scope are both fine. in the case of a SCT / Mak there's almost no dewshield (I think?) so personally given their tendency to dew more quickly I'd go to 2x aperture from the end of the scope.

Thanks Shane, I get the feeling with my newt I've done all I can with dew-shields. Looks like something more is required...

[DrRobin]

Dew tends to settle from up above so the more you are pointing up the worse it generally is. If you are pointing towards the horizon with a dew shield it would take a long while for it to collect on the secondary.

Although a secondary heater is not available a heater band around the base of the dew shield will warm the air in the dew shield/end of the scope and prevent the dew from forming, at least that is the theory.

I keep my scope in a shed so it is at or near to air temperature, that way the cool down time is kept to a minimum.

[DrRobin]

I use an old hair drier on both my SCT and the secondary of my Newt. I have never caused any damage yet, but it's heat output is down on what it was, hence the reason it is in my shed and not been used to dry Mrs DrRobin's hair.

Apart from anything else too much heat will cause the optics to expand, so just be careful and you will be okay.

I bought a 12V hair drier for defrosting the car.... ....somewhere between useless and a waste of space comes to mind. It is just about okay to defrost the washer jets but that is about all. I can't imagine using it to dry your hair, it would take forever. Still it might be okay to remove the dew from delicate optics.

[Moonshane]

I am puzzled as to why you are getting dewed up so quickly. do you live near water or anything that might increase humidity locally? (literally) the only time my 6" f11 has dewed was when I left the focuser plug out once when I went in for something to eat. not leaving it pointed up when not in use is also good advice as above. other than that, I'm a bit scuppered.

[DrRobin]

My SCT will dew up in about an hour for around 50% of the nights I observe. I live about a mile away from a river, it seems to depend on humidity of course. When humidity is over 90%, most of the time it seems, then the dew forms quickly. If it is dry air at around 70% then it will go nearly all evening without any dew. My Newtonian isn't as much of a problem as you would expect.

I keep a towel in the shed, on a cold night the outside of the scope and mount are very wet and need to be towel dry before finishing off with the hair drier before been put away.

[Planetesimal]

I am puzzled as to why you are getting dewed up so quickly. do you live near water or anything that might increase humidity locally? (literally) the only time my 6" f11 has dewed was when I left the focuser plug out once when I went in for something to eat. not leaving it pointed up when not in use is also good advice as above. other than that, I'm a bit scuppered.

There may be increased local humidity, I'm not sure. I don't live near a river / lake or anything but we are fairly rural, backing onto fields. The local soil is deep heavy clay which holds a lot of water and takes a long time to dry out, so that may be a contributing factor. Whatever the reason, it happens every time I go out to observe so hairdryer it is!