Dew tape position for a reflector?

[Notty]

Very rookie question, but for my 8" reflector, would I position the heating tape outside the tube, parallel to where the secondary mirror is, or slightly below or above it? Would this be effective enough to prevent dew on it, or do I need to be looking at a home made resistor type affair of the like I have seen which heats the mirror plate directly (which I'd rather not having read about people nearly setting fire to their scopes!)

Many thanks

[sidelight]

Hi, I place mine between right at the front of the scope over the dew shied (or under it when in was a foam one) and have never had a problem with the secondary dewing up.

Cheers

Ross

[Laurie61]

Hi, if you want to protect the secondary on a newt the dew heater has to go on the secondary. The larger dew tapes are for SCT design scopes which do go around the front of the tube. 

[Notty]

Ok thanks guys, but are these two contradictory replies? I've got a large band that goes around the circumference of the scope, and I know you can get smaller bands which go around eyepieces/finder guilders etc, but I have not seen anything which is specifically for the secondary, I wonder can some one maybe post me a link to an example one so I can see it? I can't even think of how it would attach, let alone what do you do about the wire which will be right in the path of the light?

Thanks

[sidelight]

I can only tell you my experience, with a large band around the top of my scope and a dew shield fitted I've never had a problem with my secondary.  I did buy a mirror heater but never fitted it as I  would have had to deal with the wires on the spider and I never needed it.  I can only suggest to try it and see.

Cheers

Ross

[NGC 1502]

If you have a standard Newtonian reflector, the secondary mirror is much more prone to dewing than the primary that's deep down at the lower end of the tube.

You may be able to reduce secondary dewing by extending the upper end of the tube with a 'camping mat' dew shield, or perhaps by pointing a 12v hair dryer down the focuser after removing the eyepiece.

But the best solution is a proper dew heater like the ones in this link http://www.kendrickastro.com/newtonian.html#Secondary  (scroll down).

The heat is very gentle,  to keep the secondary just above ambient temp.

There are DIY versions if you are handy.

HTH, Ed.

[Laurie61]

Hi all, I think there is a bit of confusion as to the scope type ? I am assuming it's the 200 pds in your sig that your talking about which is a Newtonian scope. These have a secondary held centrally in the front of the tube by a spider( four thin vanes) To stop these dewing you can use a hair drier and blow warm air to remove dew, as and when required, or fit a heater to the back of the mirror. An SCT uses a heater band wrapped around the tube, this warms the front corrector plate and the secondary which is protected inside the closed tube. An SCT type dew tape will not stop the Newtonian secondary from dewing up as the heat generated is not being applied to the mirror. This is a Newtonian secondary heating unit - 

https://www.astrosystems.biz/dewgrd.htm

[Notty]

Thanks guys! I didn't know about those secondary mirror heaters they definitely seem the way to go.

[sshenke]

Hello everyone, sorry to bring this thread back up, which I came across when trying to find a solution for the dew problem. I have a skywatcher 130PDS (Reflector) and have faced lots of problems with dew recently. As you can see from the attached picture, it is the primary mirror that suffers from heavy dewing. I am sure the secondary mirror has dew as well, but I think the primary seems to be the most difficult to fix. I have used hair dryer, but as rightly pointed out above, it is only a short fix. Can someone offer any suggestions to stop dew forming on the primary mirror? Thanks

[carastro]

I have never bought one myself, but I think you can buy a dew heater that sticks to the back of the primary mirror.

There is a lot of talk about it on cloudy nights but all the links seem to be in America.  Hopefully some-one will have a link to something more local.

Carole 

[tooth_dr]

Would a fan be effective? There is a fan attached to the primary of my reflector and I’m just curious would the steady flow of air help. I’m imaged on Sunday night with the fan on without dew but that may have just been favourable local conditions 

[sshenke]

it has been absolutely horrendous for me here. Can only manage 30 minutes of imaging ( i am in the Midlands) because of dew building up. 

Carole, how do you manage without any dew preventing device?

[carastro]

Quote

Carole, how do you manage without any dew preventing device?

I no longer have my SW130PDS, but when I did I never used anything for the primary as luckily that never got dewed up.  My secondary I always worried about and so I used to wrap a couple of long dew heaters around the tube either side of the focusser.  Whether it did any good I don't know, but luckily I never got any dew.

With my refractors I definitely use dew heaters.

What made you think I didn't use any dew heaters? 

Carole 

Edited October 22, 2019 by carastro

[sshenke]

1 hour ago, carastro said:

I no longer have my SW130PDS, but when I did I never used anything for the primary as luckily that never got dewed up.  My secondary I always worried about and so I used to wrap a couple of long dew heaters around the tube either side of the focusser.  Whether it did any good I don't know, but luckily I never got any dew.

With my refractors I definitely use dew heaters.

What made you think I didn't use any dew heaters? 

Carole 

Thanks,  will buy some dew heater band, but i just can't find the right way to attach it to the scope, ie for the primary mirror. as you said that you never bought dew heaters,  i just assumed you never had to use them

[Alan White]

I have owned a number of Newtonians, always have Secondary dewing on long observing sessions,
but only once on the Primary Mirror.
Are you viewing to Zenith for a long period?

Ventilation via the tube may well be a fix, rather than heating the primary.

I and others have made vents in out Newts to vent the Primary.

This is the base plate that the Mirror cell is set of from.
Clearly the mirror sits in front of the holes.

Another identical scope, with different holes, belongs to another on SGL,
who has commented already.

You could try this first as a little air movement near the mirror may be all it takes to prevent the dew.

No dew since the vents added by me.

[sshenke]

2 hours ago, Alan White said:

I have owned a number of Newtonians, always have Secondary dewing on long observing sessions,
but only once on the Primary Mirror.
Are you viewing to Zenith for a long period?

Ventilation via the tube may well be a fix, rather than heating the primary.

I and others have made vents in out Newts to vent the Primary.

This is the base plate that the Mirror cell is set of from.
Clearly the mirror sits in front of the holes.

Another identical scope, with different holes, belongs to another on SGL,
who has commented already.

You could try this first as a little air movement near the mirror may be all it takes to prevent the dew.

No dew since the vents added by me.

That's really helpful advice, thanks Alan. yes i have been doing zenith viewing for a long time, ie 2 to 3 hours at the most though. That was mainly to get good guiding results. I will certainly give it a go. Will need to learn how to do it properly without breaking anything.

[Alan White]

No wonder it is dewing if its at Zenith that long.
Best thing is observe and then drop away from Zenith so dew is not falling
into the primary or indeed lens on a refractor. 

Are you observing or imaging?

[sshenke]

13 hours ago, Alan White said:

No wonder it is dewing if its at Zenith that long.
Best thing is observe and then drop away from Zenith so dew is not falling
into the primary or indeed lens on a refractor. 

Are you observing or imaging?

Imaging. It would take about that long for my target object to come into view because of obstacles. until then i would just trial image some other dso, as i am very new to this. 

[Alan White]

I think the location you are imaging in is the real problem.
Dew heater on the primary or fans i think lies in your future.
Good luck with this.
Please let us know how things go and what option you take.

[sshenke]

hello all, i am not getting any further with solving this problem. I have looked at the option of adding vent holes behind the primary mirror, if i understood it correctly, but there isn't anything behind the mirror stopping contact with the outside air. As you can see from the attached picture, the only thing that was there was just a piece of circular cardboard, which i have now removed. 

i tried to buy a dew heater band that would go around the ota tube just near the primary mirror, but flo had informed me that this would not work at all as the mirrors of a reflecting telescope do not make direct contact with the outside tube and hence no heat can pass through to the mirror. They also said that they have no suitable dew heaters for reflecting telescopes.  Really frustrated and would be grateful if anyone has any suggestions for me. Thanks. attached is another picture if dew buildup last week. This happened after about an hour of taking the scope outside.  Location wise, i really cannot think of my rear garden to be any different to others'.