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Ross1204

Really Bad Dew On Secondary Newtonian 130pds Mirror. Dew Shield Not Helping. Help!

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Hello,

So as the title of the thread says I am consistently suffering from bad dew formation on the secondary mirror (Skywatcher 130pds) which has stopped multiple nights of imaging after barely an hour or so.

I would like to think I have done a fair amount of research on the topic but unfortunately I am still unsure of how to deal with the problem. I initially brought a dew shield which I believe has slightly prolonged the inevitable for a short while but unfortunately has not solved the problem. After the shield didn't help I brought a wrap around dew heater (https://www.365astronomy.com/dew-heater-strip-for-newtonian-telescope-secondary-mirror-small.html) which is unfortunately too big for the secondary mirror stalk.

I have also seen that Kendrick and some others do stick on dew heaters which are glued to the secondary itself but again I believe the mirror on the 130PDS to be too small for their smallest product so yet again this is not the correct approach.  

So after these failed options what is left for me to try?

Maybe a dew strap around the OTA at the front? Look in to making my own strap?

Lastly I have read people use a 12v hairdryer to remove dew but as I am not doing any visual astronomy I don't think this would be a good solution as removing the camera would mean re-focusing every so often wasting imaging time.

Any help would be much appreciated as this problem is causing quite a lot of bother by ruining many imaging sessions. 

Ross

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I don't know how big the mirror is on the 130pds, but I would be looking into building a resistor based heater to go directly on it at this stage if a commercial one isn't available.

This site has some great information on building your own, and you can control how much power then.

http://www.deepskywatch.com/Articles/newtonian-dew-heater.html

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I also have a 130PDS. I soldered together a resistor ladder type dew strap and fitted it around the front opening of the OTA and it works at treat. I control the temp using a PWM adjustable controller 

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I have sequestered one of my wife's old hair-dryers after similarly frustrating sessions. It's a total game-changer, and if it contaminates viewing slightly because it's heating up the secondary and/or the spider-vanes, although I haven't noticed it,  it's something I can live with for the sake of making the difference between that and seeing nothing at all.

Cheers, Magnus

Edited by Captain Magenta
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The previous owner of my 200p fitted a resistor block to the back of the secondary and ran a thin wire out along the spider vanes to be powered by a 9v battery. I've never used it but he said he could stay out till early morning without any build up on the secondary. I will take a pic later this evening.

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6 hours ago, upahill said:

I don't know how big the mirror is on the 130pds, but I would be looking into building a resistor based heater to go directly on it at this stage if a commercial one isn't available.

This site has some great information on building your own, and you can control how much power then.

http://www.deepskywatch.com/Articles/newtonian-dew-heater.html

Thanks for reply.

Thanks for providing the link I shall definitely take a look. I have never really made anything like that before so hopefully its not too steep of a learning curve as would like something pretty sharpish. 

6 hours ago, BigRD said:

I also have a 130PDS. I soldered together a resistor ladder type dew strap and fitted it around the front opening of the OTA and it works at treat. I control the temp using a PWM adjustable controller 

Thanks for reply. 

Now when you say 'front opening' of the OTA is that just positioned like a standard dew strap around the circumference? Or located somewhere around the secondary?

5 hours ago, Captain Magenta said:

I have sequestered one of my wife's old hair-dryers after similarly frustrating sessions. It's a total game-changer, and if it contaminates viewing slightly because it's heating up the secondary and/or the spider-vanes, although I haven't noticed it,  it's something I can live with for the sake of making the difference between that and seeing nothing at all.

Cheers, Magnus

Thanks for reply.

Yeah I have also looked in to getting one as really cheap and already have the ability to power outside. How do you actually use it though, do you direct the heat in to the front of the scope or down the focuser directly at the secondary? The thought of removing the camera to directly target the secondary seems like it could get boring quick, not so much maybe if your doing mostly visual as removing a eyepiece isn't as much of a big deal.

5 hours ago, david_taurus83 said:

The previous owner of my 200p fitted a resistor block to the back of the secondary and ran a thin wire out along the spider vanes to be powered by a 9v battery. I've never used it but he said he could stay out till early morning without any build up on the secondary. I will take a pic later this evening.

Thanks for reply.

Ok that sounds interesting, I like the idea of powering with just a standard 9v battery. With regards to the picture it is entirely up to you, don't feel you have to if too much hassle.

Edited by Ross1204

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Re: the hairdryer:

I found that simply pointing the airflow at the secondary at an angle from the front of the scope through the spider was enough to very quickly clear it. The airflow "wraps" around the whole assembly quite well. Being very careful of course not to touch anything with it. Similarly for the eyepiece itself, I suffer from eyepiece fogging at the same time, and blowing the eyepiece worked just as well.

Cheers, M

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5 hours ago, Ross1204 said:

Now when you say 'front opening' of the OTA is that just positioned like a standard dew strap around the circumference? Or located somewhere around the secondary?

I fitted the dew strap around the circumference. It's effective enough to keep the dew away and low cost too. ?

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16 hours ago, Captain Magenta said:

Re: the hairdryer:

I found that simply pointing the airflow at the secondary at an angle from the front of the scope through the spider was enough to very quickly clear it. The airflow "wraps" around the whole assembly quite well. Being very careful of course not to touch anything with it. Similarly for the eyepiece itself, I suffer from eyepiece fogging at the same time, and blowing the eyepiece worked just as well.

Cheers, M

OK thanks for letting us know it can work via the main OTA opening. The thought of not having to remove the camera is good to know, although it is a damn sight easier to see how dewed up the secondary is if you remove the camera. I guess it would help to let the scope settle for 5mins or so to let any tube currents equal out also. 

15 hours ago, bottletopburly said:

Try fitting a  pc fan to the back of the primary blowing air up ota.

Thanks for reply. 

Now assuming the fan wouldn't provide any heat to aid dew removal just the very fact air is circulating in the tube can be enough? Maybe OK for mild dew accumulation but not heavy build up? 

Also not entirely sure how I would fit such a fan, Google to the rescue..... 

13 hours ago, BigRD said:

I fitted the dew strap around the circumference. It's effective enough to keep the dew away and low cost too. ?

Would I assume you placed the dew strip over the thicker white section as surely the spider fixings get in the way in getting the heater closer to the secondary? 

Kinda like this picture. IMG_20181006_122832.jpg.4b555b7559ab83a8d8d2dbe2e4b8e8d9.jpg

Edited by Ross1204

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2 hours ago, Ross1204 said:

Thanks for reply. 

 Now assuming the fan wouldn't provide any heat to aid dew removal just the very fact air is circulating in the tube can be enough? Maybe OK for mild dew accumulation but not heavy build up? 

 Also not entirely sure how I would fit such a fan, Google to the rescue....

130PDS Cooling fan

 

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As has been suggested, you could build your own solution. If you don't want to build your own, contact Kendrick with the measurements of your secondary and they may be able to make a suggestion in regards to their products. I had questions about my setup as well and they were very helpful. Not that you need it, but I can vouch for their products. I have one on my primary and secondary of my dob. I also have one for my Telrad and my eyepieces. They work really well.

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On 06/10/2018 at 14:55, bottletopburly said:

130PDS Cooling fan

 

Thank you for going to the effort to get this picture, very much appreciated. I can definitely see the benefit of circulating the air inside the scope its just I can't help but think I will also need some heat somewhere.

 

On 06/10/2018 at 18:47, david_taurus83 said:

As promised:

20181006_174044.jpg

20181006_174052.jpg

20181006_174100.jpg

  

Thank you for going to the effort to get these pictures, very much appreciated. I have never seen a secondary heater like that before so has definitely opened my eyes and I'm sure the wider community who may stumble across this thread. Looks nice and neat also with the power cable firmly secured to the spider vane, zero interference to the final image.

On 06/10/2018 at 20:11, Buzzard75 said:

As has been suggested, you could build your own solution. If you don't want to build your own, contact Kendrick with the measurements of your secondary and they may be able to make a suggestion in regards to their products. I had questions about my setup as well and they were very helpful. Not that you need it, but I can vouch for their products. I have one on my primary and secondary of my dob. I also have one for my Telrad and my eyepieces. They work really well.

I have emailed Kendrick to see what they can do but I believe it is their Thanksgiving holiday so hopefully get a response when they are back. Looks like it might well need to be custom job because the secondary on the 130pds is obviously on the smaller size and from what I have seen most of the off the shelf secondary heaters are too big.

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Hi Ross

I encountered something similar and went with the DIY option. I put it on my blog a couple of years ago. The first part is just a basic resistor ladder, but scroll down a bit to the part where I made one for the secondary.

http://www.astromadness.co.uk/p/diy-dew-heater.html?m=1

It's based on 12v, not 9v though.

Good luck!

Tony

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I've struggled with the same issues. I bought a collar type dew srtip from 365 Astronomy (wraps around the secondary stalk) but it tended to change the profile of the secondary mirror (which affects star shapes in images). I emailed Altair Astro to see if they have something that would do the trick (the ones on their site are too big) but that petered out. In the end I've gone for the DIY option, with a  loop of 4 x 24 ohm resistors that is taped to the back of the secondary mirror. In testing it runs a bit hot, but I'll make a plug to connect it to the dew controller that I use for everything else and vary the voltage appropriately. Othr advantage is that  can use thinner wire than commercial dew heaters, which won't unduly increase the profile of the secondary strut that I run the wire across. Not a lot of work and it should do the job nicely.

Billy.

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Hi all. Has anyone tried the heater pad around the circumference of the tube to any great degree of success. I too am getting hit with dew on the XX14G so would be interested in success stories. I ordered the Kendrick Split heater but that is a few weeks away. I will post the results once it arrives. I did try a 12v hair dryer, but it barely heats up and was only good for the eyepieces..

 

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On 05/10/2018 at 21:12, bottletopburly said:

Try fitting a  pc fan to the back of the primary blowing air up ota.

I like this idea, gentle air flow through the tube assembly can prevent dew formation on the optical surfaces or at least slow it down.    It’s a solution I’ve adopted for my own scopes.

Ed.

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