Secondary mirror heater

[gajjer]

From what I've read, the problem with dew is mainly with the secondary mirror.

So my solution is to fit a small heater to it. I have yet to wire it up and build a pwm controller.

There should be two pictures attached.

The first shows the parts and a view of the type of resistor used.

The second shows it in situ ready for wiring. I plan to use enamel copper wire super glued to one of the arms - the one in line with the focus.

Comments welcome.

cheers

gaj

[Rustysplit]

Very neat piece of work.

Makes my glorified dew strap wrapped around the stalk feel under-dressed

[gajjer]

The proof of the pudding will be in the heating!!

Still got to wire it up and build the pwm controller.

Happy so far though.

More pictures will follow

cheers

gaj

[RikM]

Looks a neat solution gaj. I have seen a similar arrangement with mosfetts stock to the back of the secondary rather than on the stalk.

[astronymonkey]

Hi Gaj,

looks really neat but i was wondering what was preventing short circuits as the stalks arnt insulated, and would a superglued bond be a poor conductor rather than a soldered join.

From your post you mention enamel wire being used for the feed, so are you using the body of the tube as some sort of return ?

Looking good though,

CHeers

[umadog]

I've tried this too and came to the conclusion that secondary mirror heaters are finicky things. The goal is to heat the back of the secondary just the right amount but not too much. If I understand your images correctly, it looks like you're heating the secondary stalk. I'm afraid that's probably not going to do what you want, though. The secondary is attached to the stalk via an adhesive pad, which will act as an insulator. So the effect of your heater will be to heat the stalk and the spider but you'll get hardly any heating of the mirror at all. Worse still, the warm stalk and heater will warm the surrounding air and you will get an enormous heat plume in your light-path. You'll see it when you de-focus--it'll be very obvious. I was amazed how little heat is needed to produce such a heat plume.

The way to approach this problem is to heat the back of the secondary by direct contact (as Rik has done). Use several contact points to evenly distribute the heat and reduce localised hot-spots. Then insulate the rear surface of the heater to a) avoid the heat-plume; b ) reduce the current you need; and so c) reduce the chances of altering the shape of the secondary due too high a temperature. The fancy solution is a heater kit (http://www.astrosystems.biz/dewgrd.htm) inserted into a shrouded secondary holder (http://www.astrosyst...ges/spider5.JPG) that acts as an insulator. The less fancy solution is to attach resistors (or similar) to the back of the secondary with thermally conductive epoxy. As you say, a PWM can be used to regulate power. Sumerian use some kind of resistive wire for this: http://www.sumeriano...d-detail-01.jpg The insulation doesn't look as good, though, and they just hook up the wire to a 9V battery.

Edit: The other thing you could try is to use a dew-shield. Basically, anything you can do to reduce radiative heat loss from the back of the secondary.

[ajohnson]

I have also built dew heaters for my Newtonians, I went for a direct heat approach as opposed to heating the stem. I have done my 12" and 10" Imaging scope, so far both are working well. The basic principle I used was to stick resistance wire to the mirror back and pass a current from my home built heater controlled (basically a toy railway pulse controller). The biggest problem I have had was heat insulation, originally I used foam backed material used for underfloor heating but have settled on thick card, this is used to back the mirror and prevent heat loss into the OTA tube (and tube currents) and has the happy side effect of reducing the amount of current required.

I cut a template from paper after tracing the mirror onto it and then use this to cut a card insulator, this needs to be slightly undersize on the outside and oversize around the mirror stem.

The wire (28swg Constantan wire from Maplin) is wound into a zigzag pattern to increase coverage, this also increases the length of wire used and it's overall resistance, using ohms law (lots of links on the internet) this increases the wattage of the heater.

Finally I solder on a thin phono cable as a supply, this is about 1.5mm diameter and is coaxial I find this works best purely to reduce the impact on the light path routing it across the mirror spiders.

Finally I use a long length of black gaffer tape to secure the feed wire in place along it's whole length, again I have found spot fixing allows the cable to sag and you get weird diffraction spikes on images.

Cable routing before & after as seen via the eyepiece.

[Merlin]

If the diagonal is mounted in a cell and not just glued to a stalk, the cell can be filled with cotton wool, packed round a torch bulb, This was recommended by one-time astronomy populariser James Muerdin.

.

[RikM]

I have also built dew heaters for my Newtonians, I went for a direct heat approach as opposed to heating the stem. I have done my 12" and 10" Imaging scope, so far both are working well. The basic principle I used was to stick resistance wire to the mirror back and pass a current from my home built heater controlled (basically a toy railway pulse controller). The biggest problem I have had was heat insulation, originally I used foam backed material used for underfloor heating but have settled on thick card, this is used to back the mirror and prevent heat loss into the OTA tube (and tube currents) and has the happy side effect of reducing the amount of current required.

SDH_02.JPG

I cut a template from paper after tracing the mirror onto it and then use this to cut a card insulator, this needs to be slightly undersize on the outside and oversize around the mirror stem.

SDH_05.JPG

The wire (28swg Constantan wire from Maplin) is wound into a zigzag pattern to increase coverage, this also increases the length of wire used and it's overall resistance, using ohms law (lots of links on the internet) this increases the wattage of the heater.

SDH_08.JPG

Finally I solder on a thin phono cable as a supply, this is about 1.5mm diameter and is coaxial I find this works best purely to reduce the impact on the light path routing it across the mirror spiders.

SDH_10.JPG

Finally I use a long length of black gaffer tape to secure the feed wire in place along it's whole length, again I have found spot fixing allows the cable to sag and you get weird diffraction spikes on images.

SDH_12.JPG

Cable routing before & after as seen via the eyepiece.

SDH_13.JPGSDH_14.JPG

That's the sort of thing I am planning for my 10". Do you see much difference in the diffraction patterns in images compared to without?

[ajohnson]

That's the sort of thing I am planning for my 10". Do you see much difference in the diffraction patterns in images compared to without?

 I' don't see a lot of difference if any, I was concerned that one spider had a slightly thicker profile with the cable running under it, hence the use of a coaxial wire to keep the cross section down but I have not noticed this in any images and particularly not when viewing. I have attached an image of M45 taken last night for you to judge for yourself.

[Tommohawk]

Hi all. I realise this thread is a bit old - but thought it better to attach to this rather than start a new one as it's the same topic.

I had a lot of fun(!) last night with cloud, aircraft lights, mid-session meridian flip, and finally when the last of the late flights had landed and I got going at 1.30am - condensation on the secondary.

So I was looking at secondary heaters - Gaj - did you get anywhere with your master plan?

The idea on post#7 - A Johnson - looks fab. But I did some quick maths and at 4.2 ohm/metre, even if I could cram 10cm on there would be 0.42 ohm, which at 9v would give "V" squared over  "R" watts = 193W.

I'm wondering if I could do some crumpets on that? I guess you keep the controller turned down a bit. Did your sums work out like that?

Also - have you any way of monitoring whats going on - without some kind of fancy thermostat how do you know how much juice to give it? I cant really see the 2ary without taking the camera off and then I lose focus.

Hopefully you're still tuned in!

[ajohnson]

Hi Tommo,

The straight maths on my heater is about right and at maximum on the controller it fries wiring and the insulator. I use a Maplin 3-15v motor speed controller ( I forget the stock code but a photo of the control box is attached) I use it on a very low setting which seems to keep the secondary warm but not at the egg frying temperatures that it could achieve. An LED dimmer from e-bay will do much the same trick, you definitely need some form of voltage / current control or you will fry the circuit.

[Uplooker]

Hi,

Is this the Maplin motor speed controller you were referring to? i used one of these when I built the dew strip for my SCT.

http://www.maplin.co.uk/p/mfa-panel-mounted-speed-regulator-module-rn41u

Ian

[Tommohawk]

Hi Tommo,

The straight maths on my heater is about right and at maximum on the controller it fries wiring and the insulator. I use a Maplin 3-15v motor speed controller ( I forget the stock code but a photo of the control box is attached) I use it on a very low setting which seems to keep the secondary warm but not at the egg frying temperatures that it could achieve. An LED dimmer from e-bay will do much the same trick, you definitely need some form of voltage / current control or you will fry the circuit.

Hey thanks for follow up post! I'm thinking of using your idea but with finer wire for higher resistance, and aiming for a heating power of a few watts, as per the Kendrick split heater pads.

Also, on the basis that I'm only running at perhaps 4 W max, I was going to control with a thermostat rather than a PWM or similar. So the heater will be either on or off, but regulated by the stat - so I should be able to set the stat for say 2 deg over the dew point and leave it. I reckon that by putting thermal insulation over the heating pad,  the mirror glass should hold enough heat to prevent too much oscillation.

I guess ideally I would have stat with built in regulator, but I cant find a single unit that does this, and dont want to overcomplicate.

What do you think? Also, does the nichrome wire you used have any insulation, like motor winding wire, or is it bare?

[Uplooker]

When i build a dew heater for my 8" SCT I insulated the nichrome wire with 1mm heatshrink that had a 50% shrunk diameter. I think I got it from CPC.

Ian

[Tommohawk]

When i build a dew heater for my 8" SCT I insulated the nichrome wire with 1mm heatshrink that had a 50% shrunk diameter. I think I got it from CPC.

Ian

I plan to use double sided adhesive thermal tape on the mirror back, stick the bare nichrome wire to this, then stick the stat sensor, then cover with another layer thermal tape to hold the sensor, and cover the whole lot with insulation. I guess the heater wire doesnt need insulation on the heater pad itself, but just as far as the next connection.

I was hoping to insulate the vanes from the mirror holder and from the OTW, and use the vanes as conductors - but I think I this may prove problematic at the mirror holder end, so will probably use wire taped along vanes.

Heat shrink will be useful - ta for the tip.

[Live_Steam_Mad]

I ran into this same problem of my SkyWatcher 200P 8" F5 Newtonian dewing over heavily on the secondary mirror. So I decided to wrap my eyepiece dewheater around my secondary stalk and WOW is it ever effective. Even though it draws only 7W at 12V, it clears the heavy dew within a mere couple of minutes. I bought mine from ;-

http://www.ebay.co.uk/itm/KEENOVO-Dew-Heater-Bands-for-3-3-5-Finder-Telescopes-Eyepieces-Optics/282039789170

  ...described as "KEENOVO Dew Heater Bands for 3"~3.5" Finder Telescopes Eyepieces Optics+RCA Plug". I keep it powered off, and only activate it when I notice dew forming on the secondary, and as soon as the dew clears I switch it off again.

Pix ;-

https://www.flickr.com/photos/65636244@N04/albums/72157688544088875

Regards,

Alistair G.