RC secondary mirror dew heaters - options?

[fwm891]

Hi, I have an Altair Astro CF RC8 and want to fit a dew heater to the secondary mirror assembly and I'm wondering what others have done DIY or buy?

I like tinkering so the DIY option is high on the list (just what sort of DIY) also like the look of the kendrik? stick on jobs but don't really want to spend lots of dosh!

Any thoughts please...

[fwm891]

Initial thoughts for a DIY solution.

Used this dew heater designer for resister based projects:

Dew_Heater_Designer.xls

[Earl]

the comercail one is quiet expensive, and im not impressed with the wires, if the spider could be a conductor of sorts to power somethin on the secondary that would be cool.

[opticalpath]

That's quite neat, Francis.

I have seen designs that just use resistors soldered in series, the whole string being threaded into a length of black rubber tubing. But I think that would be less suitable for a small radius such as a secondary housing. I presume that one desirable feature would be a low profile, if the heating device wraps round and increases the area of the central obstruction.

I would be very tempted to use a few turns of thin resistance wire .... which for insulation can be fed into thin PVC sleeving or 1 mm diameter black heat-shrink tubing.

For a finder scope objective, I wrapped several layers of gummed brown paper tape around the end of the tube (gummed side out) to make a semi-rigid tight fitting band, and included a spiral of a few turns of thin nichrome wire between the layers. That works well and is very low profile.

Adrian

[fwm891]

Hi Adrian,

I've been looking at nichrome wire options but it seems to need such a long length of nichrome to get the correct resistance which makes it almost impossible to wrap around the back of the secondary. I'm going to have a play with the resistor version first as the resistors were really cheap. I will try backing the resistors with cork to insulate them from the elements and butt the resistors themselves against the secondary housing with a layer of electrical insulation tape just in case. A resistive foil would be much better if such a material exists.

[fwm891]

Further thoughts...

Embedding the resistors in a cork ring so they are just proud of the surface, and taking the end wires through the cork ring to a second cork ring with the pos/neg cable embedded in the second ring. External cables can then exit between the cork rings and along a spider vane.

Using cork should retain enough flexibility to allow the assembly to be fitted without removing the secondary assembly (hopefully)

[opticalpath]

Another way would be to make a simple ladder with one resistor for each rung and two parallel bare wires for the side rails of the ladder.  Then wrap the ladder around the circumference of the secondary holder (so each resistor lies parallel to the axis of the cylinder), join the ends of each rail where they meet and solder a power supply wire to each rail.  It would be very easy to work out the spacing and to solder up the ladder and trim the resistors on a flat surface, and the finished product would have a very low profile.  Just need an insulating layer of tape underneath before you wrap and another on top to finish.

Adrian

[skybadger]

Hi Francis

What about one of those usb cup heaters ? They are resistive disks inside - or the one I took apart was. I wonder what I did with it  ? I need it now for my Cass secondary and I also need a heater for the 12"primary with all this dew I'm getting.

Fibre glass observatories seem to let the dew in - my wooden one never did.

Mike.

[fwm891]

Mike - I didn't know they existed!

Adrian,

That's essentially what I'm doing except I'm building my 'ladder' into the cork (cork coaster) so the resistors are proud of the cork surface and touch the back of the secondary mirror housing. I then take the resistor leads through the two cork rings and solder to the pos/neg cables at the back. This then keeps all the heat from the resistors where its needed.

Now ordered a set of cork coasters (cheaper than tiles). Just need to work out now how to route 18 angled slots with 20° spacings...

[fwm891]

Hi Francis

What about one of those usb cup heaters ? They are resistive disks inside - or the one I took apart was. I wonder what I did with it  ? I need it now for my Cass secondary and I also need a heater for the 12"primary with all this dew I'm getting.

Fibre glass observatories seem to let the dew in - my wooden one never did.

Mike.

Was the element something like this:

http://www.ebay.co.uk/itm/Durable-USB-Heating-Heater-Winter-Warm-Plate-for-Shoes-Golves-Mouse-Pad-ED-/301323473105?pt=UK_Computing_ComputerComponents_MouseMats&hash=item46284750d1

?

[redtail]

Have you thought about coiling nichrome around a wick then placing the wick in a shrink tube.

http://www.ebay.co.uk/itm/Atomizer-Coil-Rebuilding-Kit-for-Ego-Nova-CE5-CE4-V3-SILICA-WICK-KANTHAL-S002/321207469317?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D27550%26meid%3D7c1ea10d783a468ca1b4305ab741c288%26pid%3D100005%26prg%3D11355%26rk%3D5%26rkt%3D6%26sd%3D321466939831&rt=nc

[opticalpath]

Hi Adrian,

I've been looking at nichrome wire options but it seems to need such a long length of nichrome to get the correct resistance which makes it almost impossible to wrap around the back of the secondary. I'm going to have a play with the resistor version first as the resistors were really cheap. I will try backing the resistors with cork to insulate them from the elements and butt the resistors themselves against the secondary housing with a layer of electrical insulation tape just in case. A resistive foil would be much better if such a material exists.

I've found the same thing with nichrome, Francis, but I noticed that Kanthal (FeCrAl) wire has a much higher resistance per metre and looks to be a lot more convenient than nichrome for smaller items.  For example, 34 SWG Kanthal is 81 Ohms per metre so at 12 Volts supply you would need only about 60 cm to provide 3 Watts heat.

http://www.ebay.co.uk/itm/A1-Kanthal-Wire-0-15mm-34AWG-0-2mm-32AWG-and-0-3mm-28AWG-in-varied-lengths-/271650740553?pt=UK_Collectables_Tobacciana_Smoking_LE&var=&hash=item3f3fa55949

Adrian

[skybadger]

The first one I took apart had a alloy disk with a circular heating element on it. But that link in eBay looks great for 2ndaries. Looks like you can cut it to shape too. Following on I found heater tape in meter lengths from Russia on eBay so that solves the primary mirror heating issue too.

M

[gajjer]

Hi Francis

I designed my own heater for an 8" Newt. I used resistors as the heaters and used enameled copper wire to take the connections to 4mm sockets on a plate  mounted on the tube. The enameled copper wire is then super glued along the spider. I don't know what your physical arrangement is but I would think you could do similar. The resistors were mounted with good thermal contact to an aluminium ring. I think I used thermal paste then glued up the hole at the end.

You need very good thermal contact. Any air gaps or cork in between the resistors and what you are going to heat is bad news.

I was surprised by the delay in getting heat into the aluminium and seeing a temperature rise in the mirror. Ideally I would have put a temperature sensor on the mirror and close loop controlled it.

It may be best to get something running and see how it performs.

I use a PWM control to set the power level manually. Initially set high then turned down after a rapid warm up.

Hope that helps.

Cheers

gaj

[fwm891]

Thank Gaj. Like you I will take my wire along a spider vane (or two) and connect outside the tube back to the power box via phono's. My tube heaters usually go on at the start of the night regardless but the secondary needs a little extra help occasionally.

I'm using the cork as an insulator to stop heat loss at the resistors to the night air. The resistors will be pressed against the secondary's back plate (or Kanthal wire suggested by Adrian above as that will make construction much easier).

Thanks

[opticalpath]

What does the secondary support look like, Francis? I was imagining some sort of cylindrical structure, and that you might even just wrap the thin Kanthal wire around the cylinder close to the mirror.

I've always wondered if there's any effect on the image quality, having a few watts of heat like this up near the top of the tube in the optical path.  Outside insulation could be important, as you described, to reduce 'wasted' heat leaking out in all directions.  

Adrian

[fwm891]

What does the secondary support look like, Francis? I was imagining some sort of cylindrical structure, and that you might even just wrap the thin Kanthal wire around the cylinder close to the mirror.

I've always wondered if there's any effect on the image quality, having a few watts of heat like this up near the top of the tube in the optical path.  Outside insulation could be important, as you described, to reduce 'wasted' heat leaking out in all directions.  

Adrian

Hi Adrian,

It has quite a large central boss. The only exposed area is a ring with inside diam around 76 mm and an OD of about 92 mm. There's quite a bit of clearance between the rear of the secondary holder and the spider vanes - hence the thickness of cork there to insulate that area.

[skybadger]

Why isn't this going inside the mirror housing?

M

[BlueAstra]

I used a Kendrick secondary heater on a similar scope, as shown in this thread

Graham

[fwm891]

I used a Kendrick secondary heater on a similar scope, as shown in this thread

Graham

Hi Graham, Like the copper strips to take power to the heater - I'll pinch that  idea if you don't mind

[fwm891]

Image 1 shows the cork coaster being used to mount the heater resistance wire to. The layout shows the coaster with outer ink ring the OD of the secondary holder, middle ink ring - the line the heater wire is tacked to and the inner ring the inside edge of the heater.

Image 2 shows the resistance wire and the method of attachment to the cork using small wire staples.

Image 3 shows the reverse side of the cork coaster, the cross-over of the staple ends and the two ends of the resistance wire emerging.

The reverse side will have two more layers of cork added to act as insulators.

The 0.15 gauge resistance wire arrived today and a quick check with a multimeter showed I needed 27cm of the wire to give me the 22 ohm resistance needed. Meaning I could run a single strand of the wire around an 4.3 cm radius which sits inside the diameter of the secondary housing.

Next steps are to stick the extra cork insulation layers in place, cut to shape and attach the power feed wires... tbc

[skybadger]

Having been prompted by this thread and checking out ebay, I found a HK site that sells carbon pads that fit nicely with a spot of trimming behind the secondary. Resistance is about 8 Ohms and they get just warm enough from a 5v USB supply; so I'll be fitting one each to the Newt and the Cass.

I also found a russian site selling braided carbon heater tape 1" and 2" wide and got a length to wrap around the primary to keep condensation at bay while the dome is shut by keeping the primary above the dew point - just.

I am a bit concerned that any heating of the primary is going to destroy the figure.

Mike

[redtail]

Having been prompted by this thread and checking out ebay, I found a HK site that sells carbon pads that fit nicely with a spot of trimming behind the secondary. Resistance is about 8 Ohms and they get just warm enough from a 5v USB supply; so I'll be fitting one each to the Newt and the Cass.

I also found a russian site selling braided carbon heater tape 1" and 2" wide and got a length to wrap around the primary to keep condensation at bay while the dome is shut by keeping the primary above the dew point - just.

I am a bit concerned that any heating of the primary is going to destroy the figure.

Mike

Can you give links to the pads and tape please

[skybadger]

Pads http://m.ebay.co.uk/itm/301323473105?txnId=1044854013020

Braid http://m.ebay.co.uk/itm/321297654628?txnId=1362093271011

Try removing the txnid part of you can't reach them.

Mobile eBay is a bit useless

Mike

[Jonk]

I have been reading this which is useful.

As the current drawn at 12v is so small, I was thinking copper tape can be used along the sipder veins?

i have an IR temperture gun so will run some experiments. One downside to the idea is copper tape is reflective, so may need to paint it black.