AUTOSTAR INFORMATION Last updated: 26 February 2004

Autostar heater pad

Date: 2/22/04, 03:21
From: Stephen Bird (stephen.bird@btconnect.com)
It is a simple task to make your own heater pads, I have made all of
mine including the heater tapes for the scope and a large heater pad for
the lap top.

All you need to do the job is:
 
Materials
1. A sheet of plywood big enough to make the largest pad or tape you
plan to make (you reuse it to make all of the pads or tapes you need).
3/8 inch (6mm) or thicker will do.

2. A large reel of duct tape.

3. A large pack of 1 inch (25mm) panel pins.

4. A roll of adhesive backed black baise (the furry velour material like
you see on pool tables). You can get it from Craft or Harware Stores.

5. Black foam or neoprene pads big enough for the pads or tapes you are
going to make and around 1/4 inch thick (6mm). I got neoprene for making
scuba diving wetsuits (semidry suits actually) from the local SCUBA
shop, they ordered it for me cut to size as they get made to measure
suits from a local supplier. The other source for the foam is a Craft
shop, but it is quite thin so you need several sheets to make it 1/4"
(6mm thick).

6. A roll of enamelled copper wire 34swg or 40swg. you can get this from
someone like Radio Shack in the US, Radio spares or Maplin in the UK.

7. 5 amp twin core black electrical flex. Get from RS or Maplin.

8. Shrink wrap sleeving, 1 piece 2mm diameter, one piece 8mm diamete or
similar. Get from RS or Maplin.

9. Phono plug if you have a multi-outlet heater controller that uses
phono sockets, or a cigar lighter plug if you are using one of those
power stations that we tend to see with scopes these days. Get from RS
or Maplin.

10. A tube of Aquasure adhesive from the SCUBA Diving shop. (Its is very
flexible very sticky adhesive).

11. Soldering iron, solder, tools various (your toolbox!).

12. Adhesive backed velcro, elastic and other oddments.

13. A cloudy evening with nothing on the TV worth watching, (not
difficult, we have 250 channels of nothing worth watching at $60 per
month for the privaledge!).
 
A Bit of Math
OK, to make them you need to know or measure the resistance of the
enamelled copper wire and how many watts you want the heater tape to be,
so you can work out the length of wire needed to produce a pad / tape of
the required heat output. for heater tapes this is easy, just go to the
website of someone like Kendrick in the US or BC&F in the UK and they
tell you the heat output they use for the tapes they would sell you! But
for the Autostar I use a 5 watt pad and for the two pads for the lap top
I use 12 watts for each pad. (But I have an adjustable temperature
controller, so I can turn the heat up and down so the heat output is not
too much of an issue for me.)

34swg (0.236mm) enamelled copper wire is around 0.41 ohms per metre
(0.125 ohms per foot).

40swg (0.125mm) enamelled copper wire is around 0.77 ohms per metre
(0.24 ohms per foot).

To work out the length of enamelled copper wire for a heater pad / tape,
use the following formulae, which assumes you are powering the pad from
a 12 volt battery or power source:

First you need to work out the resistance: Resistance  = 144 / wattage
of pad or tape

E.g., for a 5 watt pad, Resistance = 144 / 5 = 28.8 ohms.

Then you need to work out the length of wire for that resistance, for
the thicker 34swg wire you will need a lot longer piece of wire, and for
the thinner 40swg wire a shorter piece, the choice is yours. I use the
thick stuff for long taps and pads and the thin stuff for eyepiece and
finder tapes.

The length is found as follows: Length = Resistance Required /
Resistance per unit length of the wire you intend to use

E.g., for the 40swg at 0.77 ohms per metre (0.24 ohms per foot), and a 5
watt pad needing 28.8 ohms as above;

Length in metres = 28.8 / 0.77 = 37 metres or Length in feet = 28.8 /
0.24 = 120 feet.
 
Construction
Mark out on the plywood board the size of pad / tape you want and tack
using panel pins some of the duct tape sticky side up in strips if
necessary, an area about 1 in (25mm) larger than the size of pad / tape
you want. (Size of the foam / neoprene you are going to use).

Put a row of panel pins each end of the taped area, at 1/24 in or 1mm
intervals or similar will do, just inside the size of the pad tape you
are making (1/4 in / 6mm inside will do).

Leaving about 1in (25mm) of enamelled wire spare for terminating at the
first pin on one row of panel pins, wind the wire backwards and forwards
across the matrix of pins like mowing the lawn. you should work across
and down the rows of pins as you go, when you get to the bottom, just
work back up again. The idea is to finish at the same place you started,
so having the length of wire a little longer than you need and missing
out a few left right runs on the last of the wire would be a good idea.
As you wind the wire, it sticks to the duct tape, and you can push it
down on to the tape to keep it fairy low profile, so don't wind it to
tightly around the panel pins, if you wind it too tight, first you can't
get the pins out and second it can damage the enamel and short the
heater element out. Once you are done, you will have two loose ends of
wire in the top left corner. Push the wire firmly down on to the tape,
PRAY!, then pull the panel pins out, CAREFULLY!!. But not the ones
holding the tape down to the board!!!

Next attach the 5 amp twin core flex to the two loose ends, cutting and
making the soldered joins just inside the aread of the desired pad /
tape size. you need to sand / file the enamel to get to bare copper for
a good connection. Carefully sleeve each wire, and overall sleeve the
joint with shrink wrap sleeving. Use a hair dryer to shrink and make the
join secure. I actually, carefully, thread the twin core flex through a
hole in the duct tape to ensure it exits from the top / non-hot side of
the pad / tape.

Cut a piece of the black adhesive backed baise the exact size of the pad
/ tape you are making, and stick it over the wire / duct tape
construction on the board, being careful to ensure it overlaps evenly
around the edges. The black furry baise is a nice smooth surface that
won't scratch whatever it is put on, and is the HOT SIDE.

The pad / tape is taking shape.

Now, again being EXTREMELY CAREFUL, cut, using a sharp craft knife, the
excess duct tape away from the pad, around the black baise. Don't cut
the wires or your fingers! You can now remove the assembly from the
board and you have a sandwich of duct tape, heatar wire and black baise,
with a tail of twin core power flex. At this point cut the power flex to
length and terminate it with your phono plug or cigar plug. (If using a
cigar plug, get the fused type and put an appropriate size fuse in
there, THIS IS IMPORTANT. A 12 volt rated fuse of around 1.5 to 2 times
the current the pad / tape draws is required). Here is some math to do
this:

Fuse Current Rating = (1.5 x Wattage of pad) / 12, OR = (1.5 x Wattage
of pad) / 12

E.g., for a 5 watt pad  / tape;

Fuse Current Rating = (1.5 x 5) / 12 = 0.42 Amps, OR = (2 x 5)  / 12 =
0.83 Amps.

So a fuse of between 0.42 and 0.83 Amps is good, say a 0.5 or 0.75 Amps
would do.

If you don't use a fuse and a short circuit occurs, then you could get a
fire, so you have been warned!

Next, the foam / neoprene insulation layer. Cut this to a size slightly
bigger than the pad / tape. If it is a pad for the Autostar or Laptop
then you may need an edge to fasten it to the Autostar or Laptop in some
way. Strategically placed adhesive backed velcro or elastic loops will
suffice. You could even put the Autostar into one of thiose waterproof
mobile phone bags from the boat shop.

If you got this far, you are fairly handy anyway and can work something
out!

Stick the neoprene  / foam pad/s (remember you need 1/4 inch or about
6mm for the non-hot / insulated side of the pad), to the duct tape side
of the "heater sandwich", using the Aquasure adhesive. You need to be
careful with this stuff, it oozes out everywhere, sticks to everything,
and doesn't come off anything! It takes 24 hours to set, and you need to
keep the whole thing under pressure. So put the pad on the board, put a
heavy book on it while the Aquasure sets, some clean paper between the
two helps to protect the book. Once set, trim the edges of the pad wioth
the craft knife to remove any Aquasure that oozed out under pressure.

FINALLY, Aquasure "out-gasses" when it "cures", so leave it to
thoroughly cure for a couple of days. THEN connect it to the 12 volt
supply and leave it on for an hour or two, well away from your scope
before you use it in anger, or you could find it damages the optics.

For corrector plate tapes and eyepiece tapes, you need to use adhesive
backed velcro (and Aquasure as the velcro adhesive is not very good) on
the tape ends, with carefully measured tape heater element lengths and
an overlap of duct tape / baize / neoprene (foam) in the sandwich to get
them to fit tight around the scope / eyepieces.

Hope this helps.

I'll post some photos when I next make some pads / tapes up. You can be
really sad with these things, and have pads for your eyepiece case, and
even your viewing seat, hat, gloves, socks, etc., (only joking!!, but
the uses are endless once you get your production line going).

I guess when I get time I could tell you about the heater controller as
well, but that is a little more complex.

So much to do, so little time!

Regards

Stephen Bird

Subject: Mighty ETX Heater Pads
Date: 2/24/04, 06:35
From: Niall Saunders (niall@njs101.com)
To: stephen.bird@btconnect.com
Hi Stephen,

And thanks for the really useful, well documented, information about
making up heater pads and heater tape.

Just a couple of quick questions (before I head off to Maplins and my
local neoprene retailer!!):-

1.) You suggest 'Aquasure' as the glue. Is there any reason for not
using a contact adhesive such as "EvoStick"?

2.) What about the 10mm thick sleeping mats that are available from
'Outdoor / Camping' retailers? (With these being much thicker than the
Neoprene, you might not have to 'layer' as seems necessary when using
Neoprene - in fact, for the objective end of the telescope, could you
not maybe use the stiffness of the camping mat to make a 'combined'
heater / dewshield in one unit?

3.) For the eyepieces, once you have made a heater tape on the sticky
side of a single piece of (hook or loop) Velcro - say by making a simple
'zig-zag' of the appropriate length/thickness or wire (giving a tape
that is still long enough for the eyepiece - or viewfinder - diameter) -
could you not then simply stick the 'opposite' piece of Velcro over the
zig-zag (using extra glue, if needed) to give you the required tape; and
then use the foam insulation manufactured for water pipes to give you
extra insulation? (This insulation is available in a wide variety of
diameters and wall thicknesses - at almost any good hardware store - and
a piece of card stuck to the outside will also give you the basis of a
dew shield for your viewfinder).

I look forward to hearing about your 'heater controller'.

Cheers,
Niall Saunders
Aberdeen

I tried a number of adhesives, and the only one that was truly flexible,
not affected by the continual heat up and cool down cycle, and stuck to
neoprene and other fabrics was Aquasure. I have used it extensively in
diving, and it even helps waterproof and stick together the seams on a
tent half way up a mountain in blizzard conditions. So it comes highly
recommended. No reason why if you prefer another adhesive, you should
not experiment with it, just be careful of the outgassing by solvent
based products.

For the flat heater pads for the Autostar or PC, any suitable insulated
padding material is fine. For heater tapes, particularly those around
small diameter eyepieces, if it is too thick, it will not fit tight to
the surface it is wrapped around and you will get uneven heating. So in
this case I found 7mm neoprene was about the thickest that avoided this.
Also the neoprene for dive suits is fabric backed on one side, so tends
not to deteriorate. Again though, no harm in experimenting. I "chamfer"
each end of the heater tapes where the velcro is attached to ensure a
snug fit around the eyepiece or corrector plate.

The combined heater dew shield sounds interesting, but of course, you
would only want to apply heat in the narrow band around the corrector
plate or you will get some interesting images through the heat haze.

Adapting the eyeiece design as you describe sounds good in theory, can't
see why it should not work. My design merely follows the same principles
as the commercial heater tapes, which can easily be removed /
reinstalled for changes of eyepiece and stand up well to wear and tear.

The mark 1 heater controller uses a square wave to pulse a JFET on and
off, works great for a fraction of the cost of the commercial product.
The mark 2 on the drawing board now is a temperature differential unit
so that you can set the temperature of the heated surface at a
predefined level above dew point to prevent tube currents. Only really
necessary for the corrector plate on an ETX. But as I said, so much to
do, so little time!

Hope this helps.

Regards

Stephen Bird 

Hi again Stephen,

I hear all your comments - and accept that you have actually tried these
tests - whilst I am still very much in the 'thinking' phase!

I know that, in Antarctica, at -40C I found that EvoStick (contact
adhesive) was not that successful, and that we used another product
instead (an American product, whose brand name I cannot recall).
Certainly, I will agree that, if Aquasure works on Neoprene, and that is
what you are using, then Aquasure it has to be.

I had also mis-read your original article, thinking that you were having
to 'layer' the Neoprene to achieve a suitable thickness, whereas in fact
you were talking about the thinner foams available from craft shops. As
far as the 'eyepiece-tapes' are concerned, I still wonder whether the
actual heater tape could be best made out of nothing more than the two
strips of Velcro, with the zig-zagged wire held between them. You attach
the tape to the eyepiece (or viewfinder OTA) using the power of the
Velcro, and THEN you slip over the tube of pipe insulation.

For the viewfinder, the tube of pipe insulation could be used to support
a dewshield of nothing more sophisticated than black paper or card (and
would also hold a heater tape in place).

I also fully agree that the main OTA should have the heater tape just
'behind' the corrector plate (for SCTs anyway). It may actually be far
easier to use your design of heater tape (with Neoprene, Velcro and Duct
Tape) to actually hold the tape in place, and then to use a 'matt black'
camping mat s a combined dewshield and heat-tape insulator. If the foam
from the camping mat is 'too shiny' then the 'velour' material could be
used to line that as well.

I can foresee a possible issue with the enamelled wire when making up
the smaller heating pads (eyepieces, viewfinders, etc.) where it is just
not practical (or even possible) to get enough wire length into the
smaller dimensions available with these pads. Would there be any
advantage in considering the 'working eyepiece' + 'viewfinder eyepiece'
+ 'viewfinder objective' as a single element - and calculating wire
length for, say, a 6 Watt heater, and then distributing the required
length of wire as three 'shorter' lengths, between the three heating
tapes. Obviously these tapes would need to be connected in 'series'
(supply into one, then from the other end, as an input to the next, and
so on, finally returning to the supply 'ground'). I just don't
immediately see how you would sensibly get 120 feet of wire round an
eyepiece!

Finally, on your pulse-width modulated JFET. That is exactly the path I
was going to follow, but was going to use a PIC microcontroller to
achieve the PWM, and was going to use two simple diodes as the
differential temperature sensors. All you would really need then is a
potentiometer to 'set' the temperature differential, and a bi-colour LED
to indicate the mark/space ratio being applied to the JFET (the 'yellow'
segment being held 'on' whenever the 'primary' sensor temperature is
'well below' the 'ambient' sensor temperature, and the 'red' segment 
indicating the 'heating' pulse ratio being applied to the tapes - normal
operation would be seen by occasional red flashes, excess heating by
long red flashes, and insufficient heating by the yellow section being
illuminated at the same time as the red flashes). No reason why
component count should be much more than $20, and if anybody needed a
pre-programmed PIC chip, another $20 for the effort of doing that.

As for 'so little time' - have you tried using the available time after
midnight, and before breakfast? Ever since coming across Mike's site, I
find that this time is perfect for increasing your knowledge. Sleep is
so over-rated anyway!

Cheers,
Niall

Niall did of course correctly point out that to reduce wire length in
the eyepiece and finder tapes that they should be wired in series, which
is precisely what I do.

In this fashion 3 x 5 watt tapes totalling 15 watts would require a
total resistance of 96 ohms. For the 32swg enamelled wire this equates
to 23.4 metres or 81 feet in total. So 3 tapes each using 7.8 metres or
27 feet, wired in series. This makes the enamelled wire easier to get
into a tape that is probably only 12.5 centimetres or 5 inches long.

I'll try and post some pictures when I next make up some pads / tapes.

Regards

Stephen Bird

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