No frames? click here

Equipment: accessoires

Telescope and Eye-pieces SkyWindow and Binoculars filters accessories

Telrad finderHomemade Dew Heater TAl/Meade Eyepiece/filter case and table
Digital camera Nikon Coolpix 775 / 4500Catsperch observing chair

Homemade dew heater and controller for the TAL 200K and Meade LX 90

In the past two years I have been observing from various locations. Every now and again I encountered the problem of dewing or fogging of the two element corrector in my TAL 200K, especially when observing from my favourite dark sky site in Belgium, which is situated in a swampy area. After two or three hours of observing, the corrector lens, which is part of the secondary unit, completely fogged up. I had to pack up the telescope and switch to binoculars the rest of the night.

Mr. Huub Willems, a fellow amateur astronomer from our local astronomy club encountered the same problem with his Meade LX90. After a few hours his corrector plate was also completely fogged. In 2002 he constructed a dew heater for his Meade LX90, inspired by a design by Chris Heapy.

When Huub came to visit me in January of 2003 we had a look at the construction of my TAL 200K and we came to the conclusion, that his homemade dew heater probably could be mounted onto my telescope as well, preventing the corrector unit from fogging.

Dew Heater mounted on TAL 200K

In June 2003 Huub installed it onto my telescope. On the following pictures you can see the result.

First the controller was mounted to the TAL 200K tube using Velcro tape. It is positioned right behind the Telrad finder, and stays on the telescope’s tube permanently.

Then the heating element (10 Watts), protected by a Teflon tube, was fitted around the outside of the telescope tube.

Dew Heater mounted on TAL 200K

Then the heating element (10 Watts) was connected to the controller using a terminal strip screw block.

Finally the controller was connected to a 12 Volt rechargeable power pack, using a cigarette lighter plug with a built in fuse.

After we switched it on and had it running on full power (10 Watts) for half an hour, the front-end of the tube was definitely much warmer than the rest of the telescope. Because of the fact that the tube and the corrector unit are connected, and both made of solid steel, I think that more than enough warmth will be transported to the corrector. The maximum power of 10 Watts probably will be far too much. Two to five Watts should be sufficient. This dew-heater should do the trick. Of course it has to be tested in the field, but I am confident that it will work. In time I will be back with my findings, once I tested it on (the swampy) location.

In the article below, written by Huub Willems, you will find a complete description of the dew heater and controller, with all the technical details. He wrote this article for the homemade dew heater and controller for the Meade LX90. However, the dew-heater and controller for my TAL 200K is an exact copy of the one constructed for the Meade LX90. The only difference is that with the LX90 the heating element is fitted in the inside rim of the scope, but on the 200K the heating element is fitted around the outside of the telescope’s tube.

Should anyone have questions about the dew-heater and controller, please let me know. I will try to answer your questions, or forward them to Huub, who will contact you via e-mail.

Finally I would like to thank him for allowing me to publish his article on my website.

Home Made Dew Heater and Controller for the Meade LX 90

By: Huub Willems

After buying my first telescope (a Meade LX90, 8” SCT) in August 2001, I became a practical astronomer. Soon I experienced the need to equip my scope with a dew prevention system. This because most of the nights I had to stop observing because of dew formation on the corrector plate. That’s why I initially made a dew shield from a camping mat according to the one described by Kevin Honeyager.
However, this solution did not appear to be sufficient all the time. Because I have some knowledge of electronics, I decided to make my own power controlled dew heater, based on a 555-timer integrated circuit.

Heating element

According to what I found on the Internet, 3 to 5 Watt power usually will be sufficient to prevent dew formation on the corrector plate of a SCT. To have some overkill and because of the good regulation possibility, I decided to lay out mine with a maximum power of 10 W. I also wanted to keep using my dew shield to reduce the influence of light pollution. That’s why I was very charmed with the design of Chris Heapy.

Fig. 1: Internal SCT dew heater of Chris Heapy.

The heating element is fitted in the inside rim of the scope and can remain there permanently, as the front cover and the dew shield can still be used with it fitted. For the heating element I used a 70 cm long 19.6 Ω/m; resistance wire (about 14 Ω, resulting in about 10 W @ 12 V). It has a thickness of 0.6 mm and because of its vulnerability and for insulation purposes, I placed it in a 2.5 inner diameter Teflon tube. The ends are fitted into a connector, to which also the outlet wires of the dew heater controller are connected.
The controller housing is so small (100x50x25 mm) that it too can be mounted permanently on the telescope tube (Fig. 2)

Fig. 2: Dew heater controller housing with 12 V power connector and regulated output wire.

Controller circuit

Core of the circuit is a Darlington transistor (TIP 120) which acts as a fast switch for the 12 V power over the heating element(s). This transistor is driven from the output of a 555-timer IC, which is connected as a free running astable multivibrator (see Fig. 3). Because of the 2 added diodes the duty cycle can be adjusted over a wide range.

Fig. 3: Controller Circuit

The output of the 555 (pin 3) has a practically fixed frequency f of about 1 Hz
(f = 1/t, with t ≈ elog2 *1.047 MΩ *1.5 µF).
With resistor R3 (47 kΩ) in combination with the variable resistor P1 (1 MΩ) the duty cycle of the output signal can be varied between 5 and 100%. The longer the output of the 555 is high in a cycle, the longer the heating element is switched on and thus will produce more heat. In the figure below this principle is shown.
By turning the variable resistor P1, the power of the applied 10 W heating element can continuously be varied between 0.5 and 10 W.

Fig. 4: Influence of duty cycle on heat produced by dew heater

It is possible to connect several heating elements parallel. The used Darlington can dissipate up to 65 W of power, which roughly corresponds to a total current of 5 A @ 12 V. However this can only be achieved when the TIP 120 is provided with a considerably larger heat sink than I used !

Furthermore one has to realise that all the connected heating elements are regulated simultaneously in the same way. Each element will produce the same percentage of its maximum power for a given position of the variable resistor P1.
If, for instance, one wants to connect another dew heater with a maximum power of 1 W for the finder scope, the electrical resistance of this heating element has to be 144 Ω (1 W @ 12 V ≈ 1/12 A and 1/12 A @ 12 V ≈ 144 Ω).
Parallel to the heating element(s) a red LED is connected. In this way the proper action of the circuit can visually be observed. The 6.8 kΩ resistor in series with it, serves to limit the LED-current (brightness).

Printed circuit board

Figure 5 shows the component side view of the PCB (scale 1:1) that I made for the controller. The true dimensions of the PCB (as marked by the dotted lines) are 83.5 x 52 mm.

Fig. 5: PCB. (component side view)

The figure below shows how the various components are placed on the PCB:

Fig. 6: Component placement on PCB.

R1: 6k8 (1/4 W) IC1: 555
R2: 10 k (1/4 W) Tr1: TIP 120
R3: 47 k (1/4 W) D1, D2: 1N4148
P1: 1M linear (not on PCB) D3: LED, 3mm, red (not on PCB)
C1: 10 nF further: TO -220 heat sink
C2: 1.5 µF (tantalum) print connector

Figure 7 shows the completed controller circuit mounted in the housing and including all additional connections:

Fig. 7: Opened housing.

Final comments

The switching nature of this type of power controllers may produce noise spikes on the power line. This is caused by the inductive nature of the heating element(s). Although these spikes generally are very narrow, they may contain a lot of energy and are hard to filter out. Spikes can disturb digital systems like computers. Therefore it is advisable to use a separate power supply (battery or power adaptor) for the dew heaters, when operating computer-controlled telescopes like the Meade LX90 or LX200. In the Yahoo IMLTUG (LX90) user group there have been many reports of “strange” scope behaviour, when the telescope and a Kendrick dew heater controller were powered from the same 12 V battery.

The warming-up time of my corrector plate heating element appeared to be approximately 6 seconds. This has simply been determined by feeling with my hands. With this time constant in mind I have chosen for a cycle time in the same order of magnitude (1 sec). This to minimize the spiking phenomenon as described above. With a cycle time of 0.1 sec for instance, the heating element will be switched on and off 10 times as much!

Further it is very advisable to place a fuse in the power supply line of the dew heater controller. If the insulating mantle around the resistance wire should deteriorate in the course of time (due to mechanical damage or the elevated temperature of use) short-circuiting of the battery may occur. Because of the large currents involved, this certainly will result in damage to your battery, power controller or even to your scope. I deliberately did not place a fuse on the PCB, because an external fuse can be replaced much easier.

At this moment I have not been able yet, to check the influence of the spiking behaviour on the performance of my LX90. However, I usually power my scope with 2x5 1.2 V 3000 mAh NiMH rechargeable C-cells (instead of the usual 2x4 1.5 V batteries), so I probably will not encounter this problem.

H.H. Willems
January 2003

Copyright © 2003