Fitting thermometers to ensure Thermal Equilibrium

If any of you have followed discussion on Yahoo C14EdgeHD group, or some of my older posts, you’ll have noticed that getting a C14 in its EdgeHD variety thermally balanced is extremelly difficult. While thermal equilibrium is not all that important while doing deep sky imaging, it is absolutely crucial for imaging planets. The reason is that for planetary imaging, you need to get as near to the theorical resolution of the scope as “seeing” of the atmosphere will let you. While the primary mirror has not reached the same temperature as the rest of the telescope, there will be convection currents of air either warming up or cooling down on the primary, and that will destroy maximum resolution to some extent.

Because the EdgeHD  telescope serie has that lens inserted into the baffle tube to correct the field of curvature of the image plane, the tube is effectively sealed and there is no room for circulating external air through the baffle tube as can be the case with “standard” C14, either forced through a fan, or natural, through convection. The vents at the back of the tube do not seem to have any cooling effect on the primary mirror whatsoever… a telltale sign of internal tube air convection currents is slowly moving areas of brightess or shadows while looking at a unfocused star. In any case, until a few days ago, I was not able to get the scope in thermal equilibrium, due to the fact that the days were alot warmer than the nights and that the primary mirror would remain “hot” for an extremelly long time as to the extent of making the scope next to useless for planetary work. These days, because the difference between daylight and nightime temperature is less, I found that the scope will reach equilibrium within about 4 hours (just in time for Jupiter!).  Now, beeing able (with a bit of experience) to tell whether the tube is thermally stable by looking through the eyepiece is OK, but it is still subjective. What this post is about is to show a non destructive way to rig two 10 dollars thermometers to establish whether or not the primary mirror has reached the same temperature as the tube…

thermor

You can see on the picture above that the thermometers used are cheap digital ones with a long sensor wire. The first thermometer is located by the corrector plate; and the sensor is taped to the aluminium tube using think silicon tape. The second thermometer measures the primary mirror temperature. there is no need to drill any holes as the wire is fed through on of the “cooling ports” and its sensor is taped to the back of the primary using the same tape as for the other sensor.

Well, as far as I can tell it works! What I did is monitor tube currents the usual way through the eyepiece (looking at a unfocused star) after installing the thermometers, and sure enough, until both sensors read  0.2 Deg C difference, some convection currents were visible. Once the primary reached ambiant temperature, I used the webcam on Jupiter and the result was one of my best picture so far…

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4 Responses to “Fitting thermometers to ensure Thermal Equilibrium”

  • Ken Lewis says:

    TEMP-est TEMPerature Equilibration System for Telescopes – For CPC Deluxe HD
    Product Number: DSP-TEMPEST-CPCHD

    Manufacturer: Deep Space Products
    For 200.00$

    RT8100 Fridge/Freezer Alarm Thermometer Dual Channel programable by ThermoWorks for 22.00$
    I found this combination is very effective.

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    • admin says:

      thx ken,
      that system will effectively allow equilibrium but the problem is that you are bound to lose collimation by having to emove the secondary mirror to install the main fan. There is no other choice unfortunately.

      Serge

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  • Ken Lewis says:

    That’s odd, in the last 6 months, I’ve never had to recollimate my secondary mirror on my c-11edge hd, I have used it often replacing it with my Hyperstar lens, and fan, guess I’ve been lucky!

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    • admin says:

      Hi Ken,
      Well, it all depends on how accurate you want your collimation to be…
      For a “ball park” collimation, as for DS imaging, I agree removing and putting back the secondary won’t make an once of difference. But, for an extremelly accurate one, as for planetary imaging (1/20th of a screw turn accuracy), there is bound to be some slight differences when installing back the secondary. In fact, because of flexions in the primary mirror assembly (and there are little of that since C14 EdgeHD mechanics are very good), collimation for planetary imaging should be performed with the scope pointing at the nearest exploitable star of the planet to be observed… Every time such observation is taking place…
      It’s a matter of judging how good a collimation you want, really.

      Serge

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