Temperature sensing for Cats...

[Robert_T]

Hi All, I've been watching and following the temperature sensing discussions for a bit now and my interest is peaked as to how something aong these lines could be usefully applied to sealed tube (both ends or back end) Cats to help determine the best times for planetary imaging.

It occurred to me (I might be a bit slow on the uptake here) looking at Bird's temp plots from Ballarat that with a little trial and error that simple measurement and logging of the tube surface and air temps, but more importantly the trajectory (gradient - whatever's the right word here) of temperature, might by itself provide a big step up in predictability of good imaging conditions without actually getting inside the tube with sensors etc. The experimentation bit would be in taking lots of quick video segements in relation to the outside temp logs and comparing between them for quality etc (subjective yes, but then image quality is) - the aim being to try and determine for a specific Cat scope the length of time it needs to be outside with temp falling at a given rate to approach equilibrium and good imaging conditions. At the very least it should be enough to show when NOT to image.

Be good to get some discussion going on this one - whether this approach might be of some use, alternatives and so on, so we can get the best out of our Cats.

Cheers,

[bird (Anthony Wesley)]

It sounds interesting Robert - you might want to collect a few nights worth of temperature data for the Cat to see how it behaves, and then you could use that to make an educated guess about the scope.

If you recorded the seeing conditions along the way you ought to be able to produce a chart that will predict when the scope is ready to go, given the measured ambient temp gradient.

regards, Bird

[Robert_T]

thanks Bird - any other takers out there?

[rumples riot]

Hmmm, maybe compare that with a scope that is cooled with an SCT cooler. The trouble is where to put the sensors. You need to open up the back of the scope and attach the sensor to the mirror and another inside the tube. You would need to drill a hole in the rear or side of the scope for the sensor' wire to pass through and then leave enough play in the wire to travel, however not too much so that it gets tangled in the focus workings. Don't know about you but I don't want to go drilling holes in my scope until my warrantee runs out in two years.

Its a good idea, but fraught with lots of dangers in relation to SCT's especially when the mirror is moving.

[Robert_T]

Hi Paul, what I was proposing was to do temp measurements only outside the tube (no-way I want to pierce my new babies either). Idea would be to plot/log outside tube temperature on several nights taking periodic avis and making and logging visual assessment of "seeing" through the scope at the same time. Then analysing this data to determine for how long a specific SCT needs to be left out with a given temperature gradient to obtain good imaging conditions - eg by looking at the profile of outside tube temp immediately before periods of good and bad imaging. It wouldn't be as precise as Birds stuff on newtonians, but after a few nights experimentation it should be possible at least to have a reasonable idea of whether imaging conditions are good or poor just by looking at the outside tube temperature profile.


cheers,

[Hitchhiker]

Robert_T, I think it would be very interesting to do the experiment and see what kind of useful info you could come up with. If you can determine the time it takes for the tube to reach thermal equilibrium under different conditions you might be able to come up with some rules of thumb for when the whole optical system is at equilibrium.

For example, your imaging experiments might tell you that (for instance) 30 minutes after the tube has reached equilibrium the rest of the optical components will be at equilibrium and planetary imaging could be attempted for a particular set of circumstances (absolute temperature, temperature gradient etc.) I have a dual temperature device that I use on my SCT - one sensor measures ambient and the other sensor is under my dew zapper band. I use this to set the dew zapper power to keep a couple of degrees above ambient. I will use these sensors to look at the cooldown profile next time I observe and post the results.

Adam.

[Robert_T]

Thanks Adam - that's pretty much what I had in mind. Good idea though about putting one under the dew-zapper as that would ensure you didn't overdo the heat more than needed.

Probably should be a different thread, but I was curious as to whether a dew zapper could be used without any image degradation (obviously it stops catastrophic dew image degradation, just interested whether heat causes any problems)?

cheers,

[Hitchhiker]

I've always wondered the same thing. It seems to me that if you're pumping heat into your corrector plate you're driving the optical components away from equalising with the ambient temperature.

Perhaps that is something else that we could test - try and image under the same conditions but with the dew heater on and with it off. I don't have enough experience at imaging yet to have formed an opinion on the effect of the dew heater - in fact, so far, when I have done imaging of the moon or planets I have been lucky and it has not been dewy. I would be interested to know what the more experienced SCT owners think of the effect of a dew heater on seeing or imaging fine planetary detail.

[Robert_T]

Hi Adam - I split off the question of dew zappers and planetary imaging into a new thread - I'd like to hear from some of the more experinced dew zappery and imagery types too!


cheers,