16 replies to this topic

[Jester]

Has anyone tried this?

I pointed off focus at a star last night to check the collimation, and I noticed that the view looked like a lava lamp, with all the thermal cells moving around in the OTA. It was quite interesting and shows that I didn't cool down my telescope enough!!

[csa/montana]

Without having a temperature prob on the Primary, I don't know how one would check the temp of their scope with any degree of success.

Carol

[llanitedave]

It would be an interesting experiment to have twin thermometers, one measuring the outside air and one measuring the mirror temperature -- and to see at what differential the thermal effects disappear.

But it's pretty academic, since the way of determining when the mirror has reached the proper temperature is to do just what Jester did: defocus a star and look for the effects on the image.

It's also a hint that a fan might be useful even on a 5.5" scope!

[csa/montana]

Dave, are you volunteering to do this when you build your new scope?

Carol

[Jester]

It would be quite an interesting experiment to do I think Dave, you could eventually calculate the exact cool down time depending on the outside temperature etc.

I was looking at getting a fan to fit to the bottom of the mirror cell, I think seeing what I did last night it would definately be advantageous!

[Asbytec]

Actually, I do that quite a bit during my observing sessions. I check collimation more than I should or need to, I guess...but I am a stickler for good performance. I often notice the out of focus star images bubbling a lot. If the bubbling is uniform, I mean...throughout the out of focus image...then I can gauge seeing (assuming the scope is cooled down.)

If I have, and I have had, fairly steady out of focus star images, but get one narrow column of rising turbulence, then I know my scope is still cooling down. I got that a lot with both of my 10" SCTs...rising heat currents I believe coming off the baffle. They were very apparent in the out of focus image and actually lasted longer than I felt they should. My C11 cools down rather quickly and those rising tube currents are not readily visible. I see them early in the session, then they disappear.

I have to assume the primary mirror is at ambient temp in an hour or so. It's easy to tell when the corrector is at or below ambient....just check for dew. LOL

But, I am not sure how to tell, when the scope is still cooling down, if a bubbling out of focus image is due to seeing or to a warm, misshapen primary. Heat rises, and that should be visible in the out of focus image. But a warping primary may not manifest itself uniquely.

Anyway, it probably doesn't matter at this point...nothing you can do about it really, unless you have some dry ice and a thermometer on hand. Just give it time to settle down before observing. If it's still bubbling over time, it's more and more likely due to seeing.

[Asbytec]

A fan, for a newt? Wouldn't that introduce tube currents, pulling air through the tube? It seems you might introduce some vibrations, too. Or you'd only use it to get to ambient and turn it off? I haven't thought about that since my last newt, years ago...

[Jester]

A fan, for a newt? Wouldn't that introduce tube currents, pulling air through the tube? It seems you might introduce some vibrations, too. Or you'd only use it to get to ambient and turn it off? I haven't thought about that since my last newt, years ago...

Yeah I would only use it to get ot ambient temperature, and then I would switch it off.

[Jon Isaacs]

A fan, for a newt? Wouldn't that introduce tube currents, pulling air through the tube? It seems you might introduce some vibrations, too. Or you'd only use it to get to ambient and turn it off? I haven't thought about that since my last newt, years ago...

A properly sized fan that is mounted so it causes no vibration is a wonderful addition to most any Newtonian. Bryan Greer of Proto-Star has really been the leader in getting the amateur astronomy community to recognize the importance of actively cooling a Newtonian. Bryan wrote some articles for Sky and Telescope 5 or 6 years ago that are definitely worth a read. He has done extensive measurement of mirror temperatures vs flow etc.

Beside cooling the mirror and scope, a fan can breakup the "boundry layer", a warm layer that sits on top of the mirror and causes problems. Bryan also has a good section on measuring the temperature of the mirror with thermocouples.

Bryan Greer: Cooling a Newtonian with a Fan

In my experience, a fan is an important step in getting the most out of a Newtonian. The fan needs to be left on at all times because during the night, the temperature normally drops and it is important that the telescope temperature trackt the ambient temperature. A fan setup with speed control is nice in this regard.

The ability to actively cool a Newtonian while it is in use is a real advantage over the SCTs and Maks, it allows the scope to reach thermal equilibrium much sooner as well as remain in equilibrium through out the evening.

Bottom line: Attention to thermal equilibrium is one of the keys to getting the good views from a Newtonian.

jon

[llanitedave]

Dave, are you volunteering to do this when you build your new scope?

Carol

The whole thing will be an experiment, Carol!

But in this case, I think I'll add a couple of fans, and then determine when the mirror is ready the old-fashioned way.

[llanitedave]

A fan, for a newt? Wouldn't that introduce tube currents, pulling air through the tube? It seems you might introduce some vibrations, too. Or you'd only use it to get to ambient and turn it off? I haven't thought about that since my last newt, years ago...

I use mine most of the night. And that's in a truss dob.

Two reasons:

1: Ambient temperature at night is rarely stable. Most nights it falls continuously, but not steadily. Sometimes there are even warming spells at random. Your mirror is constantly playing catch-up. Since the air temperature can change faster than the mirror temperature, it is rare that they will ever be the same. The fan helps minimize any lag.

2. "Tube currents" is a common term used to refer to thermal instability, but it's not technically accurate. Moving air is not the culprit. Density fluctuations (which is what thermal turbulence creates) are what causes bad seeing in the tube. What a properly mounted fan does, in addition to speeding heat transfer on the mirror, is break up individual cells of differential temperature, and "encourage" them to mix. Once the cells are gone, images stabilize nicely. So even if the mirror has NOT yet equilibrated, the fan improves seeing through the mixing of thermal cells.

Vibrations need not be a problem if the fan is properly mounted. I use stretched rubber bands to suspend my fan below the mirror cell. There are no vibrations at all, even at over 300x.

[Asbytec]

Wow, some other interesting articles linked on that page, too.

[goldenboy]

Regarding checking temps on the primary, is it possible to utilize an infrared temperature gun? They're quite inexpensive, accurate, and certain models have a tight infrared cone allowing you to take temperatures from just inside the tube of even a relatively large reflector.

Just a thought.

[goldenboy]

Here's a link to some inexpensive models.

http://www.tempgun.com/main.html

I've personally used the PE-2 model in monitoring temperatures on reptiles. They are accurate and give readings to tenths of degrees.

[Asbytec]

Apparently running a fan is a good idea, per the URL in an above post. Apparently having the right flow rate pulls the boundary layer off the mirror and keeps it off. Having a smooth air flow is better than a turbulent one. I imagine this aids with keeping the primary at ambient temperature, too. Seems the fan needs to run continuously, though.

Ah, well..I have a SCT now. Keeping the corrector warmer than ambient temperature is my problem....dew. Wonder if a fan helps with dew problems.

[llanitedave]

I've used those temperature guns quite a bit, mostly for hot rocks. If you aim one upwards at the night sky and then at the ground around you the results will be interesting. I suspect if you read the reflective surface of the mirror it may give you the night sky temperature. The mirror back might be more accurate.

[BobW]

Without having a temperature prob on the Primary, I don't know how one would check the temp of their scope with any degree of success.

Carol

Maybe one of those stick on temp strips that change colors for every 2 degs. Not real accurate but could give some idea of mirror temp if stuck to the back of the mirror. I have one stuck on my fish tank and works well within 2 degree readings