61 replies to this topic

[Derek W]

It's interesting.... this season, I've been getting my best planetary views ever with my Orion XT10.

For the longest time, I just didn't think I'd be able to get the detailed views I am getting now.

The difference? I leave my scope out before observing.... a long time. Last night I set my scope out about 6 hours before viewing Jupiter. The seeing was mediocre at best. I had my best views ever of the planet, however. I've viewed many times previously, but have never let the scope cool down this long. (I first started doing this with Saturn, and started noticing how clear my views got, having never seen jupiter in such detail, I did the same thing, and got the same results)

I've come to the conclusion that the 45minutes to an hour suggested cool down time for this scope ISN'T going to be as good as 3 to 4 hours... or longer. Bascially, the longer, the better. (although I'm sure we would hit a point where it would be "long enough")

Your views? (FWIW, if you guys want to move this to the reflector forum, that's fine. I posted here because it specifically had to do with our planetary views)

[Daniel Mounsey]

This is very true. Many people think and hour is fine, but two to three hours is better, granted a fan can speed things up. Good post.

[Blues]

I've gotten in the habit of late of taking the scope out and setting up just before sundown. This gives a reasonably long time before I go back out to observe.

Seems to be working so far though I have been giving some thought to that little fan that Orion sells for the XT8 (which is pre-drilled and tapped for mounting).

[David Knisely]

It's interesting.... this season, I've been getting my best planetary views ever with my Orion XT10.

For the longest time, I just didn't think I'd be able to get the detailed views I am getting now.

The difference? I leave my scope out before observing.... a long time. Last night I set my scope out about 6 hours before viewing Jupiter. The seeing was mediocre at best. I had my best views ever of the planet, however. I've viewed many times previously, but have never let the scope cool down this long. (I first started doing this with Saturn, and started noticing how clear my views got, having never seen jupiter in such detail, I did the same thing, and got the same results)

I've come to the conclusion that the 45minutes to an hour suggested cool down time for this scope ISN'T going to be as good as 3 to 4 hours... or longer. Bascially, the longer, the better. (although I'm sure we would hit a point where it would be "long enough")

Your views? (FWIW, if you guys want to move this to the reflector forum, that's fine. I posted here because it specifically had to do with our planetary views)

Well 6 hours is definitely a little on the long side, as after about two to three hours, unless there was a huge difference in temperature (more than 40 degrees F), or there have been large local temperature variations, a Newtonian of modest size should be pretty much cooled down. My 10 inch f/5.6 is pretty much cooled down after an hour to an hour and a half when going from inside (65F) to cold winter temperatures in the 20's or 30's (F). It sounds like you just encountered a period of improved seeing towards the end of the 6 hour period. With a simple fan blowing on the back of the primary mirror, the cooldown time can be significantly shortened (to as little as 30 minutes). Clear skies to you.

[Derek W]

2 or 3 hours may be the cutoff point... I'm not sure.(would certainly hope it doesn't take any longer) Likely before that in my views of Jupiter I had never let it cool down that long. It was proba.bly about a 20 degree drop that night.

I should really do some testing one night...

I didn't view Jupiter until six hours after I placed the scope out. (so I had no comparisons to earlier in the evening)

Jupiter was swimming at 240, and still not too sharp at 180, yet I was still seeing more banding and detail than I had to date. (again, in my previous nice nights of viewing, I had never let the scope cool for that amount of time...and the planet always looked poor... and I used to have the same problem with saturn)

[Bird]

Actually, the myth that a reflector will come to equilibrium in 45 minutes or 2 hours, or even 4 or 5 hours, can be easily popped.

It all depends on what the air temperature is doing. I got curious about this last year and built an accurate (digital) temperature sensing unit that connects to my mirror and tube and has an air sensor as well. This lets me see, to an accuracy of 0.0625 degree C, what the temperature of these things actually are.

Now, where I live (Canberra, Australia) we get temperature swings of about 15 degrees C between day and night. Typical summer day can be 30 degrees C, nightime minimum about 15C. The temperature doesn't just switch from one to the other, it *slowly* declines starting at sunset and reaches the minimum at about 3am. Houses, roads etc slowly cool down, contributing to a lot of thermal inertia.

So what is the effect of this on a scope? I have a 10" newtonian, mirror is 35mm thick, and I can tell you that no amount of passive cooling will get the scope in equilibrium with the air until about 4am - even if the scope was setup at sundown, and had any number of fans blowing on it. The air temp is dropping at the rate of about 2 degrees per hour, much too fast for a 35mm thick pyrex mirror to follow. If you have a very thin mirror, or a smaller mirror (like 6") then you might be in luck. Maybe.

This explained why I was having so much trouble getting a good high-res view of the planets. Even is I set up at sundown (8pm) and waited for 6 hours until 2am, I was still seeing heat spirals off the mirror, severe tube currents etc etc.

I have graphs if anyone needs convincing :-)

The solution was to build an active cooler. Two peltier modules and some aluminium plate and perspex is all it took, now I have a "refrigeration" unit on the end of the tube that can cool the mirror as fast as the ambient air is cooling - roughly 2C per hour.

The result? Well, have a look in the Solar System Imaging forum for some of my images of Jupiter if you want to see what can be done with a modest scope that's actually in thermal equilibrium. These images were *impossible* for me to produce only 6 months ago, before I built this cooler.

All it takes is for the mirror to be 1 degree C warmer than the air and you can forget high res detailed views of planets. 0.5C seems to be the most that my mirror can tolerate before destroying the image.

Having been through this experience, I'm now convinced that misunderstanding how critical thermal equilibrium can be is the #1 cause of poor newtonian performance.

regards, Bird

[wes]

Hi,

I totally agree with Birds comments on a Newt reaching equilibrium, for over a year now I have had a very accurate commercial grade digital thermometer with a special surface sensor attached to the back of my primary and the air temp sensor in the mirror box.
I keep my scope in an insulated utility shed next to where I observe at and on days I image or observe I will open the shed at about 3pm and start an 18" high speed fan blowing to the back of the primary and at dusk I move the scope out side and keep the fan going until about 2 to 3am (when I do most of my imaging) and at that time the primary will usually be cooled to less than one degree of the air temp, every time that I have checked it at about 10 to 11pm there is always a two to four degree difference.
When imaging planets at very high Mags like Bird and I do the primary has to be less than one degree of out side air temp and that dose not happen as soon as most people think unless you are fortunate enough to live in an area where the day and night time temperatures do not vary more than a few degrees.

Thanks,

Wes Higgins

[Daniel Mounsey]

Good post Bird, I agree. When it gets down to it, there's more involved then most realize.

[David Knisely]

Bird, can you provide some details of the cooling setup you are using? I would like to experiment with Peltier cooling for My T-Scanner (as well as for the 10 inch), so this sounds interesting. Clear skies to you.

[Bird]

David, here's a writeup that I did at the time (March last year)

http://www.acquerra....ronomy/cooling/

It's a bit out of date, and I've had almost a years worth of practice with the unit now, but it's still basically correct.

Something I did not know when I wrote that article, that I have learned since, is that the tube material (and paint, if its painted) also make a significant contribution to temperature woes. I've now switched to an unpainted aluminium tube, and my tube temp now tracks air temperature properly.

I expect that many tubes - especially those painted white with a pain that includes Titanium Dioxide (the normal "white" paint pigment) will still have problems with the tube "supercooling" several degrees below ambient air temp.

A "white" painted tube will probably run 2 or 3 degrees cooler than the surrounding air due to radiative heat loss in parts of the spectrum that's transparent to air. The tube "sees" the 3K sky temperature and tries to come into equilibrium with it.

Aluminium doesn't have this property, and it radiates in the IR at frequencies that are opaque to air.

Again, I have graphs :-)

Of course this only matters if you're using a tube-ified newt, like me. Those with truss of other open designs need not worry too much.

regards, Bird

[Daniel Mounsey]

Bird,
How would you rate the seeing at your location?

[Bird]

I used to think the seeing was totally *bleep* around here. I reached that conclusion after spending countless hours in the backyard over the last 6 years or so and never once getting a good high-power view of Jupiter, Saturn or Mars.

Now, I think that the seeing around here is spectacular. Taking control of the thermal gremlins that plagued my scope for years has turned it all around.

I'm of the opinion that fixing the thermal issues in a scope is much more important than accurate collimation. Poor collimation is easy to spot and fix *if* you're scope is in equilibrium. If its not, then the thermals will destroy the image before you have any chance to evaluate the optics.

Yet, surprisingly, a search on the net finds many good articles on collimation, down to a fine degree of science, but almost nothing about thermal equilibrium apart from the oft-repeated "leave the scope outside for an hour or so before use".

My theory is that the imbalance in articles is due to one factor that can be easily measured with inexpensive instruments (collimation) vs the other factor that can't be measured without an accurate temp sensor box (thermals). It's simply easier to write an article on collimation and expect that most people can put it to use with simple equipment.

Apologies for posting a CCD image in this forum, but I can't think of a better way to indicate what sort of improvement was possible.

regards, Bird

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[Brian Carter]

Wow that is a beautiful picture. I have been reading this thread just as I installed a fan behind my mirror (last night). of course, it is raining now. But this whole winter I have left the scope out for 3 hours and done a star test to see ubiquitous star currents. I remember one night in december the temperature in the mountains was about 10 degrees; there were still bad currents at midnight.

I can't wait to see how well it works. if this doesn't work i'll think about a peltier.

[Bird]

Good luck Brian. Your scope looks similar to mine (I have a 10" f/6 newt) so they might be similar thermally as well.

What sort of temperature swings do you get, day to night? If it's not too much (say 10C, dunno what that is in F) then a fan should be ok. Otherwise, expect to setup at sunset, turn on the fan and wait till 2 or 3 am for good seeing.

btw - this isn't just for planets. I went to a local star party a few weeks back, didnt take my scope 'cause lugging the G-11 mount is a pain. People were using things like the 2 faint stars in the trapezium for evaluating the seeing, and their scopes. They were tough to spot in most of the scope (8" to 10" newts). There was a 20" monster that showed them ok, but a bit fuzzy. Well, I couldnt wait to try this test on my scope, so the next night I set up in the backyard, turned on the peltier and gave it a couple of hours to cool down.

The 2 faint stars in the trapezium were a piece of cake, and I could see a couple of even fainter ones in the area that were totally invisible in the other scopes.

The other candidate - the Eta Carinae homunculus nebula - showed more detail in my 10" scope than I saw in the 20" the night before. Ok, it was dimmer, but the extra detail made up for it.

Now, I've spent the last 6 years collimating, collimating, and re-collimating my scope in the hope that my bad images were due to misalignment of optics, so now that the thermals are under control I find that collimation is a piece of cake. I've had 6 years practice at it :-)

So, on a very non-scientific sample size of 1 I can say that my 10" gives a better experience than a 20" if you're looking critically at fine detail. It was clear that the owner of the 20" treated it as a light bucket, and didnt care too much about collimation and not at all about cooling.

cheers, Bird

[David Knisely]

Thanks Bird. I guess I will have to look into a big power supply for the Peltier coolers. However, as for the tube, I find that insultating the tube on its interior tends to kill off some of the problems that a painted aluminum one might introduce. I obtained some stiff paper corrigations like those used in packing materials or making corrigated cardboard boxes and darkened them with a decent flat black paint. I then lined the interior of the telescope tube with these corrigated paper segments. The corrigations tend to insulate the tube from the rest of the scope, with the corrigations being open to the interior of the tube. Tube currents that brush along these corrigations in the tube will generate some counter eddy currents, resulting in the break up of some of the larger currents. That, plus running a fan prior to observation, has tended to minimize tube currents in my Newtonians (although with my new SCT, I just have to run the Cat Cooler and hope for the best). I have used this corrigation technique in a number of painted aluminum tubes, and it seems to be at least minimize the problem enough so that painting the tube exterior does not become a significant factor (i.e. you can paint the tube whatever color your heart desires). Clear skies to you.

[Derek W]

I'm pleased at how this thread has developed. Hopefully people are picking up some good hints from all the informative posts... I know I have been.

Anyhow, I'm wondering how important it is to have fans that affect the air surface of the mirror itself...

Daniel (Mounsey) showed this sort of fan action in a post on the reflector forum... (the planetary starmaster thread)

Would the ultimate be to have fans behind the mirror, and pulling in air across the mirror as well?

[jandrew]

I just found this site:

http://home.t-online...or/mirrmain.htm

which gives some numbers and formulas regarding the thermal energy
of a mirror and how fast it radiates that energy. I redid the calculations
without early rounding and using the full mirror surface area (including
side) and got a value of 98 minutes for tau for the 15 inch mirror (tau
is defined as the time required to decrease the delta temperature
difference between the mirror and ambient by 63%).

The tau numbers I got for the 15", 12", 8", and adding a 4.5" were:

15"   => 98 minutes  (using original data: 108 minutes)
12"   => 63 minutes  (using original data:  74 minutes)
 8"   => 38 minutes  (using original data:  41 minutes)
 4.5" => 29 minutes

Maybe. I put out my 4.5" on Friday just before 7pm and went out to view a
little after 9pm (say 2.5 hours). Now, this scope has a closed mirror end,
so only one surface is freely radiating, so let's double the tau to one
hour. The temperature difference was about 37C (20C indoors, -17C outside).
After 2.5 tau, the mirror would have been in the neighborhood of 3C to 4C
over ambient (give or take). I thought the views were pretty good, and
certainly no obvious macro tube currents and Saturn looked great at 90x
with cassini easy (and sharp) nearly right around, some color and one easy
band on the planet. At 150x the sharpness and color were diminished and the
planetary band much less distinct.

I was out for 90 minutes and things did improve during that time, and Saturn
took 150x much better near the end of this session. I had assumed this
little scope *must* have been fully cooled by the time I started viewing
and that any further improvement was all in the sky. Now I am not so sure.

Obviously this is just one set of numbers and one observational anecdote,
but taken along with Bird's reports and I think some temperature probes may
just need to become part of standard equipment.

andrew

[sixela]

The only thing I find strange is that people are still talking about larger cooldown times for *large* mirrors, meaning only mirrors for scopes of large aperture.

But a large diameter mirror has a larger surface, so it also exchanges more heat with the environment because of it.

So the real problem is having a *thick* mirror - more volume, but not more surface to exchange heat with the environment...

Of course larger mirrors tend to be thicker, but I think it's more important to get a mirror that's as thin as possible than to get fans (which also means getting a very good mirror cell), yet I rarely see that concern voiced often here -- not sexy enough, I guess?

You can add fans later, but putting your mirror on a diet is a lot harder...

[Bird]

The only caveat to a thin mirror is that it might not hold its shape as well as a thick mirror, or need extra care in the mounting. By "thick" I mean a 6:1 diameter to thickness ratio. A thin mirror might be 8:1 or even 10:1.

Bird

[Brian Carter]

I think it also may have a lot to do with tube construction. Mine is sonotube, and fairly long, it certainly doesn't allow much cool air in. Also how exposed the mirror is. When I went from a closed particle board mirror cell to an open cell, cooldown time dropped quickly, even though I had to close up the back to block the light (with velvet). It blocked airflow, but temperature change was faster. Now I have a fan, so we will see.

[David Knisely]

I'm pleased at how this thread has developed. Hopefully people are picking up some good hints from all the informative posts... I know I have been.

Anyhow, I'm wondering how important it is to have fans that affect the air surface of the mirror itself...

Daniel (Mounsey) showed this sort of fan action in a post on the reflector forum... (the planetary starmaster thread)

Would the ultimate be to have fans behind the mirror, and pulling in air across the mirror as well?

I like to have the fan affect air both in front of and behind the mirror. I have mine blow against one side of the back of the mirror, and with my large cell, it allows air flow past the side of the mirror and then to enter the area around the front of the mirror. This helps a bit to get rid of the minor "boundary layer" of warmer air which often sits across the front of the mirror. Some people just have the fan blow air from a side vent directly across the front of the mirror, but while this deals with the boundary layer issue, it doesn't help the rest of the mirror cool off as fast. Air should be moving past all surfaces at least to some degree if at all possible. I like at least 1/4 to 1/2 inch of clearance between the sides of the mirror and the sides of the tube. I also like mirror cells which allow as much air to get to the mirror as possible. Clear skies to you.

[sixela]

The only caveat to a thin mirror is that it might not hold its shape as well as a thick mirror, or need extra care in the mounting. By "thick" I mean a 6:1 diameter to thickness ratio. A thin mirror might be 8:1 or even 10:1.

Or 13.5:1. I'm starting to see why those scopes with a 30mm thick 406mm mirror from Norman Oldham have a reputation for fast cool down...

And yes, I'm pretty sure you do need a good mirror cell for these.

[markf]

Say,

What about cool "up" times? I'd imaging all the same issues/rules/fixes apply, but here in Texas, in the summer time, my scope will be nice and comfy at 72f/22c in the apartment until I take it outside in 98f/37c heat.

Do I add a heater to my scope at this point?

Mark

[sixela]

No, just paint it black and take it out before sunset .

There's an even more radical measure, but that may be dangerous for the optics and things aligned with the focuser axis (including any cap -- plastic melts).

[markf]

I think i know what that radical measure is; I'll pass
Mark