The Jetstream

[reeaal]

Hi all,
I'm curious to know what the jetstream looks like through the scope. Some nights I've noticed an effect that resembles looking through water, actually seeing the ripples and eddies of the current, and am not sure if it's this jetstream thing or just my 10" mirror cooling down.
Is this what it most likely is?

[Robh]

Mark. This is what I've noticed.
Let's say you were looking at the planet Jupiter through your reflector.
As your mirror is cooling down, the convection currents will result in a flaring effect around the disk. The planet looks fuzzy and "hot".
Once the mirror is acclimatised, air turbulence will have the effect of pulsing the planet in and out of focus, clear one second then fuzzy the next. I think moving wave fronts or eddies caused by the jet streams 10kms high are the probable cause. Anyone else with ideas?

Regards, Rob

[PeterM]

Hi Mark,

The useful and animated sites in this link may be of assistance.
PeterM.
http://celestialwonders.com/articles/seeing/

[reeaal]

Thanks for your replies. It looks a lot like what I'm experiencing is a combination of the mirror cooling down and the seeing effects as shown on the Jupiter animation from that link you posted. Useful site, thanks.
Mark

[Dennis]

I’ve noticed 3 types of seeing when attempting to take hi-res images of the Moon and Jupiter:

Slow, long wave length undulations with a small amplitude which give me the best seeing. Typically I can see the 4 craterlets on Plato’s floor.
High frequency, short wavelength fuzziness which destroys all details.
Fast, long wavelength undulations with a large amplitude akin to looking through shallow water. The lava lamp effect. Images look soft and smeared with no detail.

Cheers

Dennis

[reeaal]

Hi Dennis,
Yes, the 3rd description in your list is the one I was wondering about. So is that caused by winds at very high altitude, or lower down in the atmosphere I wonder?

[Dennis]

Hi Mark

The last time I saw these were with Jupiter low, gusty SW winds and with the jet stream active according to the weather map. Normally, I wouldn’t venture outdoors for hi-res imaging under these conditions.

The ‘scope was the Mewlon 180 which has an open tube design (no corrector) and it had been outside for some 2 hours.

As Jupiter got above 35-40 degrees, the “viewing through shallow, rippling water” began to diminish and the more jittery high frequency seeing predominated.

Cheers

Dennis

[reeaal]

I see. So do you know of a good link for jetsream activity in Australia?
Thanks

[DavidU]

http://weather.wvec.com/auto/wvec/global/Region/AU/2xJetStream.html

[Robh]

That site can also be accessed from the IceInSpace left top menu ...
Resources-Weather-WVEC

Regards, Rob

[Quark]

Hi Mark,

This is the site that I use to check out the current position of the jetstream

http://www.weatherzone.com.au/synoptic.jsp?d=0

When looking at any of the planets at high magnification the ripples and disturbances that you see are a combination of atmospheric and disturbances in the boundary layer of air in contact with the face of your primary mirror.

The jetstream is generally blamed but often the majority of the turbulence is being generated in this boundary layer of air at your primary. Many scopes have fans that blow air onto the rear of the primary mirror to help reduce this effect. However the only way to achieve true thermal stability is with an active cooling system. This is a system that blows cooled air, (that is air below ambient temp) onto the rear of the primary. The absolute optimum temp range for your mirror is +/- 1/2 degree C above or below the ambient temp.

I achieve this with a Peltier cooler that is capable of cooling the air considerably below ambient. Another contributor to the disturbance that you see is air currents within your tube (I am talking about Newtonian reflectors).

I find that if the jet stream is not an issue and the seeing is good, that to get absolute optimum performance from my scope I must do the following.

1) I run my Peltier cooler for about an hour and drive the primary temp about 1 degree below ambient.

2) I turn the Peltier and all cooling fans off and wait about 20 minutes for the air currents that were generated by the cooling fans in and around the primary and within the tube to dissipate.

3)Now I have a window of about an hour, as the ambient continues to drop, where my primary will be in that sweet spot within +/- of 1/2 degree C.

I don't wish to make this sound to complex and put you off but this is what I strive for to produce the best data that my system is capable of.

There is no doubt that for observational work that even just a cooling fan that is circulating air at ambient temps will result in improved performance. It really just depends on how far you want to go.

On the atm forum of IIS you can see my Peltier cooling system under a thread titles (I think) A peltier cooler for a 16" Newt.

Hope this helps
Regards
Trevor

[reeaal]

Yes, those weather links are what I'm looking for. Trevor, I'll take a look at your cooling setup on ATM but probably just need a simple fan set-up for the primary.
Thanks.

[RobF]

Another experiment you might find useful Mark is to wind your focuser well in our out focus while looking at a bright star, so that you have a diffuse white disk visible through your eyepiece.

If you do this just after setting up, you'll probably find you can actually visualise the "boiling" of air around the boundary layer over your mirror that Trevor describes. I can usually see quite larger swirls when I first set up and its much cooler outside.

As things equalise, you should find later the disk if an even diffuse light, perhaps with a bit of brightening that matches the "twinkling" of the star through the atmosphere.

None of this gives you any info about the Jetstream of course.....;)