Local (Dome) seeing

[Shiraz (Ray)]

Anyone got any good info on dome seeing, which I take to be due to highly localised thermal turbulence?

I seem to have been getting consistently better seeing results in an open enclosure than in previous partially sealed environments and am guessing that this could be due to better thermal conditions in the open. Found a good reference to dome (and mirror) seeing that indicates that dome seeing may increase by as much as 0.28 arcsec for every 1C difference between dome air temp and outside air temp (large telescopes). This is huge if it applies to our little systems. It might also suggest that dew heaters are not the smartest idea.

Any thoughts? Regards Ray

ref: http://www.eso.org/gen-fac/pubs/astc...is/node51.html

[clive milne]

After reading that paper I came to the conclusion that the main (local) source of degradation to telescope performance was the boundary layer above the primary. A dome will to some extent prevent natural air flow from removing the boundary layer.

Excellent link btw.

[Bassnut (Fred)]

Yes well, as per another thread on large scopes in small domes, thats not a good combination. Fatally critical in fact. I suspect fans in the obs to equalise temp quickly before imaging would be required as a minimum. As an aside, I have never used my dew heaters at itelescope OBS, they close if dew is present, they think its just not worth the bother opening.

[Shiraz (Ray)]

Originally Posted by clive milne

After reading that paper I came to the conclusion that the main (local) source of degradation to telescope performance was the boundary layer above the primary. A dome will to some extent prevent natural air flow from removing the boundary layer.

Excellent link btw.

Hi Clive. Yes, the boundary layer does seem to be very important, but only a few scopes (eg the CDKs) appear to incorporate front airflow over the mirror to break it up. Will have to get out the hole saw and try some front ventilation on my planetary scope. I think that it needs to be fairly rapid airflow to do the job effectively.

Originally Posted by Bassnut

Yes well, as per another thread on large scopes in small domes, thats not a good combination. Fatally critical in fact. I suspect fans in the obs to equalise temp quickly before imaging would be required as a minimum. As an aside, I have never used my dew heaters at itelescope OBS, they close if dew is present, they think its just not worth the bother opening.

Thanks Fred. Your comment on an earlier thread precipitated this one. I had noticed better seeing lately, but had not put two and two together re possible enclosure effects. I am planning an ROR with cold aircon to soak the mount and floor before imaging - do think that would be worth the effort? At present I rely on radiative cooling with the floor and mount in shade, but open to the sky for a couple of hours prior to imaging. Not too sure how it will go in warm weather, but ambient shade is pretty cold this time of the year.

[Paul Haese]

If I did not use a dew heater of both scopes I would not get any imaging done. My site has very high dew. Dew forms nearly every night. If you don't have dew control you dew up very quickly.

Seeing in the dome seems ok to me. The shutter opening is quite wide so that probably alleviates the thermals to some extent. The shape of the scope dome being a ball on the ground with no walls also probably helps control dome currents. The images I have produced with the RC12 in the dome seem sharp enough to me and not really affected by dome thermals. I could be wrong I suppose.

[clive milne]

Originally Posted by Shiraz

I think that it needs to be fairly rapid airflow to do the job effectively.

Hi Ray,
The figures I have read in separate professional publications range between 2.5 to 3m/s .... and curiously, it has been stated that disruption of the boundary layer is best achieved when the airflow is directed horizontally across the face of the optic.

[Shiraz (Ray)]

Originally Posted by Paul Haese

If I did not use a dew heater of both scopes I would not get any imaging done. My site has very high dew. Dew forms nearly every night. If you don't have dew control you dew up very quickly.

Seeing in the dome seems ok to me. The shutter opening is quite wide so that probably alleviates the thermals to some extent. The shape of the scope dome being a ball on the ground with no walls also probably helps control dome currents. The images I have produced with the RC12 in the dome seem sharp enough to me and not really affected by dome thermals. I could be wrong I suppose.

thanks Paul. there is no arguing - you are getting very good results. I have no feel for the effects of domes (or other enclosures), but it seemed to me that it might be possible to improve resolution by a little bit if one could sort out what was damaging it in the first place. I also have trouble with dew, but have generally been able to image using only a low power heater in the airstream on the back of the mirror (about 0.4C increase in air temperature). It has always worried me that I can spend hours getting my planetary scope mirror down to ambient to maximise resolution - but for DSO it is necessary to heat the mirror up!

We can always be wrong , but these forums can provide many well informed opinions to challenge ideas, so appreciate your feedback.

Originally Posted by clive milne

Hi Ray,
The figures I have read in separate professional publications range between 2.5 to 3m/s .... and curiously, it has been stated that disruption of the boundary layer is best achieved when the airflow is directed horizontally across the face of the optic.

Thanks Clive. will give it a try.

regards Ray