Wind speed limit based on your experience

[Hector]

Hi all,

 

   I'd like to make a query on your experience regarding wind speed limits of your system.

 

    I'm actually considering the DDM160 (with the pier) with a 40-50cm open truss scope, which seems a good option to "fight" against strong winds. The enclosure that will host the whole thing is not a dome but a kind of building with open roof, so the exposure to wind is considerable, and so the wind intensity itself; this information is critical for me.

 

   Given that it seems there aren't many DDM160 registered users with enough statistics yet to give a number (if I am wrong, please, let me know), I ask for your impressions with the DDM85 to have a lower limit.

 

   I'm really interested in wind speed limit values, if it is possible to you to give them.

 

   I really appreciate your comments.

 

 Best regards.

 

   Héctor.

[wemartin]

Hector, I have a DDM85 with a ASA16N astrograph. It is mounted on a Pier Tech raising pier in a roll-off roof observatory. When the pier is up the telescope is fully exposed to the prevailing wind. The principal issue is how gusty the wind is rather than strictly wind speed. At my semi-urban site we can get gusts of 2-3x the average wind speed. I find that winds of less than about 6kph have little effect on star fwhm in a 120sec exposure. With gusts of 10kph the stars are trailed in the wind direction to about 2x the fwhm. Above 10kph constant speed I don't even try imaging but webcam 'lucky imaging' is possible if you are patient. I have set the limits in the weather sensor to 5kph and have had good success in automated photometric observations(so far...).

 

Regards

 

Bill

[Hector]

Thanks Bill,

 

  this is useful for me.

 

  I forgot to mention that my interest is the performance of the mount with relatively short exposures; within the 30-60s exposure time regime.

 

Many thanks.

[P.Keller]

Hello Hector,

 

the wind speed is not the important factor, its how gusty the wind will be.

The reason is, that the mount can handle a constant load very well (it will push against the wind torque until the measured encoder position is according to the calculated target position. The mount does 1000 measurements per second but if it gets really gusty it won't be able to correct it fast enough.

Also, I think that the flexure of the tube caused by the windload will be a dominant factor.

[RamaSpaceShip]

Hi Philipp,

 

1000Hz is fast enough. The problem to correct wind gusts is to predict the wind speed before issuing the command to the motors, and also to have powerful enough motors. I worked on mount (not for astronomy) software for ships, and I had to deal with a sea surface which is not as stable as the ground. It creates a much worse trouble that the wind gusts. The only way to have a perfect stability of the mount, even in the worse sea conditions, was to be able to predict the sea level 10ms in advance so that the order to the motors is able to counterbalance the effect of the sea level change. And it worked.

 

I think that you can implement something equivalent in your mount software. I mean, you can not only correct the effect of the wind gusts, you can better predict them and counterbalance them. And your sampling speed is perfect to do this. The real limitation is in the power of the motors.

 

Best regards.

 

Bernard

[P.Keller]

the power of the motors is indeed another limitation to correct very strong gusts because you would need a very fast acceleration which needs more power than the mount can apply. Of course this depends on telescope weight.

But as I said, the problem with tube flex comes first and will limit your ability to work in strong wind conditions anyway.

 

 

And your sampling speed is perfect to do this. The real limitation is in the power of the motors.

 

Best regards.

 

Bernard

[GeorgeCarey]

1000 readings per second? I thought it was 100.

[Hector]

the power of the motors is indeed another limitation to correct very strong gusts because you would need a very fast acceleration which needs more power than the mount can apply. Of course this depends on telescope weight.

But as I said, the problem with tube flex comes first and will limit your ability to work in strong wind conditions anyway.

Are those tube flexures expected, let's say, with the ASA 400? I mean, the performance of an open truss type telescope under wind gust is likely to be better than a close tube concept?

 

  Many thanks  for your answers.

[MichiP]

Hi Hector,

 

an open truss is only limiting the the effect of wind wind for tube flex but it does not eliminate. The question is, does it make sense to start an imaging session at such stormy weather where tube flex produced by wind gusts will take effect?

 

Regrads,

Michael

Edited October 21, 2013 by Michip

[P.Keller]

Are those tube flexures expected, let's say, with the ASA 400? I mean, the performance of an open truss type telescope under wind gust is likely to be better than a close tube concept?

 

  Many thanks  for your answers.

 

my experience is that a truss tube is MUCH better than a closed tube if it comes to wind problems.

But if you have a truss newtonian with a big light shield opposite to the focuser and a long tube it can also be bad.

Small truss tube Cassegrains should perform best.

But if you have more than 4BFT I would consider a dome.