Domes - how much benefit - or just a better mount?

[Paul Haese]

Originally Posted by clive milne

...if you opt for a commercial dome, there is a manufacturer in Europe that produces perhaps the best value for money dome that I am aware of. Check out Mark's observatory at Arkaroola for example.
...

Yes the same one I have. ScopeDome. Certainly the best dome systems getting around. Good wall thickness, cool in summer, good hardware for automation, good price, great back from the manufacturer, and software is not bad either. Only down side is the umbilical cord, but even that is not too bad.

[ChrisM]

Ray,

Another vote for a dome in a windy location. My dome is very exposed to the gusty NW winds and has withstood well over 100 kph. I installed a good dozen safety hooks which are 'engaged' 24/7 to retain the dome.

I don't use the dome in anything above about 25 kph wind since the open shutter is not that secure. For normal observing though, the lower hinged shutter is closed, so the scope is well protected from both wind and LP.

My scope has a FL > 3500 mm and combined with a solid (AP) mount, the wind is not an issue. Further details at http://www.iceinspace.com.au/forum/s...ad.php?t=36222

Cheers,
Chris

[Shiraz (Ray)]

Originally Posted by Capricorn1(Tom)

I just like the idea of a obs...if you relocate..can take it with you....I was going to build a structure out of block work...toilet and shower in it...I have building skills....but my darling wife has ordered me a Skyshed pod...I thought fantastic...there are other products on the market...but am happy what my wife has done...cheers Tom

thanks for the advice Tom

Originally Posted by DavidLJ

Wouldn't be without my 10ft Home Dome for quids. Plenty of room for 'scope and ancillary equipment. Easily accommodates small groups of visitors. Slot and rotation not automated – just manual but that's never bothered me. Never had any problems with wind. Outside can be dripping wet with dew but equipment inside still dry. Big wide slot with sliding up-and-over shutters and zenith exposed. No noticeable thermals even when computer and monitors are working. And of course permanent mounting of 'scope means only 5 minutes max setup time. Yes, bought domes are pricey but because they make it so easy to get out and use your 'scope and because they should last for many years the cost is spread over a great many viewing/imaging sessions. My recommendation – If you can, get a dome.

thanks David - permanent set up sounds great

Originally Posted by clive milne

Ray,
A couple of random thoughts in no particular order....

Within certain limitations, AO might be your most cost effective solution.

If you are concerned about theft or vandalism, making a dome out of a water tank would largely deflect unwanted attention.

if you are concerned about the thermal effects of a dome structure then insulate the structure (including the floor) There is a secondary benefit in that the process of dew formation requires a certain amount of thermal mass to which the water vapour must donate its latent heat in order to condense.

Better than an insulated dome is one that has its temperature (and humidity) actively controlled using air conditioning (I would power it with solar panels) so that the entire structure is at, or below night time temperature when you open it in the evening.

if you opt for a commercial dome, there is a manufacturer in Europe that produces perhaps the best value for money dome that I am aware of. Check out Mark's observatory at Arkaroola for example.

and lastly... nothing beats a remote, scripted telescope set up under a dark sky.


Best.
-c

thanks for the advice Clive. Following an earlier discussion, I revisited AO, but eventually decided that it couldn't be done because CA from the refractive element was excessive at f4 . I got no answer when I asked a manufacturer if their AO could be used with fast scopes.
It can get pretty hot here, so your advice on air conditioning is valuable.
I guess a robotic scope is the eventual aim, but for now, I still enjoy being involved in the nitty gritty of the imaging process.

Originally Posted by Paul Haese

Yes the same one I have. ScopeDome. Certainly the best dome systems getting around. Good wall thickness, cool in summer, good hardware for automation, good price, great back from the manufacturer, and software is not bad either. Only down side is the umbilical cord, but even that is not too bad.

thanks Paul

Originally Posted by ChrisM

Ray,

Another vote for a dome in a windy location. My dome is very exposed to the gusty NW winds and has withstood well over 100 kph. I installed a good dozen safety hooks which are 'engaged' 24/7 to retain the dome.

I don't use the dome in anything above about 25 kph wind since the open shutter is not that secure. For normal observing though, the lower hinged shutter is closed, so the scope is well protected from both wind and LP.

My scope has a FL > 3500 mm and combined with a solid (AP) mount, the wind is not an issue. Further details at http://www.iceinspace.com.au/forum/s...ad.php?t=36222

Cheers,
Chris

Thanks very much Chris. We regularly get winds well over 25 kph, so will have to pay attention to dome/shutter strength when open - good advice.

thanks very much for all of the very practical input guys - appreciated. Regards Ray

[clive milne]

Originally Posted by Shiraz

thanks for the advice Clive. Following an earlier discussion, I revisited AO, but eventually decided that it couldn't be done because CA from the refractive element was excessive at f4 . I got no answer when I asked a manufacturer if their AO could be used with fast scopes.

Ray, I'm not sure that chromatic aberration is going to be significant for an element that has zero optical power in an f4 converging light cone. There will be a small amount of spherical aberration induced, sure, but even that probably isn't going to be at a level where prime focus imaging suffers.
fwiw) here is a ray trace of an f4 system where the secondary mirror had been replaced with a prism:

http://www.cloudynights.com/ubbthrea...ordiagonal.JPG

The optical thickness of an AO element is going to be less than 20% of the above example with ca reduced proportionately.

In any event, Peter Ward is using an AO unit on his Honders at f3.8 so that should be proof enough of the real world application.

The spherical aberration component would be simple enough to fix (all be it unnecessary) by over correcting the primary to compensate.. I wouldn't bother though.

[clive milne]

If you can manage to coax Bratislav out of hiding, I'm sure he could give you a better analysis of the subject. .....

[Shiraz (Ray)]

Originally Posted by clive milne

Ray, I'm not sure that chromatic aberration is going to be significant for an element that has zero optical power in an f4 converging light cone. There will be a small amount of spherical aberration induced, sure, but even that probably isn't going to be at a level where prime focus imaging suffers.
fwiw) here is a ray trace of an f4 system where the secondary mirror had been replaced with a prism:

http://www.cloudynights.com/ubbthrea...ordiagonal.JPG

The optical thickness of an AO element is going to be less than 20% of the above example with ca reduced proportionately.

In any event, Peter Ward is using an AO unit on his Honders at f3.8 so that should be proof enough of the real world application.

The spherical aberration component would be simple enough to fix (all be it unnecessary) by over correcting the primary to compensate.. I wouldn't bother though.

Originally Posted by clive milne

If you can manage to coax Bratislav out of hiding, I'm sure he could give you a better analysis of the subject. .....

Thanks Clive. have convinced myself that the SA is not an issue (about 1/4 wave, so negligible). However, I think that the longitudinal CA is going to degrade the optical resolution at f4 and with small pixels, since it will produce defocused spot sizes of about 10 microns at the extremes of the spectral range (I think).
Peter's system is somewhat undersampled with 9 micron pixels, so he will not notice the effects of CA. Mine is closer to Nyquist with 4.54 micron pixels, so I am pretty sure that I will see the effects of CA under good seeing conditions - the tradeoff will be between tracking improvement due to AO and degradation of optical resolution due to CA. However, since I cannot image at all in windy conditions, AO would have to be better than no AO, even if it does slightly degrade the optical resolution . Thanks again for your help. Regards Ray

[clive milne]

I think context is required to determine the extent to which the ca is going to degrade resolution. The discussion really should include a measure of encircled energy ratio. If it is only 10 or 20% of the sampled spectrum that falls outside of your sampling limit then it probably wont even be noticeable... you also should weight that figure using the spectral sensitivity curve of the chip and keep in mind that each individual colour channel will be ostensibly free of ca assuming you refocus for each filtered image. The merit function of AO then reduces to a question of whether the bloating of star images due to ca (in the L channel only) is of greater extent than sub 10hz image wander due to mechanical & seeing issues.

My gut feeling is you will still be ahead with AO more often than not.

best,
c