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

[Shiraz]

We have had an horrendous season for wind this year and, even with a 3m high enclosure around the scope, buffeting from the gusting 20+ kts "sea breezes" has made imaging impossible for much of the time.

Think I need a dome, but how much does a dome protect against wind ? - and are there any hidden gotchas to be wary of ? Grateful for any advice.

The alternative may be a better mount in the existing enclosure - any opinions on your experience with a good quality mount in wind?

regards Ray

[Wavytone]

Which implies the seeing wasn't much good, either.

The dome should be designed to take into account the aerodynamic forces on it in a gale, and whatever you do to mount it must include some means to stop it blowing off as the lifting forces can easily exceed its weight. This should be passive so it doesn't rely on someone putting clamps in place or tying it down before a storm. Same applies to the slit cover. The forces can be very significant BTW. Bear in mind you could be liable if it blows off and damages someone else's property, or a harms a person.

the rest is pretty obvious.

The big question is how it will rotate, and manually or electrically. Plenty of info on the intertubes...

[PeterM]

Gotcha? My 2cents worth.

In suburbia it sticks out like a sore thumb that looks like it has something expensive in it, wow must take a look.... and acts like a chimney.
A rolloff roof garden shed solves both issues. Looks like a garden shed and no chimney and its cheaper.

[5ash]

I have an observatory with a hexagonal rotating roof which has a slice cut out of the top ,with a lid and one of the roof panels removable. The opening is not ideal and looks like a keyhole in shape . The roof can be opened and closed in less than a minute and rotates on a circular metal track. The inside is definitely sheltered from the wind and comfortable. I normally use either a c9.25" or SW ED120 on an neq6 for astrophotography.
Philip

[Shiraz]

Thanks guys - appreciate the very good advice and food for thought, particularly safety and security. I had not been thinking DIY, but maybe I should - will look at that forum for more ideas. regards Ray

[gregbradley]

Ray my dark site sometimes gets bouts of wind. An FSQ106ED is short and would sit below the wind as the walls are about 2.5 metres high or so. But an RCOS 12.5 closed tube I had stuck above the roof right in the wind stream and was impossible even in light winds.

So a smaller refractor for the windy nights works.

The other plan I had was to build a simple wind shield on the roof. Like a say 600mm wide x 2 metre long hinged panel that could be propped up with a bit of an angle to blow the wind up at an angle above the observatory. 2 steel or timber props fixed to some sort of bracket on the roof could work. I think that could be effective.

I have sometimes lifted and put the black plastic gear storage crates on the roof to create a wind shadow. Its been somewhat successful.

My refractors like AP140 are only somewhat affected by wind and the crates work so I only lose some subs. Hence the wind shade idea is most likely simple and very workable and no complicated dome, cost, visibility etc.

A better mount would help but will not handle it. I am using a PMX and an AP140 often and still can get wind effects and the AP only sticks above the roofline maybe 100mm.

Greg.

[AstroJunk]

One of my most memorable asteroid observations was done directly into a gale where I only opened the dome upper shutter 5cm, just enough to see the object. The observation was rock solid!

I'm a massive Sirius dome fan for many reasons including temp stability, security, weather protection and automation.

On that question of security, whilst I'm not a criminologist, your average tea-leaf seems to see a shed and breaks in looking for easy to convert to cash tools and the like. Never seen an LX200 in cashconverters.

[sheeny]

A dome will definitely help with wind protection, but may limit your options to "away" from the the wind. I imagine the POD type dome (180° open) is worse in this respect than a slot type but if you work in the half of the sky away from the wind the POD works beautifully. To some extent the POD also works into the wind by creating a picket of stiller air held in front of the dome, but only in light breezes;).

Be aware that slot type domes can be subject to turbulence in the slot until the dome cools down (the chimney affect). The POD is less affected due to the bigger opening but thermals have an effect no matter what you use.

Al.

[sheeny]

The biggest advantage to an observatory IMHO is that the scope can be left set up and polar aligned. Saves a heap of time!

I have always preferred a roll off roof type obs, but you may have some fun with council getting a roll off roof approved. I've never been a fan of a modified garden shed since most garden sheds are light/flimsy to say the least so you have to put a lot of work into them to make them a) secure and b) robust enough to withstand the worst storm you can expect to protect your scope.

When I enquired at council about the POD and showed them the pictures and drawings from the web and they said it was an exempt development - I was sold!:D

Since having the POD it has excellent wind protection away from the direction of the wind with (if you set your scope up properly) a view to the horizon - not that I've used the view to the horizon yet;). Its probably only comets that I'd chase so close to the ground.

Al.

Al.

[PeterM]

Worth reading.
http://www.skyandtelescope.com/howto/basics/3304176.html?showAll=y

[Paul Haese]

This is sort of like the chicken or the egg question.

For me having a great mount is a necessity right from the start. The mount provide the stability in high wind no matter whether you are in a dome, shed or open air. Worth the investment. I remember Peter Ward always saying the mount is everything and I agree. If you can buy either an AP, SB or Planewave mount.

So then is a dome worth it? Having both a roll off roof (automated) and a dome (currently being commissioned) these are my observations. The roll off roof is the easiest to automate. The ROR does not have to rotate and be synchronised via software which can be glitchy. However, it does expose everything to the open air and little wind protection if you have 1800mm high walls.

Domes on the other had afford protection from the wind, some dew control and look cool, but are a pain to automate correctly, show people what is in your yard and can act like a chimney. Although this last point depends on how much heat protection the dome gives (its wall thickness and painted surfaces make a huge difference here).

For me the ROR was the easiest to setup and work with. The dome system I bought into is good, has better wall thickness than the aussie one, good online backup, excellent automation equipment, but can be pain to get working in 64bit environment.

So I reckon get the mount first, then the permanent protection.

[strongmanmike]

:lol: ah Ray, remember me..? Mr Windy :P

Maaaate my dome kicks R's, it is just incredible, I am soooo happy I got one and my imaging life in a windy location is rock steady, no affect on seeing either but stops the wind in its tracks! I just cannot see a roll off roof being anywhere near as good in windy conditions even with a high end mount :shrug: unless as Greg says, you use a small solid refractor that sits well below the walls of your ROR :thumbsup:

Best thing I ever did :drink:

What mount do you use currently BTW?

Mike

[Peter Ward]

I've been running lots of different 'scopes in the same dome for about 20 years, which has seen one house move and many extreme weather events in that time. Like the duracell bunny it just keeps going and going...and despite the non-insignificant start-up cost, it has really only cost me $400 a year during that time

Apart from shielding the telescopes from the wind, it does a good job of blocking out the next door neighbour's flood lights.

Mine is fully automated (MaxDome II plus AutomaDome) ...in fact I've even used the system while I've been overseas...but mostly use the scope/CCD's via the LAN that runs to my home office....makes imaging runs in the winter very comfortable.

As for the "look". Well, yes I often get "What's that?!!" from visitors/tradies etc. Most think it's great and are amazed by the fact there are "astronomical telescopes" in there :lol:

[ehgore1978]

Same here in Moonta Bay mate lucky Im pretty guarded. Looking for a dome for next property

[Stardrifter_WA]

I totally, completely and utterly agree Mike :lol:

[Shiraz]

Thanks Greg - kind of what I suspected



Sirius certainly look the goods - thanks



Thanks Al. like the idea of a permanent setup.


That's a very interesting read peter - thanks


thanks for that advice Paul - very sound



thanks Mike - Mr Windy - might just stick :) You certainly seem to have done well with a dome. I have had limited success with the open enclosure formed by my two sheds, the neighbour's shed on the boundary and my caravan making up the fourth wall. does the dome allow the wind to flow around it without much turbulence?
I have an EQ6, but will update it very soon to carry a larger scope. Unfortunately, dec stiffness is not one of the standard specs for mounts, so it will still be a "buy it in hope" purchase.


Good point about the amortised cost Peter - at my age though there are a lot fewer years left to do that.. Local light pollution is an issue here, so that is another plus for a dome.


So it's not just this side of the peninsula... just finished fixing up the grandkid's trampoline after the last big wind blew it on top of the next door shed and bent everything.

Thanks for the vote.

thanks very much for all of the helpful comments - seems that it is worth trying to do better with wind mitigation as well as getting a better mount.

regards Ray

[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:thumbsup:

[Capricorn1(Tom)]

Adding...the benefits of a manufactured dome is quick installation time ....for me....will also be able to finally use my C14 & Hyperstar system at last...for me will be first light...on this system...all the best with your selection if you build or buy an obs....cheers Tom :D

[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.

[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

[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.

[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/showthread.php?t=36222

Cheers,
Chris

[Shiraz]

thanks for the advice Tom



thanks David - permanent set up sounds great


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.


thanks Paul

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]

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/ubbthreads/attachments/5336905-prismvsmirrordiagonal.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]

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