Pulsar Observatories 2.2m or 2.7m dome?

[aeneas]

Hi all,

I've decided to make a big investment in our next set-up: we are purchasing the C11 EDGE HD scope, mounted on CGX-L. The aim: (a) with kids (and friends) the main purpose will be viewing; (b) I will slowly start learning about AP. I will probably start with my Nikon D750 full-frame DSLR but aiming to upgrade to ZWO ASI 2600MC camera in the near future.

Ultimately we would like to build an observatory dome on our flat roof; we're thinking of installing the Pulsar Observatories but we need to decide between the 2.2m and the 2.7m model. Our roof can handle both and at the moment the price difference is not so huge that we couldn't handle it. Aesthetically from looking at the house (I made some simulations) the 2.2m would be les noticeable (which is what we'd prefer), but I worry that it would be a really tight space in there... especially since viewing will most often include several people - not just me.

Has anyone seen first-hand in real life the comparison between 2.2m and 2.7m models? In most youtube video reviews I see mainly 2.2m dome (it seems to be the more popular option) so can't really make a decent guess how a 2.7m would compare from the inside etc.

Any recommendations?

[steppenwolf]

I have the 2.2m version and with my 150mm Esprit refractor (1070mm focal length) on an equatorial mount there is adequate room for three adults who understand that the mount moves and they must not impede that movement! Most of the time though, the observatory is used remotely and is, therefore, unmanned. For one adult carrying out maintenance work there is plenty of room but regular attendance by other observers would take a little management with my equipment. Your much shorter C11 would be much easier to 'avoid' as it slews around so three/four adults would be OK.

Consider carefully installing on a flat roof with regard to warm air current from the rest of the building and for the future when you move into AP - will the mounting pier/tripod be on a solid enough footing? I cannot stress too much how important a REALLY solid mounting is for AP, even a single concrete pouring for both the floor and pier can be problematical and two separate pourings with a gap between them is the preferred method for a ground-based installation!!

[aeneas]

Hi Steve,

I just bought your two books (they arrived a couple of days ago) so I'm honoured to receive your reply and feedback!

Regarding AP - than you for your advice, and I'm starting to worry that this observatory may remain a dream, rather than turn into a reality; regarding the heat I am not worried - I have a passive house with heavy insulation and active climate control & ventilation so the house accumulates very little heat and the roof is covered with gravel that also feels cool in the evenings even on hot summer nights.

However the concrete pouring may be an issue. I just spoke with the architect who did statics on our house and he said concrete will not be possible, but we could construct very thick multi-layered OSB or plywood base to be drilled and fixated into the house anchor walls. For viewing that might suffice but I guess for AP that puts the observatory out of the question? 

[steppenwolf]

11 minutes ago, aeneas said:

However the concrete pouring may be an issue. I just spoke with the architect who did statics on our house and he said concrete will not be possible, but we could construct very thick multi-layered OSB or plywood base to be drilled and fixated into the house anchor walls. For viewing that might suffice but I guess for AP that puts the observatory out of the question? 

Hi, I hope you enjoy reading the books!

Concrete is the obvious choice for a ground-based observatory but I am no architect so I am unable to offer any advice on your roof other than to say that if the mount isn't as solidly mounted on your flat roof as it would have been on the ground, you are likely to run into issues with vibration and other movement while you are operating the imaging system in the future.

The 25th post (Page5) of this thread tells the very sad story of how sensitive the pier/ground equation is and this poor guy did a really good job of everything else!

There may well be a construction solution for this but I would urge you to explore the requirements very carefully and try out a proper imaging session or two on the roof in the open before you consider a permanent observatory installation.

I hate having to dampen spirits like this ☹️ but mounting is the most important aspect of AP and I hope you do find a way of resolving the potential issues.

[aeneas]

35 minutes ago, steppenwolf said:

The 25th post (Page5) of this thread tells the very sad story of how sensitive the pier/ground equation is and this poor guy did a really good job of everything else!

There may well be a construction solution for this but I would urge you to explore the requirements very carefully and try out a proper imaging session or two on the roof in the open before you consider a permanent observatory installation.

I hate having to dampen spirits like this ☹️ but mounting is the most important aspect of AP and I hope you do find a way of resolving the potential issues.

Thank you for highlighting John's thread. Very interesting read - and inspiring (although tough) journey. It seems in his case a big issue was a very noisy (poorly executed) motor engine and then a direct transmission of that noise via dome wall to a poorly isolated base was a major issue. So buying a new model with (hopefully) an efficient Pulsar's own motor drives would be "cleaner" in this respect, but still, I appreciate how sensitive all this is and the question is whether a base can ever be built solidly enough if it is not concrete (which is not an option in my case). ...Certainly will need to explore this much much deeper (and have some expert architectural and static advise) before I take the plunge.

[ollypenrice]

I have to endorse Steve's comments, not that they need any endorsing from me.

I'd also highlight some issues regarding the C11 as a deep sky imaging instrument, especially with a DSLR or small pixel CMOS camera. It's very important to understand sampling rate, AKA your image scale in arcseconds per pixel. I'm sure you'll find this covered in Steve's books but there is no point in over-sampling, which means having too much focal length for the size of your pixels. Note that Steve uses a six inch refractor. So do I. I've also used a 14 inch catadioptric and found no real advantage over a smaller refractor now that pixels are small as well.

However, the C11 makes a great planetary imaging scope because in fast-frame planetary imaging you can beat the seeing with ultra-short exposures. In deep sky imaging you can't.

I do have a large SCT here and like it for visual, a 14 inch Meade (using a cable kindly made for me by Steve!) but I don't consider this scope useful for deep sky imaging.

Olly

[Peter Drew]

My first recommendation would be to go for the 2.7m dome if the extra cost is affordable, an observatory is never too big, nearly always too small!  I operate an almost 9m dome and we still bump into each other in the dark.  You say that the main interest, certainly initially, is to share visual viewing with children and friends.  Despite the genuine concerns expressed about stability, as an outreach specialist I can tell you that the children and friends will be thrilled by the views from a 11" SCT despite some shortcomings.   The imaging dimension is a completely different consideration, both Olly and Steve are seasoned World class imagers and are naturally very sensitive about mounting requirements and telescope choice for imaging.  An 11" SCT would be fine for lunar and planetary imaging but most imagers would recommend something of shorter focal length to stand a chance of reasonable success for DSO's.  A small, short focus APO refractor mounted on the 11" might be a reasonable compromise.  Good luck with whatever you decide to do.     🙂 

[aeneas]

Thanks guys! ...I am aware SCT will probably not be the most optimal set-up for DSO imaging; but based on all I've read so far it is a great "all-rounder". I am a complete novice to AP - never even tried; so for us the first "growth" will be in considerably improved viewing pleasure and learning more through that. I would hope to "play" a bit with a DSLR to learn some of the basics (expecting to get some good tips in Steve's books)    ...if AP starts pulling us in, I would certainly think C11 SCT will not be the only scope but would be eventually joined by a decent refractor. Considering the overall costs here in the initial set-up, I don't think another 1-2k for a refractor should hold us back. But we should first see whether we're even ready for a proper AP

[Rusted]

As the builder of a second storey 3m/10' dome I agree with Peter & others here.
Domes are never big enough and you have no corners to place the inevitable fidgets.

You can't insist on people remaining still. Particularly children waiting for a view.
People move about. Any movement is catastrophic to image stability.
Even the wind will be conducted through the structure.
Any pier resting on a raised floor will shake the image violently. Guaranteed!

You need a solid, isolated pier built off solid ground and clear of the obs. floor.
Pre-cast chimney blocks are good being solid, strong and compact.

My solution was a massive pyramidal pier built on widely spaced concrete footings.
All it takes to short circuit the floor isolation is a trapped cable between the pier and desk.
In all other respects the timber and plywood pier has been a "huge" success.

If you really want to impress then buy an astro camera and good quality, 27" or larger monitor.
Place it where everybody can see it easily.
Seeing a razor sharp Moon or the Sun, the equivalent of three feet across, is mind blowing.
Far more fun than staring at dim things.
Which an educated eye cannot eve see in the few seconds in impatient queues.

Unless your house is white don't choose a white observatory dome.
It will stand out like a sore thumb from miles away.
Pulsar do a mid, sage green. Not the earlier dark grass green.
A green dome will vanish against a green or dark building backdrop.
It will look more like a round bush or conifer which the eye will pass over without pause.
 

 

[aeneas]

2 hours ago, Rusted said:

[...Any pier resting on a raised floor will shake the image violently. Guaranteed!...]

[...My solution was a massive pyramidal pier built on widely spaced concrete footings.
All it takes to short circuit the floor isolation is a trapped cable between the pier and desk.
In all other respects the timber and plywood pier has been a "huge" success. ...]

[...Unless your house is white don't choose a white observatory dome...]

Hi Chris, thanks for the tips. You have an absolutely amazingly beautiful and inspiring observatory! 

I agree I should not build the pier on a raised roof floor - that is why I am discussing with my architect how to fix the pier directly into the house's anchor walls (I'm not sure if that is the correct english expression - but the main load-bearing walls that carry the house statics)... 

It seems from your construct that building a concrete pier on top of wooden pyramid construct gave a solid result and the wooden foundation does not reduce the stability? When looking at how my pier could be created, it would also mount onto a very solid anchor wall foundation, with the only exception being that the architect says I cannot pour concrete pier but could get away with multiple layers of OSB or plywood planks. Perhaps with an additional anti-vibration insulation layer in between the planks or something... Hmm...

[Rusted]

Thanks. I don't think you will get away with anything cantilevered from the house structure.
Anti-vibration damping is not easily achieved and you could be building a springboard!
Overall sponge packing under a pier will just flatten to solid material under load. Tried it! Don't bother.
I have even tried car valve springs under concrete slabs.
If they don't flatten under the load the pier will have its own, very undamped, low frequency resonance!!

I have concrete blocks buried in deep gravel supporting my 5 meter tall, isolated, pyramidal pier.
These have adjustable height brackets which are bolted to the 100x100mm sloping pier timbers.
An unlikely and very bulky design for most situations but it suited me fine as a lone observer/imager.
I climb up inside my pier via a stepladder from the ground floor and a trapdoor.
Not recommended for anyone else! Too dangerous to have a hole in the floor!

 

What is immediately below your intended observatory site? An occupied domestic room?
Can you tolerate a free standing, block "chimney" in the middle of that room? Very unlikely.
If there is a structural wall below the middle the observatory you could build a pier off that.

A hole in a ceiling would never get past building regs in an occupied room.
A garage below would be seen as a serious fire risk!
A further difficulty is your SCT will need a very tall pier for comfortable viewing.

Much more seriously though:

The interlocking "jigsaw puzzle" sponge floors are popular for observatories.
Double the thickness with non-coinciding joints? You now have fair, footfall isolation.
It would feel weird to walk on but you might get away with it.

Your pier would be mounted on a large and heavy steel plate.
With four conical spikes in the corners to isolate right down through the carpeting. Hifi speaker style.
You could lay sponge industrial grade carpet underlay on top of your "jigsaw" sponge floor.
Add a nice pile carpet? This is easily tested out for isolation before you build anything at all.

[aeneas]

On 01/10/2020 at 13:38, Rusted said:

Thanks. I don't think you will get away with anything cantilevered from the house structure.
....

What is immediately below your intended observatory site? An occupied domestic room?

...

A further difficulty is your SCT will need a very tall pier for comfortable viewing.

Much more seriously though:

The interlocking "jigsaw puzzle" sponge floors are popular for observatories.
Double the thickness with non-coinciding joints? You now have fair, footfall isolation.
It would feel weird to walk on but you might get away with it.

Your pier would be mounted on a large and heavy steel plate.
With four conical spikes in the corners to isolate right down through the carpeting. Hifi speaker style.
You could lay sponge industrial grade carpet underlay on top of your "jigsaw" sponge floor.
Add a nice pile carpet? This is easily tested out for isolation before you build anything at all.

I fully agree cantilever is not an option - I did not even consider that. What I have in mind is to fix the onto the structural walls below; I made a simple sketch - not sure how this will be understood, but this is a birds-eye view of my roof; the three red lines horizontally mark the heavy structural trams below the roof structure. I was thinking to make foundation with a heavy steel cross (blue lines) that is rooted into these structural trams; then on top of that I could build the base for the observatory - first made out of steel plate then maybe layers of plywood or the jigsaw floors that you mention. Do you have a link to what you mean by that? I can't fully imagine.

Regarding the "pier" - I was not considering building a pier for the scope; I was planning to just use the Celestron CGX-L mount (or possibly the slightly smaller CGEM-II). That gives me the flexibility to take the whole thing quickly apart and move to an offsite location (e.g. high up in the mountains) for some alternative viewing location.

I've made two circles there - a 2.4m diameter required for the 2.2m Pulsar dome or a 3.0m diameter required for the 2.7m Pulsar dome. Looking from below the house, I made a mock-up with a similarly sized tent and from below the house the observatory would not be seen at all. One would need quite a bit down the road for it to be seen from the top of the roof and it would not really stick out much. Wife agrees that even the 2.7m mockup looked completely acceptable. See the photo attached; I will leave the photo on for a bit for people to see the idea then remove for privacy reasons.

If only we could be sure we could make this as vibration-proof as possible. It will be some endeavour, time, cost etc. and if it turns out well, this would be a dream come true. If, on the other hand, we end up with a shaky construct that is useless for any serious observation, well... I would not like to end up there

I think we could make it work, but will require some serious planning beforehand and any help from the community here would be greatly appreciated to help us avoid big mistakes!

 

Edited October 4, 2020 by aeneas

[aeneas]

it's been a while since any post. Any comments on my thinking above? Would really appreciate any input here at this stage as we are conceptually working on how to implement this. Much appreciated!

[Rusted]

It would be handy if somebody could report on the effectiveness of the interlocking sponge flooring.
Can one isolate the movements of those present using this material over floorboards or plywood?
It would certainly solve all sorts of potentially costly problems.
I have no experience of this matting, myself, so simply offer the idea for discussion.

Going back to the supporting structure:
Deeper joists will really stiffen up the observatory floor compared with standard joist depths.
For the cost of a couple of extra inches/cm depth you gain enormously.
Your architect/structural engineer will confirm. Width of joist has relatively little effect.