Sydney Observatory needs your help.

[strongmanmike (Michael)]

Man the Oddie was just unbelievable in good seeing, one Mars opposition back in around 1998 (?) it looked like a Damien Peach photo, even my 6 year old son who was with us that night was making out all the major surface details.

I first used the Oddie in early 1984 as a 16 Yr old, my mate Attila had been using it for a year previously as part of the ACT college access (was that how you accessed it too?) including the Zeiss camera. We used the Oddie as a guide scope for our C5 and Cold Camera ....ah, sigh...those were the days

Definitely a great scope for a public observatory, if you had a housing large enough of course.

Mike

Originally Posted by Wavytone

Hehe... I remember that 16" well, Mike... it was at the back of the Tradies Club in Dickson.

When it opened I think half of CAS were wondering WTF ? while the other half were laughing at the waste of putting a telescope like that in that awful location. I walked away shaking my head in disbelief when it opened.

After restoring its predecessor (Colonel Oddie had a 4.5" Cooke first, on a very beautiful mount which was originally used to do the site survey leading to the selection of Stromlo for an observatory), subsequently I had its big brother (the 9" Oddie at Stromlo) to myself one night a week, for several years before you. Both were great examples of what a refractor could do, and it is little wonder it is being replaced with a slightly smaller and faster APO.

The trouble is, nobody makes 'scopes like that as a "commercial off-the-shelf" item - they're all one-offs built to order. Even in Oddies day.

[dannat (Daniel)]

The old 8" from 1874 still gives excel planatary & lunar views with the old Brandon eyepieces, much crisper on planets than the 12", we have it on a German EQ, but eyepiece location can be tricky, we have small steps & put the tube both over & under the mount...the public are very happy with a planet view & lunar close up, galaxies in the city are a waste iPod time, even carina neb is hopeless in Melb city, Sydney would be the same, orion is ok

[GOTO (Geoff)]

On behalf of the staff I would like to thank everyone for their ideas and input. Please note we have not committed to anything at this point and we have received some very good suggestions which we are following. We have no intention of wasting your money.

I will take a picture of the dome soon and put it here for you all to see.

FYI, we have several scopes including a 12inch and 16inch dobs which we trundle out every now and then for major events. I love the view they give but on busy nights, despite telling everyone that they easily out perform the north dome scope (heat from the building is another major problem), there won't be a queue for them but a massive one for the domes, why? People love the domes and the experience they bring.

Some here have mentioned the quality of the view. Optical quality is one thing but varying staff focus is perhaps a bigger more problematic issue. If anyone can suggest a fast reliable way we can standardise visual focus, again please let me know. Video is not an option as a replacement.

Overall the view through the 16inch SCT has been good for most objects. It's a solid B+ imo. Why don't we keep it? We may well do so but we are investigating options within a budget.

Again thank you for the discussion.
Kind regards
Geoff
geoffw@phm.gov.au

[gregbradley]

APM do a folded APO as well. Dietmar uses one. As I recall its 9.25 inches and will reach deep sky galaxies.

Greg.

[Barrykgerdes]

Originally Posted by GOTO

Some here have mentioned the quality of the view. Optical quality is one thing but varying staff focus is perhaps a bigger more problematic issue. If anyone can suggest a fast reliable way we can standardise visual focus, again please let me know.
Again thank you for the discussion.
Kind regards
Geoff
geoffw@phm.gov.au

Hi Geoff
Yes focus is a problem for multiple users. When I am at astronomy nights with my scope. I have two methods.

1. I set the preliminary focus with my glasses on. This sets the start position and works for most viewers but if the viewer has noticeable thickened glasses their focus without glasses will be quite different. When there is sufficient eye relief they can usually view the standard focus with glasses on. But will always require a refocus with glasses off.

2. I have a LX200 that I fitted a mechanical counter to the focus knob. (I am lucky in that my scope has very little image shift with the standard focus knob). This allows re-setting to a standard position when the focus has been adjusted. The precicision of the scale is such that the scale even registers the difference between cold and warm focus by as many a 10 divisions. This method can be fun if you can see what glasses the observers are using. You can soon learn the offset for varying degrees of long sight and short sight by looking at the thickness of the lenses and set focus before they look into the eyepiece.

Barry

[Satchmo]

What a bizzarre thread - we go through all of this and end up with everyone agreeing a sub 10" APO would be best- when the original problem was 16" being not enough aperture ? Perhaps if we keep discussing we can come to the conclusion that the Earth is actually flat ?

Refractors are not immune to the laws of physical optics - a 9" scope can never be any more than a quality 9" 'scope. I have seen a stock commercial 10" Newt trounce a 7" Starfire refractor on Jupiter as any good 10" scope should .

[gregbradley]

Don't Televue sell diopters for their eyepieces for this issue of varying focus?

Greg.

[gregbradley]

Refractors are not immune to the laws of physical optics - a 9" scope can never be any more than a quality 9" 'scope. I have seen a stock commercial 10" Newt trounce a 7" Starfire refractor on Jupiter as any good 10" scope should .[/QUOTE]


I am sure we've all had experiences with various compound scopes where they excelled. But the usual experience is its very hard for a compound scope to match unobstructed beautifully figured triplet optics.
No obstruction seems to be the big difference.

Also not everyone likes diffraction spikes. Obviously compound scopes big strength is large aperture cheaper than APO optics which are usually limited by availability of glass and difficulty of manufacture. Its a common remark that 6 inch APOs cut through the light pollution better than any other design. Perhaps that is due again to the lack of obstruction and better contrast.

Greg.

[Satchmo]

Originally Posted by gregbradley

But the usual experience is its very hard for a compound scope to match unobstructed beautifully figured triplet optics.
No obstruction seems to be the big difference.Greg.

As long as you don't mind seeing even the brightest deep sky objects visually as small dim `contrasty' fuzzy blobs in a typical sized
APO. Lets call a spade a spade.....

[Wavytone]

Greg nice try but you've missed two subtle points.

Firstly, the reason smaller aperture scopes often cope better with turbulent seeing is that the dimensions of the microthermals are typically of the order of 15-40 cm. In average seeing a 10cm scope will show discernible diffraction rings and an Airy disk in conditions where a 20 cm or larger scope will just show a fuzzy blob. The atmospheric effects are easily seen if you defocus on a bright star. In daytime I've flown through this kind of turbulence at heights up to 3000 metres on my paraglider, you can feel the turbulence on your face and it makes for a bumpy ride, and it's also one of the reasons no-one likes to fly paragliders over urban areas.

The seeing on most nights at Observatory Hill is so poor that it does imply a large aperture scope is going to be unsatisfactory. A site survey to assess this quantitatively would be constructive, before making any decisions to commit to any telescope.

Secondly, the reason refractors perform so well is that having no central obstruction at all and with good optics, they will deliver a textbook diffraction pattern with the maximum energy concentrated in the central Airy disk (making it brighter) and with comparatively less energy spread into the diffraction rings. Any scope with an obstructed light path (Newtonians, cassegrains etc) has a less intense central Airy Disk, and more light spreading into the rings. The effect is quite noticeable on a side-by-side comparison of two scopes working at the same magnification, and in particular this has a lot to do with why a smaller refractor often matches a rather larger reflector What's more it is dependent on transmitted power, which is image intensity squared, so what you think of as a relatively small obstruction has a more significant effect on the perceived intensity of the central peak than you evidently think - and also says lot about why a smaller refractor can indeed match a larger aperture reflector. There is plenty of optical theory to back this up, FWIW. For example, while an 8" SCT may provide a significantly brighter image on an extended object, most 8" SCT's cannot out-resolve a good 6" APO on double stars or the bright planets.

The only designs that will outperform refractors are the all-reflecting unobstructed designs such as the schiefspieglers, with the advantage of perfect achromatism. However they're not appropriate in this application for many reasons.

[alocky (Andrew lockwood)]

The main reason a refractor will show an Airy disc when a nearby reflector doesn't is because of its piddling aperture, not its magic powers of trans-physics resolution. I have yet to see a refractor (and I own a few) that gives a 'textbook ' as in Suiter's textbook star test. In fact, owing to the inherent spherical and chromatic aberration in even an 'exquisitely figured' triplet it is very hard to interpret the star test.
Likewise, one of my 4" f15 achromatic scopes will split Antares - only because the secondary star falls in the minimum between the central dot and the first airy ring.
Again, Mark is bang on the money. The average punter at a public observing night is not impressed by the concept of averted vision- my experience is that less than 5% will 'get it'. Nor will they remark on the glorious airy disc or high contrast views. After watching plenty of people go through Perth obs over the last few years, the C14 usually gives the best 'wow', and the Meade 16 a close second, usually because of the target on display.
Pity a gem can't be managed in the dome. I'm now worried about when our Lx200 gives up the ghost.
Cheers,
Andrew.

[Satchmo]

A couple of points relating to previous posts:

1) In practise ,its a complete myth that a larger scope is somehow going to show less than a smaller one in inclement seeing. A smaller scope will show superficially sharper images , but will never show more than a bigger telescope, and the larger scope will always generally show more even poor seeing.
A classic memory I have of this - an APO showing a crisp small dim image of Jupiter with some festoons at the `just visible' level. At the same time a mediochre Coulter 17.5" showed a large bright softer looking image with breathtaking level of detail in the belts due to colour perception and image scale - numerous features of all different colours because there was enough light to activate the cone cells which were completely invisible in the 7" APO and the 17.5" was operating no where near its resolution limit.

2) Central obstruction issue is way overrated. Any larger telescope with a visual optimised secondary below 20% suffers negligible effect on the Contrast Transfer Function according to Dick Suiter . Similarly spider vanes although creating a visible spike do not significantly effect planetary contrast unless exceeding 1/128 the aperture in thickness according to Suiters Star testing book. Another red herring !

My 14" Newt with 20% obstruction and standard 3 vane spider , has shown salt and pepper granulation inside certain markings at Mars opposition which I have been able to correlate with photos from the HST....

[Wavytone]

Mark, I'm quite sure it would - in good seeing. Likewise I had the chance to observe Jupiter and Saturn several times with the old Reynolds at Stromlo (a 30" f/15 Cassegrain) between photometry assignments - likewise it was staggering.

The trouble is, the seeing at observatory hill is not good and it has 3-4 magnitudes of light pollution, so to be perfectly frank any telescope there is just a MUSEUM PIECE, not something anyone will use for serous observations of anything.

I used to live at Waverton across the other side of the harbour and I tried observing from Observatory Hill, Balls Head and Berry Island several times but gave it away because the seeing was invariably poor. In 4 years I never saw what I would describe as good seeing above 6/10.

Where I am now on the ridge in Killara there is often a NE sea breeze in the evening bringing laminar flow and this makes a HUGE difference, at least good views of the planets are possible with my 7" on many nights.

As for the effect of central obstruction I'll do the maths and post some charts so you can see. I don't agree with you on that score.

[gregbradley]

Did you mean Mark? You're arguing my case here.

Greg.

Originally Posted by Wavytone

Greg nice try but you've missed two subtle points.

Firstly, the reason smaller aperture scopes often cope better with turbulent seeing is that the dimensions of the microthermals are typically of the order of 15-40 cm. In average seeing a 10cm scope will show discernible diffraction rings and an Airy disk in conditions where a 20 cm or larger scope will just show a fuzzy blob. The atmospheric effects are easily seen if you defocus on a bright star. In daytime I've flown through this kind of turbulence at heights up to 3000 metres on my paraglider, you can feel the turbulence on your face and it makes for a bumpy ride, and it's also one of the reasons no-one likes to fly paragliders over urban areas.

The seeing on most nights at Observatory Hill is so poor that it does imply a large aperture scope is going to be unsatisfactory. A site survey to assess this quantitatively would be constructive, before making any decisions to commit to any telescope.

Secondly, the reason refractors perform so well is that having no central obstruction at all and with good optics, they will deliver a textbook diffraction pattern with the maximum energy concentrated in the central Airy disk (making it brighter) and with comparatively less energy spread into the diffraction rings. Any scope with an obstructed light path (Newtonians, cassegrains etc) has a less intense central Airy Disk, and more light spreading into the rings. The effect is quite noticeable on a side-by-side comparison of two scopes working at the same magnification, and in particular this has a lot to do with why a smaller refractor often matches a rather larger reflector What's more it is dependent on transmitted power, which is image intensity squared, so what you think of as a relatively small obstruction has a more significant effect on the perceived intensity of the central peak than you evidently think - and also says lot about why a smaller refractor can indeed match a larger aperture reflector. There is plenty of optical theory to back this up, FWIW. For example, while an 8" SCT may provide a significantly brighter image on an extended object, most 8" SCT's cannot out-resolve a good 6" APO on double stars or the bright planets.

The only designs that will outperform refractors are the all-reflecting unobstructed designs such as the schiefspieglers, with the advantage of perfect achromatism. However they're not appropriate in this application for many reasons.

[Wavytone]

I'll suggest we are in violent agreement concerning the seeing at Observatory Hill.

The problem is the choice of telescope. As I indicated before what is needed is a museum piece to impress the public, not something for serious observations of anything.

Geoff's hints confirm my own experience from living at Waverton - its going to be limited to the moon, the bright planets, bright double stars and a handful of nebulae, clusters and maybe 5 galaxies for the really interested. A couple of filters might help to but the light pollution form the harbour bridge, though I'll confess I didn't try this.

Perhaps the odd passing comet near the zenith (Halley).

Considering the French government has just passed a law requiring businesses in Paris to cut unnecessary lighting I'd suggest the observatory could do rather more to champion the cause against unecessary lighting - starting with the Harbour Bridge and the SCG. Just turn it OFF. A green initiative that no-one can argue against.

[gregbradley]

Yes.

Tak Mewlon may be a good choice as it takes in most of the points being talked about in this thread. It has a small central obstruction, its got really small spot sizes, its compact and would most likely fit on a fork mount and it regularly gets rave reviews as a visual instrument. Plus its a decent aperture.

Whether or not it would be held back by the weak seeing I suppose all scopes are in the same boat there.

Greg.

[Wavytone]

Greg, the problem isn't the OTA. Lots of choice provided the observatory can accept commercial grade stuff, and the choices haven't changed in 70 years.

The challenge is the mount and electronics - finding a solution with a life >15 years and not being dependent on a company in need of life support (Meade, Celestron) is a real problem.

The alternative is to accept "cheap and cheerful" as the amateurs do, and chuck it out every 10 years. This decision is a lifecycle cost issue and involves the long-term cost of finance available to the observatory and a whole lot of other issues unknown to anyone here.

There is merit to the concept of buy whatever cheap gear is available at the time and throw it out after 10 years, vs a telescope for the long haul. Instinctively amateurs are driven to the cheap stuff as optically it CAN perform just as well, and you have the advantage of much better observing locations with good seeing.

Most of you put optical performance as the highest priority and this is where the trouble starts - the problem here is that optical performance is almost irrelevant and takes a very third-place after cost, and aesthetics.

Ultimately the bean-counters will prevail. What is important is to present the long-term cost forecasts for each alternative for say 20, or perhaps 50 + years.

Time to revisit Decision theory - 101.

[gregbradley]

There are lots of high quality mounts these days. More now than 5 years ago.

Astrophysics, Takahashi, PME, PMX, Losmandy, ASA DDM, Planewave.

Of course these are all GEMs. But whats the advantage of an Alt Az mount?

For $10,000 at the moment you can get a PMX. It holds up to a 14 inch RCOS.

Astrophysics have the AP900 which is similar and a new AP1600.

These high end mounts should last longer than 10 years.

Greg.

[Wavytone]

All of which pose exactly the same problem the observatory has now with the 16" regarding proprietary hardware and obsolescence.

I'm rapidly coming to the view that the only way to get around it is either:

a) settle for a low-tech equatorial mount that tracks, with an RA worm drive and little else (similar to the Losmandy mounts), forget about GOTO and all the rest. The point being that if the supplier dies, there's a fair chance a simple and reliable drive train can be can be cobbled together out of parts available locally. Keep the OTA, keep most of the mount and no software involved.

b) if you adopt auto-guiding, push-to, GOTO or computer interfaces, at least adopt what is becoming an industry standard with some chance of being able to replace parts when they fail. But - here's the nub - there are no industry standard interfaces > 10 years old. That means be prepared to dispose of the whole scope after 10 years because the available technology will change radically (and incompatibly) in that time. Changing the mount almost certainly will mean a new OTA and vice-versa.

[Bassnut (Fred)]

Hehe, its gone full circle to what Geoff wanted in the 1st place. A 17in CDK with Mathis fork mount (no head banging GEMs). Off the shelf, reliable, lookin good and proper just standin there, visual only, wow factor bright closeups. He just wants to have a peek though a 17in CDK before purchase.