I think all of us in here have put to much effort into what an expert would see through the scope. The bottom line is, the people looking through the thing wont know the difference. It may as well be a giant kaleidoscope to them. I suggest that what needs to be taken into consideration is not optical quality or design, but what the scope looks like.
They want a bright pink fishing lure that was designed to catch the fisherman, not the fish. From here it would all come down to preference, but I love the look of big folded scopes on equatorial forks.
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 .
My experience is based on direct night after night side by side comparison from a light polluted commercial precinct in Canberra (The Trades Club) ie very similar to Observatory Hill (Not Wiruna) between a super high quality refractor with an equally super high quality and very expensive 16" F5 Newtonian. It is indeed very relevant to the discussion since we are talking here about potentially replacing a 16" scope, just as we had at the Canberra Observatory...aaand we essentially did replace the 16" and we replaced it with a 7" APO! simple As I said and based on extensive experience at a Public Observatory equipped with the some of the best scopes available including a C14 - for a public observatory in a heavily light polluted site a 7 -10" APO on a quality well known and therefore well supported mount, would be my 100% recommendation, without question and although sporting a smaller aperture and therefore dark sky abilities, to the lay person an equatorially mounted 8 or 10" refractor on a GEM will visually impress miiiiiiiles over a weird looking 20" Dob (that requires a ladder) ... and in spades!
Takahashi or Astrophyics mounts will be supported well into the foreseeable future I am sure.
Mike, Hence my remark several posts ago about doing a site survey. I have no idea whether the observatory has done this, quantitatively. Never mind the mount, it is a separate issue.
As before this is a MUSEUM. Aesthetics and long-term lifecycle-cost count for a lot more than optical performance. Nobody in their right mind would do research at this site.
Mark - regarding the refractor vs the reflector argument the maths does support Mikes claim - and my own experience: given an APO refractor with good optics it does indeed take a reflector of 1.5X the aperture to match or do any better. Fast dobs optimised for faint fuzzies (not high power) are simply not relevant due to the light pollution. At the moment I am wrestling with Bessel functions in Excel to draw the graphs for you.
Dome considerations aside- the scope that everyone wants to have their picture taken next to after the sky tours at Perth obs is the 12" Calver - a 100 year old lump that still works beautifully, and has a bit of history attached to it. Sounds like the sky is so bad at Sydney that maybe something nice to look at rather than through would be the go- maybe you should try and prise that 5" Unitron away from Unipol on this forum? Now that's what a real telescope looks like!
I'm surprised by the apparent level of funding available too, given the instrument under consideration - Perth Observatory could learn a lot from Sydney Obs when it comes to securing funding!
Cheers,
Andrew.
Yep - work of art that. Wasn't he trying to sell it recently?
But the Cavler is 100 years old and played a role in confirming relativity- that's what gets the punters excited.
Cheers,
Andrew
You don't seem to have really been absorbing anything that I have written in this thread. In modern times, the F ratio of the scope is irrelevant to whether the sky is light polluted or not. With the right combination of modern eyepieces ,a fast F ratio scope is capable of the same range of exit pupils/ magnifications as a longer one - so the F ratio is irrelevant is not a consideration when we are choosing a scope for a light polluted sky. Perceived sky background field brightness is a function of magnification/ exit pupil and not of the F ratio. Do you agree with that ?
Please explain to me exactly what you mean by a ` fast dob optimised for faint fuzzies' not high power ' and maybe we can get somewhere.?
The contrast transfer function of a compound telescope is equaled by an instrument having an unobstructed aperture equal to the diameter of the scope minus the diameter of the secondary mirror so the factor in a typical visually optimised scope would be 1.2X the refractor aperture not 1.5 X...this has been written up in Sky and Telescope a few times.
Quote:
Originally Posted by Wavytone
. Fast dobs optimised for faint fuzzies (not high power) are simply not relevant due to the light pollution. At the moment I am wrestling with Bessel functions in Excel to draw the graphs for you.
The sky in Canberra would be far better than Sydney Observatory. A 6" scope of any kind would show next to nothing but the Moon and Planets.
Quote:
Originally Posted by strongmanmike
My experience is based on direct night after night side by side comparison from a light polluted commercial precinct in Canberra (The Trades Club) ie very similar to Observatory Hill (Not Wiruna) between a super high quality refractor with an equally super high quality and very expensive 16" F5 Newtonian. It is indeed very relevant to the discussion since we are talking here about potentially replacing a 16" scope, just as we had at the Canberra Observatory...aaand we essentially did replace the 16" and we replaced it with a 7" APO! simple As I said and based on extensive experience at a Public Observatory equipped with the some of the best scopes available including a C14 - for a public observatory in a heavily light polluted site a 7 -10" APO on a quality well known and therefore well supported mount, would be my 100% recommendation, without question and although sporting a smaller aperture and therefore dark sky abilities, to the lay person an equatorially mounted 8 or 10" refractor on a GEM will visually impress miiiiiiiles over a weird looking 20" Dob (that requires a ladder) ... and in spades!
Takahashi or Astrophyics mounts will be supported well into the foreseeable future I am sure.
I think all of us in here have put to much effort into what an expert would see through the scope. The bottom line is, the people looking through the thing wont know the difference. It may as well be a giant kaleidoscope to them. I suggest that what needs to be taken into consideration is not optical quality or design, but what the scope looks like.
I think Peter has hit the nail on the head with this response. If I was to go to the Sydney Observatory as a total novice I would rather look through something that looks the part. I expect that most of the guests are there for the enjoyment of the history of the building and its contents and would rather look through a 9 or 10 inch refractor that looks like it came from the 1800s than the latest gizmos that are coma or chroma free. Most would probably find a bit of colour-fringing a desirable view, rather than perfect pinpoints of light.
Whilst The Great Wetherell Refractor does sport a TEC200 objective made from Unobtanium, to most Sydney Obs. tour guests, it could just as well be a 10" achro doublet, then at least the guest would see things like astronomers of old.
I asked the question about whether or not a Mewlon 300 would be suitable.
I got this very pertinent response:
Hi Greg,
I visited Sydney Observatory in October last year with the group of Urania (Antwerp/Hove, Belgium). I own a Mewlon 300 that I acquired used some years ago. This is a splendid telescope with excellent optics and good mechanical construction, this is valid for the other Mewlon diameters as well. The 300 needs a good mount and preferably a permanent mounting (26 kg, OTA only). It reaches thermal equilibrium quite fast (you can remove the back end covers for increased ventilation, but I would not recommend that when you have visitors using the telescope as the main mirror is exposed partially when you remove the side covers). As all telescopes it will suffer from local seeing and the more so since it has a 300 mm aperture, it will suffer more than a 6 inch APO but it gathers more light as well. I don’t think you could afford a 300 or 250 mm APO if this exist at all, but a Mewlon comes very near that I think with much less expensive glass and a manageable length. See my website http://www.astronomie.be/Tranquility.Base/ . The Mewlon 250 would also be a good choice, lighter and less expensive than the 300!
Best regards,
Geert Vandenbulcke
Also if you are interested in a historical type telescope there is one at Windsor - Tebutts Observatory. I don't know if they are willing or not but its worth an ask. Its a big ole refractor that's brass and stuff and looks the part.
Mark, I'm reading but I'll try to stay back on topic.
Firstly no-one makes "off-the-shelf" observatory grade telescopes under 40cm designed to last several decades, small enough to fit in the dome at the observatory. Any thing like that will have to be a custom one-off and its quite clear this will be beyond the budget.
That leaves amateur-grade equipment (up to 40cm aperture) which poses exactly the same problem they have now when it fails (which it will). Within this bracket the only off-the-shelf solutions with fork mounts are Meade/Celestron/Questar, the rest are all either dobsonian, or german equatorial on the basis of choosing any of various OTA and mount alternatives - but it will be a german equatorial.
Which brings us full circle to the initial premise of retaining the existing Meade 16" OTA and putting that on a commercial german equatorial.
Either raise the budget substantially to allow for a one-off custom telescope made to fit, or forget about the fork and live with the Meade OTA (or another OTA) on an equatorial, or go dobsonian.
None of these fit the initial needs Greg indicated, which implies the needs must change.
They may as well buy another latest model LX-200 as they are only USD $13,500 new. The driven ALT AZ configuration is as close to ideal as can get in that setting when the public are concerned. With the low balance point ,the eyepiece doesn't change height much and the eyepiece orientation is always constant, compared to use on a fork or german EQ which would require a lot of constant star diagonal rotation for convenient viewing on different objects.
Sell the old tube and mount will offset the cost, and this time buy a few of the critical circuit boards boards now, so as to increase the longevity /service length of the instrument. In 10 years time they might require narrow band filters with an image intensifier eyepiece and the raw aperture will be irrelevant.
I can't think of any more cost effective solution- the 16" is supposed to represent state of the art technology in contrast to the old refractor, putting in a second refractor would not be true to this aim.
Mark, I think this is where they will end up - another 16" or maybe that 20".
I've been waiting to see if anyone else would twig to the concept of buying "through-life spare parts" which should include at least 1 spare of each type of electronic assemblies, switch/sensor and cable assemblies, motors and any mechanical parts prone to wear such as clutches, springs and bearings, and consumable parts and materials such as cork gaskets or lubricants that may need to be replaced during a major overhaul (I'm thinking of whatever is used to mount the corrector plate, and the lubricants in the mount and focussing mechanism). This applies to not just the mount but the OTA as well.
It is not unusual to ask the supplier to provide a spare parts assessment to determine the quantities from a reliability analysis, and buy an initial set of spares followed by a larger set of spares a few years later.
Another useful technique is thermal imaging of the electronics to identify the components that are running hot, and obtain spares of these (usual this means power supply components), as they're often the first to fail. Making changes (adding heatsinks) to alleviate this is sometimes done.
There are other issues, too. If it becomes necessary to attempt repairs at the level of components on the circuit boards, it is necessary to know what components contain firmware, whether there are licences/keys in embedded software, and whether these are invalidated by conducting component level repairs. For example, with Windows XP embedded, it is not possible to obtain new licence keys and entire CPU assemblies may have to be trashed.
The observatory should make sure they acquire any special tools and documentation needed for fault diagnosis, disassembly/repair/reassembly, beyond what would normally be found in an optical/electronic workshop.
Meade should have a fair idea by now of this, at least for the 16".
An "end of life" spares purchase should be made too, prior to the cessation of the support from Meade for the scope, or the prospect of Meade ceasing to do business. In some industries suppliers will provide key customers with advance notice when it is time to make an end-of-life buy, as it helps them clean out the remaining stocks of bits & pieces when production has ended.
The experience of the previous LX200 would be fairly useful in these respects.
The sky in Canberra would be far better than Sydney Observatory. A 6" scope of any kind would show next to nothing but the Moon and Planets.
Meeah...perhaps marginally better, The Canberra Observatory was in a shopping/restaurant hub of the city though, so it was pretty crapola...
In any case, as I have said a few times now () it was my direct experience in a public setting, under heavily light polluted skies, that the 7" Starfire gave better views of pretty much everything, stars, PN's, nebula and even galaxies, compared to the 16" to the public eye at least, heck, the skies were so washed out it wasn't worth the effort to have people climbing the ladder to the Newtonian focus of the 16" almost everyone was bitterly dissapointed....the scope did look bloody amazing though and was a fantastic showpiece for the 7" Starfire and most visitors thought they were looking through the 16" anyway...unless I told them
Mike
Last edited by strongmanmike; 07-02-2013 at 06:17 PM.
If I may add my 5 cents... I worked for 10 years (1993-2003) as a public night astronomer at Wellington's Carter Observatory and managed to rack up many hundreds of hours at the scopes there. My experience backs up both Mike's posts (and a few others) above.
There are two main telescopes housed in the domes there - a 9 inch (22.5cm) F/15 Cooke-Apo refractor (now 9.75 inch doublet) and a 16 inch (40cm) F/15 Boller & Chivens Classical Cassegrain. The long focal lengths meant even with long eyepieces we go moderate magnifications. Higher mags darken the sky background while stars look just as bright as they did before. Double stars and clusters were the main event if there were no planets or moon around. Focal ratio itself may not make a difference to the sky background but a long focal length scope will reach decent magnification with a longer focal length eyepiece, and they will generally have better eye relief, which is essential to a public observatory that will have eyeglass wearers who need to leave their glasses on.
The public all wanted to see the refractor. It was big, old and impressive looking. It looked like what the public thinks a telescope looks like. The Cassegrain was just weird to them.
The seeing is also notoriously bad in Windy Wellington, so you'd never want to push the magnification too high anyway. Only a few nights a year would the Cassegrain outperform the refractor. The diffraction meant stars were never quite as punchy in the Cas as they were in the refractor either.
My opinion: Get an apo refractor of moderate size and focal length, so it's not too hard to get your eye to. Also get a couple of smaller scopes of differing focal lengths, and two reasonable cameras and have a wide-field and a close up of whatever you're viewing on some TV screens nearby. If people can't look through the main scope, at least they see something. Even they do get to look through it they have something to see while waiting in line. Also: Don't worry about the mount being a German Equatorial - just use a short counterweight bar and some very heavy weights. Have the thing up on a pier, you'll be fine. You can always put glow-in-the-dark star stickers on the counterweight assembly.
Meeah...perhaps marginally better, The Canberra Observatory was in a shopping/restaurant hub of the city though, so it was pretty crapola...
Mike
Mike - You seem to be completely mis-informed about the quality of Sydney skies here. Canberra skies would be way way darker than Sydney CBD in the heart of a population of 5 million- you are lucky to see more than a few brighter stars by naked eye. When was the last time you stood on Observatory Hill and looked up at the sky? I contend that a 6" telescope would be completely useless there.
awe come on, looking in an eyepeice is a completely different experience, regardless of how crappy. If I turned up just to look at a friggen screen Id walk away disappointed. Its not about what im looking at in this case, its how (for the experience Geoff is on about).
I think you're being a tad unfair. I think you need to understand that many people new to the observatory may just as easily walk away from the eyepiece not having seen anything anyway and be equally disappointed. At least something, even if on a screen, may give them the hunger to follow up on the possibilities of observig what's out there, whereas a few blurry glows really doesn't incite much curiousity to the uninitiated.
It is always first preference to observe through the eyepiece, but so people won't be disappointed during their visit maybe Alex's set up is a viable alternative.
I don't think many of the people making suggestions have ever been to the observatory. One of the advertised features is to look through the big telescope at the sky.
Twenty people in the dome does not give much space for moving around.
That is the advantage of the small footprint of a 16" SCT on a fork mount that allows ease of access to the eyepiece for most people and a 16" SCT in a confined space still looks to be gigantic.
I understand they've already got an old and extremely credible looking large achromat in one of the two domes, so that's already covered. Re-reading the original post it seems that there is also nothing wrong with the 16" at the moment, other than it does not cut it in the Sydney light pollution. Although the original poster has probably long lost interest as this thread roared off at a tangent, it seems that what they need is better expectation management, and perhaps a good light pollution filter.
I doubt that increasing aperture by a few inches will produce the step-change in the visual experience of a casual visitor they are looking for.
I sometimes wonder if having the corridors and museum at the Perth Observatory lined with large and colourful astrophotos is setting up the visitors for dissapointment. Certainly these days, someone used to seeing the kind of images common in the media is underwhelmed by their first look through a telescope. Unless it's of Saturn.
cheers,
Andrew.
Perhaps if we keep discussing we can come to the conclusion that the Earth is actually flat ? 
As I said and based on extensive experience at a Public Observatory equipped with the some of the best scopes available including a C14 - for a public observatory in a heavily light polluted site a 7 -10" APO on a quality well known and therefore well supported mount, would be my 100% recommendation, without question 
and although sporting a smaller aperture and therefore dark sky abilities, to the lay person an equatorially mounted 8 or 10" refractor on a GEM will visually impress miiiiiiiles over a weird looking 20" Dob (that requires a ladder) 
... and in spades! 
) it was my direct experience in a public setting, under heavily light polluted skies, that the 7" Starfire gave better views of pretty much everything, stars, PN's, nebula and even galaxies, compared to the 16" to the public eye at least, heck, the skies were so washed out it wasn't worth the effort to have people climbing the ladder to the Newtonian focus of the 16" almost everyone was bitterly dissapointed....the scope did look bloody amazing though 
and was a fantastic showpiece for the 7" Starfire and most visitors thought they were looking through the 16" anyway...unless I told them 
