Sydney Observatory needs your help.

[GOTO (Geoff)]

Hello everyone,

Geoff from Sydney Observatory here. We presently use a 16inch Meade LX200 for our public tours. It is purely for visual use. We are looking for a larger replacement but there are a few issues:

1. We don't want a German fork mount because of counterweight arm risks, ie head-butting the weights in the darkened dome. So a fork mount is preferable.

2. We can't have a Newtonian due to the height of the eyepiece.

3. It can't be a custom made one off.

4. We want a turn key solution.

So, the only option so far, other than a new 16inch, looks like a 17-20inch Planewave on a Mathis Instrument mount. Yes we know these are imaging telescopes. Despite the short focal length and huge secondary we haven't found anything else with the eyepiece in the right position, a big aperture and on a fork mount.

However one significant issue. We haven't been able to LOOK through one yet! Does anyone here have access to a large Planewave that would be willing to let our management team LOOK through it please? We are willing to travel anywhere and would appreciate your help. You can contact me here or via email at geoffw@phm.gov.au

Conversely, if anyone has a brilliant different approach please don't be shy but remember we need the bigger aperture to try and get down to the bottom of Mag 10 from Sydney's CBD, it must be visual and we don't want to use a ladder.

Kind regards and thanks for your help.
Geoff

[bert (Brett)]

Sent private message.

Brett

[sally1jack (Phil)]

I know you have said no newt , but have you look at the new faster mirrors being produce now which will give no ladder viewing for a 20 inch dob , i know Mike lockwood produces very fine mirrors at these f ratio's.If you talked to Peter Read at SDM he would be able to give you some useful information about this.
just a thought
phil

[mental4astro (Alexander)]

A larger aperture with a fast f/ratio won't help you. It will actually make things worse.

While looking into a scope is nice, the only alternative you have for live viewing is with a video camera. Not only will it show mag 10, but fainter too in the CBD with the 16". It will also overcome the problem that most novices don't know how to look into an eyepiece, & are also expecting to see things as they do in photographs. Even if you could see a mag 10 object in your scope, the image will be very, very faint, washed out & overwhelmingly underwhelming to novice eyes.

I'm speaking from experience here. I help out at frequent star parties at Randwick Girls' High, & the only way a galaxy is visible to their inexperienced eyes is with a video camera. Even using my 17.5", unless you know what to look for, you just won't see anything.

I use a video camera on my 30 year old C8, & not only can I see colour in Eta Carina, but I've also managed mag12 globular clusters in the LMC, all from Ranrwick.

It's not cheating - it is really the only way to overcome your problem of light pollution. Plus, it doesn't depend on a moonless night, the room doesn't need to be in the dark ( boon for insurance) and a whole room can view at the same time. As the scope slews, you see this on the screen. As air currents influence seeing you see this too. And it overcomes the issue of novices not being able to see things at the eyepiece.

You will also be able to increase the numbers of people who can view "through" the telescope to just about 100% as those who have mobility problems won't be left out because they can't see into the eyepiece.

Again, it's not cheating. It's using the right tools to overcome an insurmountable problem that will only get worse. The video camera will be able to be productive even after a 20" scope has been rendered useless from light pollution with just the 16" still being used. This aspect of not needing to get a more expensive scope with a short working life should appeal. The existing infrustructure remains, thus avoiding any expensive upgrade or costly down time - it will be functional straight away.

I'd be happy to bring my camera to give you a demo on the 16". This is the only way that you will see how effective it is. Just drop me a PM. All that's needed is a power point. There is now also a transmitter that connects to the camera to make the whole thing wireless for you.

Visual astronomy is no longer an option with DSO's for you. But a video camera will overcome this for a longer time than any bigger scope can do for you, and will open astronomy to a wider audience leaving more $$$ in your tills.

Alex.

[Wavytone]

Hi Geoff,

Appreciate the issues with safety risks with respect to things that people can bump into in the dark, or fall off (ladders).

As Alex pointed out, in the city a fast focal ratio is not a good idea as it increases the brightness of the background sky glow - you're much better off sticking with f/10 or even f/15 (as in the scope I use, which is a f/15 maksutov) and that means cassegrain optics, not a Newtonian, to keep the physical size compact.

Since this is a visual scope, the solution I'd suggest is a Nasmyth-Cassegrain, on an altazimuth fork mount. Essentially it's a Cassegrain with a diagonal flat mirror in front of the primary mirror arranged so that the light beam comes out co-axially through the altitude axis of the mount. See
http://en.wikipedia.org/wiki/Nasmyth_telescope or
http://www.millseyspages.com/astro_p...a/nasmyth.html
http://www2.l-3com.com/ios/pdf/1meter_low-res.pdf


The beauty of this is that the eyepiece is always in the same place no matter where the scope is pointing. On a large scope with a permanent mount, the fork will be sturdy enough that you can easily fit a "tractor seat" on the side of the fork so the observer can sit, riding on the fork to look through it.

Finding someone to make this configuration is going to be difficult as it will be a custom one-off.

The focal ratio is a big issue.

As Alex pointed out, a fast f/ratio increases the sky background for extended objects (nebulae, galaxies, comets) which is not helpful.You can use higher magnification to reduce the background glow, but this implies the scope should have had a longer focal ratio in the first place, say f/10 or f/12 - which would be my choice.

As you know from the Planewave example to make an f/6.7 Cassegrain the secondary obstruction is huge - the Planewave scope is intended for imaging, and it will be really bad for visual use - you will notice a big black blob floating in the centre of the eyepiece. For visual you really want f/10 to f/12, and a smaller secondary < 30% of the primary aperture.

One way to get around the constraints of a cassegrain is to build a Mersenne or relay telescope where the cassegrain focus is in front of the primary and a positive relay lens is used to bring the focal plane out where it can be reached. This permits a fast compact cassegrain (f/6 is possible) while keeping the secondary obstruction small at 25%, and the final f/ratio can be varied from say f/6 to f/15 by moving the relay lens much like a zoom camera lens. It's the same principle as eyepiece projection.

With increasing aperture at a constant f/ratio, the focal length must also be increased so the useful magnifications provided by your eyepieces will increase, too. The downside with that is that the seeing at observatory hill isn't great due to atmospheric turbulence created by the surrounding buildings and the effects on what you see will be worse with a bigger scope than a smaller one.

[h0ughy (David)]

Originally Posted by mental4astro

A larger aperture with a fast f/ratio won't help you. It will actually make things worse.

While looking into a scope is nice, the only alternative you have for live viewing is with a video camera. Not only will it show mag 10, but fainter too in the CBD with the 16". It will also overcome the problem that most novices don't know how to look into an eyepiece, & are also expecting to see things as they do in photographs. Even if you could see a mag 10 object in your scope, the image will be very, very faint, washed out & overwhelmingly underwhelming to novice eyes.

I'm speaking from experience here. I help out at frequent star parties at Randwick Girls' High, & the only way a galaxy is visible to their inexperienced eyes is with a video camera. Even using my 17.5", unless you know what to look for, you just won't see anything.

I use a video camera on my 30 year old C8, & not only can I see colour in Eta Carina, but I've also managed mag12 globular clusters in the LMC, all from Ranrwick.

It's not cheating - it is really the only way to overcome your problem of light pollution. Plus, it doesn't depend on a moonless night, the room doesn't need to be in the dark ( boon for insurance) and a whole room can view at the same time. As the scope slews, you see this on the screen. As air currents influence seeing you see this too. And it overcomes the issue of novices not being able to see things at the eyepiece.

You will also be able to increase the numbers of people who can view "through" the telescope to just about 100% as those who have mobility problems won't be left out because they can't see into the eyepiece.

It's not cheating. It's using the right tools to overcome an insurmountable problem that will only get worse. The video camera will be able to be productive even after a 20" scope has been rendered useless from light pollution with just the 16" still being used. This aspect of not needing to get a more expensive scope with a short working life should appeal. The existing infrustructure remains, this avoiding any expensive upgrade or costly down time - it will be functional straight away.

I'd be happy to bring my camera to give you a demo on the 16". This is the only way that you will see how effective it is. Just drop me a PM. All that's needed is a power point. There is now also a transmitter that connects to the camera to make the whole thing wireless for you.

Visual astronomy is nolonger an option for DSO's for you. But a video camera will overcome this for a longer time than any bigger scope can do for you, and will open astronomy to a wider audience leaving more $$$ in your tills.

Alex.

I agree use the 16" but with something like a lodestar - very sensitive small and save some money to boot. then could show to more at once "live"

[lacad01 (Adam)]

Originally Posted by h0ughy

I agree use the 16" but with something like a lodestar - very sensitive small and save some money to boot. then could show to more at once "live"

Or even a Mallincam Extreme

[Satchmo]

Originally Posted by sally1jack

i know Mike lockwood produces very fine mirrors at these f ratio's.If you talked to Peter Read at SDM he would be able to give you some useful information about this.
just a thought
phil

This piece suggests that Peter is endorsing these kind of mirrors. Peter Read has never looked through a scope with a ultra- fast mirror. He is building a few where customers like yourself are supplying their (F2.8 - F3 ? )optics by buying from the optician directly. Please correct me if I have this wrong.

The balance point no more than a few feet off the ground would not suit Sydney Observatories application of looking out of a traditional dome.

[sally1jack (Phil)]

Originally Posted by Satchmo

This piece suggests that Peter is endorsing these kind of mirrors. Peter Read has never looked through a scope with a ultra- fast mirror. He is building a few where customers like yourself are supplying their (F2.8 - F3 ? )optics by buying from the optician directly. Please correct me if I have this wrong.

The balance point no more than a few feet off the ground would not suit Sydney Observatories application of looking out of a traditional dome.

That would be correct Mark& i can see how it would not suit in this situation, i think at f3.3 20 inch can be no ladder viewing i may be wrong, i didn't intend to imply that Peter was endorsing these fast scopes & am sorry to anyone who thought i was , it was just a suggestion, i know Peter has tested f3.3 mirrors at you place with good outcomes
phil

[Wavytone]

I've had a 12.5" f/3.7... It was an interesting experiment but I wouldn't recommend such fast mirrors to anyone aiming to use it as a Newtonian - only useful for a classical cassegrain. An altaz 20" Nasmyth equipped with encoders and GOTO electronics would be quite a nice scope. With a small secondary there should be no issues designing one for f/12 and f/8 might be achievable.

[sally1jack (Phil)]

Originally Posted by Wavytone

I've had a 12.5" f/3.7... It was an interesting experiment but I wouldn't recommend such fast mirrors to anyone aiming to use it as a Newtonian - only useful for a classical cassegrain. An f/12 20" Nasmyth would be quite a nice scope.

It's not fast ratio that is the problem it 's the skill of the optician or lack of that give fast mirrors a bad reputation

[Wavytone]

Actually, no. The mirror had an excellent figure. The problems were:

a) huge secondary mirror - so big that in many eyepieces you see a big black blob swimming in the FoV;

b) range of useful magnification was very limited, largely due to the limitations with suitable eyepieces - many eyepieces won't work well with mirrors this fast, even now.

c) coma and field curvature (at the time I had this, correctors didn't exist) which were a big problem with most eyepieces except the Pretoria.

d) the fact that the fast f/ratio raises the background sky brightness, which basically defeats the point of a larger scope for looking at extended objects (nebulae, galaxies) in average skies. In really good sites with the darkest of sky, this might not be a problem but even in the Blue Mountains west of Sydney it was evident that f/5 - f/7 would have been a much better choice.

In the heart of Sydney the sky glow is much worse - and at Observatory Hill the naked-eye visual limiting magnitude is about 2.5. An f/3 mirror is about the worst possible choice.

[icytailmark (Mark)]

In the middle of Sydney your not gunna be able to see many DSO's anyways from all the light pollution. Your really limited to planets,bright nebulas, and star clusters. You don't need over 16" telescope to view them. I have a Celestron c14 and im south of Sydney. Visually i can only see planets,bright nebulas, bright galaxies and star clusters. I have found that you cant even view the bright galaxies unless you have really good night (which is rare in Sydney)

[Satchmo]

Originally Posted by sally1jack

i know Peter has tested f3.3 mirrors at you place with good outcomes
phil

Hi Phil
Its not about ladder free viewing. The dome slit at Sydney observatory would probably be chest height at the base so a Dob would be vignetted by the slit at lower altitudes. A 20" F3.3 would have you seated or knealing at 45 degree elevation- not suitable to standing crowds.

I assumed by Ultra fast you meant F2.8 to F3 . I haven't tested any F3.3 mirrors. We tested a 22" F3.6 that had been made by another vendor and refigured.

Contact me PM if you need any more info.

[Satchmo]

Most of these objections are no longer valid.

A Newtonian is clearly not suitable for Sydney Observatory due to logistics of crowd needing more constant height eyepiece and dome slit height considerations !

Any further debate about Newtonians is a hijack of the thread - I do apologise for this diversion.


I've started a new thread if anyone would like to debate Newtonians further ?

PS - Guys - apparent sky background is a function of magnification and has nothing to do with F ratio !

Originally Posted by Wavytone

Actually, no. The mirror had an excellent figure. The problems were:

a) huge secondary mirror - so big that in many eyepieces you see a big black blob swimming in the FoV;

b) range of useful magnification was very limited, largely due to the limitations with suitable eyepieces - many eyepieces won't work well with mirrors this fast, even now.

c) coma and field curvature (at the time I had this, correctors didn't exist) which were a big problem with most eyepieces except the Pretoria.

d) the fact that the fast f/ratio raises the background sky brightness, which basically defeats the point of a larger scope for looking at extended objects (nebulae, galaxies) in average skies. In really good sites with the darkest of sky, this might not be a problem but even in the Blue Mountains west of Sydney it was evident that f/5 - f/7 would have been a much better choice.

In the heart of Sydney the sky glow is much worse - and at Observatory Hill the naked-eye visual limiting magnitude is about 2.5. An f/3 mirror is about the worst possible choice.

[Barrykgerdes]

I think the discussion has drifted a little from the help request.
I have not been to the observatory for many years but what they require will be a replacement for their old LX200 16" Meade that has given service for many years (since Halley's comet) and now cannot be repaired due to no parts for the electronics system.

It is mounted in the dome on a pier that gives suitable access to the general public to view items through the eyepiece. It did at one time have a C5 telescope as a piggy back that was connected to a Meade 416 CCD. This I assume has long been upgraded but they probably will still need something similar.

I would expect that something in the 16-20 inch planewave area with manual viewing available along with the computerised viewing costing lots of dollars and someone to baby it every viewing night will be required.

I don't know of anyone at the moment who possesses such an instrument for evaluation to help Geoff out.

Barry

[Peter.M]

Originally Posted by GOTO

So, the only option so far, other than a new 16inch, looks like a 17-20inch Planewave on a Mathis Instrument mount

I have no experience with larger telescopes, or much visual observation for that matter, But you could also consider RCOS telescopes.

[tlgerdes (Trevor)]

Also, as stated as this is visual, part of the appeal for people is actually looking through a telescope. Putting a camera and displaying it on a screen, may as well go and look at Youtube.

For most people who visit the observatory this is their first time seeing a telescope let alone looking through so the "theatrics" of the experience are very important.

[mental4astro (Alexander)]

I disagree with you Trevor.

It comes down to actually seeing something in the middle of Sydney or not in this case.

But, you also flippantly mention YouTube. A couple of years back I volunteered to show the night sky to kids in NSW's only kids palliative hospital. I took my modest little C5 & my 17.5" dob, along with a modified webcam just on a hunch I had. I did not even assemble the dob that night. What that little webcam & C5 showed those no YouTube clip can replace. Those kids had no hope in hell of looking into an eyepiece. You tell those kids that they were ripped off when they saw Saturn come into view, the image shaking slowing coming to a standstill. Tell them they may as well look at a DVD when we were able to slew the camera to different parts of the moon. The thrill these kids had that night of actually seeing a scope, even a little C5, tell them to go see a YouTube clip. Tell them their experience was a fraudulent one. After what I experienced that night, I have never questioned the legitimacy, value & power of a video camera as useful tool in astronomy.

One's experience at a telescope is not cheapened if a camera is needed. Folks are smart enough to understand what a live image is. It is not ripping them of if this is the only way to show them a galaxy. As things stand now at Onservatory Hill, you have no chance in appreciating any galaxy, let lone the full expanse of M42, or even Omega Centauri to what it can really offer. To say otherwise is not understanding the situation of both the observatory & its night sky - this, coming from a die hard visual man.

A video camera also doesn't mean no direct viewing through the scope - the Moon & planets don't require a camera. But it makes all the difference in both seeing a DSO, AND appealing to the younger generations who best relate to a monitor than an eyepiece. It is easy to forget that a novice's eyes cannot see what we can!

[Bassnut (Fred)]

Originally Posted by tlgerdes

Also, as stated as this is visual, part of the appeal for people is actually looking through a telescope. Putting a camera and displaying it on a screen, may as well go and look at Youtube.

For most people who visit the observatory this is their first time seeing a telescope let alone looking through so the "theatrics" of the experience are very important.

Exactly . Geoff came to a ASNSW meeting and explained what he wanted. Off the shelf and visual only. A pic on a screen is a daft idea, completely misses the point. I think fancy ultra wide field Is daft too, a close look at the moon beats a bunch of bright high quality dots any day