It's all done with mirrors ...

[Kunama]

If a 10" F6 mirror (actual focal length of 60.3")
has the following attributes:

PV 1/31.3
RMS 1/70
Strehl 0.992

Would it be considered a reasonable mirror? :question:

[Atmos]

As long as it is nice and reflective :)

[Merlin66]

Matt,
I think you may already know, that's a very good set of specification numbers fo any mirror.

[Kunama]

Quite right Ken,
I decided I was not going to have time to build another scope at the moment and sold the mirror. The very next day Carl Zambuto emailed me the test report for the mirror.

Seller's remorse :shrug:

Anyway I think Phil will have nice views once the mirror gets to Melbourne.

[ariefm71]

Oh dear... must be this one http://www.iceinspace.com.au/forum/showthread.php?t=139048

[ariefm71]

Don't worry Matt, our CN mirror is 1/20 PV, not in the same league as the CZ but not too shabby either... :P

There's a reason I sold it to you. Now Phil needs to make a real nice structure for it!

[Allan]

For sure a good mirror, but a bit of a dinosaur in this day and age. My Lockwood mirror is over 3 times the diameter, but 20% thinner.

Carl did a recent run of ultra thin Quartz mirrors, which also coincided with starting his own coating plant. Now they would be killer planetary mirrors and worth the price of admission.

[issdaol]

Ouch ! I don't think I would have been selling that mirror myself !!

Would have made an awesome 10inch version of your current beast.

[Kunama]

If I had kept it all it would have seen for the foreseeable future would be the inside of a Pelican case. Phil already has a scope that will be a perfect fit for the mirror. Carl did mention that this mirror was guaranteed for a minimum of 50X per inch of aperture without any loss of detail if given the right seeing. Saturn at 500X in 10" scope ....... nice!

Allan, at my age a 'dinosaur' mirror seemed appropriate to own......
I am looking forward to seeing the support cell for your 30" mirror.

[N1]

Matt, grab your current beast and/or some of the Taks and get some eyepiece time! :astron:

[Kunama]

I had some excellent viewing last night, finally a perfect night, spent it in the company of the CN212 and T-Rex.

[N1]

Less than perfect up this way (Sinnie), but hoping for clear skies tonight. GFS looks promising.

I doubt the CN212 would be any worse than the mirror you've sold. Plus it's a functioning telescope.

I probably won't have time on 8/9 April, but if that changes, I might come down to your Southern Hemisphere Tak Convention.

[Allan]

Yes, very funny.

I'm looking forward to seeing the mirror cell as well. Only one day to go and I get to meet John Pratte at the Winter Star Party. He is currently making the best mirror cells in astronomy, and is one of the main reasons Mike Lockwood can make such big mirrors so thin.

[Rac]

Those results will be from using a couder mask with a foucault setup. There is a very good chance the mirror is very very good and of this I have no doubt but it will be unlikely to have a PV 1/31.3 when the whole surface is taken into account.

I wouldn't worry about all that though as that mirror will be awesome.

[Satchmo]

The measurement would be done by sampling the radius of curvature of a handful of symmetrical zones along one axis. Its all idealised to some extent and genuine Strehl ratios can never be gleaned from measuring one axis- but if the mirror is better than 1/10 wave across the whole surface area then it will perform brilliantly .

[ausastronomer]

Carl evaluates his mirrors using zonal focault testing only. Expanding on what Mark said this only evaluates the mirror over a handful of data points and then extrapolates a strehl ratio from this. The selected data points may not be representative of the accuracy of the entire mirror surface. Strehl ratios calculated in this manner will always give an optimistic strehl ratio compared to what would be obtained with interferometric testing of the mirror, which calculates the strehl by sampling hundreds of random data points over the entire surface of the mirror. The foregoing is not a negative in any way, it just means the numbers, as such, are overly optimistic. Any mirror with a true strehl over .90 is going to be very very good and will run to at least 50x per inch of aperture under good seeing.

On the practical side I own a 14"/F4.5 Zambuto mirror and it is exceptional. I have looked through several other ZOC mirrors and they have been exceptional. In summary the numbers are most likely very optimistic, but that doesn't detract from the fact that the mirror will very likely be exceptional.

Cheers
John B

[ausastronomer]

Hi Allan,

Who says John Pratte is making the best Mirror cells in Astronomy, Mike Lockwood?

Cheers
John B

[Allan]

Hello John. I just met your friend Dana at the Winter Star Party, what a nice guy. He told me about some of the observing he's done with you around Kiama.

I've had a great week here, and did get to do a lot of observing with John Pratte, Mike Lockwood, Al Nagler, Tom Peters, Howie Glatter, Joe Wambo and dozens of other fantastic people.

In all seriousness to your question, I would like to ask you if you know of anyone, anywhere who is currently making a better mirror cell than John Pratte. Have you seen his design and the research that has gone in to it?

[ausastronomer]

I haven't seen one of his cells in the flesh but I know exactly how his design works and the mechanics and theory behind it. I don't think his cells are any better than Peter Read's. Their 27 point cells are very similar in terms of pad support placement position with the main difference being that John Pratte uses whiffle tree edge supports on mirrors under 32" and a moving cable sling on mirrors over 32", whereas Peter Read uses a cable sling on all his cells, regardless of size. In addition to that however Peter uses a 27 point cell on all scopes over 24" whereas John Pratte is still only using an 18 point cell on his 25" scope. What I can tell you is that I have seen and used scopes from just about all of the custom US scope makers from the last 30 years, with the exception of JP Astrocraft and I haven't seen one yet that is as well designed and built as Peter Reads SDM's. I have no reason to suspect that the build quality of John Pratte's scopes is any better than the likes of Rob Teeter's or Rick Singmaster's. For starters no one else is using powder coated cast and machined aluminium altitude bearings like Peter has been using for the past 8 years.



Well a good bit of that research was done with Phil Townsend's mirror and your American friends are only telling you a very, very, very small part of the story in regard to that one ;)

Cheers,
John B

[Kunama]

Except Kunama Optical Laboratories ;) :lol:

Those Alt bearings supplied to me by Peter and powder coated locally here really are the "ants pants" of Alt Bearings

[Allan]

Hey John, nice post. This isn't the place for me to divulge conversations I've had with Peter about John Prattes mirror cells. I'm sure you can ask him about that yourself. I have managed to organise Peter, John and Mike to work together, and the result will be a dob like nothing ever seen in Australia.

Also, one doesn't spend 50k ordering a scope without researching the people involved. I can honestly say I probably have more information about Phils case, from every side of the story than anyone.

[Allan]

You need to trademark that name Matt, it just rolls off the tongue.

[sally1jack]

then you would be wrong
phil

[Allan]

Hi Phil. I'm sorry, John brought up your scope in conversation, and I shouldn't have responded by discussing it here. I'll leave it at that.

[Satchmo]

I'm going to second that notion.

[Allan]

Mark, mate, what are you saying. You gave me your side of the story at IISAC. But seriously, it's best we don't do this.

[Satchmo]

( Edited for brevity )

Allan , all I can think is you may have conflated some of the info attributed to me second or third hand into that conversation, which I can understand .

[Allan]

Ok Mark, no worries.

[Satchmo]

Matt - Getting your thread back on topic- I think what you were after in questioning your Zambuto specs was something like this :

0.6 Strehl - not great - ok for low power deep sky

0.8 Equivalent of the old Rayleigh criterion - will resolve doubles to theoretical limit - good for low to medium Deep Sky observing - planets will not handle high power well

0.9 Much improved planetary performance

0.96 Pretty much as good as it gets

0.999 wishful thinking in reality :)

These all assume that the figure causing the degradation is smooth and mainly primary spherical aberration for the sake of simplicity and do not generally take into account degradations due to irregular distortions/astigmatism

Technically 0.96 Strehl mirror can actually have an unacceptable star image and lousy star test - if the figure is not smooth .

Just remember that there is nothing mysterious and high tech about getting these figures . Anyone can put a simple zonal mask on a mirror and use free amateur download software like Figure XP to produce nice figures with PV RMS and Strehl on the printout . Most of the time the tests work very well if used by a knowledgeable and honest operator.

Dont worry that your mirror thickness is antiquated - most good mirrors these days up to 24" are 1.6" thick glass and cools pretty quickly - just have some good fans and you will fine .