New Direction Mewlon 300 to Custom Newt??

[ausastronomer (John Bambury)]

Originally Posted by Shiraz

why not C14 edge.

Surely there can be no meaningful arguments on optical quality: http://www.myastrospace.com/forums/f...-poke-a-c14-at
http://www.airylab.fr/AirylabUS/inde...d=52&Itemid=57

Well there can be because the design paramaters which limit the capabilities of the SCT design as a visual instrument do not affect its ability as an imaging platform, nearly as much. The original poster clearly indicated he has no interest in imaging, so its a pointless excersize directing him to one of Darryl's excellent planetary images. Its a bit like pointing someone to an outstanding DSO image taken with a Ritchey Chretien and saying they make great visual DSO instruments because they take great DSO images. Truth is Ritchey Chretiens make exceptional DSO imaging platforms and very poor visual instruments.

The factors which restrict the performance of a SCT as a visual instrument are:-

Large central Obstruction (>30%)
Additional Air to Glass surfaces (Corrector plate and star diagonal)
Fast F2 primary which introduces field curvature
Narrow filed of view which limits DSO observation.
Closed tube design which inhibits thermal equilibrium of the optics
Corrector Plate which acts as a built in dew magnet.

If you think a SCT can equal a good Newtonian or refractor as a visual instrument you have found a way to defy some of the basic laws of physics.

It's worth noting that some of the factors which affect the SCT design as a visual instrument also affect the corrected Dall Kirkhams like the TAK Mewlon and the Planewave. ie large central obstruction (~30%), additional air to glass surfaces (sub aperture corrector and star diagonal), a closed back mirror cell which inhibits cooling; and a narrow FOV.

Greg Bradley who owns a 17" Planewave has already posted that it isn't a good visual platform. This is due to the reasons I stated above. While the TAK Mewlons have a smaller CO than the Planewave, they still have a 30% (or greater) CO which has a significant affect on the MTF curves.

Cheers,
John B

[ausastronomer (John Bambury)]

Originally Posted by ariefm71

John, I think what makes Tak Cassegrains great (apart from the 1/20th wave optics)

Firstly that is subjective because we don't know how the optical quality is tested. Is it 1/20th wave RMS, 1/20th wave Peak to Valley, or 1/20th wave at the wavefront? However, we do know that TAK optics are excellent irrespective of the numbers, so you don't need to quote them. It's all pretty academic anyway because the skill of the observer and seeing conditions make optics better than about 1/10th wave at the wavefront superfluous. Just for the record my 14" SDM has a 1/40th wave Zambuto primary and a 1/30th wave Protostar Quartz secondary. My 10" SDM with Suchting primary has optics that match the 14".

Originally Posted by ariefm71

I still think the Tak will have an advantage over newtonian due to this, unless you observe from a very dark location.

If that's the case you have found a way to defy some of the basic laws of physics. See my previous post. The Mewlons are very good, there is no doubting that, but they still come up fractionally short of a well set up premium Newtonian due to the large central obstruction and the other factors I mentioned. They are IMO a better visual platform than an equal aperture SCT.

Cheers,
John B

[Shiraz (Ray)]

Originally Posted by ausastronomer

Well there can be because the design paramaters which limit the capabilities of the SCT design as a visual instrument do not affect its ability as an imaging platform, nearly as much. The original poster clearly indicated he has no interest in imaging, so its a pointless excersize directing him to one of Darryl's excellent planetary images. Its a bit like pointing someone to an outstanding DSO image taken with a Ritchey Chretien and saying they make great visual DSO instruments because they take great DSO images. Truth is Ritchey Chretiens make exceptional DSO imaging platforms and very poor visual instruments.

The factors which restrict the performance of a SCT as a visual instrument are:-

Large central Obstruction (>30%)
Additional Air to Glass surfaces (Corrector plate and star diagonal)
Fast F2 primary which introduces field curvature
Narrow filed of view which limits DSO observation.
Closed tube design which inhibits thermal equilibrium of the optics
Corrector Plate which acts as a built in dew magnet.

If you think a SCT can equal a good Newtonian or refractor as a visual instrument you have found a way to defy some of the basic laws of physics.

It's worth noting that some of the factors which affect the SCT design as a visual instrument also affect the corrected Dall Kirkhams like the TAK Mewlon and the Planewave. ie large central obstruction (~30%), additional air to glass surfaces (sub aperture corrector and star diagonal), a closed back mirror cell which inhibits cooling; and a narrow FOV.

Greg Bradley who owns a 17" Planewave has already posted that it isn't a good visual platform. This is due to the reasons I stated above. While the TAK Mewlons have a smaller CO than the Planewave, they still have a 30% (or greater) CO which has a significant affect on the MTF curves.

Cheers,
John B

Errr yes, I agree that a Dob mounted Newtonian would be best, but Phil asked for something that should be mountable on an EM400 for visual use. In my experience, that immediately rules out a big Newt - it is physically very inconvenient to use a Newt on an EQ mount and the OTA would present unacceptable wind loading.

Accepting that Phil does not sound keen on the idea of a Dob and that a visual Newt is not practical on an EM400, the only thing I could think of that would give an advantage over his existing Mewlon 300 was a C14edge, which has a fully corrected field and should provide a bit better views than the Mewlon 300 (~same obstruction, bigger aperture) and be an acceptable load on the EM400.

the links to Darryl's image and the interferometer results are not pointless - they were an attempt to pre-answer the inevitable responses that Celestron optics could not possibly be as good as Tak optics - the links show that the C14 optics more than hold their own (the C14edge tested by the French lab was significantly better than the Mewlon 300 they tested and Darryl's image was clearly produced by some near-perfect C14 optics).

[ausastronomer (John Bambury)]

Originally Posted by Shiraz

Accepting that Phil does not sound keen on the idea of a Dob and that a visual Newt is not practical on an EM400, the only thing I could think of that would give an advantage over his existing Mewlon 300 was a C14edge, which has a fully corrected field and should provide a bit better views than the Mewlon 300 (~same obstruction, bigger aperture) and be an acceptable load on the EM400.

Well I have to agree. If he is hell bent on keeping the Takahashi EM400 mount then the C14 Edge HD may be his best option. A medium to large Newtonian on an equatorial mount is not fun to use in any way shape or form. Rotating tube rings help (if its a tubed scope), but it isn't how I like to observe. To be perfectly honest I think the visual astronomy world has progressed forward from using large equatorial mounts for visual astronomy, regardless of telescope design. I have observed with some of the best and most skilled visual deep sky observers in the Southern and Northern hemispheres and none of them use an equatorially mounted scope for their main observing work, and haven't done so for a couple of decades.


Knowing what I know and having used just about all of the contenders, I would be dumping the Mewlon and the EM400 mount and getting a 12" to 16"/F4 to F5 Truss Dob with Servocat and Argo Navis on it. He would probably end up with some cash back and end up with a premium visual telescope. I am not sure why Phil is scared of dobs. They have tracking and go to and all the niceties you can get on any other telescope design. With the faster optics and more modern scope designs a ladder is a thing of the past. I am only short and can observe at the zenith seated using my 14"/F4.5 SDM and stellar observing chair. That scope blows the doors off a 12" Mewlon and cost about 1/2 as much as a Mewlon 300 and EM400 mount.

Originally Posted by Shiraz

the links to Darryl's image and the interferometer results are not pointless - they were an attempt to pre-answer the inevitable responses that Celestron optics could not possibly be as good as Tak optics - the links show that the C14 optics more than hold their own (the C14edge tested by the French lab was significantly better than the Mewlon 300 they tested and Darryl's image was clearly produced by some near-perfect C14 optics).

Damien Peach has been demonstrating the quality of Celestron Optics as the worlds best solar system imager for at least the last 10 years, using a variety of Celestron scopes.

Cheers,
John B

[issdaol (Phil)]

Thanks for all the new suggestions.

Having had multiple SCT's before and comparing them to the optical quality, mechanical and electronic quality of Mewlons I don't think I would ever go back to using an SCT.

From talking to a couple of other Mewlon 300 owners that also had 14 inch Celestron's the visually image quality does not compare despite the added aperture.

The only issue I have with Newts/Dobsonians is maintenance with collimation as well as ease of observation, as I am aware that optical quality, provided investment is made in top class mirrors, combined build quality, in the case of systems like SDM, can be achieved.

I would really need to look though one of the new breed fast SDM's to see how they compare and talk to owners about their experience with observation (standing or seated i.e. no ladders ) and maintenance.

[AlexN]

If you are worried about maintenance and collimation the worst thing you could possibly do Is buy a fast Newtonian with a truss tube that requires disassembly to transport.

As has been said. The world's best planetary images (and planetary imaging is extremely demanding of optics considering we are talking about imaging at 12 to 15 METRE focal lengths) are imaging with celestron C14's. Paul H. On these forums produced some extremely good images with his c14. Damien peach needs no introduction either.

I tend to feel a lot of people will bag out cheaper scopes in an attempt to convince themselves that purchasing that high priced astro-physics/Tak was a worth while decision.

For visual work or planetary work, a c14 is top of my wish list. That's not to say I think the Mewlon is rubbish. Just that 989.7cm2 of mirror area vs 706.8cm2... Well... Physics rarely tells lies. Larger mirror. More light. More light, more chance of seeing faint details.

I know quality of optics makes a big difference but having owned a c11 myself. I can tell you now the view through a large, high quality sct is sensational. Especially when the seeing supports the focal length.

You know if you sold the Mewlon and em 400 you could probably buy a cpc14 and a 16" dob and have the best of both worlds.

[issdaol (Phil)]

Originally Posted by AlexN

I tend to feel a lot of people will bag out cheaper scopes in an attempt to convince themselves that purchasing that high priced astro-physics/Tak was a worth while decision.

I tend to feel that a lot of people, that bag out people that purchase scopes like AP or Tak, like to convince themselves that their compromise was a worthwhile decision

Seriously though, I know that SCT have improved a lot, but there are still draw backs to them and build quality and reliability is still variable and not up to what I personally want in a scope.

A extremely high quality Dob/Newt like John B has mentioned is definitely something worth considering as the only compromises then become collimation and potentially needing to use a ladder to depending if I consider one of the new fast dobs or not.

Will have to see if I can get my hands on one for a night observing and see what I think first though

[AlexN]

Fair enough. As is always the way, taking scopes for a test drive is the definitive answer. I do know however that a fast Newtonian will likely one serve to drive you mad. I had a 10"f/3.8 for a short while and it was superb when precisely collimated but the slightest shift and the scope was of no better quality than a cheap department store scope.

For the record too Phil, I was at no point bagging out anyone who buys pricey gear nor was I trying to come to terms with my compromises. I've never been one to compromise when I want something.. I've had Tak scopes, Ive had a TMB apo, an apm with TMB lens, had the Williams optics 110/5.6 TEC apo. I've had custom built newts for imaging, rc's, intes micro 8" mak-cass for visual work. That said I was never shy to buy a cheaper alternative ifbthat alternative could outperform the competition. That's why I bought my C11. Sure, I could have hunted about for a 12" Mewlon, or built a classical cassegrain, but after heaps of discussions and the obligatory test drive or two the C11 was the best option. And at the time the AUD was at parity with the usd so with a bit of shopping around and bargaining with a couple of shops I picked up my C11 with the dew shield,a heater strap, the 6.3 reducer and a piggyback setup for my small apo for $3600. The money saved was funnelled into a Moonlite motor focuser. The scope was a dream and all up cost me less than half the price of my other options.

[issdaol (Phil)]

Originally Posted by AlexN

Fair enough. As is always the way, taking scopes for a test drive is the definitive answer. I do know however that a fast Newtonian will likely one serve to drive you mad. I had a 10"f/3.8 for a short while and it was superb when precisely collimated but the slightest shift and the scope was of no better quality than a cheap department store scope.

For the record too Phil, I was at no point bagging out anyone who buys pricey gear nor was I trying to come to terms with my compromises. I've never been one to compromise when I want something.. I've had Tak scopes, Ive had a TMB apo, an apm with TMB lens, had the Williams optics 110/5.6 TEC apo. I've had custom built newts for imaging, rc's, intes micro 8" mak-cass for visual work. That said I was never shy to buy a cheaper alternative ifbthat alternative could outperform the competition. That's why I bought my C11. Sure, I could have hunted about for a 12" Mewlon, or built a classical cassegrain, but after heaps of discussions and the obligatory test drive or two the C11 was the best option. And at the time the AUD was at parity with the usd so with a bit of shopping around and bargaining with a couple of shops I picked up my C11 with the dew shield,a heater strap, the 6.3 reducer and a piggyback setup for my small apo for $3600. The money saved was funnelled into a Moonlite motor focuser. The scope was a dream and all up cost me less than half the price of my other options.

Sounds like you have gone through a lot of scopes :-) Astronomy seems to do that to us all lol

I had similar experiences but my last SCT was a C11 which was the best SCT OTA I had used to date. Was very nice on planets.

Then used a friends Mewlon 250 which made the C11 look very poor comparatively. Plus the outstanding build quality and secondary focuser were a dream to use.

So almost went and got a M250 until I was introduced to someone that had the Mewlon 300 which blew the doors of both of them. Especially on Planetary and Lunar visuals. So though what the hell may as well go the whole distance and get the M300.

If I could have gotten the same usability, build quality & optical quality in a cheaper alternative then certainly would have done so as I certainly don't agree with paying premium dollars for a label, but there are not many alternatives options with the same combinations.

It may be that I keep the M300 and buy a custom newt/dob with zambuto mirrors just for wide field... will have to investigate !!


Cheers

[ariefm71 (Arief)]

Zambuto doesn't do sub f/4 mirrors. A large SDM with Mike Lockwood mirror is in my dream list..

Originally Posted by issdaol

It may be that I keep the M300 and buy a custom newt/dob with zambuto mirrors just for wide field... will have to investigate !!


Cheers