Budget RC announced at NEAF

[jase (Jason)]

Ah yes. Thanks for clarifying Fahim. Next time I'll read the fine print. Was thinking they were hyperbolic. Got a little confused.

[Satchmo]

I guess its alll a trade off. Some people might feel the 4X greater photographic speed @F4.5 was more appealing .

And Fahim is right , these are Newt optics. I was simply indicating that "1/12 wave' probably meant surface RMS rather than wavefront if their Newt mirror specs were anything to go buy.

Originally Posted by jase

Help me out here... looking at the GSO parabolic mirror sets, they list the 16" (AD015) as a 1800mm FL @ F/4.5. Huh? only 1800mm? Great for Nebulae, but very far from a galaxy hunter. Sheez, don't think I'd bother. If I was to purchase a 16" RC, I'd want it to at least 3000mm FL. A fast focal ratio is great, but imaging at slow focal ratios isn't a bad thing with the sensitivity of today's CCD camera. Am I missing something...Mark?

[Satchmo]

Originally Posted by duncan

i hope you are right. But even if you are not lets all hope and pray that these measurements are all equal across the board
that to me seems the most important thing. lets get a standard measurement across the board. Then maybe ametures can make a viable decision.
C'mon all you manufacturers getn your act together!!!!!!!!!!!!!!!!!!:thumbsup :

Most of the amateur market thrive on cut-throat cheap prices on their optics. Thorough testing , certification , and paperwork on optics can cost a lot, in the case of these cheaper optics as much as the optics themselves. I'll bet that if the mass production makers were to pull the occassional mirrors randomly off the production line , officially certify them and charge occordingly few people would buy them. The majority of people seem to be happy to buy cheap if they think they can get away with it. This may be a fair gamble with smaller mirrors.

On the subject of wavefront quality on these RC's , I don't think wavfront quality on a heavily obstructed instrument is as important as the quality of the tube assembly , how well it can be collimated and mainintained and the mounting. For example the final `focal plane ' Strehl on 1/4 wavefront 40% obstructed scope is 0.68 and a 1/10 wavefront 40% obstructed scope is 0.73 . Not much difference really.

The following text from the Star Instruments web site ( supply the optics for RC Optical telescopes) indicate that they think 1/4 wave system is good enough for prime focus photograpghic systems...

" OPTICAL QUALITY

STAR INSTRUMENTS guarantees a minimum of 1/4 wave front, 1/20 wave r.m.s. on all systems.
STAR INSTRUMENTS continues to be concerned with the false advertising claims being made by amateur optical suppliers who claim 1/10 to 1/20 wave optics. These claims tend to confuse the amateur astronomer into believing you must have 1/10 wave optics "

I think the new Chinese entrants on the market will be capable of that. Its weather the mechanics of the tube assemblies do the job properly that will make or break them in the ameteur market . Its fair to say also that the low prices will attract many more people to the RC market who may be less experienced and have lesser standards than someone who remorgaged their house to take their astrophotography to the next level...

[CoombellKid]

Originally Posted by duncan

hi Sachmo.
i hope you are right. But even if you are not lets all hope and pray that these measurements are all equal across the board
that to me seems the most important thing. lets get a standard measurement across the board. Then maybe ametures can make a viable decision.
C'mon all you manufacturers getn your act together!!!!!!!!!!!!!!!!!!:thumbsup :

I dont think it quite works like that in the mass produced market. They will
independantly test their mirrors randomly and peg the results to anything
coming along behind. Checking on occassion to see if they're still producing
their stated wave front.

I think a 16" (AD015) as a 1800mm FL @ F/4.5 would make a great galaxy
hunter for a visual observer , and I'll add... with not a great deal of funds.

regards,CS

[Roger Davis]

For en"light"enment and for information go read:

http://www.galaxyoptics.com/ImageQualityDiscussion.html

[Satchmo]

Ah Roger, don't go complicating the thread by opening another technical `can o'worms' :-)

[Terry B]

Originally Posted by avandonk

Sounds like the GSO RC's. It won't be perfect but at least it will be affordable. One could have lots of fun fixing the shortcomings and then put endless drivel on 'how to' on many sites!

What did a 16" DOB cost a couple of years ago?

Bert

Just need to find out who the Chinese manufacturer is and buy direct from them without a label. Then it probably will be affordable.

[Starkler (Geoff)]

Originally Posted by Satchmo

The majority of people seem to be happy to buy cheap if they think they can get away with it. This may be a fair gamble with smaller mirrors.

Who oh who can one talk to to get a high quality small newtonian mirror?

[Satchmo]

Originally Posted by Starkler

Who oh who can one talk to to get a high quality small newtonian mirror?

I was referring to the smaller Taiwanese Dob scopes.They very adequately fill a niche for 'bang for buck'. 10 years ago the completely USA made dobs by Meade , Coulter and Celestron showed optics with far more inconsistency in quality than the Tawanese jobs IMO.

If you want a `Zambuto' there are no free lunches there

[Starkler (Geoff)]

Originally Posted by Satchmo

If you want a `Zambuto' there are no free lunches there

Mark my question was meant to be taken literally and is prob off topic for this thread.
The only maker I know of happy to produce high quality sub 10" mirrors is Royce in the usa.

[Doomsayer]

It will be interesting to see how these cheap RCs turn out over time. I am building two carbon truss RCs based around cheaper pyrex Star Instruments 12.5"f6.7 optics. Star Instruments are one of the suppliers for the RCOS instruments, as many here will be aware. My dealings with the optician there impressed upon me that one of the most important aspects of the RC figuring process is the mirror material. They cannot guarantee high accuracy with pyrex because of its relatively uncontrolled thermal behaviour in the optics prodn lab - whereas with the zero expansion material they can reliaby produce very high accuacy mirrors.
I have also learnt that large to very large secondary obstructions and oversize baffling are a fact of life with the RC design in practise. Bringing the f ratio down below f8 or 9 increases field curvature and adds to the secondary obstruction in general. I am looking at around %55 secondary baffle obstruction with my f6.6 optics. The high magnfication factor of the secondary adds another degree of difficulty with the RC. I still expect the Corrected Dall Kirkham designs will prove to be better large flat field performers at the the budget end of the cassegrain astrograph market.
cheers
guy

[Satchmo]

Originally Posted by Doomsayer

My dealings with the optician there impressed upon me that one of the most important aspects of the RC figuring process is the mirror material. They cannot guarantee high accuracy with pyrex because of its relatively uncontrolled thermal behaviour in the optics prodn lab - whereas with the zero expansion material they can reliaby produce very high accuacy mirrors.

Guy , I'd have to disagree with that one. Fine annealed Pyrex, not too thick ratio is perfectly capable of taking on a relaible 1/10 PV wavefront figure. As long as the optics are within half a degree C of the surrounding air they will give a reliable reading for the optician and of course the figure is the same regardless of ambient temperature. Using Sital would make the opticians job easier ,as less equalising time between figuring runs. I thought the low cost Star Instruments RC sets were Pyrex ?

Anthony ( Bird) Wesleys latest Jupiter is pretty good testimony of the optical quality achievable with Pyrex..
http://www.iceinspace.com.au/forum/a...se.php?a=42799

I am looking at around %55 secondary baffle obstruction with my f6.6 optics. The high magnfication factor of the secondary adds another degree of difficulty with the RC.

You'll need enhanced coatings on those optics : the total light transmission will only be around 55% otherwise ( 30% light loss is due to the large obstruction).

[Doomsayer]

Yes they are pyrex.
I am only repeating what the optician at Star Instruments said. They have no plans to use pyrex or its equivalent in the future -possibly another alternate zero expansion material - I note that all sizes of their cheaper pyrex optics are now sold out. Both mirror sets I have had 98% coatings done in the US.