Hi Bobby & All,
Quote:
Originally Posted by Bobbyoutback
That was an excellent post !
Just a question regarding say a F/5 system verse a F/6 with both having the same aperture , the faster scope will need to be more accurately collimated ' may suffer coma & be harder on eyepieces plus with the larger central obstruction = softer contrast , would you agree 
Cheers
Bobby |
Apologies to Yoda but there is no "more accurately" here, either the 'scope is collimated or not. There is no "try". An f/5 is no more difficult to collimate than an f/6, or an f/8 for that matter because the same techniques and/or tools are used. Yes, simple eyepiece designs are tolerated far better in slower 'scopes than fast ones -- they show fewer and less obtrusive aberrations.
Having said that, the slower system is a bit more tolerant of slight mis-collimation than the faster one. You would be hard pressed to pick the difference between slightly imperfect and perfect at say f/6, but at f/4, the difference is fairly obvious. Though I used a traditional collimation tool (Cheshire) until recently, I have now moved to a barlowed laser that is simply quicker and more convenient in use (particularly when you are on your own and the focuser is 3m away from the collimation nobs bolts).
I know there is a movement among observers toward faster (large) Newtonians and is is becoming increasingly common to see (particularly in the U.S) 'scopes like say a 24" f/3.5 or even faster. Mel Bartels recently finished a 25" f/2.8.
Yes, there are some attributes that make such a system attractive like having both feet on the ground while you observe (nearly all the time), not having to invest in tall, cumbersome difficult to transport ladders etc. If collimated well, and the 'scope holds collimation well at all angles, a coma corrector (like a Paracorr) can deliver very good images, well corrected over a wide field. There some drawbacks however (isn't there always)?
These very fast Newtonians do require a quite large secondary mirror and this will play a role in contrast degradation particularly on planetary images when the seeing is either very good or excellent.
Last year I purchased a 25" f/5 classic Obsession second-hand in the U.S and had it shipped out here. The guy who sold it to me used the money to put a deposit (yes, just a deposit) on a 24" f3.3 from New Moon telescopes that is now nearing delivery. That 'scope has a nearly 7" secondary mirror (yes, you read that right a 7"). The 'scope will only ever require one step to observe when at zenith (a very nice thing) but, (this is the down-side) is also 29% obstructed. The mirrors in these ultra fast 'scopes cost a lotta-lotta money once the f/stop becomes less than f/4 because they are so difficult (time consuming and labour-intensive) to figure and parabolise. But, horses for courses, he lives in an urban environment and all his observing involves a 45 minute drive + set-up. So I can understand (even if I don't agree) with his choice.
Okay, the 'scope I bought is probably a bit longer in focal length than I'd want. If I could have everything and money were no object I would manufacture something bespoke for myself, I'd pick a 25" f/4.3 because you can still use a 4" secondary mirror and the central obstruction is kept below 20% **and** you don't absolutely need a coma corrector at that f/stop (It would probably be desirable though) ... and you're one or two steps down the ladder than f/5.
The other thing is, that being a truss-tube dob (getting back to that "tolerant" thing), f/4.3 while still pretty twitchy collimation-wise is quite a bit more forgiving than f/3.3. Truss-tube dobs unless they are built like battleships do suffer a little bit of sag at different angles of elevation. For f/3.3, it would need to be as tight as a drum to be collimated at all angles.
But, given my own circumstances, the 'scope I purchased (and it is the first telescope I "bought" in my life) doesn't need to travel (happily, I live under excellent skies), it stays permanently assembled and just rolls out to observe. I spent a few hundred dollars on a sturdy, well-engineered ladder so I feel quite secure even four and five steps up in total darkness. One very nice thing about f/5 is that the secondary mirror is just 3.5" resulting in a teensy-weensy 14% central obstruction. Save for diffraction spikes on bright stars, the images are genuinely refractor-like. At f/5 coma is only an issue at the very edges of low-power fields without a coma corrector.
The other nice thing is this: I landed this 'scope that was almost brand-new (had only been assembled and used a half-dozen times) with every Obsession-extra, bell and whistle etc (Nexus DSC + Servocat etc) (Galaxy mirror 0.94 Strehl ratio tested and certified), shipped to Australia, tax paid for ... $17,200
The f/3.3 New Moon 'scope (brand-new) he bought cost all-but $30,000 AUD + Servocat + DSC + import taxes and fees + shipping etc -- so about $41K AUD if delivered here.
I'm happy to walk up and down a few ladder steps for the rest of my observing career for that magnitude of saving.
Before I end, it would be remiss of me not to commend to all readers the excellent advice provided by
Alex Massey above that undoubtedly took some time to prepare and type it. It is all good stuff and well worth knowing. These relatively inexpensive Chinese or Taiwanese Dob/Newtonians are really quite good value for money, can be improved upon and made to work even better very cheaply and generally provide a whole lot of bang for buck. As you can tell, I am a fan of Newtonian telescopes -- I make no apology for that. They really are a great choice for both beginning or advanced/expert observers alike.
Best,
L.