Quote:
Originally Posted by Astro_Bot
In my 10" LX200-ACF, I've had 420x on a night of good seeing and could have gone higher if I had a shorter focal length eyepiece (the image was crystal clear and steady at that power).
|
That's a $5,000 telescope you have there. That's a bit more than the 10" collapsible tube Skywatcher I recommended which has GOTO and tracking for $1,700
Quote:
Originally Posted by Astro_Bot
I've looked through 10" and 12" dobs of GSO and Skywatcher origin at many a star party and really can't say I'm impressed, even with the larger 12" aperture - I'm not saying they're bad, but they're not as good as you claim them to be.
|
There's the rub. It takes a lot more effort and know how to set up a Newtonian telescope properly, so that it can perform at it's maximum capability. I would guess probably 75% of the 10" and 12" Newtonians you have looked through at star parties haven't been properly set up. In addition a lot of people have the know how but just can't be bothered to set their newtonian telescope up properly. I assumed Rob would know how to set the scope up properly on the basis that he already owns a Newtonian. If he doesn't he lives in Sydney with ready access to plenty of people who would be happy to help him. What I can tell you is that a lot of people with much larger Newtonians (ie well over 20") don't have the know how or make the effort to set their telescope up properly. 3 seconds at the eyepiece of their telescope quickly tells you that. I don't say anything, I just don't waste my time looking through it again. I know of one person who owns a premium 18" telescope who has nfi how to set it up and collimate it, despite being told it wasn't right. Consequently his scope will badly underperform every single time he uses it.
There is no argument from me that its a lot easier to get a SCT performing at its best than a Newtonian. Generally a SCT comes collimated out of the box and it will rarely move.
Quote:
Originally Posted by Astro_Bot
"On the other hand, at every star party I've been to, I receive complements on the relative quality of the view through my SCT and frequently hear things like, "Wow, you've got the best view here". I don't call that "mediocre optics".
|
Unless you want to go home disappointed I suggest you never set it up anywhere near my 10" or 14" SDM's, Rick Petrie's 14" Skywatcher, or Rod Berry's 20" SDM
Quote:
Originally Posted by SkyWatch
I assume that when John talks "physics", he is referring to the large secondary of the SCT's that causes a loss of contrast and therefore slightly poorer view to an equivalent newt or refractor. This however would only be if the newt had an optimised (minimum size) secondary
|
Hi Dean,
There's a bit more to it than just the size of the secondary central obstruction (CO), although that in itself is important enough. In addition the SCT, ACF or MCT have an additional 2 refractive surfaces, as well as the 2 similar reflective surfaces of a Newtonian.
I have attached a table which shows the light loss comparing an unobstructed 10" aperture, to an optimised Newtonian with an 18% CO, to a mass produced Newtonian with a 25% CO and a SCT design with a 34% CO. Some SCT models have the CO down as low as 32%, some are as high as 38%. Some Newtonians have a lower CO as well. As you can see from the table the Optimised Newtonian has an 11.5% light gathering advantage over the SCT design and the mass produced Newtonian has an 8.2% light gathering advantage over the SCT design. The experts who have conducted laboratory tests on human visual perception will tell you that it takes a 5% change in light intensity to be perceptible to the human eye. In both cases the numbers are well in excess of this. Notwithstanding any perceptible difference in the view on bright targets it can make a significant difference in the observers ability to detect targets on the verge of visibility for a given aperture. In essence a 10" Newtonian will see "deeper" on threshold targets than a 10" SCT.
In addition the larger central obstruction and additional refractive and reflective surfaces of the SCT design cause further degradation of the image quality and reduce contrast. Each optical surface in a system introduces a small degree of light scatter and diffraction. The less of these surfaces the better.
If you have a look at Damien Peach's Optical Simulation you will see the effect of a change in Central Obstruction from 20% to 30% in part 2 of the simulation examples. It's not huge but noticeable.
http://www.damianpeach.com/simulation.htm
This website explains the effects of central obstruction on the MTF curves
http://www.telescope-optics.net/obstruction.htm
In addition to the central obstruction effect the additional refractive surfaces cause further image degradation.
Cheers,
John B