I will be using it for a variety of tasks.
Viewing, imaging, comet hunting, supernova hunting etc.
Just wanted the best gear for it, no mucking about with mount scope etc.. Just turn on and go.
Greg,
The smaller scopes are more popular, as price is a major step up for the 20". This is why there are more images from the smaller scopes.
There are a quite few 20" scopes around, but are more used in universities and professional use, thus not many imaging and posting on the net.
With nearly 40% central obstruction, how does this scope compare to a top-quality 18" Newtonian with 20-25% central obstruction in terms of contrast, resolution, sharpness and overall clarity under moderately light-polluted sky?
With nearly 40% central obstruction, how does this scope compare to a top-quality 18" Newtonian with 20-25% central obstruction in terms of contrast, resolution, sharpness and overall clarity under moderately light-polluted sky?
Visually, it might have a run for its money against a top quality 18" newtonian... That said, photographically, you'd be hard pressed to find an 18" newt to give as good resolution, an image circle and flat field of the same size without increasing the central obstruction to around the same size.. I think the CDK's are about as good as you can get when looking for an all round scope... they excel in imaging, however they do offer pleasing views through an eyepiece, something that other similar designs do not (usually) Its rather hard to find a good all rounder these days.. the CDK's have slid nicely into that void and filled it right up.
With nearly 40% central obstruction, how does this scope compare to a top-quality 18" Newtonian with 20-25% central obstruction in terms of contrast, resolution, sharpness and overall clarity under moderately light-polluted sky?
In Moderately light polluted skies, the CDK wins hands down. Even my 22" Newtonian pales..
Why ?, well, yes the 22" newtonian does have better contrast, but when light pollution is amist, then forget it.. The light pollution brings up the contrast level anyway, so you loose that edge (As for the 18", dont matter at all, the CDK would have better contrast due to the larger area of glass.).
The corrected views produced from the CDK or RC, produces a sharper better defined view all over the field, as coma and other distortions are gone. Unless you add a field corrector to the newtonian, it will loose here too. Sharpness, overall clarity, the CDK's and the RC's will produce the best images (Refractors aside when equal aperature). Again, you need to correct for distortions on a Newtonian, and in fact the Classical Cassegrain can produce pin point stars, as long as its corrected.
Trevor - I'd take the parkes dish, provided someone could make the bottom of the dish 99% reflective, and turn the transmitter into a 2ndry mirror that would be SWEEET!
I Also Prefer Optical Scopes But First Ill Need A Decent Imager My Nexstar 6 Se Has Never Seen A Ccd Or Countrie Skiesbut Thats Why I Got A 12 Inche No Need For A Ccd Then
Im 13 Years Old I Must Be The Luckiest Kid In The World
The corrected views produced from the CDK or RC, produces a sharper better defined view all over the field, as coma and other distortions are gone. Unless you add a field corrector to the newtonian, it will loose here too. Sharpness, overall clarity, the CDK's and the RC's will produce the best images (Refractors aside when equal aperature).
A corrected DK ( CDK ) is just Dall Kirkham cass with a coma correcter/ field flattener ( typical coma in a DK is twice as bad as the equivelent Newtonian) . You wouldn't do astrophotograghy with a DK without a corrector, just as you wouldn't do it with a Newtonian without a corrector.
Although a 25% obstructed instrument will always deliver a higher _range_ of contrast levels than a 40% obstructed one, I think the point is moot for instruments operating at the prime focus for deep sky imaging.
I will be using it for a variety of tasks.
Viewing, imaging, comet hunting, supernova hunting etc.
Just wanted the best gear for it, no mucking about with mount scope etc.. Just turn on and go.
Greg,
The smaller scopes are more popular, as price is a major step up for the 20". This is why there are more images from the smaller scopes.
There are a quite few 20" scopes around, but are more used in universities and professional use, thus not many imaging and posting on the net.
Theo.
Well you've made an excellent choice. I know Roland Christen was very impressed with them and he is not easily impressed.
From my checking out on the net it seems to me that these CDKs rule the skies at the moment for performance and value for money.
Must be hurting RCOS sales. 20 inches, better performance at half the price.
Hey Greg,
Well, the RC's just need the flattner to be competitive performance wise, but additional benifits with the RC like the glass and manufacture used for the mirrors etc etc, just place the price too far out for most.
This CDK design has only been around a very short time, and thus the acceleration of sales and reported abilities.
One, as i mentioned in an earlier post, was its collimation, and ease of adjustments. This was paramount to me if i was to enjoy the investment.
An example (Painful too by the way) was 2 nights ago. I walked into my dark observatory and assumed i parked the scope the other way (Pointing east), so while walking, i banged my head hard against the front ring of the scope (Was pointing west). Jeepers it hurt, and i was sure i must have knocked the collimation out. A check yesterday verified it was out by a bees d%$%^, so again i spent 2 minutes re collimating it by adjusting 1 screw to centre the secondary back in the middle. Image is attached on the ease of adjustment. Didnt need to be done, but because its so easy to do, i became a fuss pot, and did it. Still got the bump on my forhead to remind me of the event.
Sales will only effect he 20" and below sizes, as i have only heard of one 40" bieng made in the U.S.A for a University, so sales should be OK on the larger RC scopes.
I have been enjoying my time visually at the moment, and havent spent much time imaging. Which is amazing, as i spent 90% of the time imaging before, now its reversed. If your ever in Melbourne, pop in and checkit out if your seriously looking at getting one. Thats what i did before i got one.
Just hanging out for the x 0.66 reducer, which will make my scope into a 20" f4.5 widefield Astrograph.
I have been enjoying my time visually at the moment, and havent spent much time imaging. Which is amazing, as i spent 90% of the time imaging before, now its reversed. If your ever in Melbourne, pop in and checkit out if your seriously looking at getting one. Thats what i did before i got one.
Just hanging out for the x 0.66 reducer, which will make my scope into a 20" f4.5 widefield Astrograph.
..18" scopes are way cool guys but you are comparing apples and oranges 
but Thats Why I Got A 12 Inche No Need For A Ccd Then
