Hi
The optics are from what I've heard, quite good, but its the other bits that could pose some issues
James had done a nice review on this scope here http://deepspaceplace.com/at12in.php
It's still good value for the price, if you can correct the other issues
Using this calculator the required diagonal offset is approx 5.56mm which is within the error margin of the measured value. The mirror is undersized, and doesn't even provide 100% illumination at the center of the image. Whilst this is not a big problem, I have replaced my secondary with a larger one (4 inch).
A 4" secondary, that's scary!
The original 3.1" secondary provides an acceptable field for visual use. A popular fast system with the imaging crowd, its characteristics might be a little less happy for the visual observer.
I am looking at upgrading to a 12" f4 bintel ota and was wondering if the optics were as good as they claim.
Rick
That's a very broad question, but generally speaking yes the optics are as good or better than claimed (usually "diffraction limited"), bearing in mind the claims are not the same as those being made by the premium brands (eg Strehl Ratio of .96 or better from Mr Royce).
I use a Vxien 8" R200SS astrograph for visual and astrophotographic purposes interchangeably, but maybe that has something to do with the extreme accuracy of their optics?
I use a Vxien 8" R200SS astrograph for visual and astrophotographic purposes interchangeably, but maybe that has something to do with the extreme accuracy of their optics?
I have a suspicion Vixen source optics from a multitude of makers and perhaps only re-figure at best. There's nothing wrong with a fast Newtonian, other than coma which can be corrected. Or the larger obstruction than reduces contrast and limits them as regard planets and double stars.
But I like planets and double stars, so my 6" F8 Newt has only a 1 inch secondary. That 6" scope certainly splits double stars way better than a 4" Vixen fluorite. But because there is still an obstruction the images are "softer" than in the fluorite.
It is a fact that exceptionally fine optics can make up for some of the loss of contrast by obstruction.
Thank all for the info. I am currently using an old equtorial pier mount with a 8" f6 primary (it could easily carry a 16' newtonian ota if I could find one) I have recently refurbished it with a coultar optical mirror set 1/8 wave and the images look pin sharp. It seems to me that fast optics are pre-requisite to avoid digital noise. My ideal choice would be a 12" f6 newtonian ota but I would need a bigger car or a dome. Large non dob newtonians seem to be hard to find.
I have a suspicion Vixen source optics from a multitude of makers and perhaps only re-figure at best. There's nothing wrong with a fast Newtonian, other than coma which can be corrected. Or the larger obstruction than reduces contrast and limits them as regard planets and double stars.
But I like planets and double stars, so my 6" F8 Newt has only a 1 inch secondary. That 6" scope certainly splits double stars way better than a 4" Vixen fluorite. But because there is still an obstruction the images are "softer" than in the fluorite.
It is a fact that exceptionally fine optics can make up for some of the loss of contrast by obstruction.
Indeed. Providing a 100% fully illuminated field of the required size for imaging (usually greater than a 1" diameter) will necessitate a certain secondary size. Not because of optic quality but simply because of physics.
The calculator that James links to in his review of the AT12IN calculates the fully illuminated field size for a given set of telescope dimensions (mirror size, focal length, distance from secondary to the focal plane). "quality of the optics" does come into the equation. He determined that the secondary was too small to provide 100% illumination even at the centre of the focal plane, so he upgraded. I am thinking of doing a similar upgrade to my GSO 200mm f4 (AT8IN) for the same reason.
Anyway, the point im trying to make is that a newt "optimised for photography" will require a larger secondary than would otherwise be necessary for visual use. A larger secondary means lower contrast for visual. Precision optics (Vixen, Orion etc) may help to make that contrast reduction less obvious, but it cant take it away.
Indeed. Providing a 100% fully illuminated field of the required size for imaging (usually greater than a 1" diameter) will necessitate a certain secondary size. Not because of optic quality but simply because of physics.
Precision optics (Vixen, Orion etc) may help to make that contrast reduction less obvious, but it cant take it away.
Well to me precision reflector optics is something of the order of AP's Mak.
For a high contrast Newtonian, something like a 15% obstruction by Dia is recommended. My 6" newt has an 18% obstruction.
My 10" newt about an 21% obstruction. AP's Mak has a has 23% obstruction, which is starting to get kind largish. But I wouldn't expect my 10" to deliver more contrast than an AP Mak. In fact I'd expect the Mak to do better because of relative optical perfection in everything else.
But then a 6" newt with an extreme FL or F/12 an very average optics and a tiny obstruction can compete with the best. You could buy a 5" Tak tube for $6000 and find cheap long FL newt costing $300 could give it a run for the money.
Going back to the original issue, on the face of it the 3.5" Diag should give a 1 inch fully illuminated field. But my guess that is the nominal size, and after the bevel and secondary holder he gets only a 3.1" real Diag. This only gives 0.5" full illumination.
I suspect the nominal 4" gives him a real working 3.5".
point taken in respect to 'precision optics'. No question about the quality of the gear you refer to there. my comments related more to the previous post that implied 'optical quality' could somehow overcome the physical limitations of a scope optimised for visual or imaging and instead allow the said scope to excel at both...
Back to the GSO secondary size, I have found with my 200mm f4 that the position of focal plane is actually further away from the secondary than it really needs to be, and at that distance my secondary (according to the OWL Diagonal calculator) doesn't actually give 100% illumination even in the centre of the field. The secondary is simply too small. The solution is either a different slightly longer optical tube to bring the focal plane closer to the secondary, or a larger secondary (or just accept that the scope is performing less efficiently than it needs to). If that was the case with James' 300mm f4 too, then I guess that would necessitate the secondary mirror upgrade.
Yes you have it right. For both the 200mm and 300mm, the default image plane is a long way out from the tube, thus it needs a bigger secondary. I fixed it on the 300mm with a bigger secondary (the focal plane was in the perfect spot for my camera/corrector combination).
On my 200mm version, I moved the focuser + secondary down the tube a bit to fix the problem.
Meade made them in the 1980's 16" f4.5 before dob's kicked of. I nearly bought one but was put of by the cardboard (sono?)tube.
Yep there dam big its called the starfinder 16" image below. I had a starfinder eq mount myself and they are built that tough its crazy, they make the eq6 look like a kids toy in terms of build. The only issue is the PE and that leaves them only good for visual. They also made a dob called the starfiinder but just add eq to your search.
Hi Wulfgar , I have a 5 inch f/13 reflector ( 1710mm ) I built in NZ many years ago and she has a 3/4 inch diagonal and on the moon and the planets the only scope that comes close and its my Takahashi Mewlon 210 ( 2410mm ) , a long focal length reflector's are very special .
Here is a photo of my 5 inch ( Goldie )
Brian.
Quote:
Originally Posted by wulfgar
I have a suspicion Vixen source optics from a multitude of makers and perhaps only re-figure at best. There's nothing wrong with a fast Newtonian, other than coma which can be corrected. Or the larger obstruction than reduces contrast and limits them as regard planets and double stars.
But I like planets and double stars, so my 6" F8 Newt has only a 1 inch secondary. That 6" scope certainly splits double stars way better than a 4" Vixen fluorite. But because there is still an obstruction the images are "softer" than in the fluorite.
It is a fact that exceptionally fine optics can make up for some of the loss of contrast by obstruction.
Last edited by brian nordstrom; 18-12-2012 at 08:58 PM.
Reason: spelling
Hi Wulfgar , I have a 5 inch f/13 reflector ( 1710mm ) I built in NZ many years ago and she has a 3/4 inch diagonal and on the moon and the planets the only scope that comes close and its my Takahashi Mewlon 210 ( 2410mm ) , a long focal length reflector's are very special .
Here is a photo of my 5 inch ( Goldie )
Brian.
It's long known standard. An F12 Newt with average optics can take on the best. That long focal length isn't as sensitive to optical faults as short focal lengths are. That's something fast Newtonian fans need to be aware of......................your F4 optics have to be good to work well. Any figure errors get multiplied by the fast optics. For that matter a similar rule applies to fast refractors.
Originally I used a 3/4 inch Royal secondary and a recessed eyepiece collar. But I got tired of Hex keying the eyepieces into the collar to change them, so I shifted the main mirror up an inch and put in the one inch secondary.
The downside of course is the step ladder required for Newtonian focal lengths over 50".
Hi Wulfgar , I have a 5 inch f/13 reflector ( 1710mm ) I built in NZ many years ago and she has a 3/4 inch diagonal and on the moon and the planets the only scope that comes close and its my Takahashi Mewlon 210 ( 2410mm ) , a long focal length reflector's are very special . 
OOPs , here is the 3/4 inch diagonal , sorry .
