Why no Dall Kirkhams?

[Paramount]

Hi all
I don't know if this is the right place to post this but as it is related to imaging I thought I'd put it here. I am currently using a TMB 115 f7 for all my imagng and next year I am planning on getting A 10" RC scope (an Astrosib as it is now available where I live). As I always look around before I buy anything of that expense and while doing so I have noticed several firms doing optimised or corrected Dall Kirkham telescopes which according to all the advertising the design is ideally suited to imaging and in most areas out performs the RC scope. I appreciate that with a RC scope a field flattener will have to be added into the occasion especially if a large ccd chip is going to be used, this is something the corrected Dall Kirkhams don't need as the corrector is already built in.
My question is this, looking at all the forums and the multitude of fantastic and varied images posted, how come does it appear that none of the imagers are using optmised or corrected Dall Kirkhams? There are refractors, newtonians, SCTs, RCs, Cassegrains, camera lenses, Maksutov variations but no ODKs or CDKs
I would be very interested to see peoples views
Best wishes
Gordon

[Satchmo]

Originally Posted by Paramount

My question is this, looking at all the forums and the multitude of fantastic and varied images posted, how come does it appear that none of the imagers are using optmised or corrected Dall Kirkhams? There are refractors, newtonians, SCTs, RCs, Cassegrains, camera lenses, Maksutov variations but no ODKs or CDKs"

Hi Gordon, Corrected DK's are a fairly recent phenomenon in physical form, although I remember some designs flying around on the interenet as much as 10 years ago amongst ray tracing enthusiasts.

A classic DK has typically twice the coma of the equivelent Newtonian . For example an F8 DK has the same coma as an F4 Newtonian. More importantly the field is curved in the opposite direction to a Newt, so a number of off the shelf coma correctors fro Newts won't work with them.

The corrected DK uses typically two to three elements above the focal plane to correct coma, and they are typically free of astigmatism at the edge of the field. An RC without a corrector lens set ( field flattener ) , has no coma but significant astigmatism at the edge of the field.

Corrected DK's are easier to make than RC's as they have a much less corrected elipsoidal primary instead of the RC's hyperboloid, and the secondary is spherical rather than hyperboloid. You might wonder why corrected DK's aren't covering the planet.

The answer is that to achieve this performance there are significantly tight tolerance on the lens centering and spacing. I believe 0.2 mm on centration in some cases. Only high end manufacturers can build tubes that will enable the kind of stability that the corrected DK demands. Don't expect any of the Taiwanese manufacturers to be making them any time soon

[avandonk]

Yep like everything in the Universe there is no free lunch. The trick is to get a compromise with the things that generate the optical faults and make sure no one parameter dominates. If there was a simple perfect scope we would all have them!

Only one thing will overcome this and that is the inherently exponential escalating cost of manufacture to attain near optical perfection.

Every design has its strengths and weaknesses. The thing is to get something that works best for your imaging interest. If something is not generally used then there could be practical reasons.

I can see your problem. How do you better a reasonably fast well corrected refractor? A TMB no less.

Not easy!

Bert

[sjastro]

Originally Posted by Satchmo

The answer is that to achieve this performance there are significantly tight tolerance on the lens centering and spacing. I believe 0.2 mm on centration in some cases. Only high end manufacturers can build tubes that will enable the kind of stability that the corrected DK demands. Don't expect any of the Taiwanese manufacturers to be making them any time soon

The decentration tolerance for the primary mirror is <0.1mm, the secondary is 0.15mm max.

I had the misfortune of owning a corrected Dall Kirkham. It was by far the worst system I have ever come across in terms of optical performance and general quality.

The corrector lens was mounted independantly of the primary mirror and despite the fact the secondary mirror was spotted it was impossible for the end user to fine adjust the secondary mirror.

The primary mirror of course had no optical centre and it was anyone's guess how much the primary mirror might have been decentered.

The biggest shame of all is that it was an Australian product.

Leaving aside the quality and design issues, the very fine tolerances for corrected Dall Kirkhams makes it very difficult to obtain maximum performance.

Regards

Steven

[Gama]

Originally Posted by sjastro

The decentration tolerance for the primary mirror is <0.1mm, the secondary is 0.15mm max.

I had the misfortune of owning a corrected Dall Kirkham. It was by far the worst system I have ever come across in terms of optical performance and general quality.

The corrector lens was mounted independantly of the primary mirror and despite the fact the secondary mirror was spotted it was impossible for the end user to fine adjust the secondary mirror.

The primary mirror of course had no optical centre and it was anyone's guess how much the primary mirror might have been decentered.

The biggest shame of all is that it was an Australian product.

Leaving aside the quality and design issues, the very fine tolerances for corrected Dall Kirkhams makes it very difficult to obtain maximum performance.

Regards

Steven

Steven, i think your scope was either a dud, or not a good variant of the whatever design it was.
First, the spacing for the mirrors is actually +/- 1mm, not .1 etc. I dont know the scope you are reffering to, but the specs i give is for the Planewave 20" CDK.
This distance is maintained by moving the secondary, as the primary and lens group are fixed and aligned by laser at the factory.
To get collimation takes around 5 minutes. The use of the supplied ronchi screen in a ronchi occular mounted on the back gets the distance smack on in minutes. Why minutes ?, because you need to move to the other end to adjust the collimation screws, and walking back and forth takes a little time.
The CDK will outperform the RC. Theres also much speculation that the RC can never be collimated perfectly due to its inherent tight design requirments. But then you also have those that disagree and find collimation a breeze. I favour the ones who know how to collimate and get the collimation right, as to those that say it cant be done right.
I also remember reading your post (Which mysteriously has disapeared from IIS), but from memory your scope (Wont say the name of the scope) is a variant of some type corrected cassegrain as it does not give the optical design specification.

I have used Rc's in the past, and i have used CDK's recently, and i have done collimation on both of them, with the RC being more fussy, but still able to do it.

Originally Posted by sjastro

The primary mirror of course had no optical centre and it was anyone's guess how much the primary mirror might have been decentered."

The CDK has NO optical axis because it uses a spherical secondary, and so has a huge centering tolerance. So unless the mirror falls right out of the frame, its still centred. But as noted, the spacing needs to be right. But having said that, i have looked thru some CDK's when the collimation was a little out, and the views were unoticable. So again, i would look at the manufacturer of the scope and design.

Originally Posted by sjastro

I had the misfortune of owning a corrected Dall Kirkham. It was by far the worst system I have ever come across in terms of optical performance and general quality.

This is totally uncalled for, as you can't generalise your problem with all CDK's, especially when the optical design of the dud scope was not stated as a CDK.
There are many manufacturers of CDK's around the world now, and again with their own differences in optical design. But that does not mean all are the same design and build.
Of course i can only comment on the Ceravolo and Planewave system, as this is the design i have studied and sifted thru all the data, personally inspected and used, and in the end, ordered one after seeing it in action by different ownwers. Now just a couple more months for delivery.

Theo

[Satchmo]

Originally Posted by Gama

The CDK has NO optical axis because it uses a spherical secondary, and so has a huge centering tolerance. So unless the mirror falls right out of the frame, its still centred.

Only the secondary mirror has no optical axis if it is possible to tilt it exactly in translation of its front surface ( which is never in reality possible) . The primary mirror optical axis and correcting lens set must in a corrected DK be initially centered to the optical axis by a fraction of a milimeter. 'Decenter tolerance ' is _not_ the same thing as 'optical spacing' tolerance : you seem to confuse the two.

[Gama]

Originally Posted by Satchmo

Only the secondary mirror has no optical axis if it is possible to tilt it exactly in translation of its front surface ( which is never in reality possible) . The primary mirror optical axis and correcting lens set must in a corrected DK be initially centered to the optical axis by a fraction of a milimeter. 'Decenter tolerance ' is _not_ the same thing as 'optical spacing' tolerance : you seem to confuse the two.

It was the secondary to the main mirror i was talking about, just bad syntax i suppose .
Which is why i said the main mirror and lens group is laser aligned.

Theo

[netwolf]

I am surprised given the less demanding spherical primary that this type of scope is not mass produced by Chinese manufacturers. Instead they have gone for the RC. Who makes an affordable one?

[sjastro]

Yes I was sold a dud telescope and no I wasn't implying that a CDK design would lead to poor performance. Perhaps my original statement was misleading, what I meant to say was that poor quality in manufacturing resulted in poor performance.

I have the design specs of the proto type which state it is a CDK and amongst other things the decentration and spacing tolerances.
I have also been informed that even the slightest decentration of either the primary and secondary mirrors is disasterous to performance.

As much as I would like to show the design specs in detail, they are not current, and I have learnt (as have the administrators of this group), this particular manufacturer has a very itchy finger in threatening people with legal action (which was why my original article was removed).

If you're interested in the design and what it is called these days I suggest you contact Damien Jones at Prime Optics.

Regards

Steven

[Gama]

Steven, does it say who designed the optical system ?, because the CDK from Cerovola is his own design, and the Planewave design is from David Rowe.
I also agree with you that if the quality is poor, your accuracy will suffer, which in turn effects performance. This is where design comes into it, and again as you said, if either mirror was out by a bee's knees, then good bye sweet views, wheras other designs allow a better tolerance in some areas.
This is why many RC users tell you these new cheap RC will not be able to handle collimation or ccuracy etc, etc. But if done right, it should. But time will tell how good they are.
Plus Fahim, the CDK's have only come out recently, where as more people recognise the RC name and the stigma behind it. So it would probably be more a business decision i would say. Bit like the old Beta and VHS, didnt matter if Beta was better, they still opted for the VHS.

I'd like to hear from someone who has one of the new cheap RC's, see how they perform.

Theo

[sjastro]

Originally Posted by Gama

Steven, does it say who designed the optical system ?, because the CDK from Cerovola is his own design, and the Planewave design is from David Rowe.
Theo

Theo,

Prime Optics located in Queensland were the supposed designers. They set the design tolerances for optical spacing and decentration. Whether they were involved at the concept stage I don't know.

Interestingly the telescope was initially advertised as a CDK. After being in possession of the scope for a couple of months (by which stage I knew I had a real disaster on my hands), I was notified by the manufacturer to stop publicly referring to it as a CDK. Suddenly it became a "unique design".

You can draw your own conclusions.

Regards

Steven

[Gama]

Originally Posted by sjastro

Interestingly the telescope was initially advertised as a CDK. After being in possession of the scope for a couple of months (by which stage I knew I had a real disaster on my hands), I was notified by the manufacturer to stop publicly referring to it as a CDK. Suddenly it became a "unique design".

You can draw your own conclusions.

Regards

Steven

Funny you mention that, i read about another manufacturer that advertises his scopes as a Modified Dall Kirkham" (MDK), again here the unique name and yet more versions of the DK.

Theo