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Why no Dall Kirkhams?

GordonH

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GordonH

GordonH

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Posted

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

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barkis

barkis

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Posted

As far as I know the main mirror is an ellipsoid, and the secondary is a sphere. According to Texereau, the coma is 2 to six times as large as in the true Cassegrain. The quote also says that is not too serious an objection in a visual instrument, that is always of limited field width.

The Spherical secondary was chosen because it was easier to test than the Hyperboloid.

The Ellipsoidal Primary, is an under corrected Paraboloid. I have no knowledge of what corrections to either, or both mirrors, that would eliminate coma at the edge of the field. Perhaps a further correction towards a parabola, I don't know.

At the end of the day, it's how it performs as an imaging system that tells the story.

Ron.

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Whippy

Whippy

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Orion Optics UK do a range of Dall-Kirkhams, they are advertised as "optimised" so I'm not sure how true they are the the original design.

http://www.orionoptics.co.uk/ODK/odkrange.html

Just looking at the prices says it all. My entire imaging setup cost a fair bit less than the cheapest model. I've seen one of these in the flesh at Astrofest earlier this year and they do look something special but at those prices, they can stay special!

Tony..

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Beyond_Vision

Beyond_Vision

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Orion Optics UK do a range of Dall-Kirkhams, they are advertised as "optimised" so I'm not sure how true they are the the original design.

http://www.orionoptics.co.uk/ODK/odkrange.html

These were on display at Astrofest 2008 in London. They looked fantastic in the flesh but are very pricey. Hence there are probably not that many in circulation yet. They would be on par with a normal F10 SCT with a 0.63x focal reducer in terms of speed being F6.8 with a built in Field Flattener.

Regards

Kevin

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GordonH

GordonH

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Hi All

Thanks everyone for the response to my question which appears to have provoked some interesting discussion. I think when the time comes I will go down the RC route and the Astrosib 10" for several reasons

1. Cost wise it is cheaper than the CDKs offered by Planewave and Cerovolo and although the Orion Optics ODK is comparable in price it is only just out and no review or results have been seen from it yet

2. I didn't want larger than 10" because of physical size and weight considerations (I have already put my back out twice putting the C14 on the Paramount - one of the reasons for selling it next year)

3. I wouldn't consider the Astrotech RC as I am always very wary of claims made for much cheaper products especially when just out on the market

I will let you know how I get on and post subsequent images

Best wishes

Gordon

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philj

philj

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In the eighties before the take off of the popular SCTs, some good DKs were produced by Fullerscopes, if you wanted a cassegrain relatively cheaply this was about the only way of going about it then. I built their 6" DK/Newtonian and what a great scope it was.

Why DKs cost premium prices now I dont know?

Phil

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