I've been making Corrected Dall-Kirkham astrographs commercially for a while and prospective buyers often ask if the instrument is also good for planetary imaging. Unfortunately I have to disappoint them by explaining that the 50% linear obstruction, while perfectly ok for deep sky imaging, disqualifies the CDK for planetary imaging.
So I decided to design a complementary instrument of the same aperture, that is as good as theoretically possible for planetary imaging.
The case for the Dall-Kirkham:
My main criteria in deciding the type of instrument were, aperture, focal length, secondary obstruction size, thermal behavior, portability and maintenance (collimation).
The aperture had to be set at 250mm as that is the maximum I can manage with my equipment.
Focal length for this aperture needs to be around 6000mm for imaging at the theoretical limit, but that would mean an f/ratio of 24, quite possible but not very nice from a manufacturing point of view.
A typical SCT telescope has a linear obstruction of at least 32% and I wanted to do a bit better than that.
Thermal behavior means that the instrument needs to track ambient temperature changes well without trapping warm air or cooling too easily below ambient and getting dewed up.
Through portability I mean a weight of not much more than 10kg so that the instrument can be taken to locations for catching planetary events or avoiding local cloud.
Ease, and retention, of collimation, and also focus drift, can be dealt with using carbon fiber and precision mechanical construction.
All the above criteria come together nicely in a 250mm f/16 Dall-Kirkham OTA.
One other very important aspect in favor of the D-K is that the optics can be made to a very high standard, rivaling the venerable Newtonian. This aspect is a bit hard to explain for people other than mirror makers.
Last edited by Stefan Buda; 10-09-2024 at 06:32 PM.
I知 very much looking forward to seeing this come together. A cassegrain will be one of my retirement projects so I値l be interested to learn here. A couple of questions to start with:
I知 assuming you prefer the DK because the classical cassegrain has the complexity of the hyperboloid secondary and simple optics are easier to build to a high standard?
I致e read some say the spherical secondary on the DK does not need collimating, although it must be well centred. Is that true?
I知 very much looking forward to seeing this come together. A cassegrain will be one of my retirement projects so I値l be interested to learn here. A couple of questions to start with:
I知 assuming you prefer the DK because the classical cassegrain has the complexity of the hyperboloid secondary and simple optics are easier to build to a high standard?
I致e read some say the spherical secondary on the DK does not need collimating, although it must be well centred. Is that true?
Rod.
You are spot on about optical complexity. That is why I compared the D-K to the Newtonian. One of the great advantages a Newtonian has over any other reflecting telescope, and few people realize, is the fact that the secondary mirror can be mass produced to very high precision without any hand figuring. Thus the typical Newtonian has only a moderately aspheric mirror that needs hand figuring. The next in line of simplicity is the D-K with its spherical secondary that can be figured against a not too difficult to make spherical test plate. The primary of the D-K is faster than a typical Newtonian primary, but the departure from spherical is only about 3/4 of the equivalent Newtonian's primary.
The D-K does need to be collimated but the collimation is far easier than any other Cassegrain configuration. The reason is that the secondary mirror doesn't have an optical axis, only a mechanical one. Any line that goes through its center of curvature can be regarded as an optical axis. And that means it doesn't even have to have its mechanical axis accurately centered on the primary mirror's axis.
The first metal parts made are the spider vanes. I had to start somewhere.
For the spider assembly I decided to use tension joints, rather than shear joints likes all the commercial OTAs use. Tension joints can't slip or rotate like shear joints and can be made more accurate and more stable, although more expensive.
I don't have a sandblasting setup to texturize the parts before anodizing for a matte finish so I tried something different: I put all the vanes (I made two sets) into a small tumbler, with chunks of carborundum, and I let the tumbler spin for a couple of hours. After that all the sharp edges were eroded away and the flat surfaces looked like they were sandblasted. Quite happy with the result.
The OTA will be a truss type and have a length of 805mm from the front ring to the backplate. The focuser will add another 70mm. From the focuser, another 79mm to the focal plane.
I paid $300 for aluminium and $170 for carbon tubes. Looks like the total cost of materials for the OTA structure will be around $700.
The Pyrex cores I accumulated from trepanning CDK primary mirror blanks are coming in handy for making DK secondary blanks. I just have to reduce the diameter of one and slice it into two discs to get two blanks or a blank and a grinding tool.
Hi Stephan. The more I read of this project the more intrigued I become. What diameter will the secondary be, in regards to the central obstruction and also what will the focal ratio of the primary be, I imagine somewhere about F3 going by the length of the truss tube.
I have a couple of these, very thin, diamond blades. I think they were made in Russia but I can't remember where I got them from. They are so flimsy that you need to clamp them between two metal discs. If properly clamped they can make less than 1mm wide cuts. In this case the clamping arrangement is a bit too small, to allow for deeper cuts, and the rim of the diamond disc wobbles a bit, making the cut a bit wider.
The RPM of the tool is only around 2000 as I'm driving it with a low RPM motor. The rotary table is hand driven.
Last edited by Stefan Buda; 14-09-2024 at 03:44 PM.
I know your focus is astrographic...Any chance of a visual emphasis Bulon?
The DK is definitely not an astrograph and it is as good as it gets for visual planetary.
It is complementary to the CDK, as I mentioned earlier in the thread.
[QUOTE=Stefan Buda;1617066]The DK is definitely not an astrograph and it is as good as it gets for visual planetary.
I am presuming that it would still be good for planetary and lunar imaging though, which is what I'd focusing on with it, if I do manage to obtain one.
The DK is definitely not an astrograph and it is as good as it gets for visual planetary.
I am presuming that it would still be good for planetary and lunar imaging though, which is what I'd focusing on with it, if I do manage to obtain one.
Of course it is. You don't need an astrograph for planetary imaging, but you do need one for deep sky.
For planetary imaging only two aberrations need to be corrected, and those are the spherical and the chromatic aberrations. Coma, field curvature and astigmatism don't come into play with the small field of view needed.
Astrographs actually compromise a little bit on the spherical aberration but not enough to matter at, say, one arc second resolution, but that amount of compromise becomes detrimental at, say, 0.4 arc second planetary contrast.
. I dont think Takahashi will complain!
