Quote:
Originally Posted by gregbradley
Yes that sounds good. The CDK design is great but the corrector lens perhaps tends to make a bit of bright central area that puts a lot of pressure on getting the flats just right.
Otherwise my CDK is great. The vignetting is still there even at a dark site. Just less so.
Fans blowing across the mirror as well as from the back may help.
The secondary shroud should be baffled. In the case of CDK their shroud is thicker at the base than it is along the outer edge by about 5mm.
The inner tube should be well baffled as should the corrector. Originally my CDK was very prone to flare from bright stars just outside the field of view. The baffling largely fixed that and now its quite good. But my AP Honders has amazing baffling and I have never seen a flare with that. It also seems to help it cut through light pollution so I think baffling is critical in modern scopes. The current set of baffles were made with 3D printing.
Carbon fibre struts with those nice aluminium knuckles ensures rigidity. My CDK holds focus pretty much night after night. I would not trust an aluminium tube, carbon fibre is the go.
A strong focuser is a must as these large format cameras, filter wheel, focusers etc are super heavy.
My CDK has a reducer which I have used at times. The problem with it is lack of backfocus. So only a selfguiding camera would really work with it otherwise its back to a guide scope and the potential problems that makes. Its a shame as the corrector works really well and can really pick up some photons!
So more backfocus at least with the reducer. Without the reducer the amount of backfocus seems fine.
One of the advantages apart from small spot size out to the edges is the fact a CDK is quite easy to collimate being a simple primary mirror.
Another point of engineering importance is the primary mirror mounting. The owner of CDK told me its one thing to make a mirror to diffraction limit on a bench and another to have one to that standard mounted. So the mounting is a critical aspect and the mirror should be supported in multiple places without pinching or stressing.
Of course the mirrors should be ground to as high a precision as possible and a low expansion type glass or thin type mirror like some make now would be desirable.
Greg.
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Thanks Greg,
I have made two D-K scopes from scratch, and used them successfully (10" and 16"). Also I completely reground the mirrors of an ODK10. That gives me some confidence that I will get it right.
Interesting to hear from the owner of CDK about the importance of making the primary mirror happy in its mounting, because you say that there are fans blowing across the mirror. I made that mistake too with my 16" D-K and it took two years of imaging until I discovered that those fans were making my primary go astigmatic due to uneven cooling. The effect would probably be negligible for a 12" f/8 astrograph, but not acceptable for a 16" planetary scope.