I asked because the only CDK corrector lens assembly I have ever seen in real life was off a Planewave 20" and it was an extremely deep meniscus doublet that I believe was aspheric - so this seemed to me to be a horribly complex lens assembly that even the experts were getting wrong !
My understanding was that ultimately they changed to a European optician to get it right.
Quote:
Originally Posted by Stefan Buda
I am keeping tabs on labour and material costs and I will be able to tell you how much you would need to fork out.
Same as I did for the Honders: I cut up a few slabs of optical glass and turn the pieces into lenses using normal lens making techniques - see some pics on my earlier phlog: http://www.iceinspace.com.au/forum/s...=151040&page=2
In fact I tried to cut some corners by producing an optical design that used an off the shelf Edmunds PCX lens, but I abandoned the idea because the Edmunds lens is available only for 50mm diameter, which is not enough.
They have a larger version of that particular lens, but it comes only with single layer AR coatings, and that would not satisfy the last point of the above listed requirements.
I asked because the only CDK corrector lens assembly I have ever seen in real life was off a Planewave 20" and it was an extremely deep meniscus doublet that I believe was aspheric - so this seemed to me to be a horribly complex lens assembly that even the experts were getting wrong !
My understanding was that ultimately they changed to a European optician to get it right.
As far as I know Planewave use a 3 element corrector made of 2 different types of glass, but my guess is that the only reason for that extra complication is to achieve the large field needed for 36x36 sensors.
As far as I know Planewave use a 3 element corrector made of 2 different types of glass, but my guess is that the only reason for that extra complication is to achieve the large field needed for 36x36 sensors.
Hi Stefan,
I know that in my planewave CDK 12.5, there are 2 elements, but I can't speak for the larger scopes.
I know that in my planewave CDK 12.5, there are 2 elements, but I can't speak for the larger scopes.
Josh
Hi Josh,
That's interesting, are you sure it is not 2 groups of 3 elements?
I have not tried, but it may be possible to have 2 elements at f/7 and achieve the extra large field. Or maybe they did go aspheric with one of the lens surfaces, as rally suggested earlier in this thread.
That's interesting, are you sure it is not 2 groups of 3 elements?
I have not tried, but it may be possible to have 2 elements at f/7 and achieve the extra large field. Or maybe they did go aspheric with one of the lens surfaces, as rally suggested earlier in this thread.
I am sure I have taken them out several times to clean, only 2 elements convex faces towards each other.
I am sure I have taken them out several times to clean, only 2 elements convex faces towards each other.
Thanks for that - it is good to know for sure.
Meanwhile all the pieces for the mandrel have come together.
After searching far and wide, this morning I purchased some polyester film from Eckersleys, to be used as a barrier layer on the mandrel.
Hi Greg,
Thanks for the observations regarding vignetting. If you have any other recommendations, please don't hold back.
My design, as it stands now, has no vignetting up to an APS-C sensor size, and less than 20% at the corner of a full size chip. That is supposed to be quite good.
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.
Last edited by gregbradley; 07-04-2018 at 05:30 PM.
I studied the site and they talk about a pair of lenses which is a bit fuzzy, but I see that their design is f/8, not f/7 as I was thinking, so quite doable with 2 lenses, and they don't even need any aspheric, I'm sure.
But I did not like their misrepresentation of the facts when they are comparing the simulated spot sizes of their CDK with an RC telescope of the same parameters. The CDK is an astrograph while the RC is a telescope.
The RC becomes an astrograph only with the addition of a field corrector.
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.
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.
Wow, I did not realise that fans could have that effect. I only mentioned the fans blowing across the mirrors as the latest CDKs have that (4 of them) plus the 3 or 4 at the back.
Wow, I did not realise that fans could have that effect. I only mentioned the fans blowing across the mirrors as the latest CDKs have that (4 of them) plus the 3 or 4 at the back.
Greg.
In its original configuration, my 16" planetary had 7 fans. Three of them on the back plate in sucking mode, an the other four were removing the boundary layer. I tried the cross blowing ones in various combinations of sucking and blowing but in the end I had to give up and remove them.
The astigmatism problem could only be revealed by the Roddier test.
With Zerodur primary it should not be a problem even for a large planetary scope. The attached image shows the boundary fans still in place.
Quote:
Originally Posted by Atmos
I know a good number of planetary imagers that use a newtonian have boundary layer fans for the primary. Would you expect the same issue?
Yes, I would definitely expect the same problem when the ambient temperature is dropping. However not blowing away the boundary layer may result in poorer images. A planetary scope needs to be designed very carefully regarding thermal issues.
Yes, I would definitely expect the same problem when the ambient temperature is dropping. However not blowing away the boundary layer may result in poorer images. A planetary scope needs to be designed very carefully regarding thermal issues.
That's very interesting. I would never have thought that cooling fans would cause astigmatism by blowing air locally on a glass surface. I guess it makes sense, the bigger the volume of glass the more amplitude in change.
So the challenge is to mount a large chunk of glass so it keeps its position mechanically but still has room to expand and breathe. How do you do that? Cement it with a compound that has the same or very close properties as the glass you're using. Like a crumple zone?
That's very interesting. I would never have thought that cooling fans would cause astigmatism by blowing air locally on a glass surface. I guess it makes sense, the bigger the volume of glass the more amplitude in change.
So the challenge is to mount a large chunk of glass so it keeps its position mechanically but still has room to expand and breathe. How do you do that? Cement it with a compound that has the same or very close properties as the glass you're using. Like a crumple zone?
In this case, with the cross blowing fans, it is simply a matter of uneven cooling that sets up thermal gradients within the mirror, and that leads to asymmetrical figure.
Fans behind the mirror are not as problematic because there is enough turbulence within the small space to even out the temperatures in a symmetrical fashion.
That is a MONSTER scope! 
