gregbradley
28-09-2010, 06:07 PM
I have been using my Planewave CDK17 now for a few months and have gotten a feel for it now.
I thought I would write it up as others are looking at getting one these scopes.
It is called a CDK as that stands for corrected Dall Kirkham. Corrected as it has a doublet corrector
just before the focuser that corrects aberrations. The Dall Kirkham design has been made famous by Takahashi with their
Mewlon series. The problem with Dall Kirkham is offaxis aberrations. Hence the narrow corrected field of a Mewlon (I believe Tak has brought out a reducer or corrector now for the Mewlons). Corrected Dall Kirkham is a popular design for astrographs these days with several manufacturers making one. The main mirror is like a Newt so it is easy for the manufacturer to make one very accurately and to a high standard which is good to know. By contrast RC mirrors are harder to make.
Firstly, fit and finish is good. The carbon fibre bits are very nicely done and the aluminium parts etc all seem very well made.
I can't fault the fit and finish except the electronic focuser has cable ties and wires and could do with a nice case to hide all that. Its a bit exposed. Luckily it works well so all is forgiven.
The focuser is very good in that it has a bevel gear and drive shaft with another bevel gear that moves the focuser in and out. I see the focuser shaft has at least 6-8 roller bearings (it may be more) so the focuser moves smoothly which it seems to do. As a result there appears to be no focuser flex which is good with a FLI Proline 16803 and filterwheel (about the heaviest camera you can get) and an offaxis guider. So that is holding about 15lbs of weight without any trouble.
The supplied adapter works well once I worked out how it went together
(it is in 3 parts and they screw together in a clever way, a bit of a jigsaw puzzle though).
The scope has 3 fans like an RCOS and these work well. It takes maybe 1.5-2 hours for the mirror temp to be close or match ambient so it would be best to turn them on early before imaging.
The scope has a simple software temperature and focus control which works well. The electronic focuser is a little unusual in that finding focus can take a while. I think the scope must have a relatively large critical focus zone and also if the optics are not cooled and brought to close to ambient it probably throws the focus off a bit. I find (like my Robofocus also) that it is better to wind out the focuser a bit too much and then bring it back in. Otherwise if you go part optimum focus and try to return it does not seem to be where you just passed it. I find though the night before's focus point seems to be the same the next night although if the temp has changed significantly it will not be the ideal focus spot.
I am not sure how all this would go with an automated focus routine, I haven't used one. But it may be a tad troublesome if the return in of the focuser is not the same as going out. Also a consequence no doubt of the heavy payload. All the same achieving optimum focus is not hard but just a bit different to my refractors which snap to focus more clearly than this does. Perhaps the increments the focuser moves are also smaller with the CDK.
Round stars to the corners so the corrector is doing its job. No signs of any aberrations.
I am finding this large aperture is more sensitive it seems to light pollution and gradients than my refractors. Even a 5nm Ha filter is showing bad gradients and a lack of contrast when there is any moon.
The scope needs 3nm Ha and also ideally super dark skies. My skies are quite dark from the zenith to the west and to the north, reasonably dark to the south and some light pollution to the east (Sydney).
I also find occassionally bright stars just out of range will flare in the image. This is usually not a problem but I do not get that with my refractors which have superb baffles. Perhaps the baffling could be improved. Perhaps also larger scopes are more prone to this??
I have not needed to collimate the scope yet as CCD Inspector showed some slight tilt which I discovered was from the MMOAG pushing the camera adapter out in one spot. I adjusted it and it seated properly and I got round stars to the corners whereas at first I got some out of focus stars in the top right corner of my images.
F6.8 at 2939mm is a nice F ratio to be imaging at as I find F9/F10 to be too dim and require too much total exposure time. Yeah I know - F ratio myth, but still for a given aperture F5 is nice F ratio to image at and F6.8 isn't that far away. I think F5 is ideal, personally. There is a reducer available at some point for this scope which will give F4.49 so that could be worthwhile.
I managed to get my MMOAG to work and that coupled with the Paramount ME (which handles this scope like it was an 80mm refractor - easily) is giving me nice round stars up to 30 minutes (the longest I have tried). I get guide stars almost every time (I did have to move the target object a bit on NGC253 to get a guide star).
I am using a Starlight Xpress Lodestar (connects instantly and runs through the USB port of the Proline - very handy and very fast and efficient, never needs rebooting etc).
On the images I have done so far I am finding the stars are very tight and only the brightest stars have diffraction spikes.
Overall I am happy with this scope but I find it needs dark skies to operate well. I am not sure how it would go for narrowband work for the reason of the gradients. I have taken several Ha images during moon weeks and they haven't been that impressive. I'll upgrade at some point to a 3nm Ha filter and see how that goes. It may well handle that.
I can see at some point I am likely to take this baby to my dark site with a portable tripod for the Paramount for a week of clear skies and galaxy imaging and see what it can really do.
This is my first really large scope (I did have a 12.5 inch RCOS as my largest before) but as far as I can see it is doing everything really well and no flaws so I'd have to rate it as being an excellent scope.
The other aspect of this scope which is unusual for "an imaging scope" is that it is also supposed to be an excellent visual scope. Tak Mewlon series is famous for their visual performance. This scope is also a Dall Kirkham with a corrector (something Tak has only just brought out for their Mewlons). This should mean excellent for visual (most astrographs are not due to the large secondary - my RCOS 12.5 inch for example was not as good as an SCT for visual). I am yet to look through it but I will be soon. Also I am hoping it will be excellent for planetary imaging as well. Again, to be tried out.
So the design is excellent to be proficient in those 3 areas which is unusual.
Planewave also have a very short waitlist which is also unusual for these larger scopes. This one was ready in 6 days
and I ordered it at a time when they advertised they had one available within 2 weeks. Service at Planewave seems to be
very good and they have a large group of happy customers overall.
Greg.
I thought I would write it up as others are looking at getting one these scopes.
It is called a CDK as that stands for corrected Dall Kirkham. Corrected as it has a doublet corrector
just before the focuser that corrects aberrations. The Dall Kirkham design has been made famous by Takahashi with their
Mewlon series. The problem with Dall Kirkham is offaxis aberrations. Hence the narrow corrected field of a Mewlon (I believe Tak has brought out a reducer or corrector now for the Mewlons). Corrected Dall Kirkham is a popular design for astrographs these days with several manufacturers making one. The main mirror is like a Newt so it is easy for the manufacturer to make one very accurately and to a high standard which is good to know. By contrast RC mirrors are harder to make.
Firstly, fit and finish is good. The carbon fibre bits are very nicely done and the aluminium parts etc all seem very well made.
I can't fault the fit and finish except the electronic focuser has cable ties and wires and could do with a nice case to hide all that. Its a bit exposed. Luckily it works well so all is forgiven.
The focuser is very good in that it has a bevel gear and drive shaft with another bevel gear that moves the focuser in and out. I see the focuser shaft has at least 6-8 roller bearings (it may be more) so the focuser moves smoothly which it seems to do. As a result there appears to be no focuser flex which is good with a FLI Proline 16803 and filterwheel (about the heaviest camera you can get) and an offaxis guider. So that is holding about 15lbs of weight without any trouble.
The supplied adapter works well once I worked out how it went together
(it is in 3 parts and they screw together in a clever way, a bit of a jigsaw puzzle though).
The scope has 3 fans like an RCOS and these work well. It takes maybe 1.5-2 hours for the mirror temp to be close or match ambient so it would be best to turn them on early before imaging.
The scope has a simple software temperature and focus control which works well. The electronic focuser is a little unusual in that finding focus can take a while. I think the scope must have a relatively large critical focus zone and also if the optics are not cooled and brought to close to ambient it probably throws the focus off a bit. I find (like my Robofocus also) that it is better to wind out the focuser a bit too much and then bring it back in. Otherwise if you go part optimum focus and try to return it does not seem to be where you just passed it. I find though the night before's focus point seems to be the same the next night although if the temp has changed significantly it will not be the ideal focus spot.
I am not sure how all this would go with an automated focus routine, I haven't used one. But it may be a tad troublesome if the return in of the focuser is not the same as going out. Also a consequence no doubt of the heavy payload. All the same achieving optimum focus is not hard but just a bit different to my refractors which snap to focus more clearly than this does. Perhaps the increments the focuser moves are also smaller with the CDK.
Round stars to the corners so the corrector is doing its job. No signs of any aberrations.
I am finding this large aperture is more sensitive it seems to light pollution and gradients than my refractors. Even a 5nm Ha filter is showing bad gradients and a lack of contrast when there is any moon.
The scope needs 3nm Ha and also ideally super dark skies. My skies are quite dark from the zenith to the west and to the north, reasonably dark to the south and some light pollution to the east (Sydney).
I also find occassionally bright stars just out of range will flare in the image. This is usually not a problem but I do not get that with my refractors which have superb baffles. Perhaps the baffling could be improved. Perhaps also larger scopes are more prone to this??
I have not needed to collimate the scope yet as CCD Inspector showed some slight tilt which I discovered was from the MMOAG pushing the camera adapter out in one spot. I adjusted it and it seated properly and I got round stars to the corners whereas at first I got some out of focus stars in the top right corner of my images.
F6.8 at 2939mm is a nice F ratio to be imaging at as I find F9/F10 to be too dim and require too much total exposure time. Yeah I know - F ratio myth, but still for a given aperture F5 is nice F ratio to image at and F6.8 isn't that far away. I think F5 is ideal, personally. There is a reducer available at some point for this scope which will give F4.49 so that could be worthwhile.
I managed to get my MMOAG to work and that coupled with the Paramount ME (which handles this scope like it was an 80mm refractor - easily) is giving me nice round stars up to 30 minutes (the longest I have tried). I get guide stars almost every time (I did have to move the target object a bit on NGC253 to get a guide star).
I am using a Starlight Xpress Lodestar (connects instantly and runs through the USB port of the Proline - very handy and very fast and efficient, never needs rebooting etc).
On the images I have done so far I am finding the stars are very tight and only the brightest stars have diffraction spikes.
Overall I am happy with this scope but I find it needs dark skies to operate well. I am not sure how it would go for narrowband work for the reason of the gradients. I have taken several Ha images during moon weeks and they haven't been that impressive. I'll upgrade at some point to a 3nm Ha filter and see how that goes. It may well handle that.
I can see at some point I am likely to take this baby to my dark site with a portable tripod for the Paramount for a week of clear skies and galaxy imaging and see what it can really do.
This is my first really large scope (I did have a 12.5 inch RCOS as my largest before) but as far as I can see it is doing everything really well and no flaws so I'd have to rate it as being an excellent scope.
The other aspect of this scope which is unusual for "an imaging scope" is that it is also supposed to be an excellent visual scope. Tak Mewlon series is famous for their visual performance. This scope is also a Dall Kirkham with a corrector (something Tak has only just brought out for their Mewlons). This should mean excellent for visual (most astrographs are not due to the large secondary - my RCOS 12.5 inch for example was not as good as an SCT for visual). I am yet to look through it but I will be soon. Also I am hoping it will be excellent for planetary imaging as well. Again, to be tried out.
So the design is excellent to be proficient in those 3 areas which is unusual.
Planewave also have a very short waitlist which is also unusual for these larger scopes. This one was ready in 6 days
and I ordered it at a time when they advertised they had one available within 2 weeks. Service at Planewave seems to be
very good and they have a large group of happy customers overall.
Greg.