I personally cannot think of a reason why your CDK shouldn't work very well as a planetary scope, all you'd need is a 3-5x barlow depending on the pixel size of your high speed camera. I imagine that a lot of planetary images aren't taken with the CDK because it is over engineered to some degree. An 18" newt would be cheaper than the CDK I imagine, for planetary a large corrected imaging circle isn't needed as a small sensor is all that is required.
Hell, for the price of your CDK and mount you could probably by something on the order of a 25" tracking dob which could achieve the same result
Basically, I think it is simply that the people buying a CDK are more interested in galaxies than planets.
Hell, for the price of your CDK and mount you could probably by something on the order of a 25" tracking dob which could achieve the same result
Actually - you'd buy several 25" dobs for the price of a properly mounted CDK17. The best planetary images I have seen are through a variety of scopes - Anthony Wesley uses a reasonably fast 16" newt, Thierry Legault, Damien Peach, these guys use 14-16" Schmidt Cassegrains.
The key is getting everything right, local seeing, which includes thermal effects around the primary, good seeing, and the patience and discpiline to spend the whole night waiting for that magic 10 minute window.
the processing is not trivial either, it's a different world to deep sky.
Rarely does the seeing approach the resolution capability of a 16", and I can only remember a handful of nights that a 25" could be wound right out to its maximum.
However, here's what a 25" driven dob can deliver with minimal talent on a night of reasonable seeing. It's nowhere as good as the visual experience on the same night, the brain and eye is still better than a camera for moving objects.
On Greg's question, I would have thought that a well collimated CDK17 on a night of good seeing would be capable of going with the best.
cheers,
Andrew.
That's encouraging so the CDK may be able to do it. I do have access to good seeing so I will try that out at some point. The best seeing I have had for planetary was one morning around 2am and visually with an FS152 Jupiter was very clear and stable in the view. Funny how these moments stay with you.
Hi Greg,
My opinion is that a CDK is not good for planetary imaging. There are several reasons for this. The most obvious one is the large size of the secondary obstruction diffracting more light into the diffraction rings and reducing theoretical resolution. A less obvious one is the thermal inertia of the large secondary assembly. It cannot be stressed enough how important good thermal control is in a planetary scope.
There is a subtle difference between an astrograph and a telescope that people tend to overlook: A telescope must be diffraction limited on axis (as a minimum) to be worth of its name, but an astrograph (even the best) does not necessarily have to be. That is why planetary imaging is done with telescopes and not astrographs. Of course, in theory, the CDK is meant to be diffraction limited on axis, but in practice, due to the manufacturing tolerances of the 6 optical surfaces and their spacing, it is highly unlikely to be. And it does not have to be because it is not used at the ridiculous sampling rate that a planetary scope is.
Stefan
Hi Greg,
My opinion is that a CDK is not good for planetary imaging. There are several reasons for this. The most obvious one is the large size of the secondary obstruction diffracting more light into the diffraction rings and reducing theoretical resolution. A less obvious one is the thermal inertia of the large secondary assembly. It cannot be stressed enough how important good thermal control is in a planetary scope.
There is a subtle difference between an astrograph and a telescope that people tend to overlook: A telescope must be diffraction limited on axis (as a minimum) to be worth of its name, but an astrograph (even the best) does not necessarily have to be. That is why planetary imaging is done with telescopes and not astrographs. Of course, in theory, the CDK is meant to be diffraction limited on axis, but in practice, due to the manufacturing tolerances of the 6 optical surfaces and their spacing, it is highly unlikely to be. And it does not have to be because it is not used at the ridiculous sampling rate that a planetary scope is.
Stefan
This is what I was thinking that planetary imaging has different requirements.
As far as a Planewave CDK goes I had a conversation with Joe Hedrick at the Gold Coast and he said the same thing. It was one thing to get optics at diffraction limited quality on a bench and another thing to achieve that in place. He said that they had worked on that and were able to achieve diffraction limited optics in place.
How true that is I would not know beyond do I get sharp images or not.
A planetary imaging run would show it up I imagine.
Its hard to imagine though an SCT outperforming a Planewave CDK. The secondary is not 50% like a lot of astrographs are. The exact % I am not sure of but I think its around the 40's.
The best planetary images I've seen this year have all come from large tracking dobs around the 20" mark. The c14's still make great images though and there is lots of proof of that.
If you already have the cdk then there is no reason not to start there and just use it. I don't think the central obstruction will make a lot of difference for imaging. Planetary imaging is more about aperture size and a bit of mirror quality, collimation and so on... as well and good seeing!!!
Its hard to imagine though an SCT outperforming a Planewave CDK. The secondary is not 50% like a lot of astrographs are. The exact % I am not sure of but I think its around the 40's.
Greg.
According to what I said in my previous post a C14 with good optics should outperform your CDK.
It is hard to convince non-planetary imagers of the importance of thermal control. Your CDK's secondary assembly has a lot more mass (= thermal inertia = chimney effect), not to mention the large spider vanes acting as heat exchangers.
My 16" UDK ( U for uncorrected :-) ) has less than 25% obstruction and only two optical surfaces that can be tested very accurately. And yet I was not able to push it to its limit for a few years because the passive cooling arrangement I used initially, was creating a slightly asymmetric thermal gradient within the primary making it astigmatic. The problem was hard to detect because it produced a wave front error of about 1/4 lambda.
It took a Roddier test to identify the problem.
And to answer your initial question: Like other people have suggested, a large Newtonian is the way to go.
According to what I said in my previous post a C14 with good optics should outperform your CDK.
It is hard to convince non-planetary imagers of the importance of thermal control. Your CDK's secondary assembly has a lot more mass (= thermal inertia = chimney effect), not to mention the large spider vanes acting as heat exchangers.
My 16" UDK ( U for uncorrected :-) ) has less than 25% obstruction and only two optical surfaces that can be tested very accurately. And yet I was not able to push it to its limit for a few years because the passive cooling arrangement I used initially, was creating a slightly asymmetric thermal gradient within the primary making it astigmatic. The problem was hard to detect because it produced a wave front error of about 1/4 lambda.
It took a Roddier test to identify the problem.
And to answer your initial question: Like other people have suggested, a large Newtonian is the way to go.
I have often though about cooling my CDK actively. Any suggestions on how to do that? One way I thought of was a portable AC unit with one of those flexible tubes. I could direct it to the underside of the scope or perhaps flow it over the top of the scope so some would go down inside it or take the shroud off and blow at it sideways just above the mirrored section of the body.
The best planetary images I've seen this year have all come from large tracking dobs around the 20" mark. The c14's still make great images though and there is lots of proof of that.
If you already have the cdk then there is no reason not to start there and just use it. I don't think the central obstruction will make a lot of difference for imaging. Planetary imaging is more about aperture size and a bit of mirror quality, collimation and so on... as well and good seeing!!!
Thanks Raymond. I have also posted this question on the CDK yahoo group site to see what others have found.
I do understand the importance of thermal control etc. SCT I suppose has a smaller secondary but the corrector plate is also large.
My AP Honders looks very similar to an SCT but it may have a larger secondary compared to an SCT. It also has better thermal control with an oversized OTA and fans.
I have often though about cooling my CDK actively. Any suggestions on how to do that? One way I thought of was a portable AC unit with one of those flexible tubes. I could direct it to the underside of the scope or perhaps flow it over the top of the scope so some would go down inside it or take the shroud off and blow at it sideways just above the mirrored section of the body.
Greg.
You have to be very careful with active cooling as you can easily make things worse. In my previous comment I mentioned that even passive cooling with fans can create problems in a large planetary scope.
SCT corrector plates are thin and not much of a problem. Glass is a very good black body radiator (unless aluminized) and a Schmidt plate would quickly cool below ambient if it wasn't for the worm air inside the OTA.
Maksutovs correctors are a different matter. Never go there for planetary.
Fortunately no one is making them large enough to be considered for serious planetary imaging.
Hi Greg - a few minutes googling reveals that your 17" has a significantly smaller on-axis spot size than a Meade 16" SCT, and given that your mount should be able to keep the planet on-axis. Nothing is likely to be damaged either so I would have to say the next step is to get away from the keyboard, grab a Barlow and a high frame rate Astro cam and give it a go!
We can theorise endlessly, but we only actually learn when we experiment.
Cheers,
Andrew.
We can theorise endlessly, but we only actually learn when we experiment.
Cheers,
Andrew.
Exactly what I've been talking about: In theory the CDK is better but in practice the C14 leaves it for dead. For reasons that I learnt experimentally and outlined in previous comments.
Exactly what I've been talking about: In theory the CDK is better but in practice the C14 leaves it for dead. For reasons that I learnt experimentally and outlined in previous comments.
I'm not questioning your results, but I don't believe for a second that there's any good reason why Greg shouldn't give it a go. It's quite addictive, and if he decides to upgrade to a C14 later he will have the rest of the gear humming!
Cheers,
Andrew.
Before jumping to conclusions you really need to test it on a bunch of different nights, so fickle is the seeing. I've had my C11 5 years and I can count the exceptional nights on a couple of fingers my point being that it takes more than just the scope to tango!
I don't want to talk Greg out of trying, I only want to dampen the disappointment by giving some reasons. Please, Greg, go ahead, I'm interested too in seeing the results and I will happily revise my knowledge if necessary.
I don't want to talk Greg out of trying, I only want to dampen the disappointment by giving some reasons. Please, Greg, go ahead, I'm interested too in seeing the results and I will happily revise my knowledge if necessary.
I think you're spot on there, and even with the best possible gear it would be optimistic to think you'd be challenging the 'pros' without enduring a steep learning curve!
Cheers,
Andrew
You have to be very careful with active cooling as you can easily make things worse. In my previous comment I mentioned that even passive cooling with fans can create problems in a large planetary scope.
SCT corrector plates are thin and not much of a problem. Glass is a very good black body radiator (unless aluminized) and a Schmidt plate would quickly cool below ambient if it wasn't for the worm air inside the OTA.
Maksutovs correctors are a different matter. Never go there for planetary.
Fortunately no one is making them large enough to be considered for serious planetary imaging.
Thanks for that.
There is a TEC Mak Cass 250mm aperture for sale on Astromart. They promote it for planetary. Not sure if its good or not. TEC make great scopes though.
Another TEC scope guy uses a 200ED for planetary and his images are usually pretty nice.
I believe you when you say a 14 inch SCT makes the best.
Don't worry I didn't take any of this excellent advice as a discouragement at all. That's why I am asking to get the advice from experienced planetary imagers.
The only planetary imaging I have done was with a C11 and a Toucam - get the idea? I thought it was good at the time but the area has moved on big time and available gear and software has improved remarkably.
I can say though at my dark site sometimes the seeing is pretty nice. 710 metres altitude and very remote with virtually no light pollution. Sometimes its windy though.
my point being that it takes more than just the scope to tango!
