What's the best planetary imaging scope these days?

[gregbradley]

I was wondering what is considered the best planetary scope these days?

I have a CDK17 which the marketing said was good for planetary but I don't think I have seen too many images using one.

Greg.

[Atmos]

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 :P

Basically, I think it is simply that the people buying a CDK are more interested in galaxies than planets.

[alocky]

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.

[gregbradley]

Nice image.

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.

Greg.

[Stefan Buda]

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

[Nikolas]

Damien Peach uses a c14 for his amazing planetary pics, (Also he goes to Barbados and has stunning night skies)

[gregbradley]

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.

Greg.

[Rac]

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!!!

[Stefan Buda]

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.

[gregbradley]

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.

[gregbradley]

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.

Greg

[Stefan Buda]

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.

[alocky]

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.

[Stefan Buda]

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.

[alocky]

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.

[Camelopardalis]

+1 absolutely give it a go :thumbsup:

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!

[Stefan Buda]

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.

[alocky]

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

[gregbradley]

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.

Greg.

[gregbradley]

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.

So what sort of camera is considered U-Beaut?

Greg.

[Camelopardalis]

I like my ZWO ASI224MC, very sensitive and low read noise. Experts with mono versions of ZWO ASI290 have been putting out some absolutely cracking results IMO.

[Rac]

For colour I like the ASI224MC with an ADC and for mono I like the ASI290MM.
I have both but haven't had a good night with the ASI290 yet. I find it more fun using the colour camera and you can get some really good results with it!
I use my DIY 20" goto dob.
Here's a link to my flickr page. All this years images are with the asi224 and the 20" and a few with my 18" f3.3 newtonian.
https://www.flickr.com/photos/46302893@N02/

[Paul Haese]

It's been a couple of years since I have produced a top class planetary image, but I have been keeping tabs and planning my return to planetary imaging albeit in a remote capacity. Staying up all night has knobs on it so my plans center around removing that problem.

Part of the above plan is to replace my peltier cooled C14. Its diameter is limiting but Damian Peach and Darryl Pfitzner are still producing the best images with these scopes. Darryl images in the SA Mallee not far from my site. That said, the corrector dewing will not meet my plans. So I am contemplating a radical change. Most likely an RC despite the seconday size and mass I am inclined to experiment and that Greg is what I think you should. Experimentation will give you good data. Seeing is always king in planetary imaging and no matter which scope you use, it will dictate your results. In the 10 years I was doing planetary I can count probaby 15 nights when the seeing was exceptional and most of them were from my current imaging site.

As to cameras. The 224MC and 174MC are highly touted. I have recently bought the 174MC but have not yet been able to test it in vain. My last camera was a 120m and will still use that until I find a new mono camera. I also have seen good results from a 290m too. Given how hard planetary imaging really is, I would recommend going with a colour camera first.

[Stefan Buda]

Raymond,

Top notch images but I'm even more impressed by those sets of extra focal images. I don't think I have seen better.

Can't advise on cameras as I'm behind in that department. I deliberately picked an older version for my current camera because of its 3.75 micron pixel size that allows me to do high resolution lunar imaging without a focal extender, at 6.5m focal length. For planetary I only need to use a 1.4x teleconverter.

Paul,

Planetary stacking software is so damn good these days that you can reach the theoretical limit of your scope in moderately good seeing. I have not experienced anything that I could call excellent seeing and yet I've been able to go down to bellow 0.2" quite often.

[ags_]

My 14"SCT coupled to a ZWO174mono with astrodon filters produced many very good planetary images, however nothing beats a correctly built Newt for contrast that's why I changed. A secondary around 20% diameter of a top end main mirror is best, but anything will work especially if you have great seeing! The best camera I have ever used is my current GS3-U3-32S4M-C a newer version CM3-U3-31S4M-CS is now much lower priced.

[clive milne]

Greg, we have a 16"DK up at the dark sky site (which often has excellent seeing) It performs beautifully on planetary, certainly better than any C14 I have ever looked through (and I have actually owned one with good optics)... fwiw) The difference in secondary ratios between these scopes is not that great (35% vs 40%)

I would not recommend the TEC Mak... you will no doubt find the aperture to be limiting.

As others have intimated, thermal management is everything.
The way the pro's do it is to house the telescope in an insulated enclosure
which is refrigerated during the day. Interestingly, telescope performance
is significantly less affected by having optics below ambient as compared to when they are warmer. The boundary layer tends to just slide off rather
than sending plumes up in to the OTA. When I visited the Max Planck observatory on Calar Alto (some years ago) the 3.5m was basically kept just above the anticipated dew point, or minimum night time temperature, which ever was greater... food for thought.

Two other things the pro's do that (most) amateurs don't are:
* Elevate the telescope above the ground... most of the seeing is within the first 10m or so. It's worth noting that often when potential observing sites are evaluated for seeing, they'll build an elevated platform for the small aperture test telescope.
* Secondly, (according to the professional literature) the boundary layer
can be disrupted by blowing air horizontally across the optics at a velocity of a couple of m/s. (Wilson et al)

The fact that massive telescopes with substrates 20cm thick or greater (like at the ESO) can perform so well even with tens of tonnes of thermal mass is testament to the fact that the strategy outlined above genuinely works.

If the scope is going to be a dedicated planetary instrument, you only need a diffraction limited field of a minute of arc (or there abouts) so a simple Newtonian would actually be the best bang for the buck. Also, being as exposures are going to be no more than a fraction of a second then the highest performance per dollar will be a large aperture Newtonian on a driven, Alt Az mount in an elevated, insulated, air conditioned observatory.

2c

~c

edit}

fwiw, here's a direct image of a planet orbiting the star CVSO 30 taken with the Astralux camera on the 2.2m on Calar Alto... separation is approximately 2 arc seconds.
Obviously, they are doing something right.
image:
http://www.caha.es/images/prel/cvso30/portada.png
info:
http://www.caha.es/the-weird-system-of-star-cvso-30-two-planets-at-extreme-distances.html

best
~c

[gregbradley]

Thanks for the answers. There's a fair bit to it.

I'll start with putting one of those recommended cameras on the CDK and see how it goes.

I have a portable air conditioner I could rig up to blow colder air onto it if needed.

Thanks to all who responded.

Greg.