Dedicated planetary telescopes

[Stefan Buda]

Looking at what is available on the market, it seems that would be lunar and planetary imagers are a bit neglected.
There is an amazing range of wide field and deep space astrographs to choose from, but when it comes to planetary, there are very few suitable instruments.
Basically one has to choose between a large Newt or a large SCT.
The large SCTs are proven performers when it comes to planetary imaging, but the optical quality seems to vary and that can be a big problem. Also it is hard to keep the corrector plate from dewing up, just when the seeing comes together. Heaters, while ok for deep sky, are out for sub arc second imaging.
Large Newts are a bit cumbersome and need to be optimized for planetary imaging to perform well, by minimising the size of the secondary, having a primary mirror that is not too thick and providing forced ventilation for the OTA. Of course, the optics need to be really good as well.
There is a third possible option for the planetary imager, but it is not well supported by the market, and that is the Dall-Kirham.
Recently I started designing a dedicated planetary 250mm aperture DK to complement my CDK250 and I would be very interested to hear from people that are intending to try lunar and planetary imaging or even just visual planetary observing.

[Atmos]

I have been meaning to, one night, use a 2.5x Powermate to do some moon and planet imaging but it’s not something I’ve had a chance to test yet.

[glend]

In my years of experience with planetary imaging, I have found it hard go go past a good Newt, and my favorite was my 10" f5 carbon strut Newt which I built, which used a GSO mirror set. It was a perfect match for my CGX observatory mount. For imaging the planet's I found the ASI 294 MC camera to be ideal, and I use the non-cooled version for planetary, because I am realtime stacking and aligning with Sharpcap and long subs for planetary are unnecessary.

Don't make It harder than it needs to be. My time spent with an SCT was a waste, constant frustration, with mirror flop, condensation problems, etc. I fail to see the attraction of SCTs for anything to do with Astronomy, except maybe they are easy to transport..

[Stefan Buda]

Colin, don't waste your time unless you are going after medium resolution lunar imaging. The large central obstruction (50%, linear) destroys sub arc second contrast. That is why you won't find cutting edge planetary images made with RCs or CDKs.


Newtonians can perform well as planetary imagers but they are hard to tame. SCTs, as you say, have problems, but some of the best planetary images are taken with C14s.

[glend]

Stefan, if you analyse the available equipment solutions, from the standpoint of cost to acquire (an often important consideration these days), a Newtonian will, or should, be the logical choice. If money is not a consideration, well of course there are other options.

[Camelopardalis]

Newts that are financially accessible for most tick all the boxes of what doesn’t work for high resolution planetary imaging… inability of spider or primary cell to hold the mirrors still, cheap and flimsy tube that bends at the thought of a slew…

SCTs are common for that use case because their sins are fairly manageable, and they don’t require superhuman strength or observatory class mounts to use them.

Fwiw I’d love a dedicated planetary scope… but it’d have to come with a kind of seeing guarantee :lol:

[Stefan Buda]

My argument is that even if money is not a consideration, there are not many options, but yes, the Newtonian is the most cost effective one.


You are spot on regarding Newts. It is very difficult to make them rigid and stable without turning the rigidity problem into a thermal problem. A good planetary telescope needs to have a small thermal inertia not only regarding the optics, but the whole structure. The Cassegrain configuration is much more suitable in that respect.

Anyway, I can provide the scope but not the seeing, unfortunately, although I may be able to include a sorcery manual :lol:

[Stefan Buda]

Not a lot of enthusiasm for planetary telescopes, it seems.

Anyway, I decided to go ahead with the DK250 project and started ordering the materials. I may start a new thread in the ATM section to document the process.

[Wilso]

Nice Stefan looking forward to following your build!
I myself use an old Intes Mk-65 maksutov for visual.
At f10 it’s good for visual. Dedicated planetary scope :shrug: I use it for a lot of objects.
How fast do you plan for a dedicated planetary imaging scope?

[Stefan Buda]

Thanks Wilso,

It will be f/16, with a focal length of 4000mm.
For imaging Jupiter it will require a 1.4x tele extender and for Mars a 2x Barlow, to achieve its full potential.

[Saturnine]

Hi Stefan, I would almost be interested in a dedicated lunar and planetary telescope that is a manageable size and weight, but it also , surprisingly, comes down to the cost as well . I have an 250mm f6 newt that I have used for the planets but it is awkward to manage mounting on the EQ6. Don't have a permanent set up unfortunately so I need something with decent aperture that is more portable.
Currently am mostly using my 180mm Mak which is fine but extra aperture would be so nice for the rare night of "good" seeing. A rough guide to pricing would be of interest. A reply by PM is fine if not wanting to advertise to the world yet.

[Stefan Buda]

Hi Jeff,

The OTA will be very portable. It will weigh around 10kg and have a length of about 950mm, including the focuser.
I also want one for myself for catching planetary events that are clouded out in Melbourne. Need to be able to throw it in the car and drive north of the divide. I can't do that with my 16" DK :lol:

[Wilso]

Would it have enough back focus for a diagonal and eyepiece for visual as well?

[Saturnine]

If the 16" doesn't fit in the car then obviously you need a bigger car, or van:question:

[Stefan Buda]

As it is now on the drawing board, the back focal length is 82mm, with the focuser fully retracted. Which is not quite enough for a 2" diagonal but it should be ok for a 1.25" one.


That would only solve the size problem, however, there is a weight one too. :)

[Peter Ward]

Apologies. Totally missed this post, but yes! There is a hole in the market for such an instrument.

Problem might be that the venerable Celestron 14 might be a commercial thorn in the side of your plans. Since Don Parker started using one with photographic
film last century (bugger... I must be old) they have produced remarkable results.

In terms of bang for buck, how would your design, on paper at least, improve on them? (e.g theoretical resolution/contrast)

[Stefan Buda]

A long time ago when Don Parker was using a 16" Newtonian, I made a 10" Dall-Kirkham as a step up from a 180mm Gregory-Maksutov that I was using at the time - and I was so happy with it that I ended up making a 16" version, a few years later.
A 10" DK cannot compete, of course, with a C14 regarding resolution but it is far more portable and can better handle the seeing. When I moved from the 10" to the 16", the good seeing nights became a lot less frequent.
A problem with C14s appears to be that not all of them are equally good, making it a risky investment. Although this opinion of mine is based on very few samples.
Another advantage of the DK is that it doesn't have a corrector plate that can dew up. At least in Melbourne, the nights with the best seeing tend to be very dewy.

[Camelopardalis]

I’m sure you have already thought of this Stefan, but what’s the plan for keeping the dew at bay? Just because the corrector plate doesn’t exist, doesn’t mean the dew remains suspended in the atmosphere.

Not being funny btw, I’m genuinely interested, but lack the means to fund an interesting scope such as this. My first Newtonian dewed then frosted up first time out bush :sadeyes:

[Stefan Buda]

A very good question.
Dew in fact does remain suspended until it contacts a surface that is cold enough for it to settle on. If a surface is a bit above ambient temperature it tends to repels the dew.
Anodized surfaces are very good black body radiators and, when pointed towards a very cold sky, quickly cool to below ambient temperature, attracting a lot of condensation.
Shiny metallic surfaces, like the mirrors in a telescope, are not good radiators, at least on their metalized sides.
Therefore a DK secondary will lose most of its heat though its back surface.
On my 16" DK, to prevent dew on the secondary, I added a 3mm thick foam disc topped by a layer of plain aluminium foil that also wrapped around the edge of the mirror. So, by slowing down the cooling of the secondary, I never had dewing problems although the mirror was still able to closely track the ambient temperature through convection cooling.

It is important to take into consideration the different use case scenarios too between planetary and deep sky observing sessions.
Planetary sessions, in my experience, never exceed a couple of hours while deep sky ones can go all night and that also helps with dew management.

[Wilso]

Corrector plate vs spider veins?

[Stefan Buda]

That is a bit complicated as it is like like comparing apples and oranges.

Have to compare transmissivity with obscuration and scatter with diffraction.

I don't know the exact numbers for the corrector plate, but the light loss due to spider vane obscuration will be 1.6% of the clear aperture, on top of the 7.4% caused by the central obscuration.
Light scatter by the two air/glass surfaces of the corrector plate is probably more detrimental than the diffraction effect of the spider vanes, considering how difficult it is to maintain the corrector in a pristine state free of any condensates.
Also most of the energy diffracted by the spider vanes falls outside of the planets image, thus not degrading the contrast on the planet.

[Tulloch]

To control dew on my C9.25" SCT I use a portable fan heater, others use a hairdryer. Just blow warm air onto the corrector plate and once the dew is gone, wait for about a minute for the corrector to get back to ambient temperature and start imaging.

This FAQ might help you in your Planetary AP journey.

https://www.cloudynights.com/topic/812022-planetary-imaging-faq-updated-september-2024/

[Stefan Buda]

I used a hairdryer too, a long long time ago when I started planetary imaging with a 180 Maksutov. Not a good idea to heat your optics when you try to image at the theoretical limit of your scope, but if you get dewed up there is no other option.

Your link is a good place to start for beginners, however, I would not recommend planetary imaging with scopes below 250mm aperture.

[Leo.G]

I have a couple of hair dryers which I never use for my hair (my son and I both have long hair to our waists (lower but I don't want another infraction even if I ### out a word) and long beards), I do however use them on telescopes, mainly only on cool but if I need the heat I run low heat, low speed air for a minimal time period. They have been my saviour for many years with astronomy through winter in Lithgow where minus 12 is common through our winter nights. Though we've just had our first winter I've not taken a telescope or camera out once, first time in over 30 years (personal problems and mind set). I'm a little annoyed with myself though, I'm sure I've missed so many beautiful image opportunities.......

[Stefan Buda]

I achieved some progress with my DK250 project and I will start a dedicated thread to document it in the ATM section.

[Stefan Buda]

New thread started: https://www.iceinspace.com.au/forum/showthread.php?t=211657

[Tulloch]

Larger scopes are certainly better, but it's definitely possible to get nice images of the planets with smaller scopes. Here are some images I've taken with my 6" SCT.

My C9.25" (with "only" 235mm aperture) is certainly capable of taking great images of the planets (they are the ones in the FAQ) in good seeing.

[Stefan Buda]

For nice images aperture doesn't matter. Your small bore images are good and they show how far you can get with a 6" aperture. But that is the end of the road for that aperture and if you are happy to stop there then no problem.
When I say 250 as a minimum, I mean something bigger than the next standard size down, which is 8". The C9.25 is a weird size but it fits my criteria as it is closer to a 10" than to an 8".