Visual lunar & planetary - Cass vs newt?

[AstralTraveller (David)]

Hi,

I'm toying with ideas for backyard observatory and scope. There are a few things I might want to do with the scope (imaging isn't one of them). For the present discussion lets assume the scope will only be used for visual viewing of the moon, planets and sun. Let's also ignore issues of convenience, ergonomics and price - I am aware of them and will factor them into my considerations. I'm only asking about relative optical performance of these two options.

1) I have a full thickness 10" f/8.2 newtonian mirror (with 2" secondary) which I believe to be at least reasonable quality. It was made for me by Astro Optics in 1975. For years it was in a poorly made scope with terrible thermal management but it worked well enough often enough for me to think the optics must be good. It could be built into a dobs - this time with a good thermal management, baffling and a motorfocus. I could also consider downsizing the secondary to 45mm.

2) I have a Vixen Sphinx head and this should be able to support a Celestron C11. (The mount is rated to 23kg, the tube is 11kg and it will be out of the wind.) Theory says that the larger central obstruction of the SC will reduce contrast more than in the newt but, on the other hand, it is 1" bigger and detail is easier to see if the object is brighter.

So, would the slight increase in aperture outweigh the greater obstruction? Which one would you sooner walk up to and look at Saturn? Thanks for your experience or conjectures.

[Atmos (Colin)]

Personally, I'd take the newt over the cass any day. That extra 1" of aperture is not likely to be noticed and the larger central obstruction likely to become more of an issue than that extra 1".

Mostly however, Cass scopes are far more likely to have thermal issues than a dob. If you put in aftermarket fans into a C11 it can cool down reasonably well but I know that my 10" LX200 really struggled in winter, it just couldn't keep up with the falling temperatures.

If you're able to regulate the temperature of the telescope while it is permanently mounted and install Tempest Fans to help regulate temperature it may be able to keep up with a newt. From what I have read any secondary below ~20-25% has little to no effect in image contrast where as a C11 has something along the lines of a 35% secondary.

[dannat (Daniel)]

long focal length newts appear to be out of favour -but get the central obstruction right & team with a good mirror youre on a winner imo.

you could buy a dob kit for the mirror -go ultralight or whatever -or you could buy a CF tube & mount it on the vixen mount -should just com in under the weight limit

[Kunama]

I like the idea of an F8.2 Dob but to build one would be quite a job, might be better to buy a used 10" truss scope and make longer trusses for it.
Then you just need to make some adjustments to get the thing to balance.

I don't know that you would gain much by reducing the secondary....?

[Satchmo]

The differences in obstruction would only be significant if the optics were equally good. Thermal management of such a long tube ( which should be made oversize as it is open ) could be significant as is the 2" thick mirror . On top of that the quality of the optics is a bit of an unknown quantity.

You would need tracking too for the long dob - always fairly expensive ( allow $2500 or so for a servo-cat and argo.)

Given all those problems my money would be on the C11- you sit down and observe and the tube should be much more stable in a breeze. Also when you inevitably get into planetary imaging with a webcam the Sphinx mount will be very precise for that .

[AstralTraveller (David)]

Thanks for the replies. They more or less coincide with my thoughts: for outright optical performance the newt should be the better performer (assuming quality optics) but every caveat to that; construction, ergonomics, stability, thermal management (though the cass also has issues and it's a 1.6" mirror in the newt) and the limitations of seeing, favours the cass. I have also found this thread where the pros and cons of all scope types are discussed at some length. Les's last comment is interesting in light of my dilemma.

My plan to put the scope in an observatory, where floor space costs money, also favours the cass. However, I have to admit to an emotional attachment to getting the newt operating well, so even if my head wins me over to the cass I suspect I'll make the dobs anyway. In either case nothing is happening for at least a few months - more likely years - so I have time to drive myself mad vacillating .

[Wavytone]

10" f8.2 vs 11" f10. Frankly not a lot of difference in terms of magnification, light grasp or resolution etc.

The big difference is size and convenience:

1. The big Newtonian will be far too long and heavy for a sphinx mount. The leverage of the OTA on the gears will seriously overload them. It would be fine as a dob, however.

Personally I wouldn't consider the Sphinx adequate for a C11 but hey, if it works for you, great.

2. Think about where that eyepiece is going to be. On the newt you'll need a small ladder half the time, or be breaking your neck trying to look up into it.

The SCT will be vastly more convenient.

[doug mc]

I went back to a Newtonian from a SCT largely because of dewing of the corrector plate, even with a dew shield. I believe that you will have to look into that problem. That large plate of glass looking straight up at the sky is a dew magnet. If you can manage that problem the C11 would be the best all-rounder of the two.

[Camelopardalis (Dunk)]

Dew management - at least in Brisbane's climate - is pretty easy to deal with using a separate heater tape (not one integrated into the dew shield) and correct placement. I find placing the heater tape just behind the thick metal band that holds the corrector on the ota itself is very effective.

Mars and Saturn were both pretty high up this year and not once did I have to pack up as a result of dew.

There was a thread on CN a couple of years ago plotting the contrast function and the detriment of the differing sizes of obstruction. The 34% obstruction appeared to cause some boost to some contrast features that more or less correlated with atmospheric details on Jupiter. I'll see if I can dig it up...