250mm Newt vs C9.25

[acme]

Hi,

I am thinking about buying a Celestron C9.25 The thing is I have OO 250mm Newtonian. What would the benifits be of owning both. If you could have either which one would you choose and why?

They both weigh in at around 9/10 kg so my mount can cope.

I need some advice before I blow my budget.

Thanks in advance.

Acme

[Merlin66]

The 10" f4.8 you have will be great for general viewing/ DSO etc etc.

The C9.25 is more compact but at f10 much slower than the Newt. (Yes I know you can add reducers etc - but...) ideally suited for planetary and double star/ globulars etc.

If you have a good 10" Newt just use a x2 barlow/ powermate to get the same benefits...

(The best scope I ever had (other than the Genesis!) was a 1/10wave 12.5" f5 Newt - there wasn't anything it couldn't do.)

[dweller25]

The 10" newt will cool much faster than the C9.25 and will have a known optical performance ie 1/6th or 1/8th wave.

The SCT is compact and easy to mount.

I would not buy a C9.25 if I had a 10" newt.

[ollypenrice]

The only reason that I can see for having the SCT as well would be for easier planetary imaging and you don't mention this. I'd keep the Newt unless you need the portability gain. (However, the 9.25 has the second best SCT optics I've ever encountered, the best being in a home made instrument by Ralf Ottow. (See Astronomy Now this November issue.)

Olly

[brianb]

the 9.25 has the second best SCT optics I've ever encountered

Mass produced SCTs are rather variable in optical quality, the real hardnuts tend to buy four or five & keep the best one. Having said that the C9.25 primary is f/2.5 rather than f/2.0 like the Meades and the other Celestrons, I don't know why they made it that way but the result seems to be that C9.25s do on average seem to be the best of the smaller SCTs, based on the appearance of the diffraction pattern. But I reckon an "average" C11 will still outresolve an "average" C9.25. C14s are not mass produced in the same way (they're hand built on demand in small batches) and are generally pretty good optically (this is reflected in the availability & price!)

Personally I'd say that the optical performance of a C9.25, C11 & a mass produced 10" Newt would be, for all practical purposes, not too different from each other, on 90% of nights the seeing would be the limiting factor for any of them & the light grasp is not disfferent enough to make an obvious difference. The SCTs are shorter & lighter, making for easier transport and an easier job for the mounting, and in my experience hold collimation pretty well. The downside is the stock focuser, which is coarse and subject to "mirror flop" ... replacing the stock knob with the two-speed geared Feathertouch replacement cures the first issue but not the second, whereas bolt-on focusers tend spoil the balance of the tube and may be prone to slippage.

If restricted to 10" - 11" aperture, money no object & no need to transport, my choice would be the Orion Optics (UK) 10" f/6.3 tube with 1/10 wave PV optics & all the available mirror cell & focuser upgrades ... on a Losmandy Titan, AP 1200 or Paramount ME. Without an observatory, I settled for a CPC1100 (fork mounted) as the largest aperture "luggable" with object tracking capability - and I'm not unhappy with my choice.

[merlinxlm]

I'm very interested in this thread as I'm buying a C9.25 next year. How would a SW 250ds stack compared to the C9.25. I'm mostly imaging DSO's with my SW 80DS pro/Olymous E510. Would the money saved buying a SW 250ds instead of the C9.25, go towards buying a good web cam imaging?

Thank you.

[Dangerous-Dave]

I've owned two SCTs and were a bit underwhelmed with their performance for the price tag. That said, they were standard f10's, the C9.25 uses a slightly different set up (longer focal length secondary) which makes it easier to mass produce decent optics. A friend has a C9.25 and we had some stunning views of M13 and M81/M82 at Dalby starcamp with it.

I expect you are familiar with the various pros and cons of the two designs, but here's my take on it anyway:

SCT: expensive, compact, better for planets, needs reducer or long fl eyepieces for deep sky work, corrector prone to misting, prone to mirror flop and mirror shift, need to star collimate but there are fudges to get around that, sealed unit so mirror stays clean, hard to put right if you get a lemon, nice viewing position

Newtonian: cheap, cumbersome, better for DSO's, need big Barlows or good short fl eyepieces for planetary work, primary stays dew free although secondary benefits from dew cap, a faff to collimate but can be done in daylight with laser collimator, focuser tends to stay put but you might have back focus issues, catches the wind, cools down quicker but prone to tube thermals, smaller obstruction so slight better contrast, fast models have high coma and need better quality eyepieces, need to be a contortionist to see through the eyepiece sometimes, mirror needs periodic cleaning and realuminising