Bolton Astronomical Society

Which Scope?
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Author:  DRatledge [ Wed Feb 29, 2012 8:16 pm ]
Post subject:  Re: Which Scope?

No one telescope can do "everything" as you suspect - that's why most will probably end up with 2. You are right that the mount is the most important item - you only want to buy that once. Telescopes can come and go. But - are you expecting to carry it out every time? If so the better and heavier mounts will put you off setting up. Are you in light polluted suburbia? Are you going to travel to dark skies? You mentioned your Canon for astrophotography so that puts additional requirements on the mount tracking quality and also a telescope with a useable field of view.

I would't rush into anything. Keep asking questions. Why not borrow some othe societies telescopes or equipment. Study the secondhand adverts on as bargains can be had. All my recent scopes have been purchased there. Above all keep coming to the meetings!!!

Author:  rwilkinson [ Thu Mar 01, 2012 9:55 am ]
Post subject:  Re: Which Scope?

Hi Phil, I'd been expecting that question! ;)
The question is a little like "what's the best car to buy" - so the answer should be "the best one(s) you can afford - maybe an Aston Martin for normal road-driving, plus a Range Rover for off-roading and carrying big loads"?

But seriously, it's true that no one 'scope is ideal for planetary and deep-sky work: for the planets you need a long focal-length (I work with an effective focal length of around 5m, obtained by projection from a Barlow lens), and for the "faint fuzzies" a wide aperture and low f/ratio.

I started out with a Chinese-built 8" Newtonian on an EQ5 (in those days it was branded Helios rather than SkyWatcher, but I think that it's basically the same design?) It gave good views, but I found it quite big and heavy for portable use (carrying up and down 3 flights of stairs!) - but it came on an EQ5 mount which I've since used with all the 'scopes I've ever owned. The mount itself was OK, but the lightweight aluminium tripod was rather too "whackery" (but I think that the new ones come with a heavier tubular steel tripod?) so I adapted an old wooden surveyor's tripod:
File comment: Helios 8" Newtonian on wooden tripod
helios.jpg [ 30.84 KiB | Viewed 11304 times ]

Then when I started deep-sky imaging (using an MX5C camera with a small CCD) I needed a wider field of view (i.e. a shorter focal length), so I tried a cheap 120mm f/5 refractor, but the image-quality was not that thrilling.
So I ended up with a Celestron C8 SCT: this has a 2m focal length, but with an f/3.3 focal reducer I'm back down to around f=620mm, and it's only about half the weight of the Newtonian.
With this "jack of all trades" combination I can manage both planetary and deep-sky imaging using my modified webcam (although the image quality using the focal reducer is nowhere near as good as a short-focus apo refractor would give).

My advice would be not to get any mount smaller than an EQ5 - and if it comes without motors, consider upgrading it straight to a GoTo system, rather than simple dual-axis motors.
Then see if you can try out a few different 'scopes on it before buying one of your own.

P.S. On my Helios 'scope, I could unscrew the 1.25" eyepiece-holder off the end of the draw-tube to expose a T-thread to attach the camera for focal-plane imaging. And I'd prefer a 9x50 finder-scope to a TelRad.
You won't need a dew-shield for a Newtonian (and don't spend any money on one for any scope, when you can easily make one from the cardboard box it came in!) - save your money for light-pollution filters!

Author:  rwilkinson [ Fri Mar 02, 2012 3:53 pm ]
Post subject:  Re: Which Scope?

poconnell wrote:
I'm not 100% sure yet on where all all this focal ratio, focal length, aperture start effecting each other and when one is better than the other.

The bigger the aperture (D), the larger your "light-bucket", and theoretically the better your resolution (but I'd reckon that resolution is more dependent on the stability of our lower atmosphere, which varies from night to night).
The longer the focal length (f), the higher the magnification, and the smaller the field-of-view: so large objects (e.g. open clusters) need short focal lengths, whereas small objects (e.g. planets) are best with long-focus optics.

Now the f-ratio (properly the f/D ratio) is just the ratio between the two.
In an ordinary (i.e. non-zoom) camera lens, f is fixed, but D is varied with the iris diaphragm, whereas in a telescope D is fixed but f can be varied by fitting focal reducers or Barlow-lenses on the back. For example I use my 8" SCT at f/3.3 for deep-sky imaging and at f/25 for planetary work.

But I think that smaller f-numbers require more curved optical surfaces, which are more difficult and expensive to make - so for a given aperture and price, the quality of a longer focal-length instrument is likely to be better.

poconnell wrote:
I liked the idea of a Telrad as I often get lost even when using binoculars. The obvious stars with the naked eye suddenly become lost when looking through binoculars as all the invisible background stars suddenly jump into view. I haven't used a finder scope but I imagined it being a lot like the view through my 10x50 binoculars.

Naked-eye finders (Telrads and red-dots) are fine if you have dark skies (like in Norfolk), but here in Bolton I have the opposite problem: the targets (and even some of the hopping-stars to get to them) aren't visible to the naked eye - so the 7x50 binoculars round my neck and the 9x50 finder on the 'scope are indispensable.

Author:  rwilkinson [ Sat Mar 03, 2012 7:22 am ]
Post subject:  Re: Which scope - and which target?

poconnell wrote:
The bit I was struggling to get my head around was the difference between viewing a planet and a distant deep sky object.

Planets require a high magnification over a very tiny field-of view, whereas deep-sky objects need a large light-grasp over a much wider field.

To illustrate the difference in brightness, I use around 1/50-sec at f/25 for imaging Mars (which is a sunlit subject, of course), whereas tens of minutes at f/3.3 are needed for "faint fuzzies".
And the Martian disc is currenly only a quarter of an arc-minute across, compared with over 200 arc-min for the M31 galaxy!

We'll be offering more insights into viewing and imaging Mars at our 17-Apr meeting, meanwhile you can review our companion presentations on Jupiter and Saturn in the Workshops..Astro-imaging section of our Members' Area.

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