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Which Scope?
http://www.boltonastro.co.uk/forums/viewtopic.php?f=8&t=234
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Author:  poconnell [ Wed Feb 29, 2012 7:56 pm ]
Post subject:  Which Scope?

I'm a complete newbie looking for advice on which scope to buy and to which options would give me the best upgrade potential.

I'm interested in viewing the Planets, the Moon and also Deep Space objects, which pretty much covers everything :) I've also always had an interest in photography. I already own a Canon EOS 450D so it would make sense to look at using that a long the way.

Initially I started out looking a Skywatcher Skyliner 200P dobsonian. However I then realised a dobsonian mount wasn't really suitable for imaging and so I started to look at the more expensive Explorer 200P on an EQ5 mount before finding out the 200P may be difficult to focus when using with a DLSR. I also read that the EQ5 mount isn't really up to holding the extra weight of a DSLR and 200P. :(

Scope wise I'm now looking at a Explorer 200PDS but its all starting to get pricey.

The 200P dobsonian seems a good, fairly cheap starter but I would probably outgrow it pretty quickly and so I am prepared to pay a bit more if needed.

The Explorer 200P + EQ5 seems a good option, but again it may not be the best for imaging and the mount may not be up to it. Cost wise it seems pretty cheap when bought as a bundle so much so that I could sell the EQ5 later to recover a lot of its discounted cost.

The 200PDS seems to be the best of this bunch on the scope side but it would be the top end of my budget even if I went for the EQ5 mount. I also need to allow some money for a few extra bits, such as a collimator, eyepieces, telrad, dew shield etc.

I've avoided looking at GOTO scopes as I see this as paying for a bunch of electronics rather than quality optics. I think Goto is something I would like to have but more of an add on later rather than as part of my initial purchase.

I'm probably looking for something that doesn't exist, a good all rounder, but I really want to buy a decent system which will visually give me value for money and also give an option of taking decent images. I don't want to scrimp on a cheap system which I then end up being disappointed with and not wanting to use but I also don't want to be one of those people who buy all the gear with the best intentions but then never use it with it next appearing as an ebay sale 12 months later.

I can see the mount is the most important component on any system but a mount without an scope doesn't really work for me :lol: but then a scope without a suitable mount isn't much better.

Would the cheaper 200P dobsonian be my best option for now ? With a view to later buying an HEQ5 and then a better scope or should I look to buy the right scope now ?

I've been using this Field of View Calculator to try to get a feel of what sort of images I can expect to see but this has just put me off considering certain telescopes and has only ended up pushing me further towards an 8" reflector.

What would you do if you were buying your first telescope today ?

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

Phil,
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 http://www.astrobuysell.com/uk/ 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:
Attachment:
File comment: Helios 8" Newtonian on wooden tripod
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helios.jpg [ 30.84 KiB | Viewed 10616 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:  poconnell [ Thu Mar 01, 2012 9:10 pm ]
Post subject:  Re: Which Scope?

DRatledge wrote:
But - are you expecting to carry it out every time?

Unfortunately for now I will have to keep taking it out as needed. I'm hoping if I stay hooked, that when she's out at work I may have an unfortunate accident with a saw which ends up with one of the sheds becoming decapitated :lol: I may pick up more knowledge on that one at the next meeting.

DRatledge wrote:
If so the better and heavier mounts will put you off setting up.

Astronomy does seem to throw plenty of hurdles in our way, doesn't it.

I have read about picking a lighter weight telescope first off, as you will be more likely to go out and actually use it than with bigger scope. This is partly why I haven't just rushed in and bought the 200P

DRatledge wrote:
Are you in light polluted suburbia? Are you going to travel to dark skies?

Yes I am in a light polluted area but I don't think its too bad. I can't see any street lights from my back garden, just a few neighbours lights in the early evening which can be quite bad and a few neighbours also have some security lights which have been a pain at times.

The bungalow at the back of me does have a large roof which shields a lot of the lower sky to the SSE but I'm far enough back to still be able to see most of the sky over it although someone in their street also has a security light which seems to be pointing straight up as it casts a huge white glow over this bungalows roof.

My garden is in a dip so I can't see the horizons at all and I probably loose the lower 10 or so degrees to the south, east and west, My own house stops me seeing North and moving to the front of the house wouldn't improve things due to street lights, trees and a fairly busy road.

As for Dark Sky sites, I haven't really thought about it and if I'm honest its probably not something I would do very much, especially at this point.

DRatledge wrote:
Why not borrow some othe societies telescopes or equipment.

Is it not a little early in my membership to be doing that ?

DRatledge wrote:
Study the secondhand adverts on http://www.astrobuysell.com/uk/ as bargains can be had.

I've spent a lot of time doing just that, but I've not seen anything so far thats within driving distance thats took my fancy. I've mainly been looking for Mounts but I've also been keeping my eyes open for some of the fancier Cats

Phil

Author:  poconnell [ Thu Mar 01, 2012 9:39 pm ]
Post subject:  Re: Which Scope?

rwilkinson wrote:
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?)


I think they are 1.75" stainless on the HEQ5 and EQ5's and 2" Stainless on the NEQ6. The EQ5 mount seems to have a bad reputation as not being the best built with bad clocks and limited weight carrying capabilities. Unfortunately the HEQ5 and NEQ6 all seem to be being bundled with GOTO which jumps the price up considerably.

rwilkinson wrote:
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

This is where it all gets a bit confusing, 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. From my photography days fast lenses were always the way to go which is probably whats got me looking at F5 newtonians.

rwilkinson wrote:
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.

I was looking at that approach with the 200P + EQ5 with a further £270 for the Goto Kit which I could add later. I probably would have jumped in and bought this scope and mount a month ago if I hadn't read that the HEQ5 should be regarded as the entry level mount for imaging.

rwilkinson wrote:
And I'd prefer a 9x50 finder-scope to a TelRad

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.

rwilkinson wrote:
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!)

I was going to make my own with some of this http://www.easyfoam.co.uk/closed-cell-p ... e-foam.php , as its also water proof, wear and tear resistant and lightweight but cardboard sounds like an cheap initial option which I hadn't considered.

rwilkinson wrote:
save your money for light-pollution filters!

Good point.


Thanks

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:  poconnell [ Fri Mar 02, 2012 6:37 pm ]
Post subject:  Re: Which Scope?

rwilkinson wrote:
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.

That makes sense I hadn't thought about Barlows and focal reducers, I was thinking only of eyepieces.

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

rwilkinson wrote:
Naked-eye finders (Telrads and red-dots) are fine.... the targets (and even some of the hopping-stars to get to them) aren't visible to the naked eye

Another good point I hadn't considered. :oops:
Most of what I've been looking at so far with only binoculars I can find via the stars I see with the naked eye. I hadn't thought about that changing once I moved up to a telescope.

Thanks

Phil

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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