* So many wanting the Skyliner 8" (200mm?)200/P Dobsonian *

[Charic]

My take on the skyliner after my very short ownership.

The Skyliner is a Newtonian designed reflecting telescope, incorporating mirrors instead of lenses to capture light. Its less expensive to make than some of its peers, but offers great value for your money. Good for deep sky observing, Galaxies, Nebula and Planets too.

Not advisable for Astro-Photography.

The Dobsonian describes the mounting system employed with this telescope, designed by John Dobson in the 60`s. People often try to compare Newtonian with Dobsonian telescopes? FACT; One is a type of telescope, The other is a type of mount. Very easy to set up straight from the box , and after a period of cooling? (all telescopes taken outdoors require cooling!) its really point and view. Ive tried the EQ mount with another telescope, Its not for me. However a motorised EQ refractor may be required when I go Astro-Photography hunting.

So how does the Skyliner work?

Light enters the open end of the telescope striking the main primary mirror. The primary mirror on this telescope is parabolic (allows all light to be correctly focused to a single point – more  on this later ?) The incoming light is reflected to a focal

point via a secondary mirror, where it forms a `real` image. This `real` image is then magnified and viewed by using an eyepiece of your choice. The image is reversed and inverted (standard - surprises some people on first use.) so not so clever during terrestrial Daylight use, but were not using this telescope for Day use.

The Skyliner has a focal length. [The focal length determines the angular field of view of the telescope]

The Skyliner has an Aperture.    [The Aperture is the Diameter of the Mirror]

The Skyliner has a Focal Ratio? [The Sum derived from dividing the Focal length by the Aperture]

Focal Length: F-1200 Aperture; D-200mm Focal Ratio: 6 [information from the telescope label- (these figures are rounded to the nearest! )

The Focal Ratio describes the `Speed` of an optical system. For visual observations (the intended use for this telescope) Focal Ratio is not as important as the Aperture or Focal Length. It is more important though if you were to get involved with Astro-Photography. The lower the f/ number the faster the scope would be, allowing more light through and shorter exposure times.

The Focal Ratio is the result of the formula F/D=? ( i.e. two telescopes here , F-1200/D-200 = 6 (f/6) & F-800/D-80= 10 (f/10) This example would suggest that the f/6 telescope is `faster` than the f/10. 

There is still a reason to be aware of the Focal Ratio for this Skyliner telescope.

To build and design telescopes with focal ratios of around say F5, they are better equipped by using a parabolic mirror, this cancels out spherical aberration. The telescopes tube length will be shorter too. That all sounds good but as the focal ratio gets faster, other problems can arise? A Newtonian reflector can suffer from Coma aberrations (aberration = not normal) The paraboloidal mirror, due to its design, cannot bring off-axis light rays completely to a single point (I haven’t looked for it on my scope yet, but its there by the nature and design of the optical system. Coma causes stars at the edge of the view to become slightly elongated, they look like little Comets heading towards the centre of view, with their tails facing out!, hence the name, Coma.-  

Coma can be corrected in high end eyepieces . (Its the main reason why so many people insist and mention the fact that "because your telescope is "fast", you need this lens" ect.... without too many words of explanation, which to me just puts ££ signs across my eyes, but now I understand the reasons for their insistence, and to be honest, there experience in using, those better lenses. The next time Im viewing, I`ll check to see if I should get better lenses, or stay with my BST Starguiders? You get what you pay for with lenses(so they keep telling me!) My initial thoughts were the telescope has done its task and presented an image of whatever quality to the focal point. The lens just magnifies that image, but Oh!-no, it can correct. Not only that, poor lenses(cheaper maybe?) can also have aberrations too. Getting the right, and sometimes, expensive lenses are going to make your system better (Maybe) The Sky’s The Limit (their offer to buy two lenses and send back the one you don’t require for a full refund is not such a bad idea if you need to `try before you buy.)

The eyepiece is also an optical system in its own right, with one important figure, Its Focal Length measured in mm. marked somewhere on the lens. The focal length of the telescope divided by the focal length of the eyepiece determines the magnification of the telescope. Shorter focal length eyepiece (low number i.e. 4mm) will give a smaller field of view and greater magnification, and a longer focal length (higher number i.e. 32mm) will give a wider field of view, but less magnification.

using an on-line calculator, Its telling me that my optimal magnification range is between 50x to 100x power. Based on my old man eyes? Its also telling me that the minimum magnification I should be using based on my Exit Pupil size is 34x power with a 35mm eyepiece. That’s pushing it a bit for the barrel size I use, so will probably stop at about 25mm. The Highest magnification it suggests is 406x power using a 3mm lens. So I’ll need to rush out and get a

3mm next(ONLY KIDDING)

Practical and Theoretical use is completely chalk and cheese unless the most perfect conditions are available, That’s why most professional observatories are High-up in the Mountains to get above the Weather, or out in Space, to help improve Weather and seeing conditions. Due to the technical issues, I will aim to use my scope between 48xpower(25mm) to 200xpower(6mm)

By purchasing the 25mm 12mm 8mm lenses I’ll get close to the limits (48x-150x) However, this game is changing all the time. I’d like to get (at time of writing) 6mm 8mm 10mm 18mm 25mm (or 32mm instead of 25mm in 2" barrel) [if Barlowed = 3mm 4mm 5mm 6mm 8mm 9mm 10mm 12.5mm 16mm 18mm 25mm & 32mm] that’s 400X down to 75x Power on a 2xBarlow.

I still suggest using the telescopes aperture to estimate the [working] highest magnification i.e. 200mm = 200x power (giving a 6mm lens) and for the Lowest usable magnification, Aperture again, divided by (your) Exit Pupil size. That gives me about a 36mm lens, then whatever you desire to fill the gaps in-between. I pretty much only use the 8mm lens at present, sight then view.

A few facts here about the telescopes physical aspect.

Newtonian (Parabolic) Reflector, Mirror 200mm (8" = 203.2mm)

Finder scope 9x50

Focuser: 2" with 1.25" adaptor.

Focal length 1200mm

Tube Diameter; 47mm

Focal ratio: F/6 (F/5.9)

Maximum mag (at a push) 400x

Resolving power: 0.56

Weight: 26.64KG (OTA=11KG, Base = 15.64KG)

Not modified yet in any way?

Pros.

Ease of Use

Value for money

Best for visual Observation

Less Pros.

requires minor maintenance – collimation, cleaning

Bulky

Cons:

Non as yet? not for me.

[kirkster501]

A good commentary.

People tend to like that Dob because it's a great combination of ease of use, aperture and cost. It's a real sweet spot.

Sent from my iPad using Tapatalk

[swamp thing]

Nice evaluation. Be aware though, high end expensive eyepieces don't correct coma. When they say well corrected it means the eyepiece is able to handle the steeper light cone of a fast optical system without introducing any optical nasties like astigmatism. They don't cure coma.

[idigitize]

Nice write up... I used one at the local club last week and the speed of setup and views were impressive.

What I found a slight niggle (which seems quite common actually) is the finder scope. I think it would benefit a lot from having a right angle finder. The straight, made it quite awkward an angle and easier is seemed to knock the scope with my forehead when trying to get my eye onto it.  As a newbie, I wondered if im missing something as virtually every  scope does have the main eyepiece and finder at right angles to each other, just not sure why.

[hippie]

I recently invested in a 10" skywatcher dob, Since owning this scope it has brought a new dimension to my viewing of the night sky's, These are fabulous scopes and i would recommend them to anyone looking to upgrade  their scope!,

EP I am useing are the BST 25mm, 18mm, 12mm, mainly at the moment,  Recently  acquired 2" 20mm tele vue Nagler EP.  

 Happy Dob viewing  nights,  Hippie

[michael.h.f.wilkinson]

Nice write-up. An 8" scope might be your first, but it can keep you entertained for a long time. Mine has lasted nearly 18 years, and I still find new stuff.

[Charic]

Swamp thing.......Cheers, I understand what your saying, that Coma is not actually cured by the lens. But it can be corrected. Correction lenses for Newtonian reflectors were designed to reduce coma in f/6 telescopes and faster, using a dual lens system (consisted of a plano-convex & plano-concave lens fitted into an eyepiece adaptor) I still want to keep to my Branded BST`s for now. Ive only looked through my telescope at the Skies, not yet actually studied what I have in my system just yet, so these faults are not noticeable by me yet. I’m sure I’ll find them one Day?

idigitize......I’m having no problems with the scope as yet. I’m still flexible enough to get behind the eyepiece. On using the 9×50 (straight through) I’ve found it easier to sight with both eyes open. The left/right image overlaps on correct alignment with the subject, lock down the scopes tension handle, and straight into the viewfinder. A correctly sighted Telrad would allow you to be further away from the eyepiece, akin to aligning an old machine gun sight. I’d probably struggle with the RA/Adaptor!

hippie....... I’m with you there with the BST`s. I own the amazing 8mm and intend to get the 12mm 25mm next, but with the ever changing rules, I’d like to own the 6mm 8mm 10mm 12mm 18mm & 25-32mm. I have the Starguider variants ( Explorers-  same brand) or Astrotech Paradigms in another brand. But they all get great feedback, and for their minimal cost and 60 degrees, happy days! or nights?