Any telescope experts ?


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#1
Hi guys...

Someone I know is about to make a telescope purchase... and was wonder what are all those extra eye pieces that come as accessories do...

Some of them have a wider degree of "Apparant Field".

Are all these extra accessories needed? Or does the standard set already include most of the stuff that one needs...

Could someone enlighten me? :D

Thanks :)
 

roygoh

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#2
Telescope basics:

The magnification you get from a telescope can be calculated by dividing the focal length of the objective lens by the focal length of the eyepiece.

For example, if the objective lens has a focal length of 900mm, and the focal length of the eyepiece is 24mm, then the magnification will be 900/24 = 37.5 times.

To change magnification for a given telescope, there are 2 ways. One is to change the focal length of the eyepiece and the other is to use a barlows lens that changes the effective focal length of the objective lens.

Most telescope users will have a set of eyepieces with different focal lengths and occasionally a 2X or 3X barlows. It is necessary to have different magnifications if one wishes to view different kinds of objects in the sky or view the same object (such as the moon) at different levels of detail.

Under perfect viewing conditions, the maximum magnification you can get is limited by the aperture of the telescope, which is calculated by dividing the focal length of the objective lens by the diameter of the objective lens.

For example, if the objective lens has a focal length of 900mm and a diameter of 80mm, then the aperture is F11. The smaller the F numner, the brighter the telescope is, and the higher the magnification it can support.

Under real life situations, other factors like atmospheric condition, quality of eyepiece, stability of telescope etc will also limit the magnification that is practically achievable.

So it is most likely not feasible to attach a 2mm eyepiece to a 900mm F/11 telescope and expect 450X magnification with a clear view.

When you look into a telescope eyepiece, you see the objects through a circular window. The bigger the size of the window you see, the bigger the apparent field of view is, and is given in degrees. The apparent field of view is mostly determined by the eyepiece design. For eyepiece with very wide apparent field of view, the actual field of view may be limited the by the telescope optical assembly itself.

There are tons of other accessories that will make star gazing more enjoyable. Not sure what other accessories your friend is getting in the package.

- Roy
 

hongsien

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#3
I hope your friend is not buying a telescope here in Singapore at the shop in the Singapore Science Center? It is very expensive there! 2-3x higher priced than when buying in the US. Buy the magazine 'Sky and Telescope' available at big bookstores here and check prices there.

I would recommend Meade or Celestron........

Hong Sien
 

Ian

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#5
Originally posted by aaronLiu
Hi guys...

Someone I know is about to make a telescope purchase... and was wonder what are all those extra eye pieces that come as accessories do...

Some of them have a wider degree of "Apparant Field".

Are all these extra accessories needed? Or does the standard set already include most of the stuff that one needs...

Could someone enlighten me? :D

Thanks :)
There's really no such thing as a 'standard' set of eyepieces, unless a telescope is bundled with a few eyepieces.

It largely depends on what you will be observing, if you are planning to be a general observer, that is a bit of planetary, lunar, some deep sky (given the light pollution in SE Asia as awful) etc then the normal Plossel occulars with a 50 degree apparent field of view will be fine.

General recommendations (based on 30 years ATM experience, 35+ years observing).

Orthoscopics (Orth/Ortho) - Best occular design for serious solar and planetary observing. Excellet for lunar work too. Narrow field of view. Typically around 40 degree apparent field of view.

Kellners (KE) - Good general purpose occulars that aren't expensive, narrow field of view that isn't always good for larger objects, slight colour fringing can be apparent. Apparent field from 35-42 degrees typical.

Plossl - Pretty much the defacto standard occular these days. 50 degree apparent field of view and reasonably good coma and colour correction.

Konig - Hard to find, but with superior performance to most plossl designs. 60-65 degree apparent field of view with low coma and excellent fringe colour correction. Not popular these days as the ultrawides from the likes of Meade and teleview dominate the market. My preferred occular for general observing.

Ultrawides. Large, heavy, expensive boat anchors that give lovely views but at great expense. Frankly not worth the money in 1.25" fittings as you really need 2" or larger occular diameters to appreciate these occulars. Apparent fields of view from 70-90+ degrees.

Symetrical Ramsden (SR) - Old old design, bloody awful and come standard with most cheap telescopes. Horrible colour fringing and optical distortion. Old high quality Ramsdens are very good though but have a very narrow field of view.

Huygenian, Huygenian Ramsden (H, HR). Again an old old design, bloody horrid to be frank, again shipped with cheap telescopes.

There are some specialised occular designs such as solid ramsdens that are excellent for planetary work (rare, expensive).


What the 'apparent field of view' actually means.

An eyepiece (ocular) has like any other lens an apparent field of vie that is determined by the field stop. The telescope optical system (lens, mirrors) also have and field of view and to calculate the actual field of view seen through the eyepiece use the following formulae.

Step 1
Calculate eyepiece magnification. As Roy's pointed out the formula is Telescope focal length in mm / eyepiece focal length in mm.

eg: 1000mm telescope fl / 20mm eyepiece = 50x magnification

Step 2
To calculate actual field of view at the eyepeice in arc minutes.

(Eyepiece apparent field of view / eyepiece magnification.) x 60

eg: 50x magnification with 40 degree apparent field of view eyepiece.

(40 / 50 ) x 60 = 48 arc minutes

50x magnification with 82 degree apparent field of view

(82 / 50) x 60 = 98.4 arc minutes or 1 degree 38.4 arc minutes.

To calculate the field of view in degrees leave of the x 60 multiplication :)

Unless you are doing purely solar or planetary observing avoid eyepieces in the 0.960 inch size, go for a scope with a 1 1/4" (1.25") eyepiece size as it's far better from not only an observing standpoint but also in terms of choice of eyepieces.
 

roygoh

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#8
Originally posted by blurblock
Got a stupid idea ..... can a camera be fitted to a telescope in any way?
There are many ways to attach a camera to a telescope.

For SLR, you can use a T-adapter to attach the camera to the telescope. The telescope objective lens becomes the lens of the camera. SO if you have a 900mm telescope, then it becoes a 900 mm lens when attached to the SLR. This is called prime focus.

Some T-adapters can accomodate an eyepiece to do eyepiece projection photography that gives higher magnification.

For digicams, there are also many adapters. The DCL-28 from William Optics is a 24-mm eyepiece that has a 28mm thread. It can be attached to a CP995 like a filter and the 995 + DCL28 combo can then be attached to any telecope that accepts 1.25 inch eyepiece.

www.scopetronix.com has a lot of other attachments for digicams.
 

Ian

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#9
Originally posted by majere2sg
http://www.buytelescopes.com/container.asp?dest=/manufacturers/wyo/WYO_bird.htm

Look good. But whats is the difference of using a telephoto convertor and telescope ?
Telescope seems to be much better unless there are some focusing issues ? :confused:
It's more than just focusing issues. Astronomical Telescopes are designed with a curved focus plane, rather than a flat focus plane used by camera lenses. This means in simple terms an increase in coma towards the periphery of the edge. It's quite noticable in bird shots taken with full frame cameras on reflecting type telescopses (ie Newtonians, SCTs etc). Also there's a minor problem with non terrestrial telescopes of image inversion etc.
 

hongsien

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#10
Hi Ian,

I have a question regarding the coma problems of reflectors:

years ago when I was young (ahum).....I made a small 11 cm mirror for a newton telescope by grinding it myself (joined a club for telescope making in Amsterdam) and it was parabolic in the end. Wouldn't this solve the problem of focus plane flatness? Btw: I never finished the telescope, the mirror was ready and got the silver reflective layer and some coating I think.

If that is the case, do you know which brands or what models have parabolic mirrors?

Hong Sien
 

Ian

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#12
Originally posted by hongsien
Hi Ian,

I have a question regarding the coma problems of reflectors:

years ago when I was young (ahum).....I made a small 11 cm mirror for a newton telescope by grinding it myself (joined a club for telescope making in Amsterdam) and it was parabolic in the end. Wouldn't this solve the problem of focus plane flatness? Btw: I never finished the telescope, the mirror was ready and got the silver reflective layer and some coating I think.

If that is the case, do you know which brands or what models have parabolic mirrors?

Hong Sien
A parabaloid solves most of the focusing issues, but doesn't solve coma unless the focal ratio is longer than about F6.

A Newtonian design is supposed to use a Parabaloid mirror, the noteable exception being the majority of f 8.0 or longer 115mm (4 1/2") Newtonians which use spherical mirrors.
 

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