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Telescope help please!

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Faerie
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« on: February 02, 2012, 14:53:57 PM »

I want to buy the S/O one for his birthday.  Wink

Some suggestions for a proper instrument that wont cost me an arm and a leg?

Affordability is max R4k, but preferably less, I'd like to take him out for supper too...  Tongue

I came across this for R3500:

Celestron 80LCM Computerised Telescope
 
Features:
••High quality 80 mm refractor
•Lightweight Computerized Mount
•Built-on StarPointer finderscope to help with alignment and accurately locating objects
•Quick-release computerized base, optical tube and accessory tray for quick no tool set up
•Sturdy aluminum tripod and accessory tray included
•Good for terrestrial and celestial observing
•Includes CD-ROM The SkyX Astronomy Software which provides education about the sky and printable sky maps
Computerized Mount Features:


•Proven NexStar computer control technology
•Database allows telescope to locate over 4,000 celestial objects
•SkyAlign allows you to align on any three bright celestial objects, making for a fast and easy alignment process
•Flash upgradeable hand control software and motor control units for downloading product updates over the Internet
•Internal battery compartment to prevent cord wrap during use
•Compatible with optional NexRemote telescope control software, for advanced control of your telescope via computer
Specifications:


•Optical Design : Refractor
•Aperture : 80 mm (3.15 in)
•Focal Length : 900 mm (35.43 in)
•Focal Ratio : 11.25
•Finderscope : Built-on StarPointer
•Mount : Motorized Altazimuth
•Eyepiece 1 : 25 mm (0.98 in)
•Magnification 1 : 36 x
•Eyepiece 2 : 9 mm (0.35 in)
•Magnification 2 : 100 x
•Star Diagonal : 1.25 Erect Image
•Accessory Tray : No Tool, Quick release
•Tripod : Aluminum
•CD ROM : ""The Sky"" X
•Highest Useful Magnification : 189 x
•Limiting Stellar Magnitude : 12
•Resolution (Rayleigh) : 1.74 arcsec
•Resolution (Dawes) : 1.45 arcsec
•Light Gathering Power : 131 x
•Angular Field of View : 1.6 °
•Linear Field of View (@1000 yds) : 84 ft (25.6 m)
•Optical Coatings : Fully-Coated
•Computer Hand Control : Fully Computerized / Flash Upgradeable
•Slew Speeds : Nine slew speeds: 3°/sec, 2°/sec, 1°/sec, .64x, 32x, 16x, 8x, 4x, 2x
•Tracking Rates : Sidereal, Solar and Lunar
•Tracking Modes : Alt-Az, EQ North and EQ South
•Alignment Procedures : SkyAlign, Auto 2-Star Align, 1-Star Align, 2-Star Align, Solar System Align
•Database : 4,000 Object Database

All of which is gibberish to me.

Some advice from the Guru's here please!
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Hermes
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« Reply #1 on: February 02, 2012, 16:40:04 PM »

I presume he would like it for astronomy, in which case an 80mm aperture is very limiting.  It would be fine for looking at the moon, nearer planets and a few clusters, but the limited ...err... scope soon turns it into a white elephant.  A better option may be a larger, used telescope.  Speak to Vincent at 011-475-3707; he may know what is available.
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Rigil Kent
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« Reply #2 on: February 02, 2012, 17:31:21 PM »

Faerie, first off, I think a telescope is a wonderful present to give someone. Each telescope has its fans to a greater and lesser degree, so what I can offer is only my opinion.

First off, important points to keep in mind when buying a telescope in general:

1. Portability - small telescopes can go with you on holiday, big ones can not.
2. The mount (tripod) must be very sturdy - remember that the higher the magnification you are going to use, the more wobbly the picture will be.
3. Maximum practical magnification is 2x the objective's diameter in mm. For example, the 80mm objective above will be limited to 160x useful magnification. It is seldom that you'll want to exceed about 200 though.
4. The magnification = Focal length of the telescope (fixed) divided by the focal length of the eyepiece. So by swopping eyepieces the magnification can be changed.
5. Achromatic refractors ( the telescopes with the big lens at the front)suffer to various degees from false colour which usually manifests itself as a violet halo around bright objects. The higher the magnification, the more pronounced this will appear. Short focal ratio telescopes are also more prone to this problem.
6. Newtonian reflectors work with a mirror at the bottom of the telescope, and effectively eliminate the chromatic aberration, but the overall view is (aperture for aperture) not as tack sharp as that offered by a refractor.
7. Decide what you want to look at - nebula and galaxies require large optics, stars moon and planets
less so. Terestrial viewing requires a Altazimuth (Up-Down, Left-Right) mount, while an equatorial mount will be more useful in tracking celestial bodies while the earth spins without the slightest regard for your tiny field of view.
8. Size matters. The bigger the objective lens or mirror, the brighter the picture will be.

Keeping these points in mind the telescope you mention in your post has a number of things going for it:

1. Reasonably priced
2. Highly portable
3. Points to objects automatically
4. Being a refractor, it will require little if any colimation or maintenaince of the optical tube assembly.
5. At a focal ratio (focal length/objective diameter) of 11.3, the false colour should be negligible.

On the down side,

1. The optics (specifically the size of the objective - the lens in front) is rather small. So while the telescope mount can be very good at automatically locating an object, you may be disappointed to find that it is very dim through the eyepiece.
2. Will require some alignment of the computerized mount before viewing. This can take as little as 5 minutes.

So here is what I think. A lot of the money that you will spend on this telescope goes towards its fancy electronics. To me, half the fun of amateur astronomy is in hunting down the faint fuzzies yourself with the aid of starmaps. In addition, a refractor such as this one is better suited to viewing planets and the moon, which really is not all that hard to find by yourself. The really faint objects for which the automatic slewing is really useful, will maybe not even be visible, especially from light polluted suburbs.

I would rather drop the electronics and the supper, and go for slightly bigger optics, and an extra sturdy equatorial mount, that tracks manually. I would look at a Chinese made 4 inch (102mm) refractor working at a focal ratio of 10, with a minimum mount quality of EQ3. Here is the type of thing.



Dobsonian scopes are reflectors mounted on a sturdy but very basic altaz mount (usually presswood), and they offer by far the best views per Rand. So my second choice would be this 150mm Dob that sells for the same price as the 80mm refractor, but will catch more than THREE times as much light!



Good luck with your (re)search  Cheesy

Rigil


 
« Last Edit: February 03, 2012, 15:39:04 PM by Rigil Kent » Logged
Tweefo
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« Reply #3 on: February 03, 2012, 09:20:33 AM »

More people were put off Astronomy as a hobby by bad telescope choice than anything else. Stay away from the Game or Cape Union Mart special. Alternative is to make your own. Not difficult and you will enjoy looking trough it far more
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brianvds
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« Reply #4 on: February 03, 2012, 11:09:22 AM »

For beginner star gazers I would recommend a good pair of binoculars rather than a telescope. And for someone who also likes bird watching, a small spotting scope might be nice - something I'll get myself as soon as I can afford it.  :-)

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Tweefo
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« Reply #5 on: February 03, 2012, 13:05:19 PM »

If you interested, I've got two "blanks". You grind them together and one become the mirror. You then build a Dobsonion mount. Lots of fun! I've build my first two scopes. Look out for the Scope X event at the War museum in a couple of months
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BoogieMonster
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« Reply #6 on: February 03, 2012, 13:16:44 PM »

Grinding a scope is something I looked into briefly in high school.

I took one look at a book about correcting problems with the mirror during grinding/polishing (which involved lasers and then measuring the scatter patterns or somesuch), and decided this may be a bit above my fireplace.

So, how hard is it, and how much time/money/misery does it really take to make a useful one?

I'd be looking at making something a bit substantial if I ever did it.
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Faerie
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« Reply #7 on: February 03, 2012, 13:21:10 PM »

Thanks guys!!!

Really, its most appreciated, and I know that he would be interested in Tweefo's suggestion, I'm not going to give him that option though, as I want to surprise him with something he's expressed many times to own.

I've still got some time, fortunately, and will explore further and report (and ask further advice) back on my progress.
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Hermes
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« Reply #8 on: February 03, 2012, 14:52:22 PM »

Grinding a scope is something I looked into briefly in high school.

I took one look at a book about correcting problems with the mirror during grinding/polishing (which involved lasers and then measuring the scatter patterns or somesuch), and decided this may be a bit above my fireplace.

So, how hard is it, and how much time/money/misery does it really take to make a useful one?

I'd be looking at making something a bit substantial if I ever did it.

The Johannesburg Centre of the Astronomy Society have weekly telescope building classes.  All get together in a workshop and work together while exchanging ideas.  There are experts who assist you in configuring the mirror.  I built my own telescope a few years ago.  The big job is grinding and polishing the mirror.  It will take you a few months.  The materials don't cost much.

http://www.astronomyjhb.co.za/
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benguela
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« Reply #9 on: February 03, 2012, 18:45:17 PM »

Definitely a Dobsonian and a pair of 7x50 (or 10x50) binoculars (no more than R450) is ideal for beginners.

Shop around, only from telescope specialists, for the largest Dobsonian you can find at your price. Try for an 8 inch.

The only ones that I know whom seem to have Dobsonians currently in stock is http://www.planetarium.co.za/ and http://shop.technopro.co.za. I see the planetarium has an 8 inch (203mm) SkyQuest for R3,965 which is cheaper than 8 inch Sky-Watcher at technopro (R4,516.00) Technopro has a bad reputation in the astronomy community for over charging and not so good after sales service.




 





 
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Tweefo
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« Reply #10 on: February 03, 2012, 19:44:17 PM »

Grinding a scope is something I looked into briefly in high school.

I took one look at a book about correcting problems with the mirror during grinding/polishing (which involved lasers and then measuring the scatter patterns or somesuch), and decided this may be a bit above my fireplace.

So, how hard is it, and how much time/money/misery does it really take to make a useful one?

I'd be looking at making something a bit substantial if I ever did it.
For you first scope you are not trying to build a replacement to Hubble. You can get great views even with a imperfect mirror. There is a book "Amateur Telescope Making" by Albert G Ingalls written quite a while ago, long before lasers and wot not. It explains grinding very clearly. To figure the mirror he use a razor blade and a light. This was written before the Dobs mount (Dobson is still alive I think) was invented, so don't follow his mount. It took me a month of grinding but I had some time for myself. It does not cost much, and like I said, I've got two blanks (6 inch), grinding material (different grades) and a secondary mirror. First come first serve - for free.
Do a 6 inch as your first project. You are learning and will make mistakes, then you can try a 8 or 10 inch. As you improve the views of course get better. I met a guy in George who made a perfect 20 inch but that's nuts.
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BoogieMonster
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« Reply #11 on: February 06, 2012, 11:07:34 AM »

For you first scope you are not trying to build a replacement to Hubble.

Darnit!

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It does not cost much, and like I said, I've got two blanks (6 inch), grinding material (different grades) and a secondary mirror. First come first serve - for free.
Do a 6 inch as your first project. You are learning and will make mistakes, then you can try a 8 or 10 inch. As you improve the views of course get better.

It's a very kind offer but it'd be wasted on me. As I said I'm not immediately considering doing this, I have lots of other stuff going on and I'm not quite ready to drop everything and spend time on an endeavour like this. I don't stargaze that much these days anyway, so it'd be unjustified to remove this opportunity from someone more serious about it.
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