Help Setting up Vivitar 76700 Telescope Pls

[alexcartman]

Hi friends,

I have always wanted to purchase a telescope and the above unit was on special so I decided to take a plunge.

The instructions only deal with assembly of the unit - doesn't tell you how to use it.

From what I have learnt so far this is a "Newtonian" telescope so has the scope on the side. 

I have no issues looking thru things on the sighting scope (but its upside down) - but I cant view anything from the main scope. 

There are a couple different items in the pack and im not too sure what they are or what they are used for...

1. 1.5X Erecting Eyepiece - what is this and what is it used for - where does it go?

2. 3X Barlow Lens - what is this - does it go in the side scope?

3. SR4mm Lens - where does this go and what is it used for?

When I look thru the main scope all I see is a reflection of my eye - when i used the 3X barlow lens I dont see anything except three lines (like a cross hair setup) and a middle circle - which is a smaller version of what I see when I look at the front of the scope.

Im really confused how to use this unit - any help would be appreciated, thanks!

Alex

[Cornelius Varley]

Hello and welcome to SGL. The items you have described are put into the focuser to draw tube so allowing you to use the telescope. Without them you wiil just see a reflection of your eye. The erecting eyepiece flips the image the correct way up for terrestrial viewing (for astronomical observing you don't need this). The SR4mm eyepiece will be the eyepiece to use at night. I am surprised that only one eyepiece is supplied with the telescope, normally two are supplied. A high magnification one (4mm in your case) and another of lower power. The barlow lens is used in conjunction with the other eyepiece to increase the magnification.

This is a link to the Vivitar 76700 manual http://www.vivitar.com/files_products/396/VIV-TEL-76700%20Manual.pdf. The insertion of the eyepieces is discussed around page 6 (assembling yout telescope) . This manual also mentions a sun filter. If your telscope was supplied with one I strongly suggest that you throw it away and also never point the telescope at the sun. Observing the sun without the correct filter can result in permanent damage to your eyes.

[Astro_Baby]

I' ll try and help.

First off the finder scope - the small one on the outside of the telescope will show things upside down.  All finder scopes (and generally speaking all astronomical telescopes do this as there is no real value in correcting an image in space - simply put if you are looking at the Moon or a Galaxy it doesn't really make much difference which way up it is and in fact the idea of up and down is  meaningless in space).

The other items you have there are 

1. 1.5X Erecting Eyepiece - this turns the image inside the scope the right way up for terrestrial viewing.

2. 3X Barlow Lens - this goes between the lens (see below) and the telescope to provide 3x power for the lens,

3. SR4mm Lens - this is the bit you look through and is called an eyepiece.  This should plug in to the side of the telescope (there will be a hole with a couple of wheels either side of it - the wheels are your focus control).  the lens goes into that hole, you look through that and focus by turning the wheels round.  

 

Your telescope specs I looked at suggest that there should be TWO lenses. A 4mm lens is pretty useless son almost any telescope.

 

Your telescope has a lens siamter of 76mm which will mean your maximum magnification would be x152  - ignore the blurb that came with your scope theres no way you will get x350 magnification and in any event most astronomy is done at about x200 anyway.

 

The focal length of your scope is 900mm which means when you plug the lens 4mm lens in you will get x175 magnification because magnification is the focal length of a telescope (in your case 900mm) divided by the eyepiece focal length (in this case 4mm) so 900/4=175.

 

To boost magnification you add the Barlow between the eyepiece and the telescope.  This will boost the focal length of the telescope by x3 so giving you a focal length of 900mm x 3 = 2700 and that is then divided by the eyepice focal length to give you 2700/4 = 675 which would be totally unusable.

 

I think your Barlow should have been more like about x2 and there should be another lens in there - a 10mm or 20mm eyepiece would be far more realistic in that kind of scope.

 

As to why you cant see anything heres what you shoukld do.....put the 4mm eyepiece into the scope and point the scope at something distant (trees, houses anything will do.  Wind the focu wheels in so that the eyepiece is closest to the scope and now look through the eyepiece (bear in miund a 4mm eyepiece is a challenge to use, even I would struggle with a high quality one, once peering through wind the focus wheels SLOWLY backwards and see if stuff comes to focus.  That should get you going.

 

I am sorry to have to say your telescope is not what I would suggest for someone new to astronomy, it seems badly specced and these types of telescopes do quite a bit to put people off of astronomy.

 

Never the less with some practice and some experience you may be able to get some use from it, even the worst scope can be a learning experience of value, you may want to consider getting something a bit better, the one I would always recommend to people starting in astro would be this http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p.html

 

Hope thats at least some help to you...

 

Mel

[laudropb]

Hello and. Warm welcome to the SGL. Astro_babe has given you a very full explanation above. I would only add that you really have to get an eyepiece of 25 or 20 mm focal length.

[Cornelius Varley]

Your number are wrong Mel. The focal length is 700mm so the magnification values will be 700/4 = 175 or 700x3 / 4 = 525. These claimed magnification are, of of course, overly optimistic and shouldn't be believed. If the telescope uses the standard 1.25" focuser better eyepieces can be purchased that will give more realistic magnifications.

[Cpaulo]

Hi there all,

I saw this post this morning and was about to pitch in my two penneth but I'm glad that some of the older and bolder more experienced members came to the rescue. However reading through the post now it does throw up one or two points for consideration that I wouldn't mind somebody clearing up; if somebody would be so kind.

When using a Barlow I understand that you effectively multiply the focal length of the scope by the value of Barlow, albeit x2 or x5 or whatever the Barlow maybe. I also get that the magnification is the focal length (multiplied by a Barlow or otherwise) divided by the size of the eye piece. All very simple, but the calculated value of magnification isn't always useable so what are the limiting factors in determining useable magnification. I have read that aperture plays an integral part a long with atmospheric conditions. Is there a rule of thumb for determining useable magnification or is it very much a case of trial and error to see what works on the night.

I apologise if I've started a new thread here or gone off topic but as I was reading above it did throw up the point about what is useable magnification and what can realistically be expected.

[Cpaulo]

Sorry,

looking at Mel's post (astro baby) I noticed the max useable magnification seems to be aperture multiplied by eight and divided by the size of eyepiece.

(76 x 8) / 4

Max useable magnification of 152. Is this the rule of thumb I'm seeking?

[Astro_Baby]

Yup - Cornelius is right - I had the scope spec down as 900mm fl but if its 700 then he is right and I am wrong.

Generally speaking a telescopes magnification is limited to 2x the size of its primary optic (either the primary mirror or the main lens.....so a telescope with a 100mm main lens would have a maximum magnification of x200 (2x 100=200). The actual magnification is carried out by the telescopes eyepiece. These come in a range of sizes and types but typically a telescope would be supplied from the manufacturer with something like a 10mm and a 25mm eyepiece. 
The eyepieces magnify an object based on the telescopes focal length divided by the eyepiece size. So for instance a telescope with a focal length of 1000mm with a 10mm eyepiece would give magnification of x100 ( 1000/10=100) while the 25mm eyepiece would give x40 (1000/25=40mm).

So how big can you get ? Theoretically a 200mm (8") Reflector can achieve x400 magnification but in reality the seeing conditions of the sky are the limiting factor and quite frequently the maximum achievable is no more than x200

[John]

Astro_baby is quite correct to point out that the maximum useful magnification is a lot less than the maximum theoretical magnification.

With a 76mm newtonian I suspect 100x is going to be as much as it will usefully take.

Another point worth remembering is that telescopes are not all about magnification. Much astronomy viewing of deep sky objects, for example, is done at low or medium magnifications.

[Cornelius Varley]

The rule of thumb for practical maximum magnification is about 50x per inch of aperture or 2x per millimetre. This is only a guideline and might be limited by atmospheric conditions.

[alexcartman]

Dear all,

really appreciate the time and effort put in to help me out.

I went thru the steps suggested by astro_baby this morning and it turns out my side scope (the one with the wheel) was broken, ie: the scope did not move up when I turned the knob.

Returned it back to the store today and got a store credit.  Might do a bit of research before making my next purchase on a good scope - I live in a highrise so mainly want it for looking at buildings / ships on the ocean as well as a bit of planet and moon gazing - not sure if there is one scope that can handle those tasks?

Im in telescope research mode now  

Alex

[Dave Lloyd]

ST80 or 102 on alt-az mount?

[happy-kat]

You might be better off with a refractor or a mak so the eye piece is at the bottom end of the telescope rather then a newtontian given your viewing situation. The refractor would have the fastest cool down but depending on what looking at might show some chromatic aberration but a longer focal length one would show less. A refractor I might expect to be less expensive then a mak.

[happy-kat]

Faulty goods you are entitled to a refund in the UK I thought.

[John]

Faulty goods you are entitled to a refund in the UK I thought.

The OP is in Australia.

You are correct though, in the UK you are entitled to a refund.

[Astro_Baby]

You are probably best rid of this so if you have cash back from the store thats good.  For what you want to do ie terrestrial work with some astro a decent pair of binos on a tripod might be a better bet.