whats wrong with my telescope? [Archive]

sky22

09-01-2007, 04:19 PM

Hi everyone, I just bought a secondhand Celestron Nexstar 4 and to tell you honestly this is the first time i am goin to touch a telescope, before i was contented looking the stars using my not so 20/20 vision. then i decided to buy even a seconhand telescope but when i tried to auto-allign it i found out that when the center of an object is center to the field of view of the telesscope it was out of the field of view on the star pointer and so the other way arround. was that a star pointer? or something more serious? Thanks alot

Dennis

09-01-2007, 06:36 PM

Hi Sky

As you have already noticed, you have a “large” telescope and a “small” finder attached to it.

The telescope tends to have a very small Field of View (FOV), a long focal length and a large diameter objective lens or mirror which is used to gather the light to view the Moon, planets, etc.
An optical finder has a much larger Field of View, a very short focal length and a much smaller objective and it is used to literally find objects, so that you can then view them in your main telescope. (We’ll talk more about red dot finders later).

Why a telescope AND a finder?
Well, your telescope’s very narrow FOV makes it very, very difficult to find objects in the sky, even the Moon sometimes; it’s a bit like looking through the end of a thin, very long straw whilst trying to find an object.

So, your finder comes to the rescue, as it has a much larger field of view. With the finder, you can usually sight along the finder and place the star in the middle of the finder’s large FOV. If the telescope and finder optical axes are aligned, the object should now be in the telescope's narrower FOV.

But, you mentioned a star pointer which I think is a red dot finder, rather than an optical finder?

Well, there are 2 main types of finder:

A much smaller telescope, e.g. 6x30, 8x50 which means x6 or x8 magnification with a 30mm or 50mm diameter objective lens. Typically the field of view that you can see through these are between 5 and 7 degrees. You could “fit” over 10 full Moon discs in the FOV of these finders.
Red dot finder – these are “naked eye” or x1 magnification finders and usually project a red dot, illuminated circle or reticule onto a glass screen through which you look to find and centre an object, such as a bright star. This sounds like your model?

Your job, made easier in the day time, is to make sure that the optical axis of your telescope and finder are lined up. That is, look at an object at least 1km to 2 kms away and centre the object in the main telescope FOV. Next, have a look through your finder – is the object centred in the finder? This should be done with the telescoped fixed and stationary - not with the motors turned on and moving the telescope.

Probably not as you have stated, so you need to adjust the pointing direction of the finder bit by bit, until the object is now centred in the finder and the telescope. You should find a couple of adjustment screws on the finder body.

If your finder is removable, each time to remove and replace it, you may have to tweak the alignment as it could shift a little.

Cheers

Dennis