Refractor unable to focus anymore.

[stephen2615]

Greetings,

I purchased a Tasco 17T in 1988 and played with it for sometime and then stored it for many years. I purchased a DSLR for my wife last year and out of curiosity, I took a couple of pics of the night sky with a number of settings. I discovered to my amazement that the pictures were very interesting especially with a length of only 28 mm. I saw a couple of groups of stars that I wanted to look closer at.

So I dragged out my old Tasco and tried to set it up looking at a house about 300 metres away. That way the spotter scope could be used to give me an idea of where to look. No matter what I do I can't get to focus the telescope. The image I see is very blurred. I used a number of eye pieces to no avail so I wonder can anyone suggest something.

I am not the most experienced person with parts of the telescope so forgive me when I try to discuss what I think is wrong. There are two "bits of glass" that sit in the objective mount. I pulled them out and gave them a clean and put them back in but nothing happened. One of the bits of glass is shaped like a double convex lens and the other looks like a plano-concave lens. The convex lens seems to sit very close into the plano-concave lens. There is a very thin plastic separator between the two lens so I expect that is how they are supposed to be separated.

I then tried to adjust the objective mount forward and back to see if it made any difference but as it can only move by about 3 mm at the most, nothing seemed to jump out at me.

As far as I can work out, there is no other reason for this blurry image. Can the lens quality deteriorate over time? I can't see any changes in the telescope over the last 12 years since I last used it. Any ideas?

Thanks

Stephen

[Robh]

Stephen,

Did you try looking at an object at larger distance. Many telescopes are designed to focus objects like stars at infinity. When objects are too close, you may run out of focuser adjustment and hence can't focus the object. Did you try focussing your lowest power eyepiece by holding it further out above the diagonal mirror insert tube. In fact, did you have the diagonal mirror in the focuser, otherwise you are shortening your distance to focus? It could also be a problem with the diagonal mirror. Have the coatings on the mirror degraded or do they look fine? If it is a flat mirror, the mirror may have been knocked off angle. Check if it is seated properly.

Regards, Rob

[barx1963]

I am assuming from the description it is a refractor?

Malcolm

[Screwdriverone]

I apologise if this seems too obvious,

Is there an eyepiece in the back of the telescope? Maybe you didnt remember to put one in?

Also, refractors generally need to have the diagonal inserted between the focuser and the eyepiece for the eyepiece to reach focus. If this is not there, you will see a blobby stars but cant get them to focus.

Your description of the front lens group seems normal, perhaps its as simple as the back of the scope not having an eyepiece and/or diagonal in it? It seems that the Tasco 17T may also have an extension tube that may need to be used...found some pics of one here (http://www.astronomy.net/forums/eyepieces/messages/523.shtml)

Cheers

Chris

[stephen2615]

Hi Rob,

I feel a bit silly now. It has been such a long time since I used the telescope, I could not remember how to use it. I used the right angled attachment (whatever it is called) and moved the eyepiece as far out of the attachment as I could and then tried to focus. Well, to my surprise (along with a few expletives) I was able to focus on a high tension power line about a kilometre away.

I put my 4 mm eyepiece in and could almost see the individual rivets on the tower. The image is pretty dark compared to the 10 mm eyepiece but I am sure it will give me some pleasure finding those stars that I found with the DSLR exposure.

All I need now is some clear nights and a couple of litres of mozzie repellent.


Thanks for the other replies. The 17T is a refractor and one day I will get myself a nice reflector and a sling shot to knock out the street lights near me.

Stephen

[Robh]

Stephen,

I'm glad you've got it sorted out.
Those street lights certainly are pesky. If only they would shield them properly.

Regards, Rob