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Telescope repair

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#1 D4rcy

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Posted 20 October 2019 - 05:01 AM

Hi all,

I’m hoping to get some advice on repairing/rebuilding an old refractor telescope.

It been siting in storage for a number of years and recently during a house move it took a tumble down a number of stairs during which the objective lenses came out.

I didn't think much of it at the time but have now tried to reassemble and am not able to achieve focus.

It doesn't make much of a difference no matter what im trying to focus on e.g. trying to look at Jupiter or a street light 200meters down the road both appear as a large extremely blurry spot of light.

I’m not opposed to paying for this be fixed up but im also tempted to use it as an excuse to invest a bit of time into a project and get more into the hobby. I have a few pairs of astronomical binoculars which suit me at the moment but the scope has a bit of sentimental value (it belonged to my dad and I have fond memories of looking at the bands on Jupiter and Saturns rings) and would like to get it usable again.

The telescope in question, as far as I am able to tell, is a Tasco 9te from the 60s/70s – this is a guess based on a bit of googling and visual comparison looks exactly like this: https://www.cloudyni...rite-refractor/

There is no branding on it but some sticker residue where this would have been.

Most of the accessories are gone all that’s left is:
    • main tube
    • two eye pieces (H 20mm and a 6mm)
    • barlow
    • Wooden tripod

I figured the issue was that I put the objective’s back in the wrong order I’ve tried re-arranging them in every combination before finding this diagram and now have it in the number 1 position (or at least i think i do based on trying to feel the difference of curve on each side of each lens):

post-266572-0-26572300-1520005705.jpg

I had also started to play with the eye piece lenses before realising this was dumb as they didn’t come during the fall and I was way in over my head i think (hope) I got these back in the correct order.

I guess my questions are
1. Is what im seeing (large blurry light spot) likely to be due to the lens order or might it be some other damage caused during the fall?

2. any tips for trouble shooting what the issue might be?
3. any advice on how to proceed beyond just paying someone who knows what they are doing?

Appreciate thoughts on this.

Thanks.


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#2 scottmm2012

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Posted 20 October 2019 - 05:21 AM

Do you have the diagonal?

Sounds like you don't have enough back focus.  At full extension of the focuser, pull out the eyepiece and see if you can bring it to focus by moving the eyepiece away from the focuser.


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#3 sg6

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Posted 20 October 2019 - 05:25 AM

1 is the usual arrangement.

The flatter face is towrads the stars, then a separator ring - assume one in there somewhere. Then the negative lens - expect this to have again 2 curved faces likely the most curved is facing the other lens so again the flatter face is "outward", or towards the eyepiece end.

 

Arrangement is therefore the "reverse" (probably).

You put the negative lens in the cell first with the flatter face in first so it is downwards.

Then the spacer ring.

Then the positive lens with the higher domed face downwards so the flatter face is looking at you.

 

Determine the arrangements and then clean everything.

Then assemble.

Then put the retaining ring in place,

 

That is the usual arrangement, there are therefore others.

 

1, 2, 3, 4 are the usual options, yours reads of 2 - separated doublet. I assume 1 and 4 are cemented.

5, 6, 7, 8 are I suppose options but cannot think of any that are arranged as such.

 

So the other lesser option is therfore 3. So try 2 and maybe 3.

Just need to determine which face is low or high and go for it.


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#4 ngc7319_20

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Posted 20 October 2019 - 07:25 AM

Maybe there is some local expert or local club that could take a look at it?  Where roughly are you located?


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#5 TOMDEY

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Posted 20 October 2019 - 07:36 AM

Associates knew I worked in an optics shop, used to habitually bring me their refractors to "tune up". The scenario was most often where they had messed with it but insisting they didn't. I'd find finger prints etc all over the optics, obvious they had disemboweled it, made it way worse... and then dragged humpty over to my shop, wanting a free favor. I'd take the time to make it right... finally got wise ... but developed this technique to discourage that buffoonery >>> I'd indeed fix the scope... but then sit on it for about six months, claiming I was "working on it" using an interferometer at work. I'd wait until they got irate and demanding I relinquish the scope --- at which point I would hand it over, and offer to fix another, whenever they liked. They never returned.    Tom


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#6 macdonjh

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Posted 20 October 2019 - 10:32 AM

Do you have the diagonal?

Sounds like you don't have enough back focus.  At full extension of the focuser, pull out the eyepiece and see if you can bring it to focus by moving the eyepiece away from the focuser.

Since the moon is full, and easy to find, using a white piece of paper to project the moon's image also works.  Also provides a much bigger target than the field lens of an eye piece.  As scottmm2012 says, that will tell you where your objective lens is forming an image, if it is.  If you didn't have a diagonal installed when you experimented, scottmm2012 has a good theory: a 1-1/4" diagonal adds something like 2" to the optical path length of your scope (a 2" diagonal adds 4").



#7 Paul Hyndman

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Posted 20 October 2019 - 11:04 AM

You may find this to be a better fit in the Classics forum... just sayin'.  wink.gif


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#8 Joe Cepleur

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Posted 21 October 2019 - 05:34 PM

A quick question about the most common reason a classic refractor will not reach focus: Does your telescope have a draw tube? If it does, it will either have a thumb screw on the top of the focuser, or a hole where a thumb screw used to be. Loosen the thumb screw, and then pull gently on the tube holding the eyepiece (or, the diagonal, if you have one). The draw tube will slide out from within the focuser, and may be six to eight inches long, maybe longer. If your telescope has a draw tube, it will only reach focus with the draw tube extended.

I may be wrong, but I believe flipped elements would more likely make an imperfect image, rather than no image. A big, blurry spot of unfocused light light would be typical of an unextended draw tube.

#9 Kasmos

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Posted 21 October 2019 - 06:13 PM

If it's a 9TE (or similar branded model), there is not a pull tube (draw tube), and the focusers only have maybe 2" of travel. I have one so I'm thinking it either needs the long image erector/barlow that came with them for terrestrial views or a star diagonal to reach focus.

9TE erector.jpg

This is the one that came with my scope. It's only 1.5x. I never used it and have since sold it, but without it I'm sure you'd never get anything to focus using straight through viewing. I just use a star diagonal for occasional terrestrial views and put up with the reversed images.



#10 stevew

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Posted 21 October 2019 - 06:50 PM

It sounds like you have put a lens in backward. #1 in your image is correct, for a standard fraunhofer doublet but the shape of the lenses is very subtle and may be difficult to see.

With a little experimenting you should be able to figure it out. 

Also astronomical telescopes don't really focus well on near objects. You are going to want to focus on something at infinity to be sure.

 

 


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#11 Joe Cepleur

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Posted 21 October 2019 - 07:00 PM

Thus the brilliance of projecting the Moon onto a piece of paper: It tests the issue of back focus without requiring either a part the telescope may not currently have, or the skill to hold the eyepiece centered in mid-air! If the Moon will not project, then it would have to be a flipped element.

Edited by Joe Cepleur, 21 October 2019 - 07:02 PM.

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#12 Mbinoc

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Posted 21 October 2019 - 11:11 PM

I have found that if you lay the lenses flat on a plain white sheet of printer paper, you can usually see the difference in bevels to get a correct sequence.

 

For me its hard to feel them by touch, but easy to identify the flint once layed flat on paper. Paper has also helped me identify the correct orientation for the crown, as sometimes the difference is minimal, but more eaily viewed when layed flat.

 

As carefull as I try to be, I have also had to reference the diagram you posted more than once when cleaning a objective. I have that image printed out, and storred in one of my books for quick reference whenever needed.


Edited by Mbinoc, 21 October 2019 - 11:28 PM.


#13 Kasmos

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Posted 22 October 2019 - 12:52 AM

The 9TE has a 700mm focal length so it's not that hard to tell which side of the crown has more curvature just by looking at the lens from its side.  As mentioned above when laying flat on paper the shadow and light will show a difference. (I tape down a piece of lens tissue nice and flat to check it on) On longer focal lengths I usually double check using the tilt test. Pressing down near the edge will make the lens rock a bit higher on one side than the other. Picked up that trick from someone here on CNs. waytogo.gif


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#14 D4rcy

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Posted 26 October 2019 - 05:37 AM

Thanks everyone for the responses its helped me a bunch.
In the end there were two issues:
1. I didn’t have the diagonal in as it wasn’t in the box of bits so I was just mounting the eye straight to the tube. 
2. The objective were not in correct order despite my own misplaced confidence that I had this correctly figured out
With the diagonal now located and some more fiddling with the lens order i’m now able to get a reasonable image.…. I swear this was like the old USB superposition joke - up (not working), down (not working), look and try up again (working!).
Thank for tips to help identify the either side of the crown to those that asked about the diagonal and extension/barlow… I’ve always just assumed the diagonal was purely just to allow a more comfortable view angle and didnt realise this actually puts the eye piece into the correct focal point.
The image isn’t great but I guess thats to be expected given its age and been subjected to my amateur repair technique. There is still a bit blurring and chromatic aberration. Despite this am able to very faintly make out jupiters bands.
Thanks again.


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#15 sdedalus83

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Posted 26 October 2019 - 11:34 AM

Is there a spacer or are the two elements in direct contact?




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