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Need help with Secondary Mirror


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Hi all,

as mentioned in a previous post I have a national Geographic 76/350 compact telescope, it maybe not the best telescope in the world..but it’s at least something I can use while I save up for a good one. However I Seem to have a problem with the secondary mirror it gets easily out of place. For example when Its center I move the telescope to look at an object but it doesn’t stay in the center  and it rotates to to which side I have moved the scope. I have a feeling that this is because it’s not colimated properly ( or at all) when I got this scope as a gift the box didn’t really come with clear instructions so have been putting it off but wanted to use it now. I’m hoping it’s an easy fix. If it does need collimating I’m hoping some of you will give me some tips on how to do it. I have taken some pictures to make it clearer on what I’m trying to say. I’ve taken two photos with the scope pointing up and the seconds mirror is center (ish) and one when it’s pointed st something and the secondary mirror has moved. I’m sorry if this sounds a stupid question but need to double check. :) Also apologies if the pictures aren’t clear.

59A8A909-2191-43F7-AE7C-030BC522D642.jpeg

45DB918C-B25F-4425-AF37-414A0C6B0B9B.jpeg

Edited by Dinoco
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Hi Dinoco,

I am far from an expert however I know enough to know the mirrors shouldn't move around freely.
I have a reflector too and my primary is held in place with three screws with another little screw next to each screw.
So there's six in total at the back. Three adjust the mirror when collimating. Three hold it in place. 

Take the back cover off your telescope and check for these screws and either tighten or replace depending upon what you find.

Hope that helps.
Pete.

Edit:
My mistake. Just read your post again and you were talking about the secondary. 
Again, this shouldn't move freely. This works along the same principles I described above only with less screws. 
If they have been turned too far they may have detached from the secondary mirror, hence this would cause it to flop around. 
You can check this by holding a smaller mirror next to the secondary to visually check or do it by feel.

WARNING: When doing anything with/to the secondary, have the scope in a horizontal position. 
Don't want to be dropping any tools or secondary mirrors down the tube onto the primary! 

Again, hope that helps.
Pete.

Edited by Redscouse
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Looking at your picture, I see 4 screws.

The centre screw is a "locking" screw. Tighten this one up and the mirror should not move.

the three outer screws are "collimation " screws. You loosen the centre screw, collimate to your satisfaction then lock down the centre screw again.

Hth, Alan

Edited by alanjgreen
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7 minutes ago, Redscouse said:

Hi Dinoco,

I am far from an expert however I know enough to know the mirrors shouldn't move around freely.
I have a reflector too and my primary is held in place with three screws with another little screw next to each screw.
So there's six in total at the back. Three adjust the mirror when collimating. Three hold it in place. 

Take the back cover off your telescope and check for these screws and either tighten or replace depending upon what you find.

Hope that helps.
Pete.

From reading the OP's comments, I read it that he stated it is their secondary that is moving... 

Whatever may need adjusting, I'd venture it isn't the primary screws that are causing the problem...

To the OP, others more adroit in secondary collimation may venture further advice, but the attached guide may help you...

Astro Baby's Collimation Guide.pdf

Edited by Stargazer McCabe
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5 minutes ago, Redscouse said:

Hi Dinoco,

I am far from an expert however I know enough to know the mirrors shouldn't move around freely.
I have a reflector too and my primary is held in place with three screws with another little screw next to each screw.
So there's six in total at the back. Three adjust the mirror when collimating. Three hold it in place. 

Take the back cover off your telescope and check for these screws and either tighten or replace depending upon what you find.

Hope that helps.
Pete.

Hi Pete,

thanks for the replie. I have tried to take the back cover off. However I don’t think it can ( unless I’m being an idiot :)) there are three screws around the tub where there should be a cover here’s a picture.

D6A9963C-4735-40C5-9219-27D34A1283B9.jpeg

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Just now, Dinoco said:

Hi Pete,

thanks for the replie. I have tried to take the back cover off. However I don’t think it can ( unless I’m being an idiot :)) there are three screws around the tub where there should be a cover here’s a picture.

D6A9963C-4735-40C5-9219-27D34A1283B9.jpeg

As per replies above, you don't need to go anywhere near the rear if it is your secondary that is moving...

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1 minute ago, Stargazer McCabe said:

From reading the OP's comments, I read it that he stated it is their secondary that is moving... 

Whatever may need adjusting, I'd venture it isn't the primary...

To the OP, others more adroit in secondary collimation may venture further advice, but the attached guide may help you...

Astro Baby's Collimation Guide.pdf

Yep. I noticed that right after posting. I'm guessing you missed the edit. :happy7:

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Sounds like the screws have been slackened off, which is allowing the secondary to flop around. The secondary mirror holder has 4 screws. The big one in the middle pulls the mirror up the tube and braces it against the three screws around it, which are used to control its angle and tilt. When the big screw is loose (careful, as if too loose the secondary can fall off and hit the primary - new scope needed) you can also rotate the mirror.

You need to get the secondary into the correct position (pointing into the focuser tube, and aliitall this much better than I could) can be found at:

http://www.astro-baby.com/collimation/astro babys collimation guide.htm

I'll be honest - it's a total pain the first time you do it. Fiddly, time consuming and annoying. However, once it's there or thereabouts you can leave it and not woory about it. Secondaries don't need collimating very often.

Billy.

 

 

 

 

 

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4 minutes ago, alanjgreen said:

Looking at your picture, I see 4 screws.

The centre screw is a "locking" screw. Tighten this one up and the mirror should not move.

the three outer screws are "collimation " screws. You loosen the centre screw, collimate to your satisfaction then lock down the centre screw again.

Hth, Alan

Thanks Alan,

before posting I did have a look online on how to colimated. But wanted to know why it was moving so much. Am I right in saying the center screw makes the mirror from moving need to read over again. And is it best to use a Allen key for this.

many Thanks.

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1 minute ago, Stargazer McCabe said:

As per replies above, you don't need to go anywhere near the rear if it is your secondary that is moving...

I did gather it was the secondary mirror :) 

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2 minutes ago, billyharris72 said:

Sounds like the screws have been slackened off, which is allowing the secondary to flop around. The secondary mirror holder has 4 screws. The big one in the middle pulls the mirror up the tube and braces it against the three screws around it, which are used to control its angle and tilt. When the big screw is loose (careful, as if too loose the secondary can fall off and hit the primary - new scope needed) you can also rotate the mirror.

You need to get the secondary into the correct position (pointing into the focuser tube, and aliitall this much better than I could) can be found at:

http://www.astro-baby.com/collimation/astro babys collimation guide.htm

I'll be honest - it's a total pain the first time you do it. Fiddly, time consuming and annoying. However, once it's there or thereabouts you can leave it and not woory about it. Secondaries don't need collimating very often.

Billy.

 

 

 

 

 

Thanks for that billy, this is why I have been putting it off trying to fix it :) but wanted to get it out. I’ll need to sit down properly when I have time and the tools to do it. I’ll have a look at the link later.

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18 minutes ago, Redscouse said:

Dinoco, See my edit at the bottom of the post. The back (primary) is fine. 
It's the secondary that needs adjusting. See Alan's post below mine also. 

Sorry didn’t see as everyone seemed to have posted at once :) 

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Just now, Dinoco said:

Sorry didn’t see as everyone seemed to have posted at once :) 

My apologies once again for initially giving the wrong advice.
I hope it didn't hinder you too much. :)

I did read your post say secondary however when it came to replying my brain decided it would have a little fun with me and changed it to primary. 
Was up late last night watching the Superbowl. (that's my excuse and I'm sticking to it :homework: )

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I may be quite wrong about this but to my eyes it would appear that the whole secondary holder has swivelled.  If the adjusting screws were just loose then then the extension (lower section) that holds the mirror itself would rock a bit but the 'back plate' against which the screws bear would remain fixed relative to the  tube.  It looks like the whole thing has moved in which case it is worth checking that the bar that supports the secondary holder isn't swivelling where it passes through the tube - or perhaps where the bar enters the holder...it could be brazed, glued or retained on a thread that has come loose.  (there are either retaining nuts both sides or a captive nut within the tube and a locking nut on the outside.

Probably just my eyes but perhaps worth checking. Best of luck sorting it out.

Edited by runoffshed
additional info
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18 minutes ago, runoffshed said:

I may be quite wrong about this but to my eyes it would appear that the whole secondary holder has swivelled.  If the adjusting screws were just loose then then the extension (lower section) that holds the mirror itself would rock a bit but the 'back plate' against which the screws bear would remain fixed relative to the  tube.  It looks like the whole thing has moved in which case it is worth checking that the bar that supports the secondary holder isn't swivelling where it passes through the tube.  (there are either retaining nuts both sides or a captive nut within the tube and a locking nut on the outside.

Probably just my eyes but perhaps worth checking. Best of luck sorting it out.

Hi, ill try and have a look, unfortunately this is what it was like when it got delivered I’m hoping it’s just needs collimating.Is this what you mean by the secondary holder ( bit confused ) :( sorry if it’s not.

image.jpg

Edited by Dinoco
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Just now, Dinoco said:

Hi, ill try and have a look, unfortunately this is what it was like when it got delivered I’m hoping it’s just needs collimating. The whole secondary  mirror can swivel 360 degrease (if you use your hand). 

Tighten the centre locking screw. Can you still move it or is it now locked in position?

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37 minutes ago, Dinoco said:

Here is a picture of the screws they seem quite high up compared to the center one. Is this normal? This is all new to me so please don’t bite my head off :D 

image.jpg

This is normal. Someone has collimated the scope (changed the angle of the mirror) by loosening the screws in varying amounts. Don’t worry about the height of the collimating screws (unless they have been loosened so much they have come out !)

First job is to read the collimation guide linked above and buy a collimation tool such as a “Cheshire eyepiece”

https://www.firstlightoptics.com/other-collimation-tools/premium-cheshire-collimating-eyepiece.html

Dont mess with the collimation until you are prepared. Just turn the whole secondary assembly while looking down the focuser hole. Turn it until you see the maximum amount of mirror through the focuser hole. Then tighten the middle locking screw. That will be good enough for now.

Now WAIT until you have a Cheshire and read and learn the collimation procedure...

Edited by alanjgreen
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11 minutes ago, valleyman said:

I was going to make the same suggestion as runoffshed. It looks to me as if it is the support arm that is turning. Is there anyway of securing the support arm where it attaches to the Tube, or is it welded?

On the other side of the tube there is a screw, but I think I have just fixed it, where the bar goes into the circle part (where the screws are located) it wasnt properly tightened. As you could see it was quite low down the bar ( I think this is what you guys were saying sorry if it was) but however it’s still going to one side maybe I’ll try and tighten the screw on the tube 

edit: the screw  doesn’t move as it’s already tight. The mirror doesn’t move as much now as before I think the scope needs to be colimated so it does move?

Edited by Dinoco
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