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Collimating a 76mm Celestron Firstscope?

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#1 penguinx64

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Posted 08 November 2016 - 08:36 PM

I was given an old Celestron Firstscope mini Dob that could use a good collimation.  The primary mirror is fixed and there is no center mark on it.  The secondary mirror has 3 adjustment screws.  I'd like to set this scope up for the kids to look at the moon at 30-50x.  I tried it last night with 3 different eyepieces.  None of them made much difference.  I could get a couple of craters to focus, but the rest of the moon looked fuzzy.  Stars are not pinpoint either.  I used another Firstscope that had great views of the moon at 46x.  I'm familiar with using the collimation cap that came with my Starblast 4.5.  That won't work without a center mark on the primary mirror.  I also have a Cheshire collimation tool. Not really sure how to use it though, especially without a center mark on the primary mirror.  Any suggestions for salvaging this scope? 


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#2 leveye

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Posted 08 November 2016 - 08:46 PM

Remove the primary and mark the center using this method....

 

http://dobstuff.com/centerdot.htm

 

The use your Cheshire. Should work great after that.


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#3 Sky Muse

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Posted 08 November 2016 - 09:01 PM

In that it's "old", I'd clean the mirrors whilst you're at it.  You can also use a notebook paper reinforcement as the center-dot.


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#4 Shneor

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Posted 09 November 2016 - 02:28 AM

Or you can use a laser collimator that shows a round grid  (sometimes called a holographic collimator) to determine the center of the primary, though it sounds like overkill for this telescope. I prefer not to mark the center of my primaries as it causes the loss of a bit of off-axis light.



#5 Sarkikos

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Posted 09 November 2016 - 08:24 AM

Be aware that the primary mirror on this scope is spherical, not parabolic, and the f/number is 3.95.  These factors will produce a good deal of spherical aberration.  So don't expect the scope to do very well at high power.  Also, it's not really a good telescope for planet or Moon viewing.  Because of the spherical aberration, the images will tend to be fuzzy.  But if you keep the power down it should not be too bad.  Just don't expect extremely sharp views of craters.

 

This little scope is better for looking at large open clusters with low power.  Try it on the Pleiades, the Hyades, the Double Cluster, Stock 2, Orion's Belt, and the Alpha Persei Cluster (Mirfak in Perseus).  It can give some decent views of objects like these.  I know because I have a Celestron 76 FirstScope.

 

Mike


Edited by Sarkikos, 09 November 2016 - 08:30 AM.

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#6 BSJ

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Posted 09 November 2016 - 08:58 AM

Like ISS said, the spherical mirror is probably the main issue. Or if it is like the one I had, with the secondary not even centered under the focuser, you won't have much luck collimating it. The screws do little, if anything. You can try bending the stalk, but that's not very 'precise' to say the least.



#7 Sarkikos

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Posted 09 November 2016 - 09:12 AM

I did have to fiddle with the diagonal stalk on mine, as well as the screws.  

 

"Collimating" a spherical primary Newt is probably somewhat of a misnomer.  Assuming the fixed primary is square to the OTA, and the focuser is square to the OTA, the best we can do is center the diagonal under the focuser.

 

Mike


Edited by Sarkikos, 09 November 2016 - 09:14 AM.


#8 penguinx64

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Posted 09 November 2016 - 12:21 PM

I did have to fiddle with the diagonal stalk on mine, as well as the screws.  

 

"Collimating" a spherical primary Newt is probably somewhat of a misnomer.  Assuming the fixed primary is square to the OTA, and the focuser is square to the OTA, the best we can do is center the diagonal under the focuser.

 

Mike

I checked the diagonal stalk.  The secondary mirror was a little loose on it.  I could easily rotate the secondary mirror assembly on the stalk without loosening the screws.  So I put in a 9mm Ortho eyepiece and looked at the moon.  I turned the mirror assembly back and forth until the moon looked good.  That did the trick!  Maybe it's not perfect collimation, but it's much better than it was.  Good enough to look at the moon 30x anyway.  Thanks!

 

I played around with some different 'star party' eyepieces that I had laying around.  One combo that worked really well was an old 25mm Kellner with a budget Seben 2x barlow.  This gives me 24x magnification, lots of eye relief, a decent field of view and it was easy to find the moon without a finder scope.  The moon looked pretty good too.  This seems like a great setup for the kids to play around with. 


Edited by penguinx64, 09 November 2016 - 12:47 PM.

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#9 Sarkikos

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Posted 09 November 2016 - 12:29 PM

Turn the little scope on some of the objects I listed.  To get a good view, you might have to wait a couple weeks until the Moon leaves the evening sky.

 

Mike


Edited by Sarkikos, 09 November 2016 - 12:29 PM.


#10 sixela

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Posted 09 November 2016 - 02:38 PM

As others have said, it's a spherical mirror scope so don't expect miracles, but if you have a Cheshire/sight tube combination, then set the secondary tilt so that the reflection of the cross-hairs is under the cross-hairs. If the primary is more or less set correctly that'll at least tend to minimise coma and astigmatism.
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#11 Harshad

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Posted 03 March 2017 - 12:49 PM

I checked the diagonal stalk.  The secondary mirror was a little loose on it.  I could easily rotate the secondary mirror assembly on the stalk without loosening the screws.  So I put in a 9mm Ortho eyepiece and looked at the moon.  I turned the mirror assembly back and forth until the moon looked good.  That did the trick!

I just got myself a Firstscope today, and can see that the collimation is off a bit; the secondary is not aligned to the focusser axis. If only I could rotate the secondary, like you could, so that it faces the focusser completely. Looseneing the screws on the secondary doesn't allow rotation, only changes the tilt.

 

penguinx64, how did you tighten the secondary? I can do it in reverse so as to loosen it wink.gif 



#12 jwestervelt

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Posted 05 April 2017 - 05:27 PM

I apologize for the slight thread necromancy, but I picked one of these scopes up as well for a new youtube channel I am putting together.  I hope you got your issues resolved.

When I received my scope, the secondary was way off.  As it turns out, it was rotated, but not along the optical axis.  Instead, the screw on the focuser that goes into the "stalk" of the secondary was just a bit loose.  I rotated the secondary along the stalk's primary axis while looking through the focuser to make sure that it was aligned.  After that, it was easy enough to tighten the screw and then do collimation with the 3 secondary screws.

What you will want to do is make a collimation cap.  I just took the focuser cap that was shipped with the scope and put a hole through the center of it.  You will find when looking through the hole of the collimation cap that you can see the support block for the secondary (the 1.5cm diameter cylinder part that has the 3 screws running through it)...

you see this...     front of scope  (=(  )  )   rear of scope  
where:
  "(         )" is the bottom of the focuser tube
  " (  )" is the secondary
  "=" is the support block for the secondary

Now if you look at the secondary, you SHOULD see the reflection of the secondary off the primary, and here you'll actually see the thinner stalk.  It should be in line with the support block
(=(-o)  )
where:
  "-" is the stalk
  "o" is the reflection of the secondary

If you notice that the stalk is above/below the support's centerline, you can twist the stalk and thus rotate the mirror to bring it into line.  I suppose a picture would be better, but I'm not at home at the moment.

 


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#13 jwestervelt

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Posted 05 April 2017 - 05:38 PM

In addition, I found that the secondary was way too far forward in the tube and was not centered under the focuser.  This made a huge mess of things and prevented good collimation.

The secondary is mounted via some double-sided tape to a block with three threaded holes for the collimation screws.  In the center of this cylindrical block, there is a short peg about 1.5mm tall.   Around this peg sits an o-ring which serves as a make-shift spring.

As you tighten/loosen the collimation screws, the secondary support rocks around this peg and compresses the o-ring... the screws act as the "pull" mechanism and the o-ring acts as the "push".  It is not a very robust design, and it doesn't allow for much adjustment before the screws are backed out enough that there is no longer a push component because the o-ring is no longer in compression. 

What I did to resolve the issue with the secondary not being centered was to remove the whole secondary contraption, replace the phillips-head screws with longer hex machine screws, and then replace the o-ring with a rather stiff little spring that would fit over the little peg on the support.  You have to pick the screw lengths somewhat carefully as the screw head must be seated against the collimation block in order to apply any "pull" mechanic, and the screw must be long enough to give you the ability to move the entire secondary backwards to center under the focuser, but not so long that it can hit the secondary mirror if threaded all the way into the support block.  You can get away with a longer screw on the side furthest from the focuser as the tilt of the secondary will allow it.

When done, you can back out all 3 screws in unison to move the secondary backwards towards the primary, tighten them all in unison to move the secondary forwards, or adjust individually to collimate.  


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#14 Harshad

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Posted 05 April 2017 - 11:30 PM

The secondary is mounted via some double-sided tape to a block with three threaded holes for the collimation screws.

 

Aha! So, if I had to rotate the secondary around the optical axis, I would need to detach it from the block and stick it again with double-sided-tape?

 

Thanks a lot for the description, especially the ASCII art waytogo.gif  



#15 jwestervelt

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Posted 06 April 2017 - 12:49 AM

That double-sided tape is pretty tough stuff.  Someone else here mentioned that they popped their mirror off to rotate it.  I gave it a bit of a tug and couldn't get it to separate.  It is a pretty thin mirror, and it is a bit hard to get a grip on it without risking touching the coating... and when the tape fails, you could end up with a secondary that goes flying across the room.  :/  If I get around to it, I'll probably pop it off by taking a razor blade and cutting the tape.


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#16 Jason D

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Posted 06 April 2017 - 01:33 AM

Use a dental flossing thread to cut (saw motion) the double sided tape and pry out the secondary mirror with minimal risk. However, the word "minimal" is a relative word when it gets to small and thin secondary mirrors wink.gif

Jason

 



#17 Harshad

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Posted 06 April 2017 - 01:48 AM

That double-sided tape is pretty tough stuff.  Someone else here mentioned that they popped their mirror off to rotate it.  I gave it a bit of a tug and couldn't get it to separate.  It is a pretty thin mirror, and it is a bit hard to get a grip on it without risking touching the coating... and when the tape fails, you could end up with a secondary that goes flying across the room.  :/  If I get around to it, I'll probably pop it off by taking a razor blade and cutting the tape.

Oh! On second thoughts, I don't think removing the tape is the right solution for my needs.

 

Inspired by your post, I got a bit more adventurous and tried to remove the secondary spider. In the process, I realized that a slight turn of the spider's stalk brought things into better alignment. I do have a self-made collimating tube, and confirmed the alignment.

 

Will do a collimation check with a star in the night. Thanks!



#18 jwestervelt

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Posted 06 April 2017 - 01:50 AM

Jason D, were you the one that said that they "unrotated" their secondary?

it's a shame that my Catseye XL and Infinity XL won't fit into the 1.25" focuser on this little scope so I could do a proper collimation.  XD

Once I get a few episodes in on my channel, I'll discuss "fixing" this scope.  Two options on the table... 1) Grind a parabolic 3" f4, or 2) Grind a Houghton Corrector.  The latter would be awesome, hilarious, and retarded at the same time.  Retarded because Houghtons are super sensitive to corrector tilt, and I'd likely not make one with a hole in the middle to allow for collimation... I'll figure something out for sure.


Edited by jwestervelt, 06 April 2017 - 01:51 AM.


#19 jwestervelt

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Posted 06 April 2017 - 03:58 AM

 

https://www.youtube....h?v=wQ3UyyGOq5U

 

 

raw, unedited video covering the mods as well as the rotation that I mentioned


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#20 Jason D

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Posted 06 April 2017 - 06:39 PM

Jason D, were you the one that said that they "unrotated" their secondary?
 

Not me...

I only commented on the topic of removing the secondary mirror from its stalk that is attached with a double sided adhesive pad.

Jason



#21 namor

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Posted 04 October 2020 - 06:26 PM

In addition, I found that the secondary was way too far forward in the tube and was not centered under the focuser.  This made a huge mess of things and prevented good collimation.

The secondary is mounted via some double-sided tape to a block with three threaded holes for the collimation screws.  In the center of this cylindrical block, there is a short peg about 1.5mm tall.   Around this peg sits an o-ring which serves as a make-shift spring.

As you tighten/loosen the collimation screws, the secondary support rocks around this peg and compresses the o-ring... the screws act as the "pull" mechanism and the o-ring acts as the "push".  It is not a very robust design, and it doesn't allow for much adjustment before the screws are backed out enough that there is no longer a push component because the o-ring is no longer in compression. 

What I did to resolve the issue with the secondary not being centered was to remove the whole secondary contraption, replace the phillips-head screws with longer hex machine screws, and then replace the o-ring with a rather stiff little spring that would fit over the little peg on the support.  You have to pick the screw lengths somewhat carefully as the screw head must be seated against the collimation block in order to apply any "pull" mechanic, and the screw must be long enough to give you the ability to move the entire secondary backwards to center under the focuser, but not so long that it can hit the secondary mirror if threaded all the way into the support block.  You can get away with a longer screw on the side furthest from the focuser as the tilt of the secondary will allow it.

When done, you can back out all 3 screws in unison to move the secondary backwards towards the primary, tighten them all in unison to move the secondary forwards, or adjust individually to collimate.  

 

had a tiny brainstorm thought, can the springs be replaced with stacking a few thin O rings onto the 3 screws (maybe found in any Home Depot?)

 

the O rings should have some push/pull like a spring 



#22 Giant8000

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Posted 24 November 2020 - 12:22 AM

I just picked one of these up. After using the method above to mark a center ring I put my laser collimator and discovered that the barrel of the focuser shifts around as you focus in and out.
As for the primary it seems if I widen the screw holes in the tube a bit and use washers on the screws, I can lock it down in the correct orientation by tighten while watching the laser target.


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