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upgrading focuser how to orient so telescope will collimate

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#1 masggid2021

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Posted 25 November 2021 - 12:36 AM

New amateur astronomer here.

I am upgrading the focuser on a 1990 era Meade Polaris114EQ reflector telescope (free to me). When I attach it to the tube, how do I orient it so it is square & perpendicular to the diagonal mirror, so the mirrors can be collimated.

I want to be sure, before I drill new holes into the tube. New focuser has 4 attachment bolts, where the original had 3. I am changing fro 0.965 in focuser to 1.25 in. The current hole in the tube is already large enough for the 1.25 in focuser.

sorry, if my terminology is not correct...education in progress and is much appreciated.

Or, am I fussing about nothing?



#2 gene 4181

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Posted 25 November 2021 - 12:50 AM

  You could  remove the secondary   and mark the other side of the tube thru the  old focuser using a laser or sight tube   on  paper or a stick on  reinforcement ring  . This way it gives you a bench mark too square up with  , then re-install the secondary and collimate.  Just a thought


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#3 Asbytec

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Posted 25 November 2021 - 02:57 AM

When I attach it to the tube, how do I orient it so it is square & perpendicular to the diagonal mirror, so the mirrors can be collimated.

 

Does your spider have three (or 4) adjustment screws for tilting the diagonal mirror and a center bolt for moving it toward or away from the primary mirror? If so, you don't have to worry much about the focuser being square to the tube itself. Just install it so the draw tube travels through the existing hole. What's important is you can tilt the diagonal mirror under the focuser so they are "square" to each other. If that is not feasible, you can always shim the focuser in any direction necessary to capture the diagonal mirror under it. 

 

Can you post a pic of your spider from the back side and a pic centered over the hole in the tube so we can see the secondary placement as it currently is? 


Edited by Asbytec, 25 November 2021 - 02:58 AM.

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#4 Garyth64

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Posted 25 November 2021 - 11:43 AM

Welcome to Cloudy NIghts!

 

You can assume that the existing hole in the tube is lined up to the secondary.  Just line up the new focuser's draw tube to the existing hole as mentioned. Mark the 4 focuser holes on the tube and drill them out  It would be smart to remove the secondary, and lay down a rag inside the tube to collect the metal bits.  The single stalk secondary housing looks like it can be adjusted if you are off for some reason.


Edited by Garyth64, 25 November 2021 - 12:54 PM.

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#5 masggid2021

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Posted 25 November 2021 - 01:46 PM

Does your spider have three (or 4) adjustment screws for tilting the diagonal mirror and a center bolt for moving it toward or away from the primary mirror? If so, you don't have to worry much about the focuser being square to the tube itself. Just install it so the draw tube travels through the existing hole. What's important is you can tilt the diagonal mirror under the focuser so they are "square" to each other. If that is not feasible, you can always shim the focuser in any direction necessary to capture the diagonal mirror under it. 

 

Can you post a pic of your spider from the back side and a pic centered over the hole in the tube so we can see the secondary placement as it currently is? 

Thank  you for your help!  My new focuser, all metal, is for a slightly bigger tube than mine. I am hoping I can shim under the attachment bolts so it sits correctly.

focuser Replacement Polaris20211125 095552
focuser Replacement Polaris20211125 095934
focuser Replacement Polaris20211125 095654
focuser Replacement Polaris20211125 095552


#6 Garyth64

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Posted 25 November 2021 - 02:49 PM

Do you know how far the focus is outside the tube?  If you have to rack that new focuser in too far, you may be blocking light from reaching the primary.


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#7 masggid2021

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Posted 25 November 2021 - 03:00 PM

Do you know how far the focus is outside the tube? If you have to rack that new focuser in too far, you may be blocking light from reaching the primary.



#8 Asbytec

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Posted 25 November 2021 - 06:02 PM

Do you know how far the focus is outside the tube? If you have to rack that new focuser in too far, you may be blocking light from reaching the primary.


That's a great point. Im not entirely sure, but it looks as though the focusers can reach the same height.

One way to check the focal plane height is to put the original focuser back on, put some cellophane tape over the top, and focus on the moon. Another way is to measure from the primary mirror to the spider, then half the tube diameter. The remainder of the known focal length will stick out of the tube. Or, sit the focuser side by side and see if they have the same travel in and out.

So, the spider is adjustable. That means it can be placed under the focuser. Or, the focuser can be shimmed to capture the diagonal mirror under it. So, that's good.
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#9 masggid2021

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Posted 26 November 2021 - 10:32 PM

Wow, You

 

That's a great point. Im not entirely sure, but it looks as though the focusers can reach the same height.

One way to check the focal plane height is to put the original focuser back on, put some cellophane tape over the top, and focus on the moon. Another way is to measure from the primary mirror to the spider, then half the tube diameter. The remainder of the known focal length will stick out of the tube. Or, sit the focuser side by side and see if they have the same travel in and out.

So, the spider is adjustable. That means it can be placed under the focuser. Or, the focuser can be shimmed to capture the diagonal mirror under it. So, that's good.

So, this is a great point, thank you Garyth64 & asbytec. Houston, I have a problem. As per the photo, the focuser extends too far into the tube. It actually extends to just grazing the secondary mount. This focuser is designed for a larger tube than I have. It was given to me & is a great focuser. I have ordered a Svbony that is for a 114 or 150 mm, supposedly all metal, with a focuser travel of 65mm.  The round base focuser is the original one. The new focuser has a square base. Much appreciation to everyone who provided a great amount of education as I travel the astronomy path. (hope this photo shows up correctly.)

Old & new focuser020211126 084102

 



#10 SteveG

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Posted 26 November 2021 - 11:02 PM

 

Wow, You

 

So, this is a great point, thank you Garyth64 & asbytec. Houston, I have a problem. As per the photo, the focuser extends too far into the tube. It actually extends to just grazing the secondary mount. This focuser is designed for a larger tube than I have. It was given to me & is a great focuser. I have ordered a Svbony that is for a 114 or 150 mm, supposedly all metal, with a focuser travel of 65mm.  The round base focuser is the original one. The new focuser has a square base. Much appreciation to everyone who provided a great amount of education as I travel the astronomy path. (hope this photo shows up correctly.)

 

 

 

No, it's racked in too far. Set them down with the bases on the same plane (like you have them here), and rack the focuser to the same height as your old focuser.

 

To me the draw tubes look the same length.



#11 masggid2021

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Posted 27 November 2021 - 12:09 AM

No, it's racked in too far. Set them down with the bases on the same plane (like you have them here), and rack the focuser to the same height as your old focuser.

 

To me the draw tubes look the same length.

Yes, the draw tubes are about the same length. However, , the new, square base, focuser when racked down into the tube extends to nearly the secondary mirror. Won't that impede light to primary?. Thanks for the observation. I'm learning!



#12 Asbytec

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Posted 27 November 2021 - 02:49 AM

Yes, the draw tubes are about the same length. However, , the new, square base, focuser when racked down into the tube extends to nearly the secondary mirror. Won't that impede light to primary?. Thanks for the observation. I'm learning!

If the draw tube extends that far into the tube, then yes it will be in the light path. But, your original focuser is much taller, so my bet is the focal plane is much higher than the short new focuser at it's minimum height. As a guess, the focal plane is probably at least an inch higher than the original focuser at it's minimum height. So, the new focuser won't be completely focused downward as shown.

 

In fact, you'll very likely have to rack it outward to reach at least the minimum height of the original focuser. Then a little higher to accommodate the focal plane. You may be operating near the outward most focuser travel of your new focuser. So, I doubt the draw tube will intrude too deep into the scope. And, since the draw tubes are the same length, you may have enough focuser travel to reach the focal plane.

 

Is the new focuser racked all the way out in the images you posted earlier? They both looked racked outward a bit. If the old focuser on the left is at or higher than focus, it looks like the new focuser might be able to reach the focal plane, too. But, it might not have enough travel to go much further out, if it's not fully out already. 

 

Edit: Wagging a guess. If the original focuser on the right is nearly racked in, and if the focal plane is about an inch above it, then I think your new focuser will probably rack upward about 2" to 2.5" to reach the focal plane. It may have about 1" of outward travel remaining. Now, you can fashion a block to raise the new focuser a little bit. You are addressing the different curvatures of the smaller tube, anyway. I mean, you can make it work. 

 

sml_gallery_385554_18313_2580620.jpg


Edited by Asbytec, 27 November 2021 - 03:06 AM.

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#13 bobharley

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Posted 27 November 2021 - 10:58 AM

You're right. I have that svbony focuser,(the square hole pattern
it will be at least half way 'out' to focus any of my eye pieces- ranging from 6mm-25 mm. No problem with it blocking light!.
I had to make the center hole larger on my xt6, and drill the 4 holes. I removed the secondary mirror, put 2 rare earth magnets inside thr tube to catch filings, and used a shop vac to clean up before and after retrieving my magnets.


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