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How to Collimate your Newtonian

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#676 georgian82

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Posted 09 July 2021 - 11:58 PM

A 1.25-inch thick 6-inch mirror sounds plenty solid, but my experience is with fine annealed pyrex, not exactly what you find today. I just don't know if three edge supports are sufficient for an equatorially mounted tube assembly. I don't think I've heard of trefoil astigmatism in the recent Mak-Newts, and I assume they're mirrors are mounted similarly. It's a puzzler...


I was wrong. The mirror is only 0.75” thick…

#677 Neuralsnafu

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Posted 19 July 2021 - 08:52 PM

I'm trying to get a Powerseeker127 to collimate correctly. I've got a laser from amazon (checked it, its pretty close to on target at 6')

 

I did get a collimation tool from celestron and it is so far impossible to get the photos I see here through it, and it shows misalignment.

 

Unfortunately this is the best photo I could get with my phones camera and my shakey hands. I had to resize for the forum so I'm sure some resolution has been lost.

 

I'm thinking that the Celestron tool is too long? As I can't see the primary mirror clips at all.

 

any help would be appreciated.

 

 

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  • PSX_20210719_204755.jpg


#678 Orion68

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Posted 25 July 2021 - 11:57 PM

I'll take a crack at an even shorter text for the procedure (not with the level of detail of Vic's though). To be as short as possible, I won't discuss the tools that let you do this, I'll just discuss what they aim to do.

1. Move the outline of the secondary¹ to be centred under the focuser (you can either physically move the secondary or tilt the focuser towards it; to pick a choice, see how centred the secondary looks from the front of the tube and see how easy it is to adjust the tilt on the focuser)

2. Rotate the secondary to make it appear as circular as possible

3. Tilt it to centre the primary centre spot reflection under the focuser, or to make a laser collimator's forward beam hit the primary's centre spot.

4. Go back to 1, until 1-3 no longer require you to do anything². Don't obsess - good enough is, so stop when you're fed up. 3. is most critical, and what follows is more critical.

5. Change the tilt on the primary.
5a. If you have a tool in the focuser with a centred pupil, make the reference (the pupil itself, or a bright Cheshire ring concentric with it) concentric with the primary centre spot reflection³.
5b. If you have a barlowed laser, centre the silhouette of the primary's centre spot in the returning light cylinder in the focuser opening.

6. Go back to 3. or 1., and repeat until you're fed up with it or no further adjustments are needed. Again, 5. is critical if you're getting too bogged down in the early steps; move along to 5. if you think the rest is good enough, but don't skip that step.

--
¹Don't confuse the outline of the secondary itself with the primary's reflection in it. Use coloured paper behind the secondary or between the secondary and the primary to make things more clear if necessary.
²The astute reader will have noticed that 1. and 3. together mean the reflection of the primary and the outline of the secondary will be concentric.
³The astute observer will see that the silhouette of the reflection of the secondary is actually not concentric, but offset along an imaginary line towards the primary. If the offset isn't on that line, you may have a small residual error in what you've done in steps 1. and 2., but it's your choice whether to ignore it or start again.

 

Hope it's ok to ask a question in this thread.

 

In #1 above it is mentioned that an option is to tilt the focuser toward the secondary. How would you do that? Is this a reference to shimming the focuser to make it square with the telescope tube?

 

Thanks sixela.

CS



#679 Asbytec

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Posted 26 July 2021 - 12:07 AM

Hope it's ok to ask a question in this thread.

In #1 above it is mentioned that an option is to tilt the focuser toward the secondary. How would you do that? Is this a reference to shimming the focuser to make it square with the telescope tube?

Thanks sixela.
CS

Not square with the tube, rather if you want or need to, you can shim the focuser to chase the secondary position approximately centered in the tube. The idea is the moving parts are square to each other rather than trying to square everything to the tube longitudinal axis. You achieve that by centering the secondary under the focuser, first. Then when the focuser axis is aligned to the primary center the reflection of the primary will be centered under the focuser, too. All three signatures are concentric (and "square") with each other.

Edited by Asbytec, 26 July 2021 - 12:10 AM.

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#680 xvariablestarx

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Posted 26 July 2021 - 12:17 AM

I'm trying to get a Powerseeker127 to collimate correctly. I've got a laser from amazon (checked it, its pretty close to on target at 6')

 

I did get a collimation tool from celestron and it is so far impossible to get the photos I see here through it, and it shows misalignment.

 

Unfortunately this is the best photo I could get with my phones camera and my shakey hands. I had to resize for the forum so I'm sure some resolution has been lost.

 

I'm thinking that the Celestron tool is too long? As I can't see the primary mirror clips at all.

 

any help would be appreciated.

 

I used to use a laser to collimate my PS127 also. Did you take out the corrector lens from the focusing tube? I think that the laser is the only way to get this scope collimated, but you can try something like a Cheshire eyepiece also, I never tried one of those on my Powerseeker.
 



#681 Orion68

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Posted 26 July 2021 - 05:28 PM

Not square with the tube, rather if you want or need to, you can shim the focuser to chase the secondary position approximately centered in the tube. The idea is the moving parts are square to each other rather than trying to square everything to the tube longitudinal axis. You achieve that by centering the secondary under the focuser, first. Then when the focuser axis is aligned to the primary center the reflection of the primary will be centered under the focuser, too. All three signatures are concentric (and "square") with each other.

Got it. thanks much.

CS



#682 Neuralsnafu

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Posted 26 July 2021 - 06:02 PM

I used to use a laser to collimate my PS127 also. Did you take out the corrector lens from the focusing tube? I think that the laser is the only way to get this scope collimated, but you can try something like a Cheshire eyepiece also, I never tried one of those on my Powerseeker.


Yes. The doublet in the focuser has been remmoved for collimation.

I think I finally got it yesterday. I mean it decided to dump rain 15 minutes after I finished so that *must* mean I did it right?

#683 Orion68

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Posted 29 July 2021 - 07:03 PM

Thanks to Vic, Don, sixela, Asbytek & others on CN I've learned about collimating my new Newt. However, there is much, much more to learn.

 

It seems that a Cheshire (which I have) is fine for the primary but I really need a sight tube for collimating the secondary.

 

Can anyone recommend a sight tube that gives consistently good results?

 

Thanks in advance and CS.



#684 Starman1

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Posted 29 July 2021 - 07:47 PM

Thanks to Vic, Don, sixela, Asbytek & others on CN I've learned about collimating my new Newt. However, there is much, much more to learn.

It seems that a Cheshire (which I have) is fine for the primary but I really need a sight tube for collimating the secondary.

Can anyone recommend a sight tube that gives consistently good results?

Thanks in advance and CS.

Catseye Teletube.
Astrosystems Light Pipe
Both in 2" if you can use 2" tools.
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#685 Orion68

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Posted 30 July 2021 - 04:16 AM

Catseye Teletube.
Astrosystems Light Pipe
Both in 2" if you can use 2" tools.

I can use 2" tools!

 

Ignore this question, I was looking at the price for the 1.25" version.

The Lightpipe cost is about 1/2 of the Teletube. I have to wonder why if both are of equal quality. The Teletube looks pretty robust, maybe that's the reason.

 

My Cheshire is an old Catseye and works fine but it is 1.25". Would I be better off going with a combo tool in 2"?

 

Thanks Don. 


Edited by Orion68, 30 July 2021 - 04:21 AM.


#686 Starman1

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Posted 30 July 2021 - 08:26 AM

The 2" sight tube has advantages due to a wider field and the ability to better see the edge of a large secondary.

A 2" Cheshire only has an advantage if the calibration of center marker to dark center in the tool is better, i.e. the larger center marker matches better with a larger dark center in the Cheshire.

I suppose there is an advantage to 2" tools in general, which is that the possible registration error of the tool in the adapter and the adapter in the focuser becomes only a possible error of the tool in the focuser.


Edited by Starman1, 30 July 2021 - 08:26 AM.

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#687 Orion68

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Posted 30 July 2021 - 04:29 PM

The 2" sight tube has advantages due to a wider field and the ability to better see the edge of a large secondary.

A 2" Cheshire only has an advantage if the calibration of center marker to dark center in the tool is better, i.e. the larger center marker matches better with a larger dark center in the Cheshire.

I suppose there is an advantage to 2" tools in general, which is that the possible registration error of the tool in the adapter and the adapter in the focuser becomes only a possible error of the tool in the focuser.

I use a centering adapter with the 1.25" Cheshire so I'm fairly comfortable the registration is good. At least I think it is.

 

I'm leaning toward the 2" Teletube at this point but the Telecat is only $16 more so it probably makes sense to go with the Telecat.

 

Thanks for your help Don.

CS



#688 Vic Menard

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Posted 30 July 2021 - 06:09 PM

...I'm leaning toward the 2" Teletube at this point but the Telecat is only $16 more so it probably makes sense to go with the Telecat.

FYI, the fully collapsed Teletube XL is essentially the same effective focal ratio (efr) as the short 2-inch LightPipe (about f/3.5). I use a fully collapsed TeleCat XL-S for my f/4 and f/5 Dobsonians (the efr is about f/3). But I prefer discrete incremental steps between the three alignment circles, which requires a shorter efr than an apex alignment, where all three circles are the same apparent size. To determine your scope's efr, click here: http://www.vicmenard...rspectives.html  and scroll down to, "Notes on matching a sight tube to your 'scope's focal length".


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#689 Orion68

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Posted 30 July 2021 - 11:28 PM

Thanks Vic, I'll check out your link on efr.

CS



#690 Orion68

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Posted 31 July 2021 - 12:50 AM

On my F5 Newtonian (Apertura) the spider was not centered in the tube when I received it, it was about 2mm off. So, I adjusted the spider vanes to put the secondary in the middle of the tube.

 

Thinking about it now, is it possible that the secondary was purposely offset to accommodate a primary mirror that was not perfectly centered in the tube? Or, is this type of mechanical precision at the factory doubtful given that this is a very inexpensive scope?

 

Does it even matter that the secondary was 2mm away from being centered in the tube?



#691 Starman1

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Posted 31 July 2021 - 01:19 AM

On my F5 Newtonian (Apertura) the spider was not centered in the tube when I received it, it was about 2mm off. So, I adjusted the spider vanes to put the secondary in the middle of the tube.

Thinking about it now, is it possible that the secondary was purposely offset to accommodate a primary mirror that was not perfectly centered in the tube? Or, is this type of mechanical precision at the factory doubtful given that this is a very inexpensive scope?

Does it even matter that the secondary was 2mm away from being centered in the tube?

It is important that the spider vanes crossing be in the center. Any offset imparted at the secondary would be relative to the centering.
Any error would simply have been careless assembly.
A 2mm error would not impede collimation, but it is a good thing you centered the vanes crossing.
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#692 Orion68

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Posted 31 July 2021 - 01:39 AM

Thanks Don, appreciate the feedback.

CS



#693 Asbytec

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Posted 31 July 2021 - 07:55 PM

Does your focuser come with an extension? If so, you may need to use it then focus inward. 



#694 Orion68

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Posted 31 July 2021 - 08:06 PM

Does anyone know if the 2" Teletube XL or 2" Telecat XL (tool only) is in stock or on backorder?




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