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How to Collimate your Newtonian

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#276 Starman1

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Posted 05 August 2018 - 07:51 PM

The other alternative, and it would work on the small primary, is to simply cement it to the cell with silicone cement.

Three blobs about 1 to 1.25" in diameter and 1/8" thick would do it.

It would allow heat to escape from most of the back of the mirror and the sides.

And it would be more than secure enough.

It looks like your tube is plenty thick enough not to deform, so the issue is the spacer washers between the 90° bend on the spider vanes and the I.D. of the tube.

It should fit snug but neither push the tube out, nor pull it in.

 

Home-made focuser?


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#277 rauber830

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Posted 05 August 2018 - 09:03 PM

Yep, homemade focuser...

 

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#278 Starman1

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Posted 06 August 2018 - 01:04 AM

waytogo.gif


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#279 MrJim

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Posted 22 August 2018 - 10:38 PM

I had a lengthy phone conversation today With Starman1, Don Pensack, about collimation and the various tools used to achieve it. It was one of the most interesting astronomical conversations I have had in quite a while and I came away with a much better understanding of the process and tools involved. Thanks Don!

 

Yep, I also placed an order with EyepiecesEtc.

 

Jim W.


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#280 Gil-Galad45

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Posted 11 September 2018 - 04:00 PM

Hello,

 

I own a Celestron 127EQ and am trying to collimate it for the first time.  I have followed the steps on YouTube videos and multiple astronomy websites and forums.  Without a center mark on the mirror, Cheshire eyepieces or laser collimators have no use to me, so I do not have expectations for perfect collimation.  I have adjusted the secondary to where the entire primary and its clips are centered.  I have also adjusted the primary to where I can observe the sweet spot in the eyepiece.  However, even in perfect focus, bright objects have a long, thin elipse protruding from them no matter how I adjust the primary.  If I shift slightly out of focus, I see two shadows of the secondary mirror.  The second shadow disappears the farther I go out of focus.  My telescope has a correction lens inside the focuser.  Could there be an issue with it to cause the poor views?  Thank you for any help,

 

Ethan



#281 Starman1

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Posted 11 September 2018 - 06:20 PM

Yes, the lens in the scope is tipped relative to the optical axis.

What you have is called a "Jones-Bird" newtonian telescope, and collimating them is notoriously difficult, even with a center marker.

But in your case, the lens is likely causing severe astigmatism.

Can you exchange the scope?



#282 Gil-Galad45

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Posted 11 September 2018 - 07:32 PM

Ok, thank you.  I looked in the focuser and main tube, and there is a way to remove the corrective lens.  I received the scope for Christmas last year, so I cannot exchange the scope.  Should I remove the corrective lens or will this make the telescope completely worthless?

 

Thanks again,

Ethan



#283 Starman1

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Posted 11 September 2018 - 08:06 PM

Ethan,

The corrective lens is necessary or the images will have severe spherical aberration.

You should remove the lens to collimate, though.

I would also put a center mark on the primary so a normal inexpensive sight tube + Cheshire combination tool $40  or a collimation cap ($10-15) can be used to collimate the scope.

Then, look at the stars (if you can achieve focus).

If the star images are round but just a bit "blobby", try then installing the lens.

If the star images then go to long ellipses, you will have to observe without the lens or figure out how to tip the lens to flat so the star images are not flared.

I would guess you'll have to observe without it, but be aware this means the focal length of the scope will be about half as long.

 

As for putting a mark on the primary, it's easy:

 

Remove the mirror and its cell from the tube by unscrewing the screws on the side of the tube that hold the cell in place on the end of the tube.

Be sure first to mark the tube and the cell next to each other so you can put it back on in exactly the same attitude because the holes may not be perfectly spaced around the tube.

 

Cut a piece of heavy paper out in a circle the same diameter as the mirror,

Fold in half and then in half again so it looks like 1/4 of a pie.

Nip off just the tip with scissors and then open the paper and make it flat.  There will be a small hole in the center.

Place it on top of the mirror and push a Sharpie pen down through the hole to make a dot on the mirror.

Then, use a pair of tweezers to place a stick-on paper reinforcement ring (the type to keep holes from tearing out in a 3 ring binder) on the mirror with the center of the hole being the dot on the mirror.

This will all be in the shadow of the secondary mirror, so no worries.  Press it down with the eraser end of a pencil to make it stick.

 

Replace the mirror in the tube and reattach the screws.  Ta da!

 

Now, for collimation.  At a minimum, a collimation cap.  For a few dollars more, a combination sight tube + Cheshire like the Celestron Collimation eyepiece.

(or Orion, or Zhumell, or any other brand).  After collimation, follow the star image inspection procedure I mention above--first without the lens, then with the lens.

If the lens really messes things up, a 2X Barlow lens may suffice to replace the lens in the focuser.  You'd use every eyepiece in the Barlow.  At least the lens will be correctly oriented.



#284 Gil-Galad45

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Posted 12 September 2018 - 12:13 PM

Thank you very much for the help!  I will try it out as soon as I can.

 

Ethan



#285 halx

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Posted 13 September 2018 - 12:10 PM

Skip the binder ring (maybe unless you have them laying around). As it will fall off eventually and at the most unfortunate moment according to the Murphy Law. You can simply draw the circle with the same sharpie. A 1/4"+ diameter thin line circle would suffice.

 

Also, I would avoid making the center dot, it will interfere with the (barlowed) laser collimation.

 

Folding thick paper to find the center is also counterproductive, as multiple creases may shift the center point unpredictably and will interfere with the alignment of the edges to the mirror, even creating a chance to scratch it with a dust particle in the cracked crease touching the surface (thin glossy tracing paper is much safer and easier to work with). The folding procedure itself is prone to human errors with thick paper (we'll keep the notion about dirty oily hands and sweaty forehead aside grin.gif as the super clean environment is a must when working with the mirror). Just recall the basic geometry from the middle school and use a square or compass method to mark the center precisely, then cut the 1/4"+ hole with the Exacto knife, ordinary knife, small seam scissors, or even a sharp drill bit, and go with the marker over its edge (using it as a template) making a perfect center ring.

That's how I do that. The template is serving me well for over a decade as I had to recoat the mirror twice already.

 

EDIT: Even easier precise center finding method: Just measure the mirror diameter, and use a compass to draw a slightly smaller (to aid with the edge centering) circle on the template paper. That's all, as the compass needle will mark the center automatically lol.gif Above methods are for the case of an arbitrary circle shape, which you might get by wrapping the mirror in paper to make a crease along its edge (which I would highly discourage from doing, unless you can see an obvious asymmetry of the cavity's edge compared to the blank's side edge).


Edited by halx, 13 September 2018 - 12:37 PM.


#286 Starman1

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Posted 13 September 2018 - 02:21 PM

1) the center dot makes no difference whatsoever to barlowed laser collimation.

It does, however, make an easily-identifiable target for the laser when collimating the secondary.

 

2) Ideally, to find the center, you don't want the paper to sag into the mirror at all.  I have done this to many small mirrors for people and I always scotch-taped the paper to the edge of the mirror so the paper was taut across the surface of the mirror and only touched the very edge of the mirror.  The mirror will not scratch.  Coatings that fragile would chip off if a fly landed on them.

 

3) a 1/4" small black ring on the primary is not going to be as valuable to a cheshire or a barlowed laser as a larger, white, paper reinforcement ring.

I don't know how long it's been since you've bought any, but they are self-adhesive now (so they can't fall off) and are even available in colors and with reflective surfaces.

I bought a sheet of them in reflective white, which is far easier to see on the mirror than black anything.  I've collimated many of the recent Chinese dobs with small diameter

black rings on the primary mirror and they are a real pain to see in the tools--any tools.



#287 halx

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Posted 13 September 2018 - 04:31 PM

You might be right if talking about small apertures and in the day light situation, Don. But with my 12" 1:5 dob and dim red laser collimator I'm having the following issues working in the dusk:

1) The problem with the dot in the center of the ring is that a dimmed laser beam (after the Barlow) wouldn't reflect well from it, it's better to keep a clear aperture for both barlowed and direct methods. With the Barlow that center dot may also distort the blurry image of the ring on the collimator target creating a chance for misinterpretation of the gap's shape in the ring on the beam return screen.

 

2) That's why I'm arguing for a geometry application vs crude folding, any crease in that paper will be getting into the way of any attempt to keep it from touching the surface and from stretching evenly.

Scratches. The paper itself is not that abrasive, yes, however, the heavyweight one is often having a rough enough surface to have a solid dust particle suddenly embedded from the environment, and it might be large and abrasive enough to make a tiny scratch.

 

3) The binder hole is 1/4" on the inside too, however, the reinforcement ring width is 1/8" and that what's not good for the barlowed laser method, as it is a) creates a too large shadow, the typical 1.25" laser collimator screen cannot fit it at 1:5 (for me), b) may require a more powerful Barlow to make the beam wide enough to cover it completely on the mirror (my actual problem as well).

 

I have no problem seeing my ring 1/4" x ~1/20" thick ring from my Cheshire at 1500mm distance I can even see the 3mm bright hole (with orange border) in it reflecting from that clear center spot (double the distance). So for a shorter scope it shouldn't be a problem either.

 


Edited by halx, 13 September 2018 - 10:40 PM.


#288 Starman1

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Posted 13 September 2018 - 04:53 PM

In a typical barlowed laser arrangement, the laser beam hits the primary mirror in a slightly rectangular red beam about 3" x1" or even larger.

When it returns to the bottom of the barlow, the red glow covers the entire bottom of the barlow.

I see this with a typical 2X Barlow in 1.25" in the typical 10-12" scope.

See: 

http://www.obsession...ation/index.php

and

https://www.astrosystems.biz/laser.htm

and

http://www.micosmos....lowed_Laser.pdf

 

Any tiny dot in the center of that would be completely hidden in the center of the large dark ring.

It is the shadow of the center marker that is used for barlowed laser collimation, and something the size of a paper reinforcement ring with a 1/4" center hole is way easier to see and use for that purpose

than something smaller, and, believe me, I've collimated my fair share of scopes with small dark rings.

The reflectivity of the very center is completely unimportant to barlowed collimation--only the shadow of the center marker is what is used.

 

With a 60" focal length in your scope, a barlowed laser should hit the mirror with a more-than-large-enough red glow to return the shadow of a center marker, whether round, triangular, square, or whatever.

 

 

Using a compass to make the cutout is fine.  If you have the wherewithal, an even heavier cardboard could be used that wouldn't sag into the mirror at all.


Edited by Starman1, 13 September 2018 - 04:54 PM.


#289 halx

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Posted 13 September 2018 - 11:10 PM

That's a modified barlowed laser scheme with the additional 2" screen by your links. I'm using the stock 1.25" diagonal screen (can't justify the need of running back and forth between the ends of the OTA while collimating) so my shadow is larger. My Barlow is also 2x but it doesn't produce anything close to a 1" beam it's more like just 1/2" (need to defocus the LED a bit, I guess). These are real problems of cheap laser collimators. Grenades I see on your pics are prob $300 a piece? One can find a working laser collimator with the diagonal and multiple brightness levels for $15 in China and make it work better than what you have linked above (no offense).

 

Barlowed+Laser.jpg

(The image is not mine, but this is about what I see, though my shadow is smaller)

 

Imagine a fuzzy center marker dot in the middle. It will interfere with the clear image.


Edited by halx, 13 September 2018 - 11:38 PM.


#290 Starman1

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Posted 14 September 2018 - 12:20 AM

The most expensive laser in the links was $139 and the 1.25" one in the last link was a lot less.

Those weren't "modified" barlowed laser schemes (the 3rd link was the article written by the inventor of the method) but the actual procedure itself.  The image is supposed to be below the barlow, i.e. before it passes back through the barlow a second time.

That's the way the shadow is sharp and dark.  With your double-pass arrangement, the shadow is blurred and additional rings occur that can be confusing.  The shadow is barely discernible in the picture.

I've used a 2X barlow on the laser in your picture and the beam hit the mirror in a red glow much wider than the center marker, so I don't know what's going on with your laser.  The laser in the picture has obviously completely covered the donut ring on the primary.

 

In the image you post, the shadow of the center marker is the dark fuzzy ring that is almost the same size as the screen.  The red glow inside that is part of the beam inside the marker and the black dot in the center is the 

source of the laser.  A dot in the center of the mirror would be invisible in the picture--it would be the same size as the laser source or smaller.  That image is not clear at all compared to the ones I posted, but it does show the red glow is larger than the screen.



#291 halx

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Posted 14 September 2018 - 03:03 PM

The "modified" is in the sense of "requiring the stock equipment modification". The simplified method (Which I've picked up from Steve Smallcombe, see my links) works perfectly fine despite the slight fuzziness of the projection, and I believe it is much more convenient for a closed OTA than the Nils's original one. The sharpie dot in the center of the mirror will be visible when you are out of the perfect collimation, that's when it will be making a confusion.

 

By the way, the $15 laser collimator! WIth the 2" adapter and 7 brightness modes! (No affiliation, just got one myself there, as mine is too bulky and I don't mind having a spare of this great tool).


Edited by halx, 14 September 2018 - 03:11 PM.


#292 Starman1

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Posted 14 September 2018 - 03:34 PM

Did you have to collimate it, or did you get lucky?



#293 Starman1

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Posted 14 September 2018 - 03:49 PM

OK, one last try:

The Barlowed laser procedure, as described by Nils Olof Carlin, doesn't depend on collimation of the laser.

It also doesn't depend on accurate registration of the laser in the Barlow.

The only thing provided by the laser is a red glow that hits the primary mirror and covers the primary mirror center marker.

If the shadow is returned to a white screen on the bottom of the Barlow, or the screen of a Blug or Tublug, then even the necessity of a barlow lens being perpendicular to the optical path

is removed.  The only important alignment is the bottom of the barlow in the focuser (assuming the hole in the cover is accurately centered).

 

Now, in the rear-view method you favor, since the shadow of the center mark passes through the Barlow once again, several other registrations become critical:

--the registration of the barlow in the focuser (if it's off, the image is misplaced on the screen)

--the registration of the laser in the barlow (if it's off angle, the red background may miss the center mark)

--the collimation of the laser (if it's off the red background behind the centermark shadow may be partially off the shadow)

--the strict perpendicularity of the barlow lens to the optical center line. (if this is off, the shadow will be misplaced and there will not be an accurate cue for collimation)

 

Is it possible that all of these will be accurate enough?  Possibly, especially if followed up by star collimation as a cross-check.

But there is no doubt that removing all of those from consideration when using the barlowed laser procedure is the genius of the procedure.

It is no accident that with Glatter or Astrosystems the screen is between the barlow and the primary center mark.

 

Frankly, lasers should not have a 45° screen in them because the screens tempt people to use the return beam of the laser to collimate the primary.

I wish someone made one of these for commercial sales:

http://www.dcnicholls.com/krupa/


Edited by Starman1, 14 September 2018 - 03:56 PM.

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#294 halx

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Posted 14 September 2018 - 04:16 PM

Did you have to collimate it, or did you get lucky?

It's still in the mail from China. Though on Amazon it goes for $30 and 250+ folks have no any serious complaints (it provides collimation screws and I'm collimating mine periodically as that old construction with the laser pointer inside is prone to sudden shifting after falling to the ground).

 

To your final notes: 
All of the "registration" issues you have described above are easy to rule out by observing the beam and the shadow while rotating the laser and the Barlow (independently) in the focuser. Just do that if in doubt. If any of them present you should first collimate your laser (I doubt either of Glatter or Astrosystems are insured by their price tags from getting out of collimation sometimes); still an issue? Ditch that broken Barlow and get another but a decent one. If that doesn't help ditch the focuser too, unless you are handy enough to troubleshoot and fix both grin.gif  

 

Precisely: The center ring shadow method works regardless of any problems you have described above they are just making the collimation work harder for a novice with the weak understanding of the physics involved. My initial intent was to convey my experience of amending all these issues tremendously by simply making the ring smaller, thinner, and with the clear center in the first place.


Edited by halx, 14 September 2018 - 04:32 PM.


#295 Gil-Galad45

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Posted 14 September 2018 - 04:50 PM

I have not had the chance to take the telescope outside yet, but, from how the inside looks, I will have to collimate it.  So what I am reading is that I should avoid a binder ring and center mark and just draw a circle in the middle from the outline of unfolded, light paper?  Is this correct or am I missing something?  Thank you,

Ethan



#296 halx

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Posted 14 September 2018 - 05:29 PM

Yep. That's my simplified method. A 1/4" (6mm) black ring should work best.


Edited by halx, 14 September 2018 - 05:30 PM.


#297 Vic Menard

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Posted 14 September 2018 - 05:48 PM

The "modified" is in the sense of "requiring the stock equipment modification". The simplified method (Which I've picked up from Steve Smallcombe, see my links) works perfectly fine despite the slight fuzziness of the projection, and I believe it is much more convenient for a closed OTA than the Nils's original one. The sharpie dot in the center of the mirror will be visible when you are out of the perfect collimation, that's when it will be making a confusion.

This approach to the Barlowed laser has been discussed in these forums numerous times (here's two:  https://www.cloudyni...blem/?p=5722718  and

https://www.cloudyni...ire-collimator/

 

 

Not mentioned is the fact that Nils Olof's Barlowed laser setup is balanced, the target is inside the focuser fulcrum while the Barlowed laser focal point is outside the focuser fulcrum. This is what makes Nils Olof's "Barlowed laser" less sensitive to registration errors (including the focuser drawtube and a 2- to 1.25-inch adapter if one is being used). Windowed lasers place both the target and the Barlowed laser focal point outside of the focuser fulcrum. (This is why the Glatter TuBlug is machined to very tight tolerances and is most consistent with higher end/precision focusers.) Nils Olof's method was intended to work for everybody.

 

For what it's worth, like the Cheshire eyepiece, the Barlowed laser multiplies the primary mirror axial error read 2X. A 1mm read error is a 0.5mm primary mirror error (f/5 tolerance for high mag performance is 0.7mm, f/6 is even more relaxed). This of course, assumes many other potentially accumulating errors have either been corrected or allowed. That said, if the end user's optical performance expectations are being met--I'm pretty much OK with whatever collimation method he's using!


Edited by Vic Menard, 14 September 2018 - 05:49 PM.

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#298 The Mad One

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Posted 14 September 2018 - 10:42 PM

As a "fast" 254mm F4 Newtonian owner I have sorted through likely hundreds of posts in dozens of threads over the collimation topic.   I've tried a few different methods over the years F5 203mm through to the current F4 254mm Newtonian.  Usually was able to achieve good collimation (total collimation... focuser, spider secondary, primary) with most.  I found the standard stand alone lase collimators to do the worst job with limited reliability (not the barlowed method...just the stand alone laser).  Of all  of the methods from the simple eyepiece type with the pin hole to my current Cat's Eye setup, the Cat's Eye to be the easiest, most  accurate, and reliable for me.  I have found for me the Cat's Eye using the Teletube XL with the Infinity Autocollimator to be the quickest, easiest, and most reliable.  The biggest twist to me is in the initial set up of the optical alignment getting the focuser to point to the dead center of the optical train.  Once that has been done the rest is a snap and usually unless you change focusers or remove and reinstall it you only go through that once in a great while.

 

I have read a lot of what Vic has put out there which was always of great help in setting up the OTA for the first run off the truck, out of the box or first fully assembled or reassembled after some part change or upgrade.  I still will occasionally run a couple of images of focus in & focus out to check my efforts as well as do a quick check with the Teletube.  Here's a focus out from last night, the focus in is still on the obs puter which is currently shut off.  PIA (Pick it apart!)... wink.gif

 

CS!

 

....  by the way the star used last nigh out @ 03:00 EST-DST (07:00 UT) was Vega!

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Edited by The Mad One, 14 September 2018 - 10:46 PM.

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#299 Gil-Galad45

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Posted 17 September 2018 - 12:10 PM

Okay, so I took my telescope out two nights ago after removing the corrective lense.  I could not get anything into even slightly off focus.  The Moon was the only object bright enough to make an appearance as a pale grey blob.  It wasn't just out of focus or out of collimation, but the image was so distorted without the lens that the light of the Moon was spread throughout multiple times the eyepiece fov.  I tried with and without the Barlow lens, all to no change.  This was a perfectly clear night with a wonderfully stable atmosphere.  So, the lens had to go back in.  I was able to reinstall it.  That allowed me to get things into near perfect view at low magnification, medium became blurry, and I didn't even try high.  Will the collimation steps listed above still work with the correction lens, or do I need a different method?

Thank you all,

 

Ethan



#300 DS-16A

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Posted 21 September 2018 - 05:39 PM

Having a little trouble using the Tectron 1-1/4" sight tube on my scope. Trying to get the bottom of the sight tube perimeter a little larger than the perimeter of the secondary holder, but appears the secondary holder is too large, if that makes sense. Am I doing something wrong? Seems pretty straightforward. Or would I have to use a 2" sight tube? Trying to collimate an older Meade DS16A, 16" primary, 3.1" secondary, 2" focuser. 




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