Reflector help for a refractor guy

Equipment Reflectors Collimation Troubleshooting
Blaine Gibbyandrea tasselliArun H
•29 replies•1.9k views
Blaine Gibby avatar
Blaine Gibby avatar
Hello everyone, I am a long time refractor user and bought a Newtonian on a whim and am now having the, I would assume, ever so common collimation struggles. I have a HoTech self centering laser collimator. After I center the laser on the large back mirror, I am then able to get it dead center on the collimator cross hair. However if I take the collimator out and then put it in again to verify that my process is repeatable, the results are always different!

Is this to be expected and that it will always be a little off and it will all wash out in the end? Or is my focuser off center or something else entirely? 

It’s a Skywatcher 10” f/4 Newtonian. I haven’t been able to produce a usable image because my coma corrector isn’t great, I finally found a skywatcher brand coma corrector in stock. But I have had some wonderful views through it under Bortle 4 skies!

Thanks again.
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Matthew Proulx avatar
Matthew Proulx avatar
Be sure that you are not tighening the thumbscrews and allow the laser or any tool to sit freely vertically in the focuser. 
The problem with using a laser is, you can't use one to align the secondary itself. If the secondary isn't aligned the laser will still show collimation but it will not be collimated. You need to use a concenter or a cheshire or some collimation cap to align the seconday. You need to be able to confirm that the laser itself is collimated, if it isn't and you rotate it, you will get a different reading.

To add to newtonian problems, the main mirrors like to shift around depending on their orientation. This will throw off the laser on the center dot. I would worry about this sort of thing only after you've exhausted all of the above.
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CCDMike avatar
CCDMike avatar
Hi Blaine,

it seems that your focuser is tilted. If you use the stock SW one, this is obvious as two screws will never hold and centre your CC.

Best
Mike
David Moore avatar
David Moore avatar
A 10" F4 Newtonian is a hard choice because of the F4, as collimation, tilt etc is harder to deal with than  F5. For visual use however under Bortle 4 skies it's a great choice. Laser collimation can be tricky as the laser can need adjusting first. I use a Cheshire eyepiece but I have an 8" F5 Skywatcher Newtonian but even with that I have flexure issues with an attached guidescope. However it does mean I can just get by without dithering. It will need careful balancing and you need to make sure the mount can cope with the weight. Also it may wobble a bit in anything over a light wind, mine does and I have a Skywatcher AZ EQ6 mount on a pillar.
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Blaine Gibby avatar
Blaine Gibby avatar
Oh good, more stuff to buy, my wife will be so pleased smile.

Does the Cheshire eyepiece have any advantages over the laser? It just seems there are so many sources of error such as the 2" adapter on the laser collimator or the focuser and it looks like there are even adjustments to be made on the laser collimator itself. 

Also why is it not possible to align the secondary mirror with a laser collimator? Every tutorial/video online shows aligning the secondary first and then the primary with the same laser. 

Thanks, I appreciate the input from you all
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David Moore avatar
David Moore avatar
I would have thought that if you could rotate a laser projecting the spot and it didn't move then it was aligned OK. If you then projected it onto the secondary and the spot hit the center of the primary then that would be a good start for a secondary cal. Before that though you would need to fit a hollow tube with a hole dead center to look through, into the focuser tube and make sure that each of the 3 retaining lugs on the primary could be equally seen.  However I do not own a laser but I do own a Cheshire eyepiece and mine needs no adjustment and none is available. All I do is use it in the daytime inside the focuser with the scope pointing south as I have noticed a little mirror movement if I have it pointing at the pole star. I can then see 2 dots if the  collimation is not too far off and I then adjust the primary to make them coincide and I am done. The secondary needs aligning first though as it may need centralizing and that can be tricky as there are several degrees of freedom. Better instructions will be available on the web and it would be best to hear from a laser owner. Collimation needs to be done very well with an F4 compared with F5. The use of an APS-C or worse a full format camera makes it harder. Focuser tilt can be another issue.
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andrea tasselli avatar
andrea tasselli avatar
Blaine Gibby:
Also why is it not possible to align the secondary mirror with a laser collimator? Every tutorial/video online shows aligning the secondary first and then the primary with the same laser.


I think the confusion here is between "collimation" and "centering". With a sight tube you "center" the secondary (an operation that is usually performed once and then never again unless you move the secondary in and out) to make sure that it indeed is square to the focuser by looking into it and making sure it (the secondary mirror) looks circular and equally distanced from the edge of the sight tube (little know fact is that the sight tube ought to have the same "focal ratio" of the main scope you are using it on). This operation is best performed without the primary at all or with a shade in front of the secondary so that the images projected by the primary do not confuse you. Then you'd use either a cheshire or a good collimated laser to center the beam on the central spot/donut on the primary. Now you've collimated the secondary to the primary and you' still have to collimate the primary back onto the focuser. Which can be achieved with a (barlowed) laser, a cheshire (to a degree) or best of all with an autocollimator.
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Arun H avatar
Arun H avatar
For a Newtonian telescope to be properly collimated, the axis of the focuser and the optical axis of the primary need to coincide at ALL points along the optical path.

Simply using a laser collimator as you are doing only assures that the optical axis of the primary and the focuser intersect at one point. So if the position of the laser changes slightly, the returned beam will be off center.  This is probably what you are seeing.

Ideally, you would use two tools iteratively to achieve perfect collimation - a Cheshire or (preferably) barlowed laser, in combination with an autocollimator. Although I own a Cheshire, I find the one I own not very useful due to parallax and so use the barlowed laser iteratively with the autocollimator. When both show collimation, you can be assured that the two axes fully coincide.

This sounds more complicated than it actually is in practice. When I first collimated my scope, it took a couple of iterations between the two tools. But now, the primary is always in collimation, and all I need before imaging is to verify secondary collimation with the autocollimator and it just takes minor adjustment to get it perfect.

I am/was a refractor user too, and migrated to a Newtonian scope (8" f/4) a few months back. Collimation has been compared to riding a bike. Once it clicks, you'll wonder what the fuss was about. And there is no comparing the light gathering power of your 10" with any refractor you could reasonably buy.
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Barry Trudgian avatar
Barry Trudgian avatar
I enjoy using a Skywatcher Quattro F4 10".  The aplanatic  CC is great.  My laser collimator has a 2" adapter.  Put it in a workmate and shine on a wall.  Turn the collimator and check it is steady in or very near the centre.

Eyeball the primary mirror through the empty focus tube.  Move your head in a circular motion and a bit to and fro to look at the edges of primary mirror and clips reflected in the secondary mirror.  Adjust the secondary as little as you can until the virw of the primary is central.

Insert collimator in focus tube andtighten with the 2 screws.  No need to be like I was at first, obsessing over a possible off centre collimator.  Fiddle the 3 secondary allen bolts until lasrr in centre of primary.  Best tip…. get Bob' knobs.

Move to rear of tube and see the target on the side of the collimator pointing towards you.  Slacken all 3 mirror tightening knobs, the smaller ones.  Turn each big adjuster knob in turn to play peekaboo with the red dot.  Just use hand and eye to estimate when the dot has reached its middle point when uou cannot see it.  Repeat for the othe 2 knobs.

Tighten each of the tightening knobs in turn until just touching.  Repeat on each for a little gentle tightening and repeat again for more gentle tightening.

Used to take me 10 minutes now usually 2 minutes.

Whatever method of focussing you use you can check the defocussed star for correct collimation.

Hope this helps.  It works great for me.

Good choice of scope.

Clear skies
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Blaine Gibby avatar
Blaine Gibby avatar
Can I get an expert opinion on this device?

Towards the end it seems to me that the center of the camera/sensor should align with the center of the primary mirror. Skip to the end to see what I'm talking about: 

https://youtu.be/00zSzDr8SXA
andrea tasselli avatar
andrea tasselli avatar
It is expensive but does the job. But so do Cheshire and autocollimation EPs and are way cheaper and do not require a PC to run. Not sure what you mean with the last bit: at the end of the exercise in the movie clip the sensor center IS aligned with the mirror axis/center.
Haakon Rasmussen avatar
Haakon Rasmussen avatar
Hallo Blaine,

I have a TS UNC F4 Newtonian, and it took me more than a year to get the collimation right. I used  the Concenter eyepiece at first, clearly  it wasn't accurately enough, so i bought the Ocal Electronic Collimator. It is attached to the coma corrector with 55m distance. After some months practicing  I now have fairly round stars all over the image. The tilt problems i had, are gone to.

CS Haakon
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Earle Waghorne avatar
Earle Waghorne avatar
Hi Blaine,

One thing to check is whether the HoTech is properly collimated. If you have the 1.25" version, simply put it into the 1.25" focuser adapter, get it on the center spot on the primary mirror and carefully rotate the adapter. The spot should stay on the center spot of the mirror. The first HoTech that I had made a circle about a cm in diameter. My supplier tested it and confirmed that it wasn't collmated. The replacement was better but not perfect and  I've gone back to my cheaper, collimatable, laser and use it with a Howie Glatter parallizer. I also use a Cheshire, which makes initial alignment relatively easy.
 
The focuser screws can tilt the laser, or Cheshire, tighten them only until the laser resists rotation. With my focuser (SW 8" f4) the SW coma corrector was also tilted by the screws and I changed to a Baader clicklock, which solved all of the tilt problems.

A strange observation is that it seems to be possible to collimate with either the Cheshire or Barlowed laser and have the other indicate it isn't collimated but with some patience you get to where both show collimation and then it really is. (I have no explanation for this observation.)

It takes some time and work to get this right but a well collimated Newtonian delivers wonderful images or views.

CS
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Blaine Gibby avatar
Blaine Gibby avatar
Ok this finally all clicked and makes perfect sense.

I am looking though a Cheshire at my secondary with something to block the reflection of the primary, my question is: does a tool exist that will tell me if I have my secondary mirror perfectly “circular” in my Cheshire. Or is it just something you have to eyeball? 

also: how do you make sure the secondary mirror is centered in the Cheshire eyepiece in respect to up and down direction as opposed towards and away from the primary mirror ?

Thanks again 
Blaine
andrea tasselli avatar
andrea tasselli avatar
You would normally check that the annulus between edge of Chesire and edge of secondary is uniform across the whole of the perimeter of the secondary, so, yes, you eyeball it.

Up and down (towards and away for the EP) directions are only controlled during the build-up phase of the reflector so you shouldn't have to worry about it. You can check the length of the arms of the spider and see whether are they of all equal length or there is an offset away from the focus location, in which case you have an offset secondary (which would be the classical arrangement for it).
DanRossi avatar
DanRossi avatar
Earle Waghorne:
Hi Blaine,

One thing to check is whether the HoTech is properly collimated. If you have the 1.25" version, simply put it into the 1.25" focuser adapter, get it on the center spot on the primary mirror and carefully rotate the adapter. The spot should stay on the center spot of the mirror. The first HoTech that I had made a circle about a cm in diameter. My supplier tested it and confirmed that it wasn't collmated. The replacement was better but not perfect and  I've gone back to my cheaper, collimatable, laser and use it with a Howie Glatter parallizer. I also use a Cheshire, which makes initial alignment relatively easy.
 
The focuser screws can tilt the laser, or Cheshire, tighten them only until the laser resists rotation. With my focuser (SW 8" f4) the SW coma corrector was also tilted by the screws and I changed to a Baader clicklock, which solved all of the tilt problems.

A strange observation is that it seems to be possible to collimate with either the Cheshire or Barlowed laser and have the other indicate it isn't collimated but with some patience you get to where both show collimation and then it really is. (I have no explanation for this observation.)

It takes some time and work to get this right but a well collimated Newtonian delivers wonderful images or views.

CS

*** Hi all,  I'm currently experiencing the same pain as Blaine. I recently purchased a GSO 6" F/6 (I chose this over a refractor of the same focal length) and getting this thing ready to go has been challenging.  I already had a collimation cap from a previous newtonian I owned, and then I thought I only needed a Hotech SCA Laser.  Of course, the laser wasn't collimated and I shipped it Hotech (at my expense, even under warranty) to get calibrated, and I'm still waiting for it.  I also purchased a collimation set (Sight Tube/Light Pipe/Autocollimator) from Astrosystems, and I'm waiting on that to arrive so I'll see how it works out.  Lastly, I'm also struggling with the backfocus space on my coma corrector, which is the GSO/TPO/Apertura/Orion/Omegon version that's out there.  I know I need to get my collimation taken care of before I can fully criticize the coma corrector, but when I use it, the subs look terrible. I'm using the 75mm backfocus space as noted, but I know it can vary (any insight from anyone would be helpful).  I'm used to the convenience (meaning no collimation) of a refractor, but I'm hoping that if I get this all nailed down that the future won't be painful.  This hobby is challenging enough as it is.  Thanks!***
andrea tasselli avatar
andrea tasselli avatar
The Astrosystems stuff is pretty good, as I have them and found them reliable for collimating even at f/4. Laser collimator I can't judge because I had mine built to spec and it can be collimated. As for the CC: at f/6 you should have pretty large FOV well corrected, around 1/2 a degree. The 3 CC I have (Baader, GSO/TS, SharpStar 0.95x) all require 55 mm of backspace so I'm not sure where this 75mm comes from.
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DanRossi avatar
DanRossi avatar
andrea tasselli:
The Astrosystems stuff is pretty good, as I have them and found them reliable for collimating even at f/4. Laser collimator I can't judge because I had mine built to spec and it can be collimated. As for the CC: at f/6 you should have pretty large FOV well corrected, around 1/2 a degree. The 3 CC I have (Baader, GSO/TS, SharpStar 0.95x) all require 55 mm of backspace so I'm not sure where this 75mm comes from.

I actually have the Orion version and it mentions ~75mm backfocus on the Orion product page.  The TPO page, for the same corrector just rebranded, says 70mm, so it seems to vary.  I also reached out to someone on Astrobin who confirmed they use 75mm...it seems to be a trial-and-error situation.

Good to hear that Astrosystems is worth the try!
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Blaine Gibby avatar
Blaine Gibby avatar
Hello, me again.  Thanks for all your wonderful suggestions. I am slowly pulling things together. 

I am collimating my f/4 Newtonian with a Howie Glatter type collimated with concentric circle projection. I have done several iterations of adjusting the secondary and primary mirror and cannot get the concentric circles to project on the wall. I get something like the image attached. However:  the projection on the reflective part of the collimating device shows concentric circles. Is there another source of misalignment? I realize it could be the ALIGNMENT of my secondary mirror to my focuser but that is purely eye-balled. 

any advice?
andrea tasselli avatar
andrea tasselli avatar
Forst thing first you need to square the secondary to the focuser. That means, using a sight tube or cheshire, make sure the secondary looks centered with the circle of the sight tube and looking round.
Blaine Gibby avatar
Blaine Gibby avatar
andrea tasselli:
Forst thing first you need to square the secondary to the focuser. That means, using a sight tube or cheshire, make sure the secondary looks centered with the circle of the sight tube and looking round.


yes that’s what I mean by eye balling the tube to the secondary.
Jacob Heppell avatar
Jacob Heppell avatar
I've owned a skywatcher F4 quattro for over 2 years now and have it very well refined. Firstly, for collimation I use a catseye cheshire and autocollimator with a hotspot on the mirror. This is the gold standard for collimation. Lasers are useless for fast scopes. For centering the focuser to the secondary, I use a concenter eyepiece. Secondly, the springs on the collimation knobs that hold the primary are very weak. These need replacing with much stronger springs. I bought C706 springs from Bunnings then trimmed them a little. Thirdly, I highly recommend a threaded connection to the focuser as I can say from experience that a 2" compression ring onto the coma corrector creates tilt and pinches the optics. I replaced the stock focuser with a 3" feathertouch.
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Arun H avatar
Arun H avatar
Fully agree with Jacob on the collimation protocol.

When I first bought my Newtonian, I contacted people whose images with these were good, and the advice was the same as Jacob.

Today, for primary collimation I use a Cheshire (Farpoint, but soon migrating to Catseye WiFi Cheshire) and for secondary, I use a autocollimator (currently also Farpoint, but soon migrating to Catseye which has the offset pupil). People like VIc Menard and Jason Khadder have done a lot of work reducing this to a science and the combination of Cheshire/autocollimator seems to be the best method. See this CN discussion:

Concise thread about autocollimators+improvements - Reflectors - Cloudy Nights

You can save yourself a lot of grief by avoiding unproven solutions and use what people who take good images use!
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Arun H avatar
Arun H avatar
Here is a step by step guide on the use of a Cheshire and autocollimator:

Collimation And The Newtonian Telescope V.4 - How To (mikehotka.com)
Philippe Barraud avatar
Philippe Barraud avatar
Well, well... Is collimating à Newtonian telescope that complicated, like a RC telescope ?
By chance, I did not know !
I collimate my Skywatcher Quattro 200/f4  in a few seconds with an ordinary laser collimator: first, I adujst the screws of the secondary mirror in order to have the red dot in the center of the circle on the primary mirror.
Then I adjust the primary so that the laser light "disappears" in the center of the target of the collimator.
And that's all. And my images are perfectly sharp with this extraordinary telescope !

Philippe

https://www.astrobin.com/qditjv/B/
https://www.astrobin.com/wpnb6o/B/?nc=user
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