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Which collimation tool (kit) to buy?

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#1 bokemon

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Posted 23 October 2020 - 07:09 AM

Hello folks,

I have a R200SS telescope which is F4. 

The secondary alignment is probably close to correct from the factory since the spider vanes are very thick and sturdy. 

The primary is a little bit out of alignment because I bonked the end of the telescope and stars at one side of the image are a little crooked.

I will be using a coma corrector with it.

I use this scope for imaging only, so I expect that after whatever collimation I do, hanging the camera and filter wheel off the focuser is going to bend it a little bit.

Ideally I would like to finish up with a "star test", but the combined tube currents and seeing are just too bad to get the nice Airy disks.  I would try using a Tri-Bahtinov mask or take images and use CCD inspector to look for coma / elongation.

To use the collimation kit requires I take off the imaging train and screw in a 42mm to 1.25" eyepiece adapter.  It is about 1.5" tall, and I can't jam anything in further than that without hitting the coma corrector.

Primary is also marked with a donut.  I don't wanna have to spot the secondary.

 

The two kits I am looks at are:

Farpoint 1.25" kit = Cheshire + laser

Astro Systems kit = sight tube + autocollimator

 

Which combo would work better given my situation?  Or maybe just one tool, or none at all?

 



#2 eyeoftexas

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Posted 23 October 2020 - 07:14 AM

There's no reason to spot the secondary.

 

You should also consider collimation tools from Catseye: https://www.catseyecollimation.com


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#3 TiSaph

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Posted 23 October 2020 - 08:07 AM

I strongly recommend a Barlow'd laser and a Catseye autocollimator with offset pupil.  With the autocollimator  you have the satisfaction of knowing the collimation is perfect, and it's substantially cheaper than one of the highest-end laser collimators.  However, they are a little more trouble to use solo, and you need something to get the primary and secondary rough adjustments correct. For that, you can get a cheap laser, improve its alignment using the v-block rotation trick, and get a cheap Barlow.



#4 philinbris

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Posted 23 October 2020 - 04:52 PM

I have been using a cheap but well aligned laser (I aligned myself on a wall 10 meters from the laser). I stick that in a 2x Barlow and I can just make out the primary mirror doughnut on the lasers 45 deg surface. This is all after sec mirror setup of course. After just using the laser only alignement, the Barlow approach showed the doughnut was a bit off. More important for F4 than F5 I imagine.

That had me so convinced this method is very good I bought a Howie Glatter Barlowa couple days ago to put my laser in. Really keen to see how that improves the return image.

Cheers


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#5 Asbytec

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Posted 23 October 2020 - 05:45 PM

 

Astro Systems kit = sight tube + autocollimator

 

From your previous thread, I'd recommend the above, especially the autocollimator for the precision you are looking for. Or the same from Catseye site above. 

 

Also, check your primary center spot for being geometrically centered on the primary. Trace a paper template the same diameter as your primary mirror, then carefully fold it into quarters. Snip a small piece from the corner, then lay the template on your primary. The perforation in the primary center mark should fall within the small hole in the center of the template. You can mark it with a tiny dot using a sharpie. Hopefully it'll be within half a millimeter or less. At f/4, your primary alignment needs to be fairly precise, especially with a coma corrector. There are other methods for checking the center marker, but this one is easy to get a quick look at it. Catseye also sells center spots and templates, they are not expensive. 


Edited by Asbytec, 23 October 2020 - 06:34 PM.


#6 e7FvPDZR

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Posted 23 October 2020 - 05:50 PM

I've had good results with Farpoint 1.25" kit (Cheshire + laser + primary mirror center decal).

 

The mirror on my old Dobsonian didn't have a center mark, so that part of the kit was very useful. The laser made aligning the secondary very easy. Then using the Cheshire (collimation cap) to check and adjust the primary is easy. I just leave the cap in the focuser so that when I move and set up the telescope outside, I just shine my red led flashlight at the primary to check the mirror and adjust it if needed.



#7 Chris Westland

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Posted 23 October 2020 - 07:41 PM

I purchased the Farpoint 2" laser + cheshire + autocollimator kit.  I initially thought it might be overkill, but then I like shiny new things. 

 

It's actually not overkill I'm finding.  First, 2" allows for tight alignment with the focuser, and no issue with slop in the 2-1.25" converter.  I started out using the cheshire, and got everything aligned, then tried the laser -- everything was out of alignment.  I ultimately figured that the problem was the secondary (the skyandtelescope guide to collimation suggests that this is the weakness of laser collimators with dobs).  The angle of the secondary has to be {0,45} degrees (2D) with the focal plane of the eyepiece (i.e., with the focuser).  I have shims on my MoonLite focuser mount that got a little way toward that; but final tuning needed to be with shims to the secondary screw mount.   With a combination of laser and cheshire, I centered the focuser on the secondary, and then shimmed the secondary holder until the primary could be centered in the secondary.  This is a tighter alignment than just a sight tube with cross hairs would give you.  Follow that with centering everything with the cheshire, and verifying with the laser collimator (which was aligned once the secondary was properly shimmed).  The autocollimator followup allows for fine tuning; I was at first concerned that it would be overly sensitive, but it's actually quite useful for final tune-up.

 

My scope is an HO 14" f/4.6 (I've reviewed this elsewhere on CN), and the skyandtelescope guide suggests that at f/4.6, you only have about a 2 mm sweet spot.  So at least for my scope, the extra attention and a quality collimation kit pay off.  The quality shows up at the eyepiece as well, as stars are now noticeably more tightly focused (no coma).

 

Star tests are hyped as the ultimate collimator, but they are a real bother.  Particularly if you have any seeing, and because you have to constantly run back to the eyepiece and rack in and out.  There is a reason to use collimators.


Edited by Chris Westland, 23 October 2020 - 11:48 PM.


#8 bokemon

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Posted 24 October 2020 - 03:53 AM

Also, check your primary center spot for being geometrically centered on the primary.

Vixen I hope is a reputable company.  Why would you center spot and etch a primary, and then get it wrong?



#9 Asbytec

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Posted 24 October 2020 - 04:29 AM

Vixen I hope is a reputable company.  Why would you center spot and etch a primary, and then get it wrong?

Okay, so it's spotted already, etched by the company. Cool. One might think they do it right. Many of us have to check it. 


Edited by Asbytec, 24 October 2020 - 05:44 AM.


#10 Jon Isaacs

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Posted 24 October 2020 - 04:37 AM

The angle of the secondary has to be {0,45} degrees (2D) with the focal plane of the eyepiece (i.e., with the focuser).

 

There is nothing sacred about 45 degrees.  There are "Low Rider" Dobs with the focuser angled down 20 degrees to lower the eyepiece height.  

 

http://www.reinervog...lowrider_e.html

 

All that is necessary is that the secondary is properly positioned and that the axis of the focus is aligned with the axis of the primary mirror.  This is adjusted using the tilt of the secondary.  

 

Jon


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#11 Jon Isaacs

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Posted 24 October 2020 - 04:45 AM

Vixen I hope is a reputable company.  Why would you center spot and etch a primary, and then get it wrong?

It happens.  

 

The centering of the center marker is critical and it's my opinion that this is a large source of collimation errors.  Actually placing the center marker within the accuracy required is quite difficult.  The collimation tolerance at F/4 on the primary is 0.013" = 0.32mm.  The center marker needs to placed more accurately than that. Doing that at home is not a trivial task.

 

For your Vixen, one would expect that it was done correctly but it never hurts to check.  

 

Jon



#12 bokemon

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Posted 24 October 2020 - 10:32 AM


All that is necessary is that the secondary is properly positioned and that the axis of the focus is aligned with the axis of the primary mirror.  This is adjusted using the tilt of the secondary.  

 

 

The Vixen R200SS uses thin sheet metal aluminum for the tube.  Once I hang the camera + OAG + filterwheel off the focuser, I think it is going to bend a little bit.  Hopefully this is not a big problem after collimation is already done?
 



#13 Chris Westland

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Posted 24 October 2020 - 08:00 PM

There is nothing sacred about 45 degrees.  There are "Low Rider" Dobs with the focuser angled down 20 degrees to lower the eyepiece height.  

 

http://www.reinervog...lowrider_e.html

 

All that is necessary is that the secondary is properly positioned and that the axis of the focus is aligned with the axis of the primary mirror.  This is adjusted using the tilt of the secondary.  

 

Jon

Understood.  Mine is 45.  The important thing is that it be square with the focuser and eyepiece focal plane.



#14 Jon Isaacs

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Posted 24 October 2020 - 08:19 PM

Understood.  Mine is 45.  The important thing is that it be square with the focuser and eyepiece focal plane.

 

Yours is at 45 but it works just as well at 46 or 47 degrees.  Shimming the focuser is really only another way to position the secondary mirror. 

 

You can precisely collimate the scope with the focuser at 47 degrees.

 

Jon



#15 Chris Westland

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Posted 24 October 2020 - 09:44 PM

Yours is at 45 but it works just as well at 46 or 47 degrees.  Shimming the focuser is really only another way to position the secondary mirror. 

 

You can precisely collimate the scope with the focuser at 47 degrees.

 

Jon

I know.  I have a standard 2ndary holder, which is 45, and my focuser mount is a standard perpendicular (just what came with the scope, see attached).  The focuser is also shimmed, and gets me close to where I needed to be. 

 

I actually didn't know that people did 'low riders', but it's clever, and you could potentially have a cascade of secondaries (with larger obstruction) to drop the eyepiece even closer to the ground.  You just have to do all of the structure work yourself; not too many commercial options as far as I know.

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Edited by Chris Westland, 24 October 2020 - 09:46 PM.


#16 Vic Menard

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Posted 25 October 2020 - 10:37 AM

Understood.  Mine is 45.  The important thing is that it be square with the focuser and eyepiece focal plane.

Actually, the only reason for a 90-degree intercept (and the accompanying 45-degree secondary mirror) is that it's the quickest way out of the light path, which helps to minimize the central obstruction. Since most secondary mirrors are made with their major axis equal to the minor axis times the square root of two, we tend to "build" our tube assemblies around a "squared" secondary mirror/focuser geometry.

 

That said, when it comes to the actual collimation of the optical components (primary mirror, secondary mirror, and focuser), as long as the "measurable" mechanicals are reasonably close ("centering" the primary mirror, the secondary mirror, and the spider, and "squaring" the spider and the focuser), the axial alignments (focuser and primary mirror) will need to be corrected to their prescribed tolerances, and the secondary mirror placement configured to pass the imaging light cone. And this is a good thing, as the critical axial tolerances for a coma corrected Newtonian would be difficult to assess and correct if squaring (focuser and intercept) was also required to meet a similar critical tolerance.



#17 Steve OK

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Posted 25 October 2020 - 03:29 PM

I've been running this around in my head the past few days, and am still uncertain about one thing.  If the focuser is not square with the tube, but all other collimation requirements are met, is the image plane still parallel with the top of the focuser (and "flat" to the sensor of a camera placed in the focuser)?  Part of my brain says "yes".

 

Steve



#18 Vic Menard

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Posted 25 October 2020 - 04:18 PM

...If the focuser is not square with the tube, but all other collimation requirements are met, is the image plane still parallel with the top of the focuser (and "flat" to the sensor of a camera placed in the focuser)?  Part of my brain says "yes".

The part of your brain that says "yes" has it right. As long as the axes coincide, the image plane will be perpendicular to the optical (primary mirror) axis. There are, of course, tolerances for the required precision. The allowable linear focuser axial error tolerance (that ensures perpendicularity of the image plane) is about 3-percent of the primary mirror diameter. Coma overwhelms defocus inside the allowed tolerance, which is why the error tolerance is smaller if the system includes a coma corrector (between 0.005D and 0.002D, depending on the corrector).


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#19 Steve OK

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Posted 26 October 2020 - 06:43 AM

Thanks, Vic!  I'll give that part of my brain a pat on the head.cool.gif

 

Steve



#20 Asbytec

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Posted 26 October 2020 - 07:34 AM

If the focuser is not square with the tube, but all other collimation requirements are met, is the image plane still parallel with the top of the focuser (and "flat" to the sensor of a camera placed in the focuser)?  Part of my brain says "yes".

 

Steve

Steve, not long ago I quit worrying about squaring anything to the tube or it's longitudinal axis. I believe the process is arduous and not necessary. Instead, it helps (me) to think in terms of being able to square the "moving parts" to each other. Primarily the secondary and the focuser because the primary mirror cell is pretty much fixed near the tube's longitudinal axis, already. This squaring happens when you center the secondary under the focuser, then align the focuser axis to the primary.

 

The optical alignment may or may not coincide precisely with the mechanical tube assembly. But, we're not after mechanical centering. We're after optical alignment. So long as the secondary is reasonably centered in the OTA, and so is the primary, then the entire optical alignment from primary mirror to the focuser will be close enough to the mechanical axis. The upper tube assembly won't vignette the light path. As VIc said, it's entirely possible to bring all optical alignments into tolerance without much concern with the tube itself. Just the moving parts... 

 

Edit to add: In my case, I center the secondary by measuring along the vanes and assume the primary cell is close enough, too. No rigorous precision required, just good enough. Then, tilt the secondary under the focuser if small adjustments are needed. If larger adjustments are needed, then I may shim the focuser a bit to "chase" the secondary in its natural position near the center of the tube. But, only in the "up and down" (y axis) direction as seen in the focuser. Fore and aft (x axis) secondary placement (toward or away from the primary) is controlled by the center bolt. Normally, no shim is needed to correct fore and aft secondary centering parallel to the tube axis. I'm "chasing" the moving parts in their "natural" position approximately centered in the tube without /much/ regard for the tube itself. 


Edited by Asbytec, 26 October 2020 - 07:48 AM.

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#21 Steve OK

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Posted 26 October 2020 - 08:46 AM

Thanks Norme!  That makes a lot of sense.

 

Steve


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#22 Starman1

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Posted 27 October 2020 - 02:28 PM

For accuracy in tools, I have found the following to give excellent results:

Sight tube for secondary placement under focuser: Astrosystems Light Pipe

Secondary alignment tool: Astrosystems Light pipe, Glatter laser, Farpoint laser

primary alignment tool: Astrosystems Light Pipe, Farpoint Cheshire, Catseye Cheshire, Glatter laser with barlow attachment, Glatter laser with Tublug,

      Farpoint laser with Tublug, any laser with Tublug, 

Autocollimator: Catseye XLKP (the best), Catseye XL, Farpoint AC.  I do not recommend an AC with a 2nd surface mirror (like the Astrosystems).

 

There may be other tools that work OK.  The ones above I have used extensively, and they give repeatable, excellent collimation.

Even better, they all agree with one another when the scope is collimated.


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#23 Vic Menard

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Posted 27 October 2020 - 03:37 PM

For accuracy in tools, I have found the following to give excellent results:

Sight tube for secondary placement under focuser: Astrosystems Light Pipe

Secondary alignment tool: Astrosystems Light pipe, Glatter laser, Farpoint laser

primary alignment tool: Astrosystems Light Pipe, Farpoint Cheshire, Catseye Cheshire, Glatter laser with barlow attachment, Glatter laser with Tublug,

      Farpoint laser with Tublug, any laser with Tublug, 

Autocollimator: Catseye XLKP (the best), Catseye XL, Farpoint AC.  I do not recommend an AC with a 2nd surface mirror (like the Astrosystems).

 

There may be other tools that work OK.  The ones above I have used extensively, and they give repeatable, excellent collimation.

Even better, they all agree with one another when the scope is collimated.

I'll add my penny.gif penny.gif

For optimal secondary mirror placement, I like the adjustable TeleCat and TeleTube. And for short focal ratios, the XLS series (EFL 2.93) is the one you want (the short LightPipe (EFL 3.43) is essentially the same length as the fully retracted Catseye XL series (EFL 3.46)).

For secondary mirror (focuser axis) alignment, I prefer the Glatter laser with the 1mm aperture stop that allows focuser axis assessment under the focuser, where I can use my glasses (or reading glasses for a lot of users) to see the alignment "up close" while I'm adjusting the secondary mirror, or a good autocollimator and a carefully decollimated primary mirror. With my vision, I can't get real "precision" with any sight tube.

For primary mirror alignment, I regularly use my Glatter with the same 1mm aperture stop--works like a Barlowed laser, and delivers good precision. But nothing beats a calibrated Cheshire, which was pioneered, and perfected, by Jim Fly of Catseye Collimation.



#24 mjgillen

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Posted 27 October 2020 - 08:21 PM

For accuracy in tools, I have found the following to give excellent results:

Sight tube for secondary placement under focuser: Astrosystems Light Pipe

Hey Don, which size do you recommend for a 12.5” f4.8 DOB? It has a 2” focuser so do you recommend the 2” light pipe or the 1.25” and why?

 

Thanks,

Michael



#25 bokemon

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Posted 27 October 2020 - 09:37 PM

For accuracy in tools, I have found the following to give excellent results:


Autocollimator: Catseye XLKP (the best), Catseye XL, Farpoint AC.  I do not recommend an AC with a 2nd surface mirror (like the Astrosystems).

Well unfortunately Astro Systems is the only one who makes a 1.25" autocollimator.




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