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How to Collimate your Newtonian

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#51 sixela

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Posted 22 April 2010 - 01:32 AM

If the star test works, is it safe to assume everything is aligned correctly, or could the alignment still be suboptimal?

It is safe to assume that the optical axis crosses the focuser axis at the focal plane, but you don't know:

-whether the focuser axis is parallel (if it isn't, the focal plane will be tilted)
-whether the secondary is placed to deliver a centred fully illuminated field.

#52 kev721

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Posted 22 April 2010 - 10:16 PM

How do you test for those?

#53 sixela

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Posted 23 April 2010 - 03:31 PM

How do you test for those?


Usually with good collimation tools. The effects on visual observation are extremely subtle, though the effects on wide field photos is usually not (especially for a tilted focal plane).

#54 kev721

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Posted 24 April 2010 - 12:32 PM

So if I can see the entire primary in the secondary, then collimate with a laser, then a star test, I should be in good shape? Anything else I might miss?

#55 sixela

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Posted 25 April 2010 - 06:05 PM

Yup. Assuming of course that you adjust primary tilt while star testing.

#56 kev721

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Posted 27 April 2010 - 03:37 PM

It seems the star tests usually look all right after I have done the laser. For the star test, please correct me if I am wrong here.

1. Polaris is usually an okay star to use, but at least try something near the pole to keep it in the field longer.
2. Use as much magnification as I can. Should I barlow or not?
3. If the circles are not concentric, pick a knob, turn it, and see which way it affects the star, then repeat with another knob.

Is that about it?

#57 sixela

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Posted 29 April 2010 - 02:27 AM

1. Polaris is usually an okay star to use,

I don't like to use Polaris: it's an 18.4" separation double.

3. If the circles are not concentric, pick a knob, turn it, and see which way it affects the star, then repeat with another knob.


You forgot one thing to mention: always centre the star perfectly before evaluating the rings.

And you can do better than throwing darts when you have to pick a knob to turn: move the knob closest to the axis from the compressed side of the rings to the expanded side, and in such a way as to move the star away from the compressed side. Then recentre the staar and evaluate again.

#58 kev721

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Posted 29 April 2010 - 11:26 AM

What star do you prefer? Since you did not mention any problems with #2, then I assume go for maximum magnification with barlow. So, that means I should still try to get something near the pole or I'll be chasing it across the sky all night, 20' at a time.

#59 uniondrone

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Posted 09 August 2010 - 02:09 PM


Hello,

I was wondering how critical (and how frequent) is the collimation of the secondary? If I can see all four mirror clips of the primary through the peep hole of the collimation cap, is that good enough? Or should I strive for the reflection of the primary to be absolutely centered?

#60 Vic Menard

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Posted 10 August 2010 - 10:35 AM

...how critical (and how frequent) is the collimation of the secondary?

As long as it's not impacting imaging or visual performance, the secondary mirror alignment is the least critical of the three alignments (primary mirror axis, focuser axis, and secondary mirror).

...If I can see all four mirror clips of the primary through the peep hole of the collimation cap, is that good enough? Or should I strive for the reflection of the primary to be absolutely centered?

For an xt10, it's almost certainly good enough for secondary mirror alignment. I'm assuming you're not also using this coarse alignment for the focuser axis.

#61 uniondrone

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Posted 10 August 2010 - 05:50 PM

...how critical (and how frequent) is the collimation of the secondary?

As long as it's not impacting imaging or visual performance, the secondary mirror alignment is the least critical of the three alignments (primary mirror axis, focuser axis, and secondary mirror).

...If I can see all four mirror clips of the primary through the peep hole of the collimation cap, is that good enough? Or should I strive for the reflection of the primary to be absolutely centered?

For an xt10, it's almost certainly good enough for secondary mirror alignment. I'm assuming you're not also using this coarse alignment for the focuser axis.


Basically, I can see the full reflection of the primary (i.e. all four mirror clips), and I have centered the reflection of the peephole in the middle of centermark on the primary. This adjustment has already given me views at first light that are better than anything my SCTs have ever given me, but I would like to know what additional adjustments might be useful.

I am not really sure how each axis is named. As far as I can tell, the possible adjustments of the secondary are rotation (does the secondary appear circular instead of oval), position along the central OTA axis (i.e. is the outline of the secondary centered in the peephole), and tilt (is the primary fully visible and reasonably centered in FoV of the secondary). Am I missing anything? Any thoughts on this?

#62 Vic Menard

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Posted 11 August 2010 - 08:09 AM

...I can see the full reflection of the primary (i.e. all four mirror clips), and I have centered the reflection of the peephole in the middle of centermark on the primary.

The first part ("I can see the full reflection of the primary...") means you should have good illumination across the fov. The second part, ("I have centered the...peephole in the middle of the center mark..."), means you have good primary mirror axial alignment.

I am not really sure how each axis is named.

There are only two. I think you already understand the primary mirror axis and how to correct it (you described it above). Correcting the primary mirror axis places the "sweet spot" in the center of the fov. It's the most critical alignment, requiring corrections less than a few hundredths of an inch for an f/5 primary mirror. Thankfully, aligning the collimation cap pupil to the primary mirror center spot (donut) is pretty easy, and the pupil/center spot alignment actually magnifies any residual primary mirror axial error 2X.

The other axis is the focuser axis. It's easy to recognize if you have a simple thin beam laser collimator. With the laser mounted in the focuser, the beam is the focuser axis. The focuser axial alignment tolerance for a 10-inch aperture (w/o Paracorr) is about three tenths of an inch (more than 10X the tolerance of the primary mirror) measured relative to the primary mirror center spot. Generally speaking, the most common way to correct the focuser axial alignment is to tilt the secondary mirror to aim the laser beam at the primary mirror center spot. If you don't have a laser, you can get similar results using a sight tube and aligning the crosshairs to the primary mirror center spot.

As far as I can tell, the possible adjustments of the secondary are rotation (does the secondary appear circular instead of oval), position along the central OTA axis (i.e. is the outline of the secondary centered in the peephole), and tilt (is the primary fully visible and reasonably centered in FoV of the secondary). Am I missing anything?

Yep. The secondary mirror alignments you've described above should get the focuser axial alignment close. But are you close enough? Three tenths of an inch sounds like a lot, but using your alignment procedure, you'll need to either perfectly center the secondary mirror relative to the bottom edge of the focuser drawtube, or split the tolerance between the two alignments (bottom edge of focuser to actual edge of the secondary, and actual edge of the secondary to the reflected edge of the primary). That's only one and a half percent of the apparent diameters--a pretty tough read. It's doable--but it's a lot easier with the right tool.

FWIW, exceeding the focuser axial tolerance causes the focal plane to tilt enough to begin to impact image performance. Lower magnifications and eyepieces with less edge correction minimize the impact. If you're getting "snap" focus with your current alignment and you're satisfied with your scope's performance, I wouldn't worry too much about it.

#63 uniondrone

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Posted 11 August 2010 - 10:43 AM


Hi Vic, Thanks for the detailed reply. It all makes good sense to me now.

One more question: how critical is the order of the adjustments? I have already aligned the cap pupil with the primary center spot. Presumably if this was done with poor secondary alignment (i.e. tilt adjustment), then the primary axis is actually aligned with a misaligned secondary. Oddly enough, conversely, aligning the focuser axis by centering the crosshair of an alignment EP with the centerspot of a misaligned primary would result in a secondary that is aligned with poorly aligned primary. It seems like a chicken and egg situation.

#64 Vic Menard

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Posted 11 August 2010 - 12:09 PM

...how critical is the order of the adjustments?...It seems like a chicken and egg situation.

The order of axial alignment is simple: first, focuser axis, last, primary mirror axis. This is because, as you noted in your post, adjustments to the focuser axis typically impact the primary mirror axial alignment--but adjustments to the primary mirror axis usually have little or no impact on the focuser axial alignment.

Getting the secondary mirror positioning fully corrected and achieving good focuser axial alignment simultaneously can seem like a "chicken and egg situation" if the alignments are not approached systematically. I won't go into detail here because it's already covered on the first page of this thread.

But remember to always correct/tweak the primary mirror axial alignment last!

#65 uniondrone

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Posted 11 August 2010 - 03:18 PM


Thanks again, Vic! As batty as all this probably drives some people, I am really starting to appreciate the degree of control and ability for DIY that a Dobsonian provides.

#66 uniondrone

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Posted 12 August 2010 - 12:16 PM


Hi Vic,

I tried using a laser collimator, but found that the loose tolerances of the fit in the focuser creates a lot of room for error. A slight wobble in the collimater gives a movement of as much as two inches for the laser reflection on the primary. The laser striking the target on the collimator can pretty much move across the entire field with a slight wiggle. Tightening it down with the set screw shows the primary to be *way* out of alignment, despite the fact that I can visually confirm good alignment using the peephole in the collimation cap.

Any advice?

#67 Vic Menard

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Posted 12 August 2010 - 02:56 PM

...I tried using a laser collimator, but found that the loose tolerances of the fit in the focuser creates...error...as much as two inches for the laser reflection on the primary.

Any advice?

Sure. First, I hope the loose fit is caused by the laser and not the focuser, because if it's the focuser, your eyepieces will be subjected to the same focuser axial misalignment--not good. Now, if it's the laser's fault (and it usually is), and the fit is really that bad, the best option, IMO, is to replace the laser. Some users advocate taping the laser barrel to get a tighter fit, but what you really need is a barrel that mimics an eyepiece barrel, that way when you secure the laser in the focuser, it's axis mimics an eyepiece's axis. I also suspect you're using a 2-inch focuser, with a 2- to 1.25-inch adapter, and a 1.25-inch laser. A better 2- to 1.25-inch adapter should improve the fit and the alignment consistency. How the eyepiece fits the adapter, and how the adapter fits the focuser drawtube, is called "registration". It's important to verify the focuser axial alignment with the locking screws secured, the same way you would use the locking screws with an eyepiece. This "fixes" the registration. If the misalignment is repeatably consistent when the locking screws are secured, your focuser axis is indeed, misaligned!

If you have a dial caliper or similar measuring tool, you can quickly determine the cause of your registration inconsistency.

...Tightening it down with the set screw shows the primary to be *way* out of alignment, despite the fact that I can visually confirm good alignment using the peephole in the collimation cap.


"Tightening it down" only secures the registration of the laser to the focuser. Remember, the laser beam defines the focuser axis. The return beam is simply a reflection of the focuser axis. The only way the return beam can be used for primary mirror axial alignment is if the focuser axial alignment is perfect. One half of any residual focuser axial error will be propagated forward to the laser beam emitter. I'm guessing this is what you're seeing when you say the primary is "*way* out of alignment". The collimation cap, which is relatively insensitive to focuser axial errors, shows the primary mirror alignment is in fact, "good", so you're back to where you started: good secondary mirror positioning, good primary mirror axial alignment, and unknown focuser axial alignment.

Of course, even without a laser, we know your focuser axis isn't grossly misaligned, even though it may be out of tolerance. As long as you're happy with your scope's performance, you shouldn't lose good observing hours worrying about what might be wrong. Learning how to collimate your scope is all about patience and persistence, an exercise best saved for rainy days and cloudy nights (pardon the pun).

#68 uniondrone

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Posted 12 August 2010 - 03:45 PM


Vic, thanks for the advice!

You are correct that the telescope has a 2" focuser with a 1.25" adapter. I will use my calipers to measure the outside barrel of the laser collimator and inside of the adapter to see how much play is there. I do have a higher quality adapter that came with my SCT Crayford. I could try using that instead. The focuser on the Dob is the stock rack and pinion focuser used in older model Orion Dobs.

I totally agree with you on the part about not worrying too much as long as the performance makes me happy. So far it has met or exceeded my expectations. My main aims are 1) to at least have a competent understanding of all major aspects of collimation, and 2) have all adjustments close enough to tolerance to know that my view is not significantly compromised.

You mentioned using a crosshair alignment tool for aligning the secondary. Although I imagine that it would be susceptible to the same registration issues, it might actually be easier to line it up with the center spot of the already aligned primary than trying to get the laser to work. Any thoughts on this? Is there any advantage in doing this, or is it just going to give the same problem?

#69 Vic Menard

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Posted 12 August 2010 - 04:57 PM

...using a crosshair alignment tool...I imagine that it would be susceptible to the same registration issues, it might actually be easier to line it up with the center spot of the already aligned primary than trying to get the laser to work. Any thoughts on this?

IME, a good laser is easier to use than a good sight tube when you're aligning the focuser axis. And you can use the laser as effectively after dark. But I still like a good sight tube for positioning the secondary mirror.

My main aims are 1) to at least have a competent understanding of all major aspects of collimation, and 2) have all adjustments close enough to tolerance to know that my view is not significantly compromised.

:waytogo:
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#70 kev721

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Posted 31 August 2010 - 11:17 PM

Has anyone tried the $44 laser collimator from scopestuff? It looks the same as the ones that cost $69 and up from other shops.

Please let me know if anyone has tried it.

Thanks!

Scope stuff laser collimator

#71 SonOfDob

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Posted 01 December 2010 - 01:35 PM

My god,... I think you guys spend more money on collimation equipment than I did on my entire 10" start to finish,... granted the price of glass has gone up :)

Seriously though,... put a dot in the middle of the mirror, line the secondary up so you see the same amount of tube on all sides when looking down the eyepeice (position yourself 6-12" back from the eyepiece tube and you can use the inner and outer openings to align your point of view), then adjust the mirror so the center dot lines up in the middle of the properly positioned secondary.

Plywood, cardboard, elmers glue, a throwaway pair of binoculars, a few inches of plumbing pipe, teflon tacks, a lag bolt and a bit of scrap leather is about all you need to build as big a **** reflector as you can find glass to grind.

A 24" dobsonian has been made start to finish in 2 days by 2 guys :),... I just wish you could still buy US navy surpluss porthole glass in bulk!
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#72 Jason D

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Posted 01 December 2010 - 01:51 PM

My god,... I think you guys spend more money on collimation equipment than I did on my entire 10" start to finish,... granted the price of glass has gone up :)

Seriously though,... put a dot in the middle of the mirror, line the secondary up so you see the same amount of tube on all sides when looking down the eyepeice (position yourself 6-12" back from the eyepiece tube and you can use the inner and outer openings to align your point of view), then adjust the mirror so the center dot lines up in the middle of the properly positioned secondary.

Plywood, cardboard, elmers glue, a throwaway pair of binoculars, a few inches of plumbing pipe, teflon tacks, a lag bolt and a bit of scrap leather is about all you need to build as big a **** reflector as you can find glass to grind.

A 24" dobsonian has been made start to finish in 2 days by 2 guys :),... I just wish you could still buy US navy surpluss porthole glass in bulk!


Why don't you start a new thread with your thoughts.

#73 SonOfDob

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Posted 01 December 2010 - 02:27 PM

Sure,.. Why not :)

#74 Jan Owen

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Posted 22 January 2011 - 06:39 PM

At THIS stage along the way, anything *I* might say is relatively meaningless.

But still, I've been in amateur astronomy for about 45 years now, more or less (probably more, since it really dates back to the late 1940's when my dad showed me a lunar eclipse through HIS telescope, given to him by his dad).

I've been using my own telescopes for a LONG time, starting with the ubiquitous 60mm department store refractor, and working up to 24", then after massive back problems, back down to 12" and less...

My only reason in pointing that out, is that, in those years, my BIGGEST take-away has been that there are THREE things that are critical for optimum viewing (assuming quality optics, of course). Seeing, telescope thermal equilibration, and collimation.

NONE of these can be under-estimated in their importance. But only TWO are directly under OUR control. Thermal equilibration, and collimation...

Vic (and several cohorts) has done the whole family of amateur astronomy a LOT of good by putting what needs to be done, and how to DO it, down on [virtual]paper, and making it readily available here (and on their Yahoo Group)...

I've learned a LOT from Vic and friends over the years, above and beyond decades of personal experience, and just wanted to take a few minutes here to thank THEM!!!

#75 CollinofAlabama

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Posted 19 April 2011 - 05:53 PM

With all due respect to Nils Olaf Carlin, Vic Menard, and Don Pensack, I'm sorry, but all your writings have been like Polish to me (and I know no Slavic tongue -- da [or duh, in my case]). Every time I started reading one of your texts, my eyes would glaze over, and in my mind an inner demon would whisper "you know, with a refractor, you don't have to worry about all this"

But then I remembered to google YouTube, where pictures (at 24 frames a second) are indeed worth a thousand words (tens of thousands when the subject is collimation). Ladies and gentlemen, the absolute best collimation guide for the collimation-averse ...

http://www.youtube.c...be_gdata_player

Now, you need some kind of cheapo laser collimator to utilize this, but most folks with a reflector these days have one. If you don't go buy one for $50 and you're in business (or, as the Canuck says in the video, pay more for a "quality" laser collimator). He uses a cheapy, and mine, the Orion Deluxe Collimator, is probably considered cheapy, too. I don't care. Collimation finally makes sense and is EASY.

"Free at last, free at last. Thank God Almighty ..." Well, you know the rest.


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