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How to collimate a refractor dedicated to imaging (ideally indoors)

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#1 robonrome

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Posted 26 October 2021 - 05:30 PM

Hi all, I have an APM LZOS 130/780 refractor with a collimatable lens cell.

 

I've never used it for visual and dont plan to, ever. It's setup with a Nightcrawler focusser and cooled camera, filter wheel, OAG etc virtually permanently attached.

 

I'm suspicious of the scopes collimation, but I'm so often battling tilt and spacing it's really hard to tell.

 

Whenever I search on refractor collimation what comes back seems oriented to those doing visual observing and so recommend star test with high power eyepiece using Polaris (which incidentally isn't an option where I live) which would entail setting scope up for visual first and second doing that dance of laying on the ground checking and then up and tweaking screws and back and finding it's the wrong way and back and back all while mosqitos buzz and the brief periods of clear sky dissappear (spent a lot of time doing this with an SCT for planetary)... and presumably a camera is as good as an eye anyway right?

 

So can I check collimation via the camera view as well as with an eyepiece (what should I be looking for and best approach) and is there a good approach for collimating a refractor indoors when it's raining so I dont waste imaging time?

 

Really appreciate any advice I can get here.

 

thanks

 

Rob

 

 

 

 



#2 imtl

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Posted 26 October 2021 - 05:46 PM

Rob,

 

You're looking for artifical star setup

https://www.cloudyni...llimation-r2798


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#3 Alex McConahay

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Posted 26 October 2021 - 05:58 PM

Just curious, Rob---what is the difference in collimation between visual and astroimaging?

 

I know there is a difference in the flatness of field---but is that affected by collimation? (I mean, if it is well collimated, is it not as flat as can be----not that a poorly collimated scope can have a good flat field.)

 

And it is common to use an artificial star to do a collimation indoors. 

 

Alex



#4 markb

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Posted 26 October 2021 - 06:12 PM

I've had an easy time collimating my tough to collimate homebuilt Jaegers 5 slip-over-tube and 6 slip-into-tube refractors with just the AgenaAstro refractor cheshire, with the crosshairs removed (they thread in), and half of the polished aluminum plug masked with tape (one of my most used CN suggestion/finds, heard about the AA refractor collimator here too).

 

A bright light on the polished aluminum plug and them just stack the reflections off the objective surfaces. The 1/2 masked off plug makes stacking so much easier since reflections with good coating are not overly bright.

 

It is extremely sensitive, so even tightening the cell to tube screws often makes a sizeable reflection stack shift. The downside of that type of moveable but no collimation screw type cell.

 

Night collimation tests looked perfect. With a screw type cell it could get that final tweak on the tracking mount, but mine needed no changes.

 

But, an artifical star for daytime testing is a breeze, too.

 

I have a Hubble Optics 5 star artificial star, but I most often use a chromed ball bearing, snapped to a magnet with black paper between the two for better contrast, in Sunlight.

 

I've also done it frequently with a green laser pointer illuminating the ball bearing. Even AZ gets overcast, and my trees shade the area at times. The GLP is easily aimed from your observing position. I've used my tube mounted GLP but the needs reaiming after a couple of collimation adjustment as the scope aim changes of course.

 

I've done this on the scts, mostly.  The Jaegers stay collimated if not banged around. . Star testing needs a certain minimum distanceto avoid false SA findings, but I've have no collimation issues from 30 feet or more.

 

I also got another resolution jump with a laser to square the focuser (3 tube screws holding the focuser), using a simple center dot target at the objective. Squaring is often said to be uninportant, but I found to the contrary. I doubt this applies on your commercial build, but some focusers are adjustable.

 

If your focuser is adjustable I would absolutely look at it. Even if it is not, a test won't hurt.

 

All of this was done in the kitchen, in daytime, with a wooden carpenters miter box providing a cradle for safe handling, but it works equally well on a mount if using a collimatable cell.

 

But one concern I would have with your triplet would be whether the triplet elements are properly centered and aligned/parallel, so if collimation does not do it that would be the next thing to look at.


Edited by markb, 26 October 2021 - 07:37 PM.

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#5 Alex McConahay

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Posted 26 October 2021 - 06:28 PM

You know, I had an idea. Not suggesting that your post is off topic, because it is on topic. But, you may get more knowledgeable answers in the Telescope Making Forum than down here in Imaging. Those people know stuff we don't know. 

 

Alex



#6 markb

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Posted 26 October 2021 - 06:30 PM

+1 on the Mod's link to Derek W's wonderful article. The Hubble Optics unit is in there, too if you want a look at it. Great units and reasonably priced.

 

I finally bought an orphaned secondary but they don't show up very frequently. A chromed ball bearing is well formed and polished and works great. I've used 1/4 to 3/4", and prefer the smaller bearings. Christmas ornaments are usually internally silvered and can give a double superimposed reflection.

 

The diffraction pattern to adjust for conentricity is the diffraction pattern used for star testing, just a bit off from focus. SCT users sometimes use the obstruction shadow but I only use the Poisson Spot diffraction pattern. It is very sensitive to changes, and works in moderate seeing and on artificial stars of course. I'm not sure if the article made that clear, without rereading it.

 

Breezes can disrupt the Airy disc pattern even in daylight testing indoors. That still amazes me.


Edited by markb, 27 October 2021 - 09:15 AM.

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#7 Madratter

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Posted 26 October 2021 - 06:59 PM

Get yourself a cheshire. They can be used to collimate a refractor.



#8 fftulip

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Posted 26 October 2021 - 07:46 PM

A chesire is easy to use but I find it hard to see for the last bit of fine tuning (my eyesight's not that great either).  Takahashi makes a collimation telescope which magnifies the concentric circles for easier fine collimation.  The issue is it's kind of expensive and requires a ridiculously expensive adapter for non-Takahashi scopes (hence I don't own one yet).  I also have an artificial star but rarely use it because there isn't really a long line of sight indoors for me, and outside it's very hilly where I live and trees and shrubs interfere with the line of sight.  I like the idea of a ball bearing, just have to figure out where to put it outdoors where I can see it.  Sometimes for rough collimation I'll use a porcelain or glass insulator on an electrical pole that's far away - it's not really a circular point source so I only use it for rough collimation.


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#9 ngc7319_20

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Posted 26 October 2021 - 08:00 PM

I would star with a Cheshire collimator tool.  That will tell you if the lens is square to the focuser, and basic stuff like that.

 

After that I would setup an artificial star about 50+ feet away.  A 1m ball bearing illuminated by a tiny lamp (LED flashlight?) works well -- ideally the lamp would be roughly in the direction of the scope and couple feet from the ball bearing.  (If the lamp is too large it can be resolved.  You can also get astigmatism if the lamp is off to the side.)  The spherical correction would be wrong, but you can still test for coma, astigmatism, and weird non-axis color stuff.  If you want the star farther away, you can put the ball bearing outdoors and shoot through an open window to the scope indoors (thermal issues maybe though?).

 

You can also setup a Newtonian running backwards with a pinhole at the Newtonian focus, and use that as a collimator.  Then you are testing are true infinity.  But the Newtonian needs to be a good one.  Probably to test a 130 off-axis you would need at 12" to 14" at minimum.

 

If you want to isolate coma and astigmatism, use a green filter.

 

I bought a APM LZOS 130/780 once and it had horrible coma on-axis.  Just looking at the lens you could see that the spacers were no longer aligned at 120 degrees.  It was like the elements and spacers had rotated.  I sent it back.  Eventually it went back to APM who fixed it.  Maybe have a look at your spacers if there is concern.  The spacers are little chunks of metal a few mm on a side near the edge of the lens.

 

If there is bad coma on-axis, I would probably send it to APM.  You can try centering the glasses yourself, but a 130 / 780 is probably a bad place to start if you haven't done it before.


Edited by ngc7319_20, 26 October 2021 - 08:03 PM.

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#10 StevenBellavia

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Posted 26 October 2021 - 08:00 PM

Hi,

 

I am not an expert in refractor collimation.

But  I have had great success collimating several refractors using a camera and an artificial star, which I feel is much better than performing this task visually, as you can zoom in much better digitally than by eye.

 

And collimation is somewhat different than tilt.  For tilt, I developed an artificial star field, which is discussed in this CN thread:

 

https://www.cloudyni...ial-star-field/

 

and also discussed here:

 

https://www.cloudyni...g-my-redcat-51/

 

For collimation, you only need a single artificial star, and the goal is concentric rings for the star at the center of the camera sensor,  I tend to work mostly while in Intra-focus, as it seems most refractors have clearer rings there.

 

Of course be careful, especially if there are metal screws involved which can crack a lens if pressing directly on the glass.

 

Good luck and happy tuning!

 

Steve


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#11 markb

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Posted 26 October 2021 - 08:03 PM

fftulip, just get a couple of super magnets and stick to a pole or anything steel, and snap the ball bearing onto it, with black dollar store construction paper underneath. Bearings are cheap on Amazon and at some hardware stores.

 

The back inner wall of a mailbox is a good one if using a focused light or GLP for illumination.

 

A long line of sight is only needed for Suiter testing and not for collimation.

 

As to the reflection stacking, try the black tape covering 1/2 the aluminum Cheshire plug first.

 

What a difference! 

 

You get alternating 1/2 donuts, an absolute breeze to align! A hearty 'Thanks' to a CNer for that tip, years ago. My Jaegers are such a pain, the half covered plug was a life and sanity saver.

 

No collimating telescope needed, usually.


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#12 robonrome

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Posted 26 October 2021 - 08:24 PM

thanks for the great input here. I had shied away from artificial star as had though required vast distance I can get in my house, but if it can work with as little as 25ft I can do that - thanks Eyal for that link.

 

Really apreciate your detailed ideas markb...will check out the AA cheshire.

 

Alex, the only difference from my pov is I dont look through my scope (and would need to find a way to put an eyepiece back on)... what I'm really asking is does it have to be visual or can I use my camera view to evaluate collimation and guide what I'm doing ... benefit also in I can see that on screen near where I'm working on the collimation screws


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#13 robonrome

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Posted 26 October 2021 - 11:13 PM

I'm going to try the artificial star approach... I take it if I go that path I dont need to do the Cheshire cat thing?

 

I understand from reading here and around that first step would be to check the focusser is orthoganal and a calibrated laser in the focuser existing dead centre of lens proves that... nto sure what adjustment would be possible on the night crawler if that's Not the case but guess needs checking... seems these lasers etc are all 1.25in, so it seems there's no escaping I need to take off camera and fine a way to attach an adaptor to to get to 1.25in... and that bothers me in that I've spent a lot to have wide screwed coupling in the imaging train to ensure everything is perfectly centred and no tilt to only go back to some rubbish adaptors to 1.25in ... is there no other way? Or is the focusser orthagonality test sufficient coarse it shouldn't matter?



#14 markb

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Posted 27 October 2021 - 12:00 AM

If your focuser is not tilt or otherwise adjustable I'm sure it would be fine, and I'd not bother except as a confirmation.

 

Yes, the focuser interface for a laser leaves much to be desired. Mine was checked for accuracy in the clicklock by rotating it before doing the objective target centering test for the focuser squaring.

 

My Baader Clicklock helped me with good clamping and concentricity, and my three edge screw attachment had a massive amount of screw side play, making squaring the focuser very important in my case. Even my old Carton 60mm f4.7 conversion of a old long tube Tasco was way out of square. If a focuser hadn't been a threat on type I doubt it would even have bothered to check it.

 

I have a 1.25-2 Baader Clicklock reducer, in the focuser 2" Baader Clicklock, and results were tested for repeatabiity over multiple insertions and rotations. 

 

Glatter makes a 2" laser and others are out there as well. My old Kendrick SCT laser has SCT threads that could be hard threaded with adapters but the Baader CLs work very well.

 

The AA refractor Cheshire, with blackened half plug is so easy to use for collimation I'd definitely start there, then. It might also might help in deciding if a triplet element might have shifted.

 

Them you can finish with the artificial star.

 

But I see no reason the artificial star won't work by itself.


Edited by markb, 27 October 2021 - 12:03 AM.

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#15 freestar8n

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Posted 27 October 2021 - 06:08 AM

By coincidence I recently had to collimate a refractor and I used a simple cheshire eyepiece to do it.  I have an old TeleVue Ranger that needed its lens cleaned, so I removed it and put it back - but realized I could see coma in the images.  It was fairly pronounced even though I thought the lens had been put in ok.

 

So I tried a cheshire and could see right away there was a problem - so I took the lens back out and made sure it was seated properly.  The lens has a very tight fit in the cell so it was pretty hard to get it in there all the way - but the cheshire let me know it was ok.

 

This was about a month ago and I took pictures of the process.  Below is a view of an iphone looking into the cheshire - and it is attached with an iphone attachment from Celestron that works pretty well.  Then I show a pic out of  collimation - and then after re-seating the lens so it is collimated.

 

The cheshire is very simple and just needs bright light to shine on the white wedged surface of the eyepiece.  This cheshire has a wire crosshair but I removed it for this operation.  The refractor has a lens cap on so you can see the reflections from the surfaces of the lens.

 

I don't know how you would make repeated small adjustments while collimating - particularly if you need to keep removing the lens.

 

Hope this helps - and good luck!

 

Frank

 

CameraOnScope.jpg

 

OutOfCollim.jpg

 

GoodCollim.jpg


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#16 robonrome

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Posted 27 October 2021 - 05:08 PM

very clear and helpful thanks Frank

 

By coincidence I recently had to collimate a refractor and I used a simple cheshire eyepiece to do it.  I have an old TeleVue Ranger that needed its lens cleaned, so I removed it and put it back - but realized I could see coma in the images.  It was fairly pronounced even though I thought the lens had been put in ok.

 

So I tried a cheshire and could see right away there was a problem - so I took the lens back out and made sure it was seated properly.  The lens has a very tight fit in the cell so it was pretty hard to get it in there all the way - but the cheshire let me know it was ok.

 

This was about a month ago and I took pictures of the process.  Below is a view of an iphone looking into the cheshire - and it is attached with an iphone attachment from Celestron that works pretty well.  Then I show a pic out of  collimation - and then after re-seating the lens so it is collimated.

 

The cheshire is very simple and just needs bright light to shine on the white wedged surface of the eyepiece.  This cheshire has a wire crosshair but I removed it for this operation.  The refractor has a lens cap on so you can see the reflections from the surfaces of the lens.

 

I don't know how you would make repeated small adjustments while collimating - particularly if you need to keep removing the lens.

 

Hope this helps - and good luck!

 

Frank

 

attachicon.gifCameraOnScope.jpg

 

attachicon.gifOutOfCollim.jpg

 

attachicon.gifGoodCollim.jpg



#17 rockstarbill

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Posted 28 October 2021 - 07:34 AM

Very expensive solution, but the Takahashi Collimation Scope is one tool I have in my Astro toolbox that I consider as close to a Swiss Army Knife as we can get. It's a Cheshire on steroids (with no cross hairs) but it can provide excellent collimation assistance on just about any scope and since it's high powered you can really zoom in and see things with excellent clarity.
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