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Old 24-10-2011, 09:03 PM
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Shiraz (Ray)
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additional alignment tool for fast Newts

Tandum (Robin) recently started a very interesting thread on aligning an f3.8 Newt. Thinking about the issues of getting it just right with such an instrument, it seemed that most Newt alignment procedures get the mirrors and focuser properly harmonised, but largely ignore the alignment of the optical system with the tube itself. To get some info on this aspect, I built up this little gadget to put a bright LED at the focus of the optics (an LED torch should work OK if it has a wide enough beam). When a sheet of translucent paper is placed over the open end of the OTA, the light column and shadows of the secondary optics can be seen. This makes it very easy to see if the light column is central in the tube and:
- if any component is optically aligned, but still not in the optimum position for peripheral rays, particularly the secondary offset, skew or longitudinal positioning,
- if any components, or the OTA itself, intrude into the light column, causing diffraction and distortion in star shapes,
- if the spider vanes are twisted or otherwise misaligned; it is easy to very accurately align the spider using a ruler directly on the shadow pattern.

It is a very simple gadget and I assume that something like it has probably been used by others in some form or other. It is a comforting confirmation if the alignment is perfect, but I managed to improve the alignment of both the secondary and the spider vanes on my f4 within 5 minutes of getting it going - it provides information on aspects of alignment that are difficult to test in any other way. Probably also worth making a version with LEDs at the positions of the corners of the CCD chip to have a look at vignetting. Regards Ray
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Old 24-10-2011, 10:09 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Ray.

What a good idea!

I'll try this on my 16". The scope has a rotating head which is always difficult to collimate, this gadget should help me track down those misalignments.

Thanks,

Justin
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Old 25-10-2011, 06:31 AM
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Hi Justin. Be interested to know what you find. I am still trying to work out the best way to use it. Regards Ray
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Old 25-10-2011, 05:26 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Ray.

I plan to do this on the next cloudy night, which there's been lots of recently in the Clare Valley!

I hope to do an observation and image of the supernova in IC 4901 tonight.

I'll keep you informed.

Thanks,

Justin.
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Old 27-10-2011, 07:09 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Ray.

Your idea works a treat!

Here are two images of the 16", it shows I think?

1.Primary Mirror tilt.

2. Primary mirror not central, although I measured and it is central to the box.

3. Most likely, my trusses are slightly off and I'll need to adjust them.

In any case it's ggoing to be a lot easier to work out the problems now.

Thanks again.

Justin.
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Old 27-10-2011, 10:12 PM
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Shiraz (Ray)
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Hi Justin. Really appreciate the feedback.

I am also finding it quite difficult to interpret exactly what the test shows - but it certainly can pick up some misalignments that do not show up with central ray collimation, but may still affect image quality.

Looking at your images, agree that the trusses would seem to be the main suspect, but at least the gadget will quickly show the effects of any changes you make to them. Might it also be worth checking the longitudinal position of the secondary? - if that is out, the normal collimation process could require that the primary be angled slightly away from the tube axis to compensate. Anyway, good luck.

Regards Ray

Last edited by Shiraz; 27-10-2011 at 10:38 PM.
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Old 28-10-2011, 01:04 AM
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bmitchell82 (Brendan)
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Thinking about what is happening with the light, you won't really find primary mirror tilt with a led in a cap at the focuser its not directional its broad beam

Primary mirror not central? mmmm still the beam spreads its not directional it doen't give you a clear cut measurement. Use a set of vernier callipers if you are very worried about it and measure the distances.... how can you be wrong ?

If you want to see if your truss's are stright eg the tubes are aligned get a piece of string.thread it past the primary mirror (use a small paper clip tilt the tube and slide it down) hold it taught, if your tube is out you will instantly see it not guess because the string should touch the tube all the way down if its bent it will either touch it in the middle or on the ends with a gap that can be seen. if its in the mm range. don't worry about it the collimation routine will take that up. This is how you would align the rear sproket to the front sproket of a push bike or even a motor bike.

A ruler will tell you if your spider vanes are straight much more reliably and quickly than a few pieces of baking paper laying over it

I commend you on trying to find a handy tool but yeah we have the tools already that do the job very well.

Brendan

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Originally Posted by tilbrook@rbe.ne View Post
Hi, Ray.

Your idea works a treat!

Here are two images of the 16", it shows I think?

1.Primary Mirror tilt.

2. Primary mirror not central, although I measured and it is central to the box.

3. Most likely, my trusses are slightly off and I'll need to adjust them.

In any case it's ggoing to be a lot easier to work out the problems now.

Thanks again.

Justin.
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Old 28-10-2011, 06:43 AM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Brendon.

I certainly agree with what your sayng about measurement, and most of this has been done. The problem with this design is that there is a whole new level of complication with a rotaing head on a truss mount.

I know with Ian Bedfords 30" telscope, (which I based my design on). He spent an extrodinary amount of time with the same problems and his engineering is perfect.

I also agree that the scattered light will not give pefect alignment, but it will make it easier to see where the adjustments are required.

Cheers,

Justin.
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Old 28-10-2011, 11:07 AM
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bmitchell82 (Brendan)
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No problems mate, but Ill garentee you that if you do that string trick you make the adjustment its done. I don't like doing things that don't have a solid base line to work from and hence if you use the string method i spoke about your tube will be aligned no ifs or buts.

Also for visual you don't need to be mm perfect it does not matter. My newts mirror is not central I know it isn't i can see that it clearly is to one side, I know the effect it has on my images as the center of the light cone is ever so slightly off to one side. But i need to make a part to sort that out properly which will happen in due course!

Good luck
Brendan
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Old 28-10-2011, 03:01 PM
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Shiraz (Ray)
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Hi Guys.

Actually a cone of light is the ideal way to align a focusing mirror - that's why the Barlowed laser was invented for primary mirror collimation.

But this device is not intended as a primary alignment tool. It is mainly for the person who has a standard f4 Newt with a CC, collimates it perfectly with whatever system and then finds that the ##$% thing still will not produce round stars on a CCD. If this story is not familiar, you will not gain anything from this discussion (and you probably don't have an f4 scope) - unless you have a non-standard scope like Justin. If it sounds familiar, you either have misalignment problems that are messing up the star PSFs or you have dud optics. This device can help you see any misalignments that are not exposed by the collimation process, before you race off and buy new mirrors.

As an example, my f4 system has a centered secondary and it relies on the offset collimation process where the optical axis of the primary is angled off from the axis of the tube to introduce the appropriate secondary offset. Works fine for visual and collimates perfectly under a star test, but the light column is offset far enough that it catches the edge of the OTA tube when imaging over a largish CCD - adding significant diffracted energy. Dim stars are fairly nice round shapes, but bright stars have odd-shaped PSF skirts which vary over the FofV (example crop below). A few minutes of playing around with the alignment tool showed what was going on and what has to be done to fix the problem - the secondary has to be properly offset and I will have to make a new secondary mirror holder. In less than an hour I had solved a problem which had been pretty much a mystery for a year.

Finally, this device actually provides an excellent way to align the secondary vanes - the projection method puts the vane shadows in the same plane as a ruler, so there is no possible parallax error - and any twist in the vanes shows up as an increase in the width of the projected shadow - dead easy and very accurate.

Regards Ray
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Old 28-10-2011, 03:47 PM
adman (Adam)
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Ray - looking at Justins images, I would imagine that you could get the appearance of an offset primary from a couple of sources:

1. If the primary itself is actually tilted
2. If the primary is not centred in the tube
3. and in the case of a truss design, if the top part of the tube is offset relative to the base due to misalignments in the trusses?

Are you able to tell using this tool which one it is?
BTW looks like a handy addition to the standard collimation tools!

Adam
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Old 28-10-2011, 05:11 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Adam.

I plan to go through scope next week.

I'm going to start with the primary mirror, rechecking it central to the box. Then I'm going to level the mirror in the box regardless of collimation.

Then I'll check the secondary mirror axis making sure it's central and parrallel to the rotating head.

I'll check again with Rays gadget.

If the mirror is stil not centerd, the trusses will be adjusted, and if all this works! I'll collimate with the laser.

Keep you informed of progess.

Cheers,

Justin.
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Old 28-10-2011, 05:24 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Brendon.

With a rotating head it's very important to have the primary mirror centred.

If you don't have it aligned with the rest of the axis you can never have good colimation.
You may not notice it visually but you definitely do withe finder scope. This being set to one point of collimation, if this is out then when the head is rotated the finder is out.

The other thing, is that this has focal length of 2.1m so a little out on collimation is a long way out with a rotaing head.

Cheers,

Justin
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Old 28-10-2011, 09:09 PM
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Hi Adam,
The primary mirror in Justin's scope is definitely tilted a bit, but it has almost certainly ended up that way as a normal outcome of the collimation process. The gadget can only show you this sort of issue, but it cannot indicate directly what the cause is (unless it is being used to identify intruding diffraction sources). However, it does give you some idea of what components to try adjusting if you identify that there is a problem (see below). I am not sure how generally useful this is going to be as a tool for Newt owners - anyone with a Newt above about f4.5 will be wondering why bother, since their systems tolerate typical alignment errors - its possibly only the owners of really fast Newts (or ones with rotating heads) who will see it as useful. Whatever, it certainly helped me to quickly understand what was going on with my own f4 scope.

Justin. If the scope is collimated, the collimation process will have aligned the primary so that it's optical axis meets the focuser axis on the surface of the secondary (and hopefully somewhere near the centre of the secondary). If the secondary is in roughly the right longitudinal position, the column of light coming out of the OTA should be symmetrical about the primary's axis (provided the LED is on the focuser axis - rotate it to check that the column does not move too much). As an aside, the LED acts as a nearly point source and it will not matter much where it is pointing, as long as it is positioned near the centre of the focuser aperture and generally aimed towards the secondary - it shares this insensitivity to pointing with the Barlowed laser.

It should be possible to walk the axis of the primary across to the axis of rotation of the head by adjusting the spider to move the secondary and then recollimating after each move - when the light column is centred after a move/collimate, the optical axis should also be centred (note that the secondary shadow should be offset a bit away from the centre of the column at this point). Provided none of the mechanical components are grossly out of square, this should be all that is necessary to get the optical axis and the axis of head rotation to coincide. At least I think this is the case...I am having some difficulty getting my head around the complications of a scope with rotating upper section! Will be very interesting to see what you come up with.

Regards Ray

Last edited by Shiraz; 29-10-2011 at 08:02 AM.
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Old 29-10-2011, 06:44 PM
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tilbrook@rbe.ne (Justin Tilbrook)
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Happy camper!

Hi,
Success!!!
Ray your gadget is a great confirmation tool.
Here’s what I did.
1. Took your advice Ray, and started with the secondary mirror making sure it was central and parallel to the axis of rotation. This took some time using the laser collimater. It turns out the laser needed collimating it, was off by 5mm over 2.1m. I also had to allow for the less than perfect fit of the laser into the focuser, I tightened and centralised as best I could. Once this was done I rotated the head and was happy to see only a 2mm wonder on the primary mirror.
2. Next I checked the primary mirror for centre, adjusted slightly.
3. Checked the trusses, all measurements were spot on.
4 Used the laser again, adjusted the primary mirror tilt till the beam coincided on the secondary and back up through the focuser. Rotated the head and only had the initial 2mm wonder, which I was very happy with.
5. Used Rays gadget again for confirmation of mirror tilt, see results in before and after images, a difinite impovement.
I’m one happy camper!
Cheers,
Justin.
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Old 30-10-2011, 08:38 AM
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Congrats Justin - really good to hear that you have had a successful outcome. Would it be worth trying to remove the residual 2mm by tweaking the secondary a tiny bit more - or is that close enough?
I agree with your assessment that this is a confirmation tool. Thanks very much for your help in sorting out just what this thing actually does. Regards Ray

I have been doing a bit of experimenting myself and conclude that, for conventional Newts:
1. this device is an adjunct to normal collimation tools - it will tell you how things are aligned and can indicate how to adjust the secondary (or possibly the focuser or primary if offset sideways), but the standard collimation procedures should always be used after any tweaking.
2. if you have a Newt with a central secondary, the light column will be tilted towards the focuser as a normal outcome of collimation. If you have a Newt with offset secondary, the light column should be central. In either case, don't mess with anything if the column of light does not run near any hardware in the input aperture or focuser extension (but remember that you need to allow half the diagonal of the CCD for clearance at the edge of the column).
3. if the light column offset is great enough to allow unwanted diffracting objects to intrude into the beam (easy to see), you can only adjust by moving the secondary and recollimating - you cannot fix anything by adjusting the angle of the primary in isolation.
4. If you are using a very fast Newt with a central secondary for imaging, it is worth considering a remake of the secondary holder to introduce some offset - turn it into an astrograph with a central light column.
5. and if you have gone to the massive trouble and expense of making one of these things, you may as well use it to accurately set your spider vanes as well.

Last edited by Shiraz; 30-10-2011 at 10:18 AM.
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Old 30-10-2011, 11:14 AM
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tilbrook@rbe.ne (Justin Tilbrook)
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Hi, Ray.

My first issue is the laser collimater fit into the focuser housing. I will probably hahe to turn up a new 2" to 1/1/4" housing to get a better fit.

Once this is done I can chase down the last few mm.

Thanks,

Justin.
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Old 25-06-2015, 08:49 AM
ntk.thava (Hamish Thava)
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I have just went through "Hell and Back" on my RC14 and discovered some stuff to share with you all. Some of you RC owners might have come across the DSI collimation procedure and have tried it. I have been trying to use it for well over a year, starting with my RC10 and have failed miserably until last night. I found out for my RC14, the on-axis coma procedure works according to the document but the off‐axis astigmatism required a slight modification to what is mentioned in this document. Instead of doing it out side of focus, I needed to do it at focus. At focus, I can clearly see the "flat stars" and "pointy stars". Since this is a GSO RC, there is not "pull screw" for the secondary mirror cell. We only have a "push screw" Therefore when they say tighten "pull screw" this means unscrew "push screw". I did this adjustment in almost real time in Maxim doing a continuously exposure of about 2s and that moment happened with just a very slight (abt 1/16th of a turn - like how we always read but never could understand what they mean ...). Like magic, the star field was balanced.

I just thought that I should share this with you all in case you might want to try it out.

Cheers and clear nights...
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Old 25-06-2015, 10:59 AM
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ZeroID (Brent)
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This system worked for me on the RN 8F8 build.
To keep weight and size to a minimum the rings are all very close to the optical axis and perimeters so any primary misalignment shows easily. Also aligning the wire spider was a mission in itself.

I had one of those Led torches like a small cylinder. It just happens to be nearly 1'25" diameter and slides in and locks nicely in the 2" to 1'25" adapter.

A piece of paper over the front of the OTA showed me where the cage and rings were intruding and got the primary aligned down the middle and also showed a minor mislaignment in the OTA rings, easily resolved. A centred cross on the paper got the secondary into the right position.

Still had to adjust the secondary position a bit as it got closer to the axis, one of the issues with a wire spider being the three axis movement any retensioning has on it. Now it's in place though nothing moves.

With the longer FL of the scope (f8 = 1600 m ) any mirror misalignment is magnified more. I found I was making too coarse an adjustment when I first started and overshooting the spot. Final tweaks were almost infintessimally small. I'm using my neoprene rubber block 'springs' which don't move so not expecting it to shift any time soon.

But the system certainly works.
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Old 25-06-2015, 12:02 PM
glend (Glen)
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Reading back through this thread it seems to be all over the place with approaches, and certainly f8 RCs are not 'fast newts' by definition but we all have issues with the light cone. As a newt and refractor builder myself , I assemble my strut rings on a bench jig and while it is set to align all the rings perpendicular to the central laser axis, only the front and rear rings really matter as they are the optical mount points. I don't agree with Brendan's 'String Theory' because the centre deflection of a tube is inmaterial as long as its not intruding into the light path, and the front and rear plates are aligned. You might have a cone error later on when on the mount depending on how you attach your scope to the mount but this can usually be tuned with dovetail screws. I do use strings for Ray Diagrams to plot baffle diametres in my refractors but in newts if you make your 'tube' of sufficient diametre they are not required. Production newts always seem to allow for a 3 degree widening of the light path towards the front simply by making the tube wider than required for that mirror, don't believe me - then use the string to test that. Tubes are always at least 12mm wider than the mirror carrier. Strings have some use in ring alignment but if your jig is straight its only confirmation of the jig.

I agree that any laser used must be perfectly collimated itself before being used and over as long a distance as you can manage - I use over double the focal length of my longest scope ( so 5 metres), with the laser needing to stay within a 10mm circle during a 360 degree rotation and point plotting. An accurate laser is essential. If you can't collimate your laser then get one that you can collimate.

Finally, if your laser is loose in the focuser tube don't just torque the lock screw as this forces it off axis and you will collimate to that. A better approach is to shim the laser tube to just squeeze into the focuser - a piece of thin plastic works for me. I found the plastic backing on adhesive velcro strips sold at Bunnings is just the right shim thickness for my GSO focusers. I recommend collimating with a laser on the bench where the laser can sit vertically in the focuser tube, this prevents gravity sag and allow accurate centering in the focuser tube - and it is essential for tuning focuser alignment if your doing that.

Just my 2 cents on the subject.

Last edited by glend; 25-06-2015 at 12:31 PM.
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