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malc-c

200P - colimation or poor optics

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OK managed to do sometesting and something is still not right, even after using the barlowed laser method.

The spider vanes are tight and using the Dion way of centering, all equidistant. I still get that same artifact.

However on pulling focus right out, it's clear that the collimation is still out, or something is off as the result is not circular

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post-10726-0-19297400-1346186197_thumb.j

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David, I have no idea !

Just removed the secondary and re-collimated again - now to me the spikes are thinner, but there is a strange offset glow around the star.... and that additional spike at 5 o'clock !! - ARRRRR !!!

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Hi Malcolm. I know you have tried most things but I'd give collimation another go.

If you follow the instructions to the letter in this link http://www.propermotion.com/jwreed/ATM/Collimate/Chesire.htm you may get results.

I know the title sounds basic, but it has helped out many. Sometimes it's a simple missed step in the sequence that messes things up and hard to trace.

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Tony, I'll give it a try. I also received a mail from David who has agreed to lend me his new collimator as featured in Dions video. So between that guide and using this precision auto-colimator it should remove the issue of bad colimation causing the strange defraction patterns. Here's hoping

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That should also add another string to your bow in solving this issue, I've heard good reviews about it.

I will be ordering a new moonlite focuser for my MN190 soon and the auto collimator you mention to accompany it.

I won't get it yet, it's to sensitive for my current Crayford focuser.

Good luck and keep us posted.

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For fast scopes you may need to be set up like this...

fast%20collimation.jpg

OK, came home from work and stripped the scope of its focuser and secondary mirror. Went back to basics and checked the squareness of the focuser using the excellent tutorial from Dion on youtube, pleased to say it was still spot on from last time. I then took my time and over a couple of hours now have the view down the cheshire looking exactly like the above illustration. This was achieved using a sight cap, cheshire and Hotech laser.

Now on to the controversial bit... David popped over and loaned me his newly acquired Farpoint auto collimator. Placing the tool in the focuser It looked way out, with 4 doughnuts in a line and with a strange crescent reflection to one side. However, carefully rotating the tool in the focuser this vanished and the doughnuts lined up ! - rotate it further round and the crescent re-appeared as did the multiple reflections. I studied the 15 or so pages of a cloudy night thread and was still none the wiser... for all I know it could be correct as one image seems to suggest a correct secondary alignment as it looked like one of the images in the thread...

So depending on where you rotated the tool it went from collimated to out of collimation... OK during a conversation David stated he has a moonlight focuser fitted to his 10" Quatrtro, but I think on a mass-produced scope like the 200P, the tolerances in the precision of the focuser and its housing is such that using such a precision tool like an auto-collimator only makes matters worse as it only serves to highlight the weakness in the stock focuser and you end up chasing your tail trying to reach the goal. Anyway, given that the view through the cheshire resembles that shown in the images above, and the final result in the link Tony listed I'm resisting the urge to tinker further (its also late and I'm tiered and that's a recipe to damage things). I'll place the OTA back on the mount tomorrow and see what results I get when we have the next clear night. If I still get strange diffraction spikes then there must be something else wrong in the optical path, one of which could be the camera or CLS filter. As David has also loaned me his 300D and LP filter I should be able to get to the bottom of these issues. I've also purchased a baader coma corrector which might also throw some things into the mix... fun times ahead :)

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Aye, once you get off the beaten path of "standard collimation" because of some fault or other life can get interesting to say the least.

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Then the mirror has to be offset from the tube centre. A little bit away from the focus tube. This is because the light cone is wider nearer to the mirror. I don't know if this is the effect in your scope. But careful measurement of the spider will show a small difference in length to/from the focus tube. Those at 90deg to the tube should be equal length.

This effect is negligible on a long FL scope, but significant on a 'fast' scope.

This explanation looks clumsy. I'm sure pictures would explain it much better. Or maybe someone else can describe it better than me?

This is a common misconception. A centered mirror can intercept more than 99% of the light cone.

The need to offset the secondary mirror away from focuser has more to do with improving DSC accuracy and avoid front aperture vignetting.

Jason

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Lack of edit is a pain!

I completely agree. I can't edit a simple grammatical mistake in my last post. Oh well...

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I think that the extra single spike is created by something else in the light path. it's opposite the spike which makes sense I think (and assuming the second photo is the same orientation). could it be that your camera focus position puts the drawtube into the light path?

I agree.

Typically, additional spikes are caused by intruding screw heads around the central the secondary mirror holder. Sometimes it shows up only on one side due to the way the secondary mirror is position with respect to the primary mirror.

Jason

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Thanks for the input guys...

@Eigen - I use a Hotech self centering laser which as far as I can tell is spot on (sorry no punn intended). I also use an Orion optics precise self centering adapter in the focuser, so the whole lot should be solid and central in the focuser. When you guys say "offset" what needs to be offset ?

I've been looking at these Infinity XLK collimation aids which removes any of the second guessing, but they are not cheap !

This is another misconception. It is intuitive to assume if the laser beam hits the primary mirror center then traverses its path all the way back that the whole setup is collimated. Not true. Positioning of the secondary mirror under the focuser can't be optimized with a single beam laser. After all, the single beam laser does not interact with the secondary mirror edge therefore how can it be used to ensure the secondary mirror edge is centralized under the focuser. That is why a holographic attachment is needed to optimize positioning the secondary mirror under the focuser.

Jason

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If the autocollimator reflections go from un-stacked to stacked when you rotate the tool then I believe that means the tool isn't made properly and can't be trusted. What you're seeing is probably due to the mirror in the device not being perpendicular to the focuser axis.

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This is another misconception. It is intuitive to assume if the laser beam hits the primary mirror center then traverses its path all the way back that the whole setup is collimated. Not true. Positioning of the secondary mirror under the focuser can't be optimized with a single beam laser. After all, the single beam laser does not interact with the secondary mirror edge therefore how can it be used to ensure the secondary mirror edge is centralized under the focuser. That is why a holographic attachment is needed to optimize positioning the secondary mirror under the focuser.

Jason

Reposting with larger font

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I agree.

Typically, additional spikes are caused by intruding screw heads around the central the secondary mirror holder. Sometimes it shows up only on one side due to the way the secondary mirror is position with respect to the primary mirror.

Jason

Reposting with larger font

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If the autocollimator reflections go from un-stacked to stacked when you rotate the tool then I believe that means the tool isn't made properly and can't be trusted. What you're seeing is probably due to the mirror in the device not being perpendicular to the focuser axis.

This is my feelings - The autocolimator is a very snug fit in the 2" focuser, which has been squared up using Dions Tutorial. I even rotated the Hotech in the Orion self centering adapter to conform the center spot as marked remained under the dot of the laser. From what I can see, the Farpoint is a tight fitting sight tube with a flat mirror drilled for the viewing hole, so in theory I would of not expected to see a change in the reflections ? Don't get me wrong, I'm very thankful of David for loaning it to me, but I'm not 100% confident in its use.

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Yeah, it sounds wrong. In fact, you don't need to square the focuser in the tube (at least not with any great accuracy). If the alignment signatures using your tools are good then that means the focuser is "squared" with respect to the primary mirror. That's the whole point of collimating the scope. If you can collimate your scope and have everything look good then squaring the focuser in the UTA is irrelevant.

The reflections should unstack minimally if at all when you rotate an autocollimator. Thinking about it again, if that's not happening then really the only possibility is that the tool is faulty.

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There's also the Farpoint Autocollimator which, while expensive, is cheaper than the Catseye stuff. Bern at MA has recently started stocking the Farpoint http://www.modernast...#accCollimation. Dion has done a video review of the Farpoint http://www.astronomy...hp?f=33&t=11787. I was up for one myself, but I've had to postpone it for a while.

Be careful when purchasing an autocollimator. All have an eyepiece case, a mirror, and all show multiple reflections of the primary mirror center spot. But that does not say much about the precision of the tool.

An autocollimator has to meet a high standard of quality and precision. If it lacks precision then it will not provide any additional collimation improvement beyond a mass produced laser collimator.

Here is a detailed criterion to evaluate any autocollimator:

1) How many center spot reflections can you see with reasonableclarity? Four is excellent. Less than four is poor. Note that two of the center spot reflections might look fuzzy. That is OK. The degree of fuzziness is dependent on vision acuity and the scope focal length. The smaller the focal length the fuzzier the two reflections. Seeing the fourth center spot reflection will enable a procedure called “CDP” introduced by Vic Menard. CDP will make it easier to align the focuser axis with high precession.

2) Unstack center spot reflections then rotate the autocollimator in the focuser without tightening any focuser setscrews. Test it at 90, 180, and 270 degrees. To validate the test, rotate the autocollimator 360 degrees and check – repeat few times. If reflections stay close to their original location after each 360 degree rotation then this test is valid. Do reflections shift significantly at 90, 180, and 270 degrees compared to their original locations? If yes then the autocollimator quality and precision is poor. It is almost impossible for any autocollimator to pass this test with 100% accuracy. This is a highly sensitive test. Small shifts by around ¼ the diameter of the center spot is acceptable. However, if the shift amount is larger than to the diameter of the center then the autocollimator is of poor quality. This test checks the squareness of the autocollimator mirror against its casing and it checks for registration errors of the autocollimator with respect to the focuser.

3) Look for distorted center spot reflections. If some of the center spot reflections are noticeably distorted then that is an indicated of poor autocollimator mirror flatness. If some of the reflections have a slightly different size but otherwise are undistorted, that is OK. Size difference is an indication the autocollimator mirror is located away from the focal plane.

4) When the scope is collimated and the autocollimator background reflection is dark, how dark is it? The darker the better. Gray’sh background is an indication of poor autocollimator mirror flatness and possibly poor reflective surface.

5) How centered is the central pupil of the autocollimator case? Uncentered pupil will introduce errors.

6) How large is the central pupil? Around 2mm is good and reasonable. Larger pupil hole will introduce parallax errors.

7) How large is the non-reflective area around the central pupil from the autocollimator mirror side? This is important. If this area is large then a tangible error will be introduced. The reason is somewhat complicated but I will try to summarize it. A large non-reflective area around the central pupil will cause two of four center spot reflections to disappear prematurely. That is, as all four center sport reflections start merging, two of the reflections will quickly and prematurely disappear and you will be left with two reflections. Stacking two center spot reflections is far from being acceptable. You need to stack a minimum of three reflections.

8) The reflective area of the autocollimator mirror has to be free of artifacts – especially around the central pupil. Again, the reasoning is somewhat complicated but I will try to summarize. As reflections merge, if the reflective area around the central pupil has rough surface or poor reflection then the background will suddenly brighten reducing contrast and two center spot reflections will potentially disappear. That will lead to confusion and poor collimation.

9) How large is the reflective surface? In general, 2” autocollimator should be used when possible. The large reflective area will make it much easier to see all center spot reflections. For 2” autocollimators, it is desirable to have thinner case walls and more reflective area.

10) Is it a single or dual pupil autocollimator? Not only the dual-pupil autocollimator,provides more accuracy but it will also catch errors explained in items 7 & 8 of the above list.

Unless an autocollimator meets the high standard explained in the above list, it might not provide more collimation accuracy.

Jason

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Many thanks for the write up Jason, very informative and a lot to think about before I decide to purchase one.

And there I was thinking this was all very simple, a new Moonlite focuser, autocollimator bish bosh and sorted.

I think I'll leave it a while and see how I get on with my tried and tested method. :undecided:

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Dion has done a video review of the Farpoint http://www.astronomy...hp?f=33&t=11787. I was up for one myself, but I've had to postpone it for a while.

I watched Dion’s video () and I have few concerns.

When stacking all center spot reflections, you need to be aware that two reflections will disappear prematurely. This will impact accuracy. Using the “CDP” method (introduced by Vic Menard) is one way to get around this issue.

I never liked the analogy between autocollimators and two opposite flat mirrors. Placing two flat mirrors opposite to each other is very different from placing a flat mirror at the focal plane of a concaved mirror. The mathematics is much more complex and very different. For those who have used an autocollimator, why do consecutive reflections of the center spot maintain the same size???? Shouldn’t they get smaller with each round trip like you would expect with two flat mirrors opposite to each other!!!! Why only four reflections of the center spot can be seen????? Shouldn’t we see an infinite number of center spot reflections!!!!! Why some center spot reflections are sharp when others are fuzzy???? Shouldn’t they all be sharp!!!!!! Why do two reflections disappear prematurely when users attempt to stack all four reflections????

Each center spot reflection has a meaning and a purpose. Do not think of these reflections as you would with two flat mirrors placed opposite to each other. Reflection 1 will tell us how far the focuser axis from the primary mirror center of curvature is. Reflection 2 will tell us how parallel the autocollimator mirror to the primary mirror is. Reflection 3 will tell us how far the focuser axis from the primary mirror center spot center is.

When the autocollimator background is darkened, is it an indication of collimation??? The answer is NO. I can darken the autocollimator with a collimation error of few centimeters. Nils Olof Carlin and Vic Menard have been posting about this misconception for years.

Jason

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I watched Dion’s video () and I have few concerns.

When stacking all center spot reflections, you need to be aware that two reflections will disappear prematurely. This will impact accuracy. Using the “CDP” method (introduced by Vic Menard) is one way to get around this issue.

I never liked the analogy between autocollimators and two opposite flat mirrors. Placing two flat mirrors opposite to each other is very different from placing a flat mirror at the focal plane of a concaved mirror. The mathematics is much more complex and very different. For those who have used an autocollimator, why do consecutive reflections of the center spot maintain the same size???? Shouldn’t they get smaller with each round trip like you would expect with two flat mirrors opposite to each other!!!! Why only four reflections of the center spot can be seen????? Shouldn’t we see an infinite number of center spot reflections!!!!! Why some center spot reflections are sharp when others are fuzzy???? Shouldn’t they all be sharp!!!!!! Why do two reflections disappear prematurely when users attempt to stack all four reflections????

Each center spot reflection has a meaning and a purpose. Do not think of these reflections as you would with two flat mirrors placed opposite to each other. Reflection 1 will tell us how far the focuser axis from the primary mirror center of curvature is. Reflection 2 will tell us how parallel the autocollimator mirror to the primary mirror is. Reflection 3 will tell us how far the focuser axis from the primary mirror center spot center is.

When the autocollimator background is darkened, is it an indication of collimation??? The answer is NO. I can darken the autocollimator with a collimation error of few centimeters. Nils Olof Carlin and Vic Menard have been posting about this misconception for years.

Jason

And again with the proper font size. I do not know why some of my posts end up with small fonts. Unfortunately, I can't go back and edit those.

Jason

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Jason, thanks for the write up. I felt quite bad suggesting that Davids Farpoint may have a build quality issue, but from the gist of your post it would seem quite common for these devices to have, shall we say, non perfect optics.

I'm curious. Whilst I appreciate the fact that I have, in comparison to some makes and models of telescope, a cheap, mass-produced device which whilst it will work will not give the results of something like a planewave etc. But when you do own $120,000 scope, where collimation must be very precise, how do they achieve this, both in the factory and in the field. My guess is that if they do use these sort of auto-colimator devices then they are probably a few thousand bucks/quids and have the precision to overcome the change of patterning when the thing is rotated through 360 degrees. - So purchasing a tool for less than £80 and expecting it to do the same sort of job is probably asking / expecting too much.

Ok, lets hope we get a suitable window to test my re-collimating efforts again, and also do comparison between my 400D and Davids 350D (with and without filters) to see if the spike I was originally getting goes away.Or of it's something I'm going to have to live with.

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The Catseye autocollimator doesn't cost much more than the Farpoint and it works perfectly. The double-pupil Catseye is all the autocollimator you'd ever need.

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Jason, thanks for the write up. I felt quite bad suggesting that Davids Farpoint may have a build quality issue, but from the gist of your post it would seem quite common for these devices to have, shall we say, non perfect optics.

Hello Malcolm,

You mentioned in an earlier post:

Placing the tool in the focuser It looked way out, with 4 doughnuts in a line and with a strange crescent reflection to one side. However, carefully rotating the tool in the focuser this vanished and the doughnuts lined up ! - rotate it further round and the crescent re-appeared as did the multiple reflections.

When you rotate the autocollimator, how far apart do reflections move away from each other? If the farthest distance between any two reflections is more than the diameter of the center spot then I would be very concerned about the quality of that specific autocollimator. After all, what would be the point of spending effort to stack reflections properly to find out all reflections are way out of alignment when rotating the autocollimator by 180 degrees!!! It would not make sense.

Jason

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