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malc-c

200P - colimation or poor optics

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A while back I flocked the inside of my 200p and spent an age trying to re-colimate the scope. It all seemed fine, but whilst I get the secondary squared up with the focuser, etc and the laser spots the centre exactly, I've noticed strange defraction patterns on the stars. I've used a mask to get focus spot on, but in this shot you can see that there are additional "spider" defraction lines (the most prominent is at 5 o'clock position)

Any ideas ?

post-10726-0-40560700-1345666503_thumb.j

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Is it possible that the spider vanes are bent or twisted?

James

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Here's the best "star focus" - not quite sure, but it looks like the colimation is out, or the spider is off axis ?post-10726-0-73725300-1345667538_thumb.j

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Diffraction spikes cause by the vanes that hold your secondary mirror. Not a lot you can do about it, Newtons do that I'm afraid

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Here's the best "star focus" - not quite sure, but it looks like the colimation is out, or the spider is off axis ?post-10726-0-73725300-1345667538_thumb.j

From the look of your pic it looks like the spider is out and not centralised

Matt

Sent from my iPhone using Tapatalk

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Yes. My understanding is that each part of the spider causes two diffraction spikes. When the spider is correctly set up, pairs of these eight diffraction spikes overlay each other, so it looks like you only have four. If you're getting eight, as your initial image suggests, I'd suspect a problem with the spider.

James

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thanks guys... looks like a stripdown of the secondary set up is in order this weekend

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A couple of assumptions here.

First the secondary mirror is central on the mirror holder.

Second. The scope build means the secondary mirror is ideally intercepting the reflection from the primary. That is no overspill, and little or no dark (non illuminated) edges on the secondary.

If the above are true, or nearly so. 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?

Looking at the photo, the secondary mirror looks to be in the wrong place - off centre. Besides the double diffraction spikes suggesting a badly set spider.

Hope this is useful.

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Hope this is useful.

Very much so... Thanks for the comments.

Given that the next few days are wet / cloudy I'm going to strip the optics and re-assemble, taking my time and using digital calipers to measure spacing. When I flocked the scope the mirror cell was removed, and I'm sure the mirror itself was left in the cell, but I will check to see if it's centred in the cell correctly

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well after some three hours of faffing around I'm giving up - I've watched so many video's on collimating newtonians - tried to fathom out loads of images and I'm still confused (dot com!)

I started by removing the spider and secondary then checked the focuser for squarness by following one of the Astronomy shed vids on you tube. Placed the spider in the tube and with some 5mm bar in place used some plasticard to centre the spider which with the laser collimator in place was confirmed by its beam striking the middle of the bar. The bar was then removed and the secondary replaced. I then used a combination of laser, cheshire and sighting cap to try and get the optics collimated. The attached images are not fantastic, (taken with a cheap webcam) but when viewed with the eye through the cheshire, all the circles are concentric, with the spider running through the middle.

Only a clear night and some more testing will show if my efforts have improved the sharpness of the images... comments wellcome, or if there are any collimation experts in the Stevenage / Letchworth area that wouldn't mind sorting my scope out in exchange for as much tea or coffee they can drink I would love to hear from them :)

post-10726-0-69940100-1345917358_thumb.p

post-10726-0-20249100-1345917378_thumb.p

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Forgot to mention, the second picture a poor attempt to overlay both images to show the centering of the circles

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Don't have anything to add that hasn't already been said.

Have you checked that the spider vanes aren't slightly bent, perhaps from a tightening. I would venture that the problem lies with the vanes and not with the rest of the optical path.

Good luck fixing the problem, let us know what the culprit was when you fix the scope.

Regards,

Eigen

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Not sure but I thought on fast newts (F5 and quicker) the secondary was supposed to be offset and not concentric (once struggled with that one on my 12" dob)?

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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?

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Not sure but I thought on fast newts (F5 and quicker) the secondary was supposed to be offset and not concentric (once struggled with that one on my 12" dob)?

Yes this is true, fast newts will have an offset secondary assembly. Opening poster, how well is your laser colimator...well colimated?

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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 !

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

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.modernastronomy.com/accessories.html#accCollimation. Dion has done a video review of the Farpoint http://www.astronomyshed.co.uk/forum/viewtopic.php?f=33&t=11787. I was up for one myself, but I've had to postpone it for a while.

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I was watching Dion's review of these collimators and he really summed it up when he said that whilst it's nice to get this precision, it's effectively wasted given the tolerances of the standard mass produced focuser. If you have a moonlight or similar precision focuser then this would be a worthy accessory, but for the "quality" of the SW optics and focuser you would never maintain the precisely stacked patterns.

I've just pointed the scope at the sun and it's resolving the structure of the few spots that are present, but the real test will be to point the scope at Vega and see what the diffraction pattern is like

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The Catseye tools are worth it. If you buy an autocollimator then buy one from Catseye. Some of the others (I don't remember which ones) aren't well made and don't work properly (i.e. they are less effective than a Cheshire) because the mirror isn't perpendicular to the tube (as it should be).

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David, thanks for the PM, and subsequent time and assistance - Hopefully if I get a chance this week I'll see how good the collimation is and post up the results

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