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malc-c

200P - colimation or poor optics

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Guys thanks for the input.

I've checked the spider veins using a length of thin cord held taught between the centre of the tension screws and it runs parallel to the veins, so as far as I can tell everything is as square and straight as I can tell with the tools available. There is no lifted flock or oversized bolts. All being well it looks like tonight will provide the opportunity for some more testing, I'm going to try some images, moving the tension screws on the spider slightly between shots to see if the detraction gets worse or better.

Jason, for some reason the second image on your post above showing me what your were referring to has come up with a "no permission" and doesn't display the picture...!!

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Jason, for some reason the second image on your post above showing me what your were referring to has come up with a "no permission" and doesn't display the picture...!!

Let me try again. I am still trying to figure out how to use the new software of ths site.

This is the photo I have been trying to upload.

farpoint1

The following animation shows you the impact of the central unreflective area. It will cause some key reflections to disappear prematurely. If the unreflective area is small then the impact will be minimal. If it is large then the impact will be larger. Autocollimators have to meet higher standard to achieve that extra collimation perfection; otherwise, their use will not add value.

R2 animation

And another more sever case showing the importance of understanding how to use the autocollimator and how the size of that unreflective area can have an impact. In the following animation you can see how the second reflection of the center spot can disappear when collimation is way off. Also note how the autocollimator background is darkened even when collimation is way off. The animation also shows you how the offset pupil of an autocollimator can catch the problem. The second reflection of the center spot does not disappear from the offset pupil in this case. I do not mean to over-complicate things. With quality autocollimators, proper knowledge of their use, and with little practice you can achieve great collimation.

P 2

Jason

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When in the editing box, click on "More Reply Options" just below the bottom right corner.

In the advanced editor click on "Browse" below "Attach Files" to the lower left.

Having selected your file click on "Attach This File"

After it has loaded - click inside the editing window - then click on "Add to post" (which will only appear after loading)

Job done :)

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On my scope the stack clearly shows four images of the centre spot and when stacked the middle goes darker.

What is the shape of your center spot?

When I rotate the alignment of the stack will separate but they all stay within view.

Can you approximate the distance between the two farthest reflections in terms of the center spot diameter?

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When in the editing box, click on "More Reply Options" just below the bottom right corner.

In the advanced editor click on "Browse" below "Attach Files" to the lower left.

Having selected your file click on "Attach This File"

After it has loaded - click inside the editing window - then click on "Add to post" (which will only appear after loading)

Job done :)

WOW!!!!! Thank you.

Let me test it... Attached is an animation showing how the second reflection of the center spot is formed between the autocollimator mirror and the primary concaved mirror.

Jason

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WOW!!!!! Thank you.

Let me test it... Attached is an animation showing how the second reflection of the center spot is formed between the autocollimator mirror and the primary concaved mirror.

Jason

post-5330-0-56775400-1346697995_thumb.gi

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Ok, it's been and interesting evening. David came over and we've been pulling the scope apart in order to resolve this..... bottom line ..... still no change !!!!

  • Spider checked for straight veins - using a taught cord against them there was no obvious signs of bending
  • Scope re-colimated (looked like Jason's avatar)
  • Star test - despite the poor seeing the rings were concentric and smooth one side of focus, but racking the draw tube inwards the star was dancing around just prior to focus, then through focus continuing inwards the shadow of the secondary was offset with rough airy rings - this was repeated with Davids 6mm Televue Radian eyepiece
  • Primary mirror rotated in its cell by one screw division - still same artifact (we had to re-colmated after the rotation used the Hotech)
  • Placed a large lump of folded paper on both East / West spider veins - no dramatic change in the subsequent image)
  • Physically displaced the secondary by loosening one spider tension and tightening up the opposite side - whilst the normal defraction spikes were doubled up, the artifact spikes remained unchanged.

(Imaged to follow)

One possible cause is dirt / stray light on the optics, so the plan is to strip and clean the optics and then paint the sides of the mirrors with black paint and then re-assemble and go through this all again :)

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Forgot to add - we've also tried Davids Canon 450d and had the same artifact was present in the test images

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I reckon you've got a Spike Fairy. :)

Blimey but you'd have thought all those checks would have revealed something!

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Tell me about it !

On leaving David mentioned that another possible area could be the focuser....this was flocked too, so maybe taking off the flock inside the draw tube will be the simplest way to rule that out !

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A thought.

Do the nature of the spikes change when the star is off-axis compared with exactly on-axis?

The on-axis star sees just the thickness of the vane, but an off-axis star intercepts the projected depth of the vane, so the vane appears thicker to this star.

Interesting discussion on CN here:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5185134/page/0/view/collapsed/sb/5/o/all/fpart/1

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Oh my brain hurts !!

Thanks for the links guys. I'll sit down with a strong coffee and have a read at lunch time !

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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?

could my original suggestion be close to the mark?

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Malcolm, I've been following this thread from the beginning and I'm intrigued to see what the outcome is, I'm sure it won't be long before you crack it (the spike that is, not the scope!!!) :grin:

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Are the spider vanes fitted properly? If they are parallel but one is twisted or kinked it could cause a diffraction like the one you are seeing.

Is it one of ours? If it is we'll arrange a solution :smiley:

HTH,

Steve

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Steve, regretfully I purchased it from elsewhere, and it's almost 18 months old so wouldn't be under warranty. If you could PM me the cost of a replacement Spider it would be appreciated

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I'll happily do that but it might also be worth contacting your supplier. The 200p is a popular model so they might have a customer-return they can cannibalise for you.

I am guessing you have twisted or kinked one of the spider vanes when you removed and refitted the secondary mirror assembly?

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Steve, there is nothing obvious, so any twist is minimal - and when we drastically pulled it to one side or placed a Vee shaped wadge of paper over the veins we didn't see any dramatic change to the defraction pattern.

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Your telescope is fitted with 0.5mm secondary mirror supports (to reduce diffraction spikes). With vanes that thin it doesn't take much twisting to effectively double it's thickness/profile when observed from the front. And the thickening would not be consistent across the full length of the vane, it would be thicker at one or both ends.

I could be wrong, without the telescope in front of me I can only guess :smiley:

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Right, day off work booked and I'm in my wife's good books as we've already gone to the places she wanted to visit, so now I can spend some time in the sun sorting out the scope. It's stripped, and I mean stripped ! - The primary and secondary are removed from their cells and we've purchased some pure soap and distilled water to follow Astro-baby's tutorial on cleaning the mirrors http://www.astro-baby.com/TAL%20Telescope%20Rebuild/Telescope%20Mirror%20Cleaning.htm

I've made a paper template and as far as I can tell the centre spot is bang on, however she downloaded the spotting template from Catseye website, but it appears that these are only available to purchase - can anyone point me to a link where the template can be downloaded from ?

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Scope has now been re-built and re-colimated. The centre spot was checked and is centred from the factory. The main and secondary mirrors are now spotless having been washed. The spider was checked for squareness and the veins for level, and to my eye there is no sign of any warping of the spider veins.

The only main change I've made was to remove the flock from the focuser tube and re-painted the inside. Not sure if the flock inside the focuser was causing anything, but it did have a slight overlap on the join.

now waiting for Vega to come out and play to do some testing

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