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malc-c

200P - colimation or poor optics

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Thanks guys...

@acey, Yes I've seen that site in the 1st link, To me the extra spike is an issue. Take a nice image of M45 and each star has the same 5th diffraction spike... it's very noticeable and IMO looks false, as if you've cut and pasted the same star over and over again !

Thanks for your kind word's regarding my work situation

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It's great news you've managed to get a replacement for the secondary. I really hope things work out well for you, and hope everything re work is sorted. :)

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If they've seen fit to replace the secondary and spider for nothing, it sounds like they suspect that's the problem. Let's hope it is. You must be the SGL scope rebuild and collimation king by now :)

James

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Great news re the scope Malcolm. Not so much about the job though, hope you get that sorted out soon.

Gaz

Sent from my U8815 using Tapatalk 2

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Well that was quite a read!

I hope that the new secondary sorts the problem out. My gut feeling would be an issue associated with the drawtube extending into the tube and stray light catching an edge and giving an extra spike, but I think Shane suggested this early on in the thread?

One thing though, make sure when taking your test shots that the mirrors are cooled properly, especially these nights when the temperature is dropping rapidly. You can get all sorts of weird warped mirror effects and distortions, especially if your primary mirror clips aren't set properly.

It might also be useful to take a photo of a bright star moderately defocused to the same amount both in and out of focus and see what artefacts show up in the donut image.

If the problem persists despite the new spider, pm me, i'm not good at remembering to come back to threads :s

Remember when testing the focuser with a laser to run the drawtube to both extremities and observe the spot movement if any, and note and change or jump caused by locking the drawtube, stock focusers tend to shift the position of the tube a little when locked.

I have had five fast newts and can recommend the Catseye twin pupil autocollimator and BlackCat chesire as a worthy investment. Combined with a Hotech and webcam you have a formidable arsenal for collimating :D

Cheers

Tim

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Well a week after sending the mirror and spider I received a parcel today. OVL had sent a nice new mirror and spider for me to fit. Strange thing is that even though the spider had been wrapped in bubble-wrap, placed in my original box, which in turn was placed in a large box filled with loads of styrene chips, the veins are bent !!

Anyway I would like to thanks both RVO and OVL for their assistance and will report back soon on how well the new parts perform

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Well the spider is in, centred by placing a strip of plasitcard against the tube and marking off the centre bolt position from each side, until the distance was the same for all 4 arms of the spider. Then fitted the new secondary and collimated the scope. Here's the result taken with a web cam (visually through a cheshire it's more precice !)

Just need to get a clear night now to do some tests to see if this has cured the problem, or not. It will certainly remove the spider and secondary from the equation if not !

Again, I would like to publically thanks Ian at Rother Valley Optics for organising the replacement, and Robert at Optical Vision for the fast tern-around and supplying the replacement parts. Quality aftercare service alround - Thank you both

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Good luck you deserve a break

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:(

Not a happy bunny - here is the 1st image again vega - worse than it was before !! - ARRRRR !!!

I'm going to completely strip the thing down at the weekend (and throw it all in the bin -....... No! ) and start from scratch one one time - If I can't then resolve the problem the scope will go up on fleabay and I'll let someone else with more patience than I have to sort it out!!

This is really bugging me... and I'm beginning to really get disheartened with this astronomy lark - I really do regret the day I chose to flock this thing

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can you take an image down the tube when you have everything on the scope and are in correct focus position. I have always been convinced this is the focuser drawtube intruding into the lightpath and said this early on in the thread. I'd be interested to see where the focuser drawtube is to at least rule that out.

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another option is flocking that has lost its grip and has risen up into the light path.

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Well it must be enormously frustrating, but at least you can rule out the spider and the secondary completely. It must be something else.

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Guys, thanks for the comments - I'm going to strip the thing right down and perform a re-assemble. I'll take pictures of where the draw tube is when things are in focus as requested, but as the focal point of the primary remains unchanged, and there is only a small amount of travel available on the secondary bolt that IMO wouldn't cause the draw tube to interfere to this degree - but I hear what you are saying and at this point open to all possible suggestions. I can confirm that the flock is fully adhered to the inside of the tube - there are no stray bits flapping about in the optical path.

I'm more inclined now to think it's the squareness of the setup - as if the tube has been pulled out of shape by the flock or, the focuser drawtube is off axis, even though I could centre the optical axis using the laser on the measured centre spot opposite.

We'll see.

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Bummer!

One thing to check when you do the reassembly - is the PRIMARY centred in the OTA?

If it's off to one side it's possible to get collimation but you may have something (such as the focuser or a mounting bolt) in the light path.

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Could it be the camera? Maybe you could try a different camera and that could sort it.

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the light can realistically only be 'diffracted' by something when it is on its way to the primary as the light cone reduces dramatically on its way from the primary. the only exception to this is possibly a baffle inside of the focuser drawtube perhaps or a bright edge etc to the drawtube. do you use a dew shield? or perhaps wires to a dew heater on your secondary? (although I suspect you won't have put such a wire on the new secondary yet if you do).

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Bummer!

One thing to check when you do the reassembly - is the PRIMARY centred in the OTA?

If it's off to one side it's possible to get collimation but you may have something (such as the focuser or a mounting bolt) in the light path.

Thanks for the tip. The primary had been removed and put back after the last rebuild, but there could always be a possibility that the mirror is not dead centre

Could it be the camera? Maybe you could try a different camera and that could sort it.

Thanks for the suggestion, however I've already tried two dSLR cameras, with and without clip filters, and with and without coma corrector fitted - no change was observed.

the light can realistically only be 'diffracted' by something when it is on its way to the primary as the light cone reduces dramatically on its way from the primary. the only exception to this is possibly a baffle inside of the focuser drawtube perhaps or a bright edge etc to the drawtube. do you use a dew shield? or perhaps wires to a dew heater on your secondary? (although I suspect you won't have put such a wire on the new secondary yet if you do).

Thanks also for the comments. There are no other obstructions on the spider - no wires or heater elements. The draw tube was originally flocked, but then I removed the flocking and sprayed the inside with matt finish paint.

I have received an offer through my local society to take the scope to a chap who's profession was working with optics, and has some dedicated test equipment so we can check the optics and hopefully get to the bottom of this issue once and for all. Having said that, please keep any suggestions coming.

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that sounds great Malcolm. good luck and I hope you get things sorted soon.

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Well.... what an interesting morning we had.

Having called in to David's we both set off to meet with Es at his Cambridgeshire workshop. On the way there David informed me that Es has been designing optics for military and private contractors most of his working life and, although he probably wouldn't admit it, is internationally acclaimed in his profession, so I was really hopeful that we would get to the bottom of this issue. Anyway, to cut a long story short, after a quick coffee, the scope was mounted on a test rig and using some of the most sophisticated test equipment, Es proceeded to test and eliminate various possible causes. I was please to hear that the primary was in good order, so that left the secondary as the main suspect. Having explained that this was recently replaced and that the resulting diffraction artifacts were different , Es tried a few other options, but then on inspecting the secondary made a discovery that could explained what was happening, and then went on to prove his theory.

The edge of the secondary on the minor axis appears slightly flat, as if the mirror was held in a jig whilst being coated. The tolerance of my scope, in relation of the focuser position with the secondary, appears such that when collimated, the edge of the secondary on the minor axis is right on the edge of the reflected light cone produced by the primary, and with this "cropped" edge on that part of the mirror being the way it is, the diffraction pattern is what occurs. The fact that this replacement has a worse diffraction pattern than my original would suggest that my original secondary was slightly larger, or only had this defect on one side.

Anyway, the fact that this issue is still present after using two secondary's suggest that there is an issue with the scope's manufacture, more precisely that it is right on the edge of the tolerances of its geometry. I would need to move the focuser a fraction nearer the edge of the tube, which in turn means the focal point is pulled back a tad further inside the tube, and then would need to shorten the draw tube to prevent it causing obstructions inside the OTA. The only alternative is to fit a larger secondary.... which is what I'm proposing to do following consultation with RVO.

Fingers crossed !

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That's fascinating, Malc. I'm surprised that they size the secondary such that even what must be a very small flaw right on the edge causes a problem, but really good to have found what appears to be the problem. Guess that means you can re-flock everything now :)

James

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James, I guess with modern mass production, there is always a chance that one scope slips through the net... I can only go by the results Es found, and hopefully RVO and OVL will be able to work with me to try and resolve this. It's a shame that I can't simply purchase a replacement OTA and be done with it....that would be the simple option, but my personal circumstances are such that I'm not in a position to do that at this moment in time.

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Could it just be that the mounting holes for the spider were drilled one or two centimetres too close to the primary? Re-drilling would mean that the secondary intersects the light cone at a slightly narrower point and would clear your problem.

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