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200P - colimation or poor optics


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Its a pity your having trouble with the actual supply of a different type of mirror - different in quality - as this would confirm the question of a good or bad quality secondary a lot better than repeated replacement of similar quality mirrors. (I know this was your original intention,)

Even if it only affected part of the huge batch of 200p's its still worth identifying and remedying the problem, especially when it may still be happening - my scope was bought 1 year ago last month (looked at the date because the mount alt block 'thing' happened with the soft metal last week).

(@steve) Isnt it more sensible to talk with the people who made the product and will be able to improve how they do it rather than the company that just distributes them after they are made?

Aenima

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You didn't buy your telescope from OVL. You need to discuss this with your supplier, not their supplier.

True. But when the retailer who is backing the customer gets the door slammed in their face by the importer / sole distributor then the retailer isn't getting the support from their supplier. I was advised by the retailer to contact the importer direct to voice my concerns. I must add that I have a good working relationship with the retailer who has been 100% supportive with this issue and are still trying to come up with a solution to resolve this issue.

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Just quote the supplier the Sales Of Goods Act, the supplier is liable & HAS to rectify this not the manafactuer.I forgot to say under the sale of goods act a product regardless of the warranty expiring should be working and or fit for purpose for a reasonable time if not they really should exchange the scope or refund you.I seem to remember that some secondary mirrors are slightly offset ( i think faster ones though).Hope you get it sorted.

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Just quote the supplier the Sales Of Goods Act, the supplier is liable & HAS to rectify this not the manafactuer.I forgot to say under the sale of goods act a product regardless of the warranty expiring should be working and or fit for purpose for a reasonable time if not they really should exchange the scope or refund you.I seem to remember that some secondary mirrors are slightly offset ( i think faster ones though).Hope you get it sorted.

A good point. Sounds like it might help.

I have no knowledge of the law regarding this but wont the supplier just say that after so much time has now passed that the warranty thing is applicable and therefor they are not in the wrong anymore? (though they- suppliers - aren't wrong as far as I'm concerned. If my company bought a bunch of stuff, then sold it on without alteration, then i'd be unhappy if the end customers only went as far as blaming me without also acknowledging the fact that I didnt make the stuff - I only sold it on )

I dont know. The blame isnt the main thing here - rectifying a problem is, but there needs to be someone who can take responibility and try to fix it.

Aenima

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The Sale of Good Act says a product should work as intended for a reasonable length of time as would be expected in the lifetime of the product.

Obviously very vague, but with a telescope its expected life span would be multiple years, a 12 month warranty on an item with an Inherent fault is irrelevant, you would be covered for multiple years of the intended lifespan of the product.

Key point its an Inherent Fault, not wear and tea/ user damage.

This is down to the retailer to either fix, or replace.

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The Sale of Good Act says a product should work as intended for a reasonable length of time as would be expected in the lifetime of the product.

Obviously very vague, but with a telescope its expected life span would be multiple years, a 12 month warranty on an item with an Inherent fault is irrelevant, you would be covered for multiple years of the intended lifespan of the product.

Key point its an Inherent Fault, not wear and tea/ user damage.

This is down to the retailer to either fix, or replace.

V Troo. The retailer is responsible, not the manufacturer and are bound by the law.

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Sounds like you need to be going down the small claims court route and let a judge decide the interpretation of the Sales of Goods Act.

Tricky thing will be gathering the independent evidence backing up your claim as you will need a report on the mirrors.

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Just as a note for anyone else reading this, I've been rebuilding my dob getting everything aligned properly recently and whilst blackening the secondary edges held the secondary mirror side up to a light to check my work. It was quite clearly possible for light to pass through the secondary from front to back on both sides of the minor axis where the aluminising didn't quite cover the front perfectly. I'm sure that can't be a good thing and blackened the back of the secondary to try to prevent light passing through in the opposite direction when in use. I've not got as far as testing it out yet and I'm beginning to wonder if perhaps I should have used blackboard paint rather than indelible marker, but it will be interesting to see what happens.

James

I have only just caught up with this thread and have suspected that the edge of the secondary that is uncoated had something to do with it. While Jason D is quite correct that the cone of light coming from the primary should not be large enough to hit that uncoated area, the INCOMING light path certainly will and if it gets through the solid bit of glass, the straight edge of the coating will cause a spike. I suggest that you follow James's advice and blacken or cover the back of the secondary. It is quite possible that your experiment in covering the spider vanes with a bent bit of card also covered the area of the secondary that is uncoated thus eliminating the rogue spike if that proves to be the case. After all this will cost nothing to try.

It is surprising that a manufacturer would be happy with secondary mirrors that were not fully coated. I suspect that if you had got a larger secondary from Synta then it would have shown the same problem.

Nigel

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Guys, thanks for the advice and comments.

@Kai. The small claims court would be the last resort. Especially as the cost of submitting the claim etc would be more than the £110 a high quality secondary from Orion Optics would cost. Plus as pointed out, under the sale of goods act it would be the retailer I would need to sue and IMO they have done everything they can to help. It might be different if retailers could go to two or more importers / distributors, but with OVL being the SOLE importer and distributor there are no other channels to get a 200PDS mirror that was originally requested.

@Nigel. Having seen the maths and the full sized diagrams, the area of the returning light cone from the primary does in fact hit that un-coated area. Also punching in the data into several on line calculators show that the stock 200P secondary provides less than 80% illumination across the 400D sensor. Installing a secondary with 63mm minor axis will give 100% and should eliminate these pesky spikes once and for all :) - OK the contrast might be affected, but then I use the scope mainly for imaging rather than visual. Whilst the larger PDS secondary might well have similar flat areas on the minor axis, the larger size should of meant that 100% of the light cone would be diverted down the draw tube without striking that area

@Earl. Looking back, I have an image taken when the scope was 3 months old and was as supplied by the retailer / manufacture. When examined this image also shows the spike was present. Contained within the meta data is the date, time temperature etc of when the image was taken. This I feel is good evidence that the fault has been evident since the day purchased as apposed to something I've done in the course of flocking the scope and collimating.

@Aenima. That's part of the law that IMO needs changing. My contract under the sale of goods act is with the retailer. But then the Retailer should be similarly protected as they purchased the item from the distributor. But if that distributor refuses to play ball that leaves the retailer in a sticky situation. I'm just pleased that the retailer is trying to do everything they can to help me out, but the simple solution would be for me to purchase a 3rd party secondary and put this thread to bed.

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Malcolm, the retailer BY LAW has to sort this out, it sounds like your scope has not been in use for a reasonable amount of timer, it's not even like you get to use it 2-3 times a week, normally if you quote the 'Sales Of Goods Act' to the retailer they think twice & will sort you a new scope. It would make no difference if the scope was £600 or £6000, you bought it in good faith & it does not perform as it should do.

I am not one of those people who, im not having a dig here btw, fiddle farts about, make the point & get them to sort it so you can enjoy what you bought for what it's intended.

I for one would not have been happy either regardless if i was friendly with the supplier.

Good luck with any progress you make though all the best.

Hi Earl btw :-)

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I do not think your analysis is correct. The pics of the brighter stars have all been "on axis" or close to, and that cone of light should not hit the edges of the secondary. Simply black out the secondary's back to check. Alternatively, you could put an iris on the end of the focusser to vignette the image so that the edges of the secondary cannot be seen and see if the effect goes away ( I don't expect it to ).

Nigel

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For those who have only recently joined this thread, First Light Optics did not supply malc-c's telescope. I wish we had, because then we could have offered a solution.

It is remarkable that a thread discussing one telescope can reach 17 pages and 300+ posts without a conclusion.

It would be easy for me to sit on the sidelines safe in the knowledge it is another's retailers problem. But in all honesty I am not convinced Malcolm's telescope optics are faulty. I posted earlier in this thread, and Jason-D has said the same a number of times, the extra spike is almost certainly related to a twisted or non-parallel spider vane, or other protrusion, in the optical path.

I am not sure how this thread will end but it is clear, for whatever reason, malc-c is not pleased with his purchase. We can all offer him advice but ultimately he needs to resolve this through his supplier.

HTH,

Steve

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Steve I think its safe to say this thread wouldn't be like it is if it was purchased from FLO, I recall you offering a full refund on the 250 I bought from FLO when I performed the stupid torch test, for a scope which was perfectly fine.

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I can confirm that the scope wasn't supplied by FLO. In fact somewhere in the depths of this thread it's clear who the retailer who sold me the scope was. I don't want this thread to turn in to bashing a particular retailer, nor do I want people to feel that Skywatcher produce telescopes that are not fit for purpose. IMO it has raised some questions on the quality control used in the secondary mirror production, but then maybe I've just been unlucky and have a lemon 200P. The fact that others have commented they too are getting images with the same issue would suggest that this issue is not an Isolated case. Why it became more evident after I flocked the inside of the tube I have no idea, only that my experience in telescope maintenance had evolved and that once properly colimated the fault became more obvious.

The one fact remains... after stripping the components of the telescope down and rebuilding it several times, after fitting three spiders in every combination possible (including dismantling the veins and swapping them around), and after performing more than 50 collimations using a variety of techniques and performed by three different people the issue still remains. I find it incredible that having changed all the components of the spider people are still suggesting that a twisted or bent vein is the cause. The only thing that remains consistent is that every mirror used has had flat areas on the minor axis, hence the reasoning to replace the mirror with an oversized mirror. This would either prove the theory that the flats on the mirrors are to blame, or rule that theory out once and for all. Replacing the secondary for an oversized one is the only thing that hasn't been checked.

I also have two people who have worked in the optics field who have tested the scope and also back the theory that it's optics rather than mechanical that's causing the issue, and they have backed up their claim by crunching the numbers and producing diagrams from those results which replicate the results I'm getting. Whilst I would love this theory to be proved right and I end up with a fully functional 100% error free scope, I won't be that disappointed if it proved Jason and those who agree with him were right after all in that the mirror is not to blame. However given everything that has been tried, if it still present after installing an oversized mirror I have no idea what else to try other than throw the OTA in the skip and get a refractor !!

I must admit that when I posted that first post asking for suggestions I had no idea that the thread would run to 17 pages and be over 300 posts, over approximately nine months.

Maybe I should of purchased the scope from FLO, but then if they had done everything RVO had done they too would be in the same position as OVL would not be offering any further assistance to them either. Maybe they would of handled things differently, but I've been happy with the support RVO have and are offering in a bid to make me a happy customer again.

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Well hello everyone, I thought it was time I put in my 2 cents worth to this discussion. I have been working with Malcolm and Es Reid on this problem for the past few months.

Going back to the beginning, Malcolm posted images showing an extra diffraction spike on bright stars. Here is one in monochrome and inverted.

8727222287_ef630d70fa.jpg

Malc_c_4_invert by drdavies07, on Flickr

You can measure the tilt angle of the spike as about 8 degrees.

8728341452_d2c4886dcf.jpg

Diagram 3 by drdavies07, on Flickr

We have replicated this effect on Es Reid's optical bench. We used an artificial star - the tip of a ball point pen illuminated by a tightly focussed halogen lamp. We could see the additional spike by eye.

So the question is: what artefact is in the light path at right angles to this spike that is causing it, remembering that diffraction spikes appear at right angles to the feature that causes them.

This is a diagram of Malc's first secondary mirror:

8727222431_0b596964f0.jpg

Diagram 4 by drdavies07, on Flickr

You can see that the top and bottom edges of the minor axis have straight edges, where the silvering stops. It appears as if the mirror was held in a jig when silvered and the edges of the jig overlapped the edge of the mirror surface, preventing complete silvering.

The following diagram - labelled Diagram 1 shows the geometry of the light cone (shown green) hitting the surface of the secondary mirror. This view is looking down on the edge of the secondary, the focuser is in the downwards direction and the primary mirror is to the right.

8727222527_92f9c63d36.jpg

Diagram 1 by drdavies07, on Flickr

This is a pretty standard diagram and the important parameter for the geometry is the F number of the scope. For our F5 200P scope the edges of the light cone are at 5.7 deg to the horizontal. Note how the light cone hits the secondary. The top part of the cone hits the secondary closer to the primary - and therefore at a greater distance from the axis of the light cone, than does the bottom edge of the light cone. This leads to the requirement to offset the secondary by shifting it about 4 mm in the diagonal direction.

The geometric centre of the secondary is therefore offset - shifted upwards and to the right in this diagram. This offset also moves the positions of the start and end points of the marks on the minor axis. I've sketched these as points A and B in this diagram.

Diagram 2, below, is a view of the secondary mirror as seen from the camera, looking down the focuser tube.

8728341614_b5160ef10d.jpg

Diagram 2 by drdavies07, on Flickr

We now see a circular secondary and we can trace the positions of the start and end of the mark, Points A and B, on its edge using the two construction lines from Diagram 1. These are shown on the diagram.

The mark now appears tilted due to the offset of the secondary and the perspective effect of looking at the secondary mirror when it is tilted away from you at 45 deg. The tilt angle of the mark to the horizontal is 8 degrees. You can try this for real by cutting out a picture of the secondary, complete with marks, and inspecting it when tilted away from you to make it appear circular. The two marks no longer appear parallel but converge.

My conclusion, therefore, is that the marks on the edges of the minor axis cause the additional diffraction spikes. The diffraction spikes arise at right angles to the projected lines of the marks on the secondary mirror as seen by the camera. Two marks lead to two spikes, like a backwards letter K.

If you should collimate the scope with the primary mirror set well back in the tube, you could have a situation where the light from a central star just misses the mark and shows no additional spike. But an off -axis star will show a spike and Malcolm has already demonstrated this.

I wasn't content to rely only on diagrams to measure the tilt angle of the mark, but have described the geometry in a set of equations. I've put these in a spreadsheet and explored what the key parameters are. My findings are:

The 200P scope has its secondary mirror at 750 mm (as near as we could measure) from the primary. This leaves 250 mm to the focus point directed down the focuser tube by the secondary. The tube radius is around 110 mm resulting in a focus point about 140 mm from the tube surface. With this geometry the light cone on the secondary is 50 mm across, nearly filling it. The 100% illuminated field on the camera sensor is pretty small - I think the 200P is, after all, primarily a visual scope. The design shows that the required offset of the centre secondary mirror is 3.7 mm.

The math shows that the tilt angle of the mark as seen from the camera varies with the actual offset of the secondary. The design calculations show an offset of 3.7 mm is needed for the 200P secondary, and the resultant spike angle would be just over 6 deg. If the offset is set to 4.5 mm, however, then the spike angle increases to 8 deg. So I can't be absolutely certain in terms of numerical values because we haven't measured what the actual offset of Malcom's mirror is.

Hope that helps.

Dave

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But in all honesty I am not convinced Malcolm's telescope optics are faulty. I posted earlier in this thread, and Jason-D has said the same a number of times, the extra spike is almost certainly related to a twisted or non-parallel spider vane, or other protrusion, in the optical path. Steve

Haven't put my glasses on yet!

Read your Member Title as Reassurance is useless!

I'd agree that the fault can be replicated by poorly aligned spider vanes, I did it my self when first collimating my dob.

However, if several people have tried over 50 times to collimate this 'scope, I'd suggest that they're pretty good at the basics by now!

I'm finding this frustrating Malcolm, you must be sick to death of the whole thing.

I sincerely hope you reach a resolution.

Cheers

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Dave, brilliant piece of work there! It got me thinking, this is not me proposing a fix but there is something that could be tried to provide a practical confirmation of your proposed cause.

Given that the effective convergent angle seen on the non-silvered/silvered border results in the 8 degree spike it would be easy to confirm this by changing that angle to 0 degrees. This could be done by masking the un-silvered/silver margins with an 8 degree divergent border. This would deliver an effective 0 degree border and place any resulting spikes behind the spider induced spikes, not a fix but proof and a temporary work-round.

Interested in your thoughts as I have the same scope (sourced from FLO but currently packed away) awaiting the completion of my obsy so I will be looking for a solution/work-round if I have the same problem.

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Haven't put my glasses on yet!

Read your Member Title as Reassurance is useless!

Oops! The "Resistance is futile..." was a throwback to when we launched Borg telescopes at FLO :smiley:

It would be easy for me to sit on the sidelines safe in the knowledge it is another's retailers problem.

I think I'll retreat back to the sidelines...

Steve

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That's a very interesting bit of work there Davies07 + Es Reid. Well done.

In my post I had assumed ( clearly I was wrong, only just now having checked with the SW web site ) that the secondary was significantly larger than the central cone of light ( as is normal practice ) and it would therefore not be affected by the unsilvered edge. I would now concur that it is that unsilvered edge as being the most likely culprit. I can easily imagine the production of these secondaries includes an aluminising jig that consists of two bits of angle spaced sufficiently far apart so as to prevent the secondaries falling off when they are put into the coating chamber. This way many secondaries can be coated at one time. They will receive both the Aluminium and Silica coatings except at the edges that are resting on the jig thus providing a small step on the surface of the secondary which provides the diffractive element. Thus all secondaries produced at this facility will have the uncoated edges as has been found so far.

My calculation for your telescope, using the figures in Davies07 post gives a 100% illumination at the focus of just 3mm dia. In view of the intended application, AP, I expect that you will need a larger secondary anyway so that should solve the problem.

Nigel

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Oops! The "Resistance is futile..." was a throwback to when we launched Borg telescopes at FLO :smiley:

I think I'll retreat back to the sidelines...

Steve

Yes, I enjoyed your Borg Collective add!

But don't retreat to the sidelines, FLO has a well deserved, highly respected reputation here on SGL and I for one wish I'd bought my 'scope from you even though I've had no problems!

Cheers

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Thats quite a breakthrough Malc, very impressive. :)

Its brought a lot of highly intelligent people to bear on a difficult to solve issue that has implications for stargazers whichever theory is correct and looks like it is really shining a light on a simple and hopefully fixable problem with the machinery that holds the mirror while coating is applied. (or similar situation in factory setting).

Thinking over the timescale and having the 'extra spike' 200p its funny to imagine our scopes being cousins, with possible 'unknown siblings' out in the world somewhere :p

It does now look like the larger mirror would be a great benefit for imagers and meticulous visual owners of those mirrors. Maybe a quick check to get an idea how many uncoated mirrors made like that could be useful to various suppliers and related retailers, even though its obviously a manufacture fault. There is a small chance that to admit to a mirror defect in a certain batch would not be a major domino effect that they might be worried about, it could be less than 50 or better....?

Anyway, for now, I'm understandably sympathetic but kinda glad your original theory had merits - it was very intriguing to watch it unfold and many clever people chipping in knowledge and experience made it stay that way for me at least :)

As usual I'll be watching this space.

Aenima

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It does now look like the larger mirror would be a great benefit for imagers and meticulous visual owners of those mirrors...

Yes, but Malcolm 'chose' to buy a regular Explorer 200p and not the more expensive Explorer 200p-ds (which has a larger secondary mirror).

When Synta noticed the steadily increasing number of 200p owners using their telescopes for imaging they reconfigured the design and released the 200p-ds with a dual-speed focuser and larger secondary mirror positioned closer to the primary mirror for a greater field of illumination. It also makes it easier for a camera to achieve focus.

Malcolm's 200p is designed primarily for visual observing, but he is using it for imaging. He has asked his supplier to send him the larger secondary mirror from the more expensive 200p-ds to 'fix' his Explorer 200p. If I were Malcolm, as soon as I knew I was going to use it for imaging I would have sold the 200p and ordered a 200p-ds. If he had then he would now be imaging with a telescope better suited to his purpose.

This situation is not as dramatic or as profound as some are saying and I am still not convinced malc-c's 200p has 'faulty' optics so am sympathetic towards his supplier.

Steve

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