200P - colimation or poor optics

[malc-c]

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

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

At the moment its still theory, and until an oversized mirror is installed and hopefully resolves my issue we can't really draw any conclusions as to how many scopes have secondaries like this that are affected in the same way.

Given the conversation I had with OVL it would appear that they are somewhat in denial that this coating issue is a problem either in manufacturing or quality control. But then I never set out to suggest this was an issue of such magnitude, I was just documenting a particular problem I have with my scope. If others like Aenima are experiencing the same issue then I would suggest they contact the retailer to seek redress.

I'm in negotiations with RVO at the moment, but either way hope to have a replacement larger secondary very soon. With luck this will resolve the issue and I can finally close this thread. But I have to agree with Aenima, it's had input from a lot of experienced and eminent people and I hope that other folk have learned a lot from whats been discussed in the many pages.

[Aenima]

I'm sorry, looking back over the post it might not be worded properly.

I hadnt meant to suggest that the 200p needed its mirror size changed in order to 'fix it' - simply that anyone with this scope might want to consider buying a bigger mirror for their 200p's especially imagers and meticulous visual observers.

Hope this is clearer.

Regards

Aenima

[malc-c]

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

Steve, whilst I note your comments, I have to disagree with some of the points. In the marketing literature and retailers websites (including FLO's) one of the key features for the 200P states direct SLR camera connections. If the stock 200P was not suitable for imaging and was solely a visual telescope then why advertise this feature. I could read your statement that you are saying that the 200P is not fit for purpose when used as an imaging scope, even though it is marketed with this facility to attach a SLR camera. Is that not false or misleading advertising ?

As for replacing the 200P for the PDS version, other than the reason stated above, not everyone is blessed with the funds to replace scopes or purchase additional scopes. If funds weren't a problem I would skip the 200PDS and opt for one of the CF Quattro's, but given my personal circumstances I'll have to make do the the scope I have.

I respect your view point and where your sympathies lie. I agree that some folk here are reading too much into this issue, and suggesting that the two scopes reported here are the tip of the iceberg. As it is, in the months this thread has been running, there would of been dozens of similar reports if it was the case. As there have been just the two it would suggest that this issue is more isolated rather than a global issue.

[gary1968]

_{I too cant believe that this tread is still going..... I watched with interest at the start. If it is any consolation Malc, yours is not the only scope to be showing this spike. I saw an image posted somewhere in the last couple of weeks that also showed this spike and I remember thinking at the time "I wonder of Malc ever got to the bottom of his problem" Gaz}

[malc-c]

Hi Gaz,

Yes, as I've already stated, I too didn't expect it still to be running when I first posted last summer !

My scope is not the only one affected. Aenima also reported additional diffraction spikes similar to the results I have been getting, and I've noticed a few other images posted on the forums have shown a slight similarity, although not as profound

[Aenima]

Steve, whilst I note your comments, I have to disagree with some of the points. In the marketing literature and retailers websites (including FLO's) one of the key features for the 200P states direct SLR camera connections. If the stock 200P was not suitable for imaging and was solely a visual telescope then why advertise this feature. I could read your statement that you are saying that the 200P is not fit for purpose when used as an imaging scope, even though it is marketed with this facility to attach a SLR camera. Is that not false or misleading advertising ?

As for replacing the 200P for the PDS version, other than the reason stated above, not everyone is blessed with the funds to replace scopes or purchase additional scopes. If funds weren't a problem I would skip the 200PDS and opt for one of the CF Quattro's, but given my personal circumstances I'll have to make do the the scope I have.

I respect your view point and where your sympathies lie. I agree that some folk here are reading too much into this issue, and suggesting that the two scopes reported here are the tip of the iceberg. As it is, in the months this thread has been running, there would of been dozens of similar reports if it was the case. As there have been just the two it would suggest that this issue is more isolated rather than a global issue.

I agree with the idea that just because there is a better and more expensive version of the 200p that to buy one for imaging is a mistake.

The odds of there being more 200p's out there with the same problem, after at least 3 unrelated purchases in recent months have been noted to have the 'flat bit' on the secondary mirror are sufficient that its not unreasonable to assume that more might exist.

Even so, i'm not blowing it out of proportion and said that "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....?"

It was an attempt to put the problem INTO proportion rather than anything else.

I also probably word things incorrectly and meant no offence to anyone involved or in the same position as the unfortunate retailers who simply sell what they get from their suppliers and do so in good faith.

Apologies.

Aenima

[FLO]

Steve, whilst I note your comments, I have to disagree with some of the points. In the marketing literature and retailers websites (including FLO's) one of the key features for the 200P states direct SLR camera connections. If the stock 200P was not suitable for imaging and was solely a visual telescope then why advertise this feature. I could read your statement that you are saying that the 200P is not fit for purpose when used as an imaging scope, even though it is marketed with this facility to attach a SLR camera. Is that not false or misleading advertising ?

Of course you can image with a regular 200p, you yourself a few posts back were saying how pleased you were with the result. There must be literally hundreds of members here at SGL who are imaging successfully with a 200p. And you can use a 200p-DS for visual too. All telescope are a compromise, no one telescope excels in all areas. That also applies to refractors, SCTs, Maks, etc. The 200p is best suited to visual (smaller secondary, higher contrast) and the 200p-DS to imaging (larger secondary, greater field of illumination) but both models can be used for both visual and imaging.

As for replacing the 200P for the PDS version, other than the reason stated above, not everyone is blessed with the funds to replace scopes or purchase additional scopes. If funds weren't a problem I would skip the 200PDS and opt for one of the CF Quattro's, but given my personal circumstances I'll have to make do the the scope I have.

Precisely. You assessed what was available, the strengths and weaknesses of each model, their prices too, and made a decision. You chose to buy a model designed primarily for visual then adapt it for imaging. Many others have done the same.

I agree that some folk here are reading too much into this issue, and suggesting that the two scopes reported here are the tip of the iceberg. As it is, in the months this thread has been running, there would of been dozens of similar reports if it was the case. As there have been just the two it would suggest that this issue is more isolated rather than a global issue.

If Dave's diagnosis is correct then it would almost certainly be global because the 200p, with it's smaller secondary mirror, is more likely to show that type of aberration than a 200p-DS. But your 200p's secondary mirror is not 'faulty'. If you had paid the extra and purchased a 200p-DS from your supplier would you now be criticising it for having less contrast than a 200p? If you had bought one and used it for visual, would you now be asking them for a smaller secondary?

I hope I don't sound unsympathetc because I can tell how frustrated you are but I really do feel you should change tack. I don't think you will find a solution to your dilemma, your 200p will never be a 200p-DS. Even if you succeed in obtaining a secondary mirror from a 200p-DS and fit it to your 200p, it will not be in the optimum position. Instead I think you should make plans to buy the telescope you should have chosen from the outset. Have a chat with your supplier, perhaps they will sell your telescope for you then offer a discount off the 200p-DS. Maybe they have a demo model or customer return. You might find a secondhand 200p-DS here at SGL (some 200p-DS owners upgrade to a Quattro).

HTH,

Steve

[davies07]

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.

Hi Tony,

I think the only way to eliminate the extra spike is to paint a thin black edge around the secondary that is just wide enough to cover the straight edges. This would effectively cause a whole set of small diffraction spikes to be generated off the painted edge, but they would all cancel out and become invisible. Curved spider vanes work in the same way. You would have to paint the whole 360 deg edge of the secondary to make this work.

The drawback with this cure would be that you would reduce the reflecting surface of the secondary, and, as has been noted, the 200P is primarily a visual scope and the secondary is already close to the minimum size it can have to produce an un-vignetted image. The trade-off in secondary size is that a small secondary gives high visual image contrast but with increasing vignetting towards the edge of the field of view (hardly noticeable visually) against a large secondary to give a larger 100% illuminated field for a camera, but at the expense of reduced visual contrast. Thus we have the 200P and 200 PDS models to meet these needs, as Steve has already mentioned.

By the way, I owned a 200P when I started out in this madness. I thought it was a great scope and learned a huge amount from it.

Dave

[Aenima]

Yes, well said, and I agree - I LOVE MY 200P -

I still do. When I bought it I was thinking about AP but not limiting my plan to visually use the scope as well, and if I purchased a new scope it would be as an addition to the 200p not a replacement.

Regards

Aenima

[nightvision]

I see your point Dave, well I am pleased I have a PDS! No spikes in store I hope!

Good luck Malcolm!

[Jason D]

Interesting analysis, Dave

But I have few basic questions:

1- Why would the problem go away then come back? The secondary mirror was not changed and the uncoated edge in question did not change.

2- What about the other edge? Shouldn't there be another spike at 8 degrees in the opposite (mirrored) direction for the central star? I am going with the assumption the secondary mirror is at minimally sized based on your analysis, hence, the central star should see both edges.

3- Shouldn't the stars at one end of the FOV see one edge and stars on the opposite edge see the opposite edge? Shouldn't the spikes count and direction vary depending on their location with respect to the FOV?

The design shows that the required offset of the centre secondary mirror is 3.7 mm.

How did you come up with 3.7mm? According to your numbers any my math, the proper offset should be around 2.5mm

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.

The physical offset of the secondary mirror mount is irrelevant. That is, whether the secondary mirror is mounted central or with an "away-from-focuser" offset is irrelevant. The actual offset between the secondary mirror geometric center and where the optical axis of the primary mirror hits the surface is practically the same.

Jason

[Jason D]

What about this old post

http://stargazerslounge.com/topic/159748-200p-colimation-or-poor-optics/page__st__100#entry1627649

Per Malcolm's experiment, there is a high correlation between the spider vanes and the spike location?

Jason

[Aenima]

.....and this is where the entire thing goes over my head.

I genuinely do not wish to encourage any conflict and hope that my interest in the discussion remains neutral. I'm still very impressed by all the science and maths that have come down on this little diffraction spike.

Regards

Aenima

[Aenima]

If I get the same spikes after all this faffing around then I'll try the 3/4 obstruction suggestion, otherwise I'm juts going to have to live with it

This is pretty much my thoughts now regarding the extra spike. Seeing everything malcolm has tried and countless collimations and re-builds, I think if I do a full collimation and the spike is still there - at least its in collimation.

Regards

Aenima

[Moonshane]

surely there is someone who lives near Malcolm and has a secondary from either a 10" f5 or a 8" f4 or even a 8" PDS that can pop round and let Malcolm try their secondary in his scope for a night? this might solve the problem once and for all as if the only thing that's changed is the secondary then this must be the culprit?

[malc-c]

surely there is someone who lives near Malcolm and has a secondary from either a 10" f5 or a 8" f4 or even a 8" PDS that can pop round and let Malcolm try their secondary in his scope for a night? this might solve the problem once and for all as if the only thing that's changed is the secondary then this must be the culprit?

That's a good suggestion, but I don't expect others to start dismantling their scoped to be fair, unless the mirror is a spare.

However I'm hoping to have the funds soon to allow me to purchase a quality secondary from Orion Optics, but at least Moonshane's suggestion would prove a larger mirror would resolve the issue before I shelled out £100 first

[Jason D]

Another question to Dave. See the first attachment. They include 4 photos with the spike in a different quadrant in each photo -- some spikes with different angles. Based on your theory, I would expect to see more consistency with the location and the angle for the spike.

The second attachment has no spikes. Shouldn't there always be a spike based on your theory?

Malcolm, why do you still suspect the secondary mirror if you managed at one point to get rid of the spike? A problematic secondary mirror should always be problematic.

Jason

EDIT: I just noticed that lower left corner photo is a reversed image compared to the other three but this does not change the points I made in this post.

[Ewan]

Hi again, i was just doing some investigating into the Airy Disc pattern of C8's & came across this which backs up Davids idea considerably regarding the coating on the secondary.

Here is the link http://www.oldham-optical.co.uk/Airy%20Disk.htm

The last paragraph explains that if the secondary is not fully coated it WILL cause extra diffraction lines, i am no expert but i hate to see others struggling with their gear just to be able to get the best out of it, hope this helps a bit.

[malc-c]

Malcolm, why do you still suspect the secondary mirror if you managed at one point to get rid of the spike? A problematic secondary mirror should always be problematic.

Jason

Hi Jason,

Basically because an oversize mirror is the only thing that has not been tried.

• All three mirrors (the original, the first replacement and the latest from China) all have one or more edges uncoated - The firts replacement had two uncoated area on the minor axis and two spikes were observed.
  
• I've had three spiders (original, first replacement and latest from China) - these have been fitted in various rotations with veins moved around the boss but the spike remained
  
• I've stripped the scope down, rotated the primary, rebuilt it and re-collimated and still the spike remained.
  
• The scope was originally placed on a optical test rig with an artificial star and precisely collimated - the spike could be observed optically as well which ruled out any optical issue with the camera or filters.
  

Bottom line is that given the consensus was that this issue was the result of a bent spider, having fitted three spiders, swapped veins around, ensured that they are true using a steel edge etc I find it very hard to accept that theory. The spider vein would have to be bent some pronounced amount to produce this spike IMO. There are guys on this forum who have fitted power resistors to the secondary for dew control and run the supply wires taped along the edge of one or more spider veins but they don't have the problem, these wires are far more an obstruction than any small twist in the vein that I cant detect.

Lastly, Dave has now produced diagrams and crunched some maths that seem to back up the theory that the cropping of the coating on minor axis of the secondary is the cause as originally suggested by Es Ried, who is a specialist in optics who has apparently designed and made high precision optics for commercial and military projects, so have no reason to doubt his findings.

[Aenima]

I'm just wondering about the movement of the spike, and trying to visualise what kind of obstruction could create the same spikey shape as a spidervane.

Can something as small as a screw head cause it? Or the edge of the focuser?

I have trouble imagining that spike coming from something that isnt spidervane shaped unless its actually from one of the four vanes reflected or shifted by something else.

Its not easy to picture, and i'm less than knowledgeable about these things and have no idea what the problem is - hence following the thread.

One thing that keeps puzzling me is with my extra spike its still only one spike, at a similar angle to malcs, but in the attempts to fix it ive wobbled and twisted all of the vanes - but still only one spike. By now my spider is pretty warped but the spike is still how it was to begin with. While bending up other vanes shouldn't I have caused more spikes to appear?

Sorry if this is adding to the puzzle or just plain unhelpful, I just want to try and get to the bottom of this but dont have enough know-how to contribute more.

Regards

Aenima

[malc-c]

Aenima,

Take a look at the secondary and see if that has a similar un-coated area on the minor axis

I follow your logic in that if you twist or bend any other viens then you would expect rogue spikes all over... but it juts doesn't happen

[JamesF]

I have to say that I have huge admiration for Malc's patience and persistence in sorting this problem out. I really hope you get to the bottom of it Malc, not only because I think such "dedication" (if you will) deserves a just reward, but also because once the problem is tied down the information will be of use to a much wider audience, both existing 200P owners who will know how to correct this problem if they come across it, and potential 200P owners who may be able to make a better-informed decision about which scope to buy to suit them.

James

[malc-c]

Thank you James,

I'm not normally this patient - It must be an age thing, 25 years ago I would of taken a sledge hammer to it !!

I think your comments are fair, and given what Steve from FLO has already stated, I hope that it helps future Explorer 200 purchasers chose the version of OTA best suited to their needs, especially if they intend to go down the imaging route rather than visual. Personally I would like to see the descriptions used in the literature and on retailers websites make it a little clearer that whilst you can connect a DSLR camera directly to the 200P, the stock 200P is primary for visual use rather than imaging, and that the 200PDS is more suited to imaging with a DSLR.

Having said that, it might well be that Aenima and I have been unlucky and have one of those scopes that for some reason or another are on the edge of the tolerances for the scopes design.

[Aenima]

I am kinda unlucky when buying online but usually thats down to trying to save money,

But with the 200p ive been pleasantly surprised with its performance and in the fact that it IS a budget set-up even for visual only users, and it isnt a 'bad' imaging scope necessarily, just one with less enhancements to the design.

I did check my secondary and can see the edge on one side of minor axis is flattened. I had assumed for a while that it didn't have any effect and still not sure now, but there is an 'edge' to the area rather than the expected curve.

Hope this is of help in some way.

Regards

Aenima

[Jason D]

Malcolm,

In your recent post

http://stargazerslou...00#entry1907205

the issue was resolved and the extra spike disappeared. If the secondary mirror is problematic then how could you have resolved the issue? You seem to overlook this important fact.

With respect to Dave's theory, I am interested to read Dave's responses to my questions which I posted earlier.

Jason