200P - colimation or poor optics

[Jason D]

What appears to be the issue with my scope is that when colimation is achieved, the secondary is not intersecting the cone of light correctly, and the resulting "clipping" is causing the refraction patterns with my scope. .

Can you elaborate?

When good collimation is achieved, the whole light cone is intercepted for central stars; otherwise, you are miscollimated.

Even if you do achieve perfect collimation, off-center stars will typically have their light cones clipped by the secondary mirror at low/medium magnification.

See my earlier post in this thread

http://stargazerslounge.com/topic/159748-200p-colimation-or-poor-optics/page__st__200#entry1702434

Jason

[malc-c]

Jason, here's a quote from the optical engineers report. In a nutshell the quality of the secondary is poor, and suffers the same issues as the original, only the original was to a lesser degree !

the prob was caused by the two uncoated areas either end of the diagonal minor axis. These appear in perspective when seen from the focus and thus give two distinct angles of diffraction that increase with decreasing F ratio each varying in intensity across the vertical (in the test orientation) field of view.

Similarly, the smeared surface gave a fan of diffuse diffraction from the line structure affecting the wavefront.

The belt and braces approach is to try an over sized secondary such as the one used in the PDS, which is where we are today... only with things the way they are it will be January before it arrives.

[FLO]

The secondary mirror from the 200p DS is not only larger than the one fitted to your 200p, it is also normally fitted closer to the primary mirror.

I don't know who your optical engineer is but you are very fortunate to have Jason D posting in your thread. He knows more about collimation than anyone else I know and is advisor to the industry.

I so wish you had bought your telescope from us Malcolm. Your thread is 12 pages long with 226 posts and over 5,000 reads. And now you say you must wait til January! It must be very frustrating for you and it gives the wrong impression to others because the 200p is probably the UK's most popular 8-inch reflector and one of the most trusted.

If I were you I'd let your supplier and OVL know about this thread.

I hope your situation soon improves

Steve

[BuzzBomb]

I've been following this thread with.interest and have certainly learnt a lot.

Whilst. I agree that Jason D does know a lot, I do cringe reading his post as he comes across as most abrupt and almost argumentative in some of his replies.

Maybe it's just me, but I am sure he would be more of an asset if he could maybe tone them down a touch

Sent from my GT-I9001 using Tapatalk 2

[John]

If Jason appears business like in his posts I put that down to his eagerness to address the issue. I'm very pleased that he posts on this forum as his knowledge of this subject is virtually unrivalled, as Steve says.

I just can't understand why the supplier did not just provide a replacement OTA though

The residual issues with the original one could then have been a matter for resolution between the supplier and the importer with no further inconvenience to the customer.

[malc-c]

Steve, thanks for your comments,

Es Reid is now retired, although he still gets work on a consultancy basis. Having worked for MDBA designing and building precision optics for military and scientific research, from microscopes to projects he simply cant talk about due to the official secrets act. I'm not knocking anything Jason has said, he has contributed a lot to this and other threads, and I welcome his advice. I agree with Buzz that sometimes he comes over as blunt and to the point, but that's just his way and I have no issue with that.

I also agree that the 200P is otherwise a fantastic piece of kit, and very popular. And given the popularity and production numbers, it's possible that one or two have a few issues, so maybe I've just been unlucky and have one that falls inside the small percentage that are out of spec for the tolerances at which these are produced.

John, I agree that it would of been nice if I could of been given a PDS secondary to try and see if it does indeed give a better performance and resolve this issue, and then let RVO and OVL sort out the mater afterwards, but there were no spares in the UK, and neither RVO or OVL apparently were in a position to remove one from a stock item. The scope was 18 months old when I first brought this to RVO's attention, and has since been flocked. It's outside the warranty period so i would not expect them to simply replace the 200P OTA. The fact that they have arranged a direct replacement secondary, and now a PDS secondary to try and resolve this matter without any charge to me is more than I would of expected. If after the PDS is fitted and I still get issues, then I'll see what RVO have to say regards an exchange of the OTA for a replacement or something else in its place.

Maybe in hindsight I may of received a different level of service or options from Steve at FLO, but what is done is done... maybe if I don't get any satisfaction with the replacement secondary and I look at exchanging or cutting my losses and getting a new OTA of some description my order may well go FLO's way.

Yes this is frustrating, especially when we've had a few of those rear exceptional seeing clear nights of late, and even though the scope is capable of producing some kind of image, it's dampened my enthusiasm to do any imaging sessions so the scope has remained idle.

[Jason D]

Steve, John, I appreciate the kind words.

Malcolm, I agree with your characterization when you stated: “he comes over as blunt and to the point, but that's just his way.” I am an engineer who works in a company culture that encourages individuals to question and bluntly/directly challenge ideas.

Buzz, I read your post but I am not going to “argue” with you

Anyway, I do not want to derail this thread. Let us get back to the subject.

Jason

[Jason D]

I looked into the photos in this post

http://stargazerslou...60#entry1669874

and I do not see any uncoated areas but that could be due to the photo brightness and angle. Can you see these uncoated areas when you look down an empty focuser?

I do not see how uncoated mirror areas can produce such prominent additional spikes? Spikes are introduced by obstacles with straight edges. Uncoated areas are not obstacles.

If the issue is indeed with the uncoated areas then I do not understand the need to use a larger secondary mirror. Why are you planning to use a larger secondary mirror? Why not recoat the existing mirror or get another fully-coated one with the same size?

Malcolm, I am not being argumentative but I am the type of person who questions why I am told.

Jason

[BuzzBomb]

I'm not looking to argue either, just stating an opinion.

Sent from my GT-I9001 using Tapatalk 2

[malc-c]

Can you see these uncoated areas when you look down an empty focuser?

Yes, and on the replacement secondary OVL sent it's more noticeable than the original. Looking face on the secondary looks like the attached sketch... the dark area on the minor axis are un-coated

Malcolm, I am not being argumentative but I am the type of person who questions why I am told.

Jason

No problem with that... and if installing the PDS secondary fails to resolve the issue, it would prove Es's findings wrong, and I dare say he'll be the first to accept that he got it wrong even with his many years of experience... I'm just fortunate that I have someone who is an hours drive away who has more experience than I have and can help with the basics as well as the real scientific stuff. I was nice to hear that when the scope was first put on the test rig that my efforts to collimate the scope were very close given that it was done visually with a cheshire / laser and MK1 eyeball

[Jason D]

I highly doubted it. Let me clarify.

1- I assume your secondary mirror is little oversized which is typical for most reflectors. Most likely none of that stars light cones will cover both edges of the secondary mirror. See first attachment. Figure "A" shows the whole secondary mirror surface is used for a given star which is a misconception. Figure "B" is more accurate for the center star assuming the scope was properly collimated. As you can see, the central star will most likely not touch the two uncoated edges. Figure "C" is for horizontal off-center stars and these will most likely not cover the uncoated edges. Finally, the stars in figure "D" cover only one of the edges.

2- If you examine the attached photo from your scope, you will not see any anomalies around the perimeters of the defocused star. The only two main anomalies are because of the primary mirror clips. If the uncoated edges will cause pronounced diffraction spikes then their presence should be evident in the defocused star – just check the main spider vanes presence.

Jason

[Jason D]

Another interesting photo. If you compare the view from your empty focuser to your defocused star image, you cam see some correlation. You can see two of the three primary mirror clips and maybe a hint of one of the uncoated areas. If that is indeed one of the uncoated areaa it is hardly visible which again leads to believe your issue can't be caused by the uncoated areas.

[Aenima]

Hello folks,

Having followed some of the thread, and saw the pic of malcolms star with one spike off, its something i've seen in my own images and assumed it was an offset spidervane.

But at this point with the 200p some of the lower end features can make perfect collimation unattainable unless you mod/upgrade things like the standard focuser, as well as obtaining top gear like laser collimators etc.

So i'm wondering if my decision to get a basic laser collimator to try and expand on the collimation I can achieve with my cheshire alone will actually be worth it if my scope is not 100percent pimped out?

Im having feelings that my 2ndary is a little off and not sure if a basic laser can help my general collimation, as a bit of the subject matter in the above thread goes over my head in parts and from my point of view some of the science can cause confusion as to how to approach my particular scope collimation in practice...

For impromtu collimation a laser - of standard quality - is as good as and or better than a cheshire alone?

Thanks,

Regards

Aenima

ps hope you get your trouble sorted out Malc

[malc-c]

I highly doubted it. Let me clarify.

1- I assume your secondary mirror is little oversized which is typical for most reflectors. Most likely none of that stars light cones will cover both edges of the secondary mirror. See first attachment. Figure "A" shows the whole secondary mirror surface is used for a given star which is a misconception. Figure "B" is more accurate for the center star assuming the scope was properly collimated. As you can see, the central star will most likely not touch the two uncoated edges. Figure "C" is for horizontal off-center stars and these will most likely not cover the uncoated edges. Finally, the stars in figure "D" cover only one of the edges.

2- If you examine the attached photo from your scope, you will not see any anomalies around the perimeters of the defocused star. The only two main anomalies are because of the primary mirror clips. If the uncoated edges will cause pronounced diffraction spikes then their presence should be evident in the defocused star – just check the main spider vanes presence.

Jason

Jason, thanks for your input. You seem to have a strong feeling that the issue is not with the secondary mirror, yet the condition worsened when the secondary's were exchanged, and as the scope has been stripped and re-assembled many many times, with the optics in never replaced in the exact same position (ie the primary has been rotated within its cell etc).

As you are certain it's not a secondary mirror issue, what exactly do you think is the issue (bare in mind that the secondary spider has also been replaced) ?

[Aenima]

star extra spike.bmp

Sorry to butt in again, my scope isnt maintained well enough to contribute to the discussion I think, but here is a picture taken with my set-up showing the extra diffracion spike.

This may or may not be relevent, although if a solution to the problem arises i'd be interested to know - just in case - as its quite annoying to spend ages with a b-mask getting the focus as good as possible as far as my equipment allows, only to have that spike intruding in every star shape.

The image is probably slightly off due to vibration/tracking but the spike is clearly visible.

HTH

Regards

Aenima

[malc-c]

To my eye, there is no refraction to the normal spikes produced by the spider that is sometimes seen in my images, and the normal spikes are fine and narrow, which to me suggest that the spider arms are true and not bent or warped as have been suggested in this thread, yet I have to agree your scope is showing the same issue. So we have two SW 200P's with the same issue.

I wouldn't want you to remove the spider, but can you comment or try and take a picture of your secondary to see if there is any area on the minor axis that seem un-coated ?

[Aenima]

Yeah, i'll give it a try, see if it shows up something visible.

After the kind of effort thats been put into the one you have, its hard for me to make any guesses, all I know is the spike looks a little familiar when I look at your image - and in mine its entirely possible that somethings not aligned right, it hasn't really been changed much from the factory - just unboxed, roughly collimated and focused with a bahtinov.

Sorry I cant be of more help, a lot of the tech is still a bit over my head but if it helps you guys solve the problem that'd be good.

As for the coating there does seem to be small areas around the very edge - on the primary it looks like each clip has little linear marks next to it, too small to make me overly concerned but I may be able to get it on an image, and the secondary has equally small areas right at the edge but these also seem too small to make me worried - compared to the dust or miscollimation and things like a small hair - - on it they seem insignificant, but again my experience is limited here.

After seeing on another forum some guy found a scope in a skip filled with sand which seemed to make little difference to the view, though this was a military zone in afghanistan and the picture was more a surprise than a precise measurement - ! - but still.....

Anyway, if I can help then I will be glad to.

Regards

Aenima

[Jason D]

As you are certain it's not a secondary mirror issue, what exactly do you think is the issue (bare in mind that the secondary spider has also been replaced) ?

I strongly believe it is either misaligned spider vanes or some protrusion by a small object along the light path such as a screw head from the OTA rim or the secondary mirror assembly – but we have already discussed these possibilities in this thread.

May I make the following suggestion:

Refer to the attachment.

1. Make a mask consisting of a long and narrow rectangle (bottom figure) – around 1” in width. Use rigid material such as cardboard. Center a bright star in the FOV and ask someone to cover then uncover one set of the spider vanes.over and over again while you observe the star. Do the same of the second spider vanes set. If the extra spike goes away then the issue is most likely with misaligned spider vanes.
   

1. Make a mask similar to what I have shown in the attachment (top figure -- I know what it resembles but let us not go there). Make the circle diameter around 3”. Bend/curve the wire to avoid a spike. Now ask for an assistant to carefully and methodically scan the whole OTA opening while you observe the bright star. Ask the assistant to stop when the extra spike disappears. Examine what is underneath the mask and identify the protrusion.
   

Jason

[Aenima]

. Now ask for an assistant to carefully and methodically scan the whole OTA opening while you observe the bright star. .

Jason

While this sounds a very thorough and precise exercise, and Malcom you might find it useful and may be able to eliminate possibilities but for me the truth is I find even star testing my collimation by eye difficult to do properly, and also I have only noticed the extra spike in photographs - I doubt my vision would pick up much through an EP I'm afraid.

I can only guess as to the cause of these strange diffraction (or whatever) spikes, assuming it's the same issue for both scopes, but will use my friends snapshot camera to get some pictures of the scope's inner bits - hopefully something will show up.

All the best

Aenima

[Aenima]

Unfortunately, after nearly 50 shots, i've still not managed to get the camera in the right position with the right settings to pick up the kind of details you're loolking for

The flash just seems to wipe out any view of the secondary mirror's surface, other than to highlight the dust or an upside-down hand holding a camera. I'm wondering if a webcam could do better, or if the images that I did get might still be of some use.

Its a lot harder than I thought to get a camera through the secondary holder vanes and get the angle and settings right while still being able to reach the button.

Sorry for the wait, i'll keep trying over christmas - especially with the bad weather - see if I can improve on the last results, meanwhile I hope whatever this problem is, your able to make some progress and also if you do have a breakthrough it'd be good to post an update here, i'm very curious to know more about the issue.

All the best - merry christmas, also -

Aenima

[spaceboy]

Sorry but I'm a very late comer to the thread and while I tried to read all the posts there is no doubt many suggestions I missed. Malc when you say it might be a size issue with the secondary mirror did you try moving the secondary closer to the focuser by tightening and loosening the spider. I did read that some scopes (although I have never known it to be the case with the 200p) require the secondary to be slightly offset (IE: shorter distance on one spider than the opposing spider.) You saying the secondary is small suggested to me the offset to be towards the focuser and so it turn making it larger in doing so ???

I know my idea is probably not the best suggestion made in the thread but if your waiting for the PDS secondary anyway it might be worth a try ??

I really hope you are able to get it sorted mate but please don't give up on the hobbie through a case of collimation OCD

[Aenima]

. So we have two SW 200P's with the same issue.

I wouldn't want you to remove the spider, but can you comment or try and take a picture of your secondary to see if there is any area on the minor axis that seem un-coated ?

Hi there Malc,

I'm finding it dificult to get a decent shot of the secondary which gives a good view of the surface, though it certainly picks up the dust.

I'm waiting on a basic laser collimator to try and improve on what my cheshire can do regarding getting it a few mm more accurate - its a fast -ish- scope and the difference might allow me to get better planetary images, but this is also coinciding with me hopefully removing my secondary (with or without spiders? - I havent worked that bit out yet.) as its not really positioned as well as it should/could be and I need to get past the fear of undoing screws . . .

Anyhow, I'm hoping to be able to take pictures and get a look at the surface, generally, in the hope it will still be of use regarding your dilemma.

All the best

Aenima

I cant verify for sure but I saw the exact same diffraction effect in another image but couldn't find details as to which scope - definitely an 8" reflector though.

EDIT: also I noticed that one of the 6 screws that make up three sets of two around the secondary holder part has come out (it was rattling inside the OTA ! so i'm hoping to put it back soon as I do the collimation 'do-over'

[malc-c]

Hi Spaceboy,

I've tried moving the secondary off axis as part of the elimination process, even whilst on a target star, to no real avail. The interesting thing was that the second secondary I was sent displays a worse diffraction pattern than the original, hence the leaning that it's to do with the poor finish / coating of the mirror. On my 200p, there is no means of moving the secondary nearer the primary other than via the center fixing bolt, it's not possible to physically move the spiders position fore and aft.

[malc-c]

Guys,

Due to family matters, redundancy, and Christmas etc... I've not really had the enthusiasm or interest in taking this further. The weather hasn't really helped, and the fact it's now been almost two months since RVO / OVL agreed to get a mirror from China has meant that my astronomy has taken a back seat for now. Apologies to Jason et all for not trying the suggestions and reporting back

I'm hoping the receipt of the mirror will re-kindle the spark and I'll hopefully get back into the hobby.... either that or the lot will be going up on e-bay and I'll be renting the observatory out as semi-furnished accommodation

[Aenima]

Sorry to hear that Malc,

I'm hoping for the more positive outcome, fingers crossed.

Take care

Regards

Aenima