200P - colimation or poor optics

[Steve Ward]

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.

But surely the secondary would still have to be in the right position relative to the focuser , regardless to an extent to where the spider holes were drilled ?

Sounds more like an undersized secondary to me , my 200PDS secondary has a slight flat on 2 edges but they are outside of the cone and make no difference to the image.

Steve.

[squeaky]

hmm... yes. No easy answer is there?

[malc-c]

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.

That's one way, but as stated, you would also need to drill and re-position the focuser so that you could colimate the optics, which would also mean reducing the draw tube so that it wouldn't obstruct the path as the focal point would also be moved further in

[malc-c]

Sounds more like an undersized secondary to me , my 200PDS secondary has a slight flat on 2 edges but they are outside of the cone and make no difference to the image.

Steve.

I think you hit the nail squarely on the head....

[Moonshane]

glad you seem to have got a result! I wonder if you could prove the theory by making a template to block the edge of the secondary (effectively reduce its diameter)? this would effectively reduce your aperture but might remove the spikes?

[squeaky]

Good idea - but easier to cut an annular ring and place it in front of the focuser. Might well provide "proof of concept".

[Naemeth]

Could you not also mask most of the aperture down? Which (I believe) in effect, it would make the secondary larger in proportion to the primary. An off-axis type mask.

[Freff]

I suggested using a gradually reducing mask at the aperture a while back, not sure if Malcolm tried it or not.

[malc-c]

LOL - you guys should be optical engineers... most of these things suggested were replicated using the test equipment.... one of things Moonshane suggested really clinched it as a small slither of tape was placed across the un-coated edge of the secondary and trimmed to maintain shape - this changed the diffraction pattern, but didn't improve it to the point where they disappeared, but Es came to the same conclusion as Steve, in that the secondary is undersized for its position in the OTA

If RVO / OVL do exchange the secondary for one used on the PDS and it doesn't resolve the issue then it will be a case of stripping the scope for spares to raise cash towards a new OTA. !!!

[Naemeth]

LOL - you guys should be optical engineers... most of these things suggested were replicated using the test equipment.... one of things Moonshane suggested really clinched it as a small slither of tape was placed across the un-coated edge of the secondary and trimmed to maintain shape - this changed the diffraction pattern, but didn't improve it to the point where they disappeared, but Es came to the same conclusion as Steve, in that the secondary is undersized for its position in the OTA

If RVO / OVL do exchange the secondary for one used on the PDS and it doesn't resolve the issue then it will be a case of stripping the scope for spares to raise cash towards a new OTA. !!!

It's merely a matter of just thinking about what it isn't and what it could possibly be I suppose .

I really hope you can get a replacement secondary as that should solve it .

[squeaky]

LOL - you guys should be optical engineers... most of these things suggested were replicated using the test equipment.... one of things Moonshane suggested really clinched it as a small slither of tape was placed across the un-coated edge of the secondary and trimmed to maintain shape - this changed the diffraction pattern, but didn't improve it to the point where they disappeared, but Es came to the same conclusion as Steve, in that the secondary is undersized for its position in the OTA

If RVO / OVL do exchange the secondary for one used on the PDS and it doesn't resolve the issue then it will be a case of stripping the scope for spares to raise cash towards a new OTA. !!!

Um...

You'd have a case under the Sale of Goods Act to claim that the telescope (or at least the secondary) is not fit for purpose and that the fault is inherent from the point of manufacture.

You have the options of Repair, Replacement, or Refund; though the manufacturer can refuse an option if the cost is prohibitive and offer one of the others.

At this point I would think that replacement of the whole telescope is not feasible, though supply of a suitable secondary is. A refund, if negotiated would not be full price at this late stage - there would be some reduction made for the use that you have had of the telescope, and this might possibly be more than your estimated "stripped for spares" value; not to mention absolving you of all the hassle of selling parts.

To get more information if your discussions with the retailer don't resolve this to your satisfaction you could ask on consumer rights forum such as this one:- http://forums.moneysavingexpert.com/forumdisplay.php?f=173 but since, at the moment, you seem to have a good relationship with them so far - I would leave that as a last resort.

[malc-c]

Thanks for the advice. I'm sure it would not come to pursuing a claim under the sale of goods act in order to obtain a decent secondary, I have faith in my supplier to work with me on this

[squeaky]

Which is why I said...

To get more information if your discussions with the retailer don't resolve this to your satisfaction you could ask on consumer rights forum such as this one:- http://forums.moneys...splay.php?f=173 but since, at the moment, you seem to have a good relationship with them so far - I would leave that as a last resort.

[davies07]

Malc,

A thought....

Is there any scope (sorry!) to adjust the primary towards the back of the tube using the primary collimation screws. The objective would be to reduce the cone diameter at the secondary just enough to miss the flattened edges on the minor axis. Such an adjustment would also shift the prime focus inwards towards the tube so cannot be taken too far otherwise you won't be able to focus the camera.

I should have thought of this at Es's where we could have tried it - but we were running out of time there.

David

[malc-c]

David,

I hear what you are saying, given that we cant pull the secondary towards the aperture without having to re-drill the tube for the focuser. From memory the primary is fairly far back, but there should be a few mm to play with, but I'm sure I've tried this with the original secondary fitted, starting with the primary cell springs fully compressed. Hopefully I'll get chance in the next few days to try re-positioning the primary all the way back in as a starting point for collimation and see if it makes any improvement.

[m0rt]

Malcom - have to say that I admire you perseverance for that perfect image! This thread has been a very interesting read.

Good on you. Whatever you think about the diffraction spike issue, the images you are producing are extremely well done. Your image of M27 is a particularly impressive example, I think.

[malc-c]

Malcom - have to say that I admire you perseverance for that perfect image! This thread has been a very interesting read.

Good on you. Whatever you think about the diffraction spike issue, the images you are producing are extremely well done. Your image of M27 is a particularly impressive example, I think.

Thank you for those very kind words.

I don't think I can ever expect a "perfect" image. I'm realistic that I have a mass produced £300 OTA on a £800 mount... but I'm confident the scope was never performing to the best of it's abilities. The good news is that RVO are working with OVL to come up with a suitable option to allow the scope to perform to it's maximum capability.

[Jason D]

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.

Only stars around the center of the FOV for a well-collimated scope will have their light cones confined to the secondary mirror surface. All other stars (majority) will have their light cones be clipped by the secondary mirror edge. Therefore, you can never avoid the secondary mirror edge unless you are at high magnification or have a huge secondary mirror -- assuming a well-collimated scope.

Jason

[malc-c]

Well I've had good news and bad news. The good news is that with Ian's help OVL are going to supply a secondary for a 200PDS for me to try FOC. The bad news it could be two months whilst its shipped from China

Anyway, yesterday I stripped the scope down and applied a few suggestions. The first was to test the colimation of the Hotech laser. Having made a rig up from 21mm waste pipe, the thing was rotated and pleased to say the dot didn't move. The focuser was completely removed and using digital calipers the centre point was calculated and re-spotted - It was right on the previous mark. The focuser was then re-fitted and using the self-centreing fitting and Hotech adjusted until the laser hit the mark opposite. The spider was installed and using the threaded rod method, set in the vertical axis, whilst using the gauge to centre in the horizontal. The maim mirror cell was pulled back until the springs were fully compressed. The secondary was then fitted and the scope collimated using the cheshire. - It looked good.

Last night it cleared up at around 11pm so I went out to see if anything had improved - Nope !

Here is a guided 180s single sub on M45 - Not only is the original additional spike still present, so are the new artifacts at 2 and 4pm positions. (I've also included a 3min sub of M42 - both are JPG direct from camera)

If using a larger secondary makes no difference then I'll probably scrap the OTA and reside myself that I'll have to take a break from astronomy as my recent change in employment status rules out replacing the OTA for some foreseeable future

[sunshine185]

Forgive me if this has already been tried or is I stupid suggestion, but could the primary mirror clips be at fault? Could a small mask be made to hide the primary clips to see if that could be the problem? I think I read a post from 2011 about the primary clips causing strange diffraction spikes.

[malc-c]

Thanks for the suggestion. When the optics were tested, we took the main mirror out and re-positioned it, adjusting the clamps to avoid pinching. regretfully this had no affect on the resulting diffraction spikes. The fact that we've replicated the condition, and that the condition was worse with the second secondary does point to this area as the most probable cause. We'll know for sure when the PDS secondary arrives an a month or so

[sunshine185]

Hope you get it sorted, sry for the primary nonsense.

[Aenima]

Only just found this thread, sorry, but wanted to ask about collimation - I have that same thing when photographing a star one of the diffraction spikes has the same extra spike - which makes focussing difficult- and i'd put it down to bad focus on my camera but it often appears in good and bad pictures...

My collimation is rough at best so i'm wondering if this is a spidervane twisted? its certainly possible with my scope?

One day I need to correct some of the more sloppy adjustments - is a cheshire not enough or is there enough quality in a skywatcher basic 200p set up to warrant aiming for perfection?

A very interesting thread, not that I fully understood much of it, but i'm interested in what happened after reading through

Regards

Aenima

[malc-c]

Colimation is probably in the top 5 of topics discussed on the forum, but I think I can safely say that the minimum tools required is a good cheshire. It helps (to a degree and IMO) to have a decent laser colimator to assist wit fine tuning of the centre axis of the optics. However you need to make sure your laser is colimated itself otherwise it will put the scope out.

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. I can't comment if your scope is suffering from the same issue, but it would be interesting to see a picture taken at prime focus through your scope, and doing a comparison. If it is the same then it might be worth referring the matter back to OVL via the retailer as it would certainly prove that this is a manufacturing issue with some batches of 200P scopes.

[Aenima]

Yes i'll get a shot of a big star - take me ages to find focus now my bahtinov mask is broke. sad face. but when the shot is right it seems to me that a spidervane is not flat, but it looked just like your star pics that i'd thought to post here.

I'll get the starshot, then post back..

Regards

Aenima