200P - colimation or poor optics

[Moonshane]

Malcolm, sorry to hear about your continued problems. As I indicated in my earlier posts in this thread, I never believed the secondary mirror was your problem. I still believe your issue relates to what I described in my last post in this thread (below). In that post I suggested you do the very experiment you did above and your findings support my theory. Do not cover only one spider vane; cover two opposite spider vanes as I described in my last post. In addition, look for localized protrusions as I described.

http://stargazerslou...40#entry1740804

Jason

I totally agree with this approach and suggested similar in about post 4. see the pic of the spike :

and compare with the pic of the well out of focus image

there seems to be something in the view opposite the spike which might be the issue although it's hard to tell what it might be. there is also something at the bottom right which is creating a spike to the left f the top spike. can you identify what this is and maybe it's something as simple as flocking poking into the light path? I think the only way to correct this issue is a systematic approach as suggested by Jason as only this way will you eliminate possibilities.

[malc-c]

The next clear night I'll repeat the covering up of the spider vein as before, with the addition of the one opposite as Jason suggests to see what effect that has. As for some other obstruction, the inside of the tube is completely flocked with no loose edges or anything obvious protruding into the optical path. The only intrusion could be the draw tube, but if you look at the recent images you'll notice the vein that seems to be the offender is at 90 degrees to the draw tube.

[Freff]

I still think ring masks (washer shape) the type of device used on a refractor to remove clip astigma, should show up something. Keep increasing the thickness of the ring even if it almost covers the entire aperture.

[Aenima]

I'm a total novice here, its really just that damn spike - its there again in moonshanes pic! What is it? The mystery is actually interesting now (no offence considering the trouble its caused you malc, sorry) but my 200p is both non-flocked and badly collimated at best and there is still the spike, so whatever it is is beyond me. o.O

Malcs scope looks brilliantly maintained and he certainly knows how to use it, yet we both have extra spikes.......thats all kinds of weird.

Hope the answer is soon in coming.

Regards

Aenima

[Jason D]

As I mentioned earlier, the issue could be due to localized protrusion into the light path. Secondary stalk screw heads or spider vanes clips are candidates as indicated by the arrows below:

It is possible to get what I indicated above into the light path. Refer to the following illustration. In the left diagram, both protrusions are not in the light path. In the right diagram, both are in the light path. The left diagram represent a scenario were the secondary mirror is lowered towards the primary which will cause it and the primary mirror to rotate upon the completion of collimation.

Jason

[dark knight]

would using a permanent black marker to paint the screws of the spider vane/OTA connection help? at least they wouldn't be as reflective.

[Jason D]

would using a permanent black marker to paint the screws of the spider vane/OTA connection help? at least they wouldn't be as reflective.

If an object protrudes into the light path, it will introduce a spike regardless of color.

Jason

[malc-c]

Uhmmm - I'm still convinced it's the mirror that's the issue, and it's a shame I was sent a stock 200P secondary rather than the larger PDS mirror which was the hole point to disprove the theory that the un-coated area of the secondary is the cause. Mainly because upon closer inspection, the new mirror has a section on one side that is flat and uncoated on the minor axis, which is exactly inline with the offending vein (see my crude sketch attached). On the previous mirror this "cropping" of the mirror was on both top and bottom of the minor axis and I had two spikes in the same plain.

I've got two choices - one is to take the previous mirror and have it re-coated so that the coating forms a complete elliptical covering with no flat edges, or source a larger secondary, remove the previous stock 200P secondary and attache the new mirror to the boss. It will at least hopefully prove the theory that its the optics that's the cause rather than intrusion into the OTA.

[malc-c]

Forgot to add - does anyone know the length of the minor axis of a 200PDS secondary ?

[Jason D]

Uhmmm - I'm still convinced it's the mirror that's the issue, and it's a shame I was sent a stock 200P secondary rather than the larger PDS mirror which was the hole point to disprove the theory that the un-coated area of the secondary is the cause.

Why don't you cut a small piece of paper and cover part of the secondary mirror as shown in the attachment -- at 90 degrees with respect to the existing un-coated area. You can tape the piece of paper without touching the mirror surface. Make sure the paper straight edge is comparable in length to the existing un-coated area. Look for a new spike at 90 degrees angle. If you see it then your theory has merits. If you don't then you can drop the theory and look somewhere else.

Jason

[malc-c]

Why don't you cut a small piece of paper and cover part of the secondary mirror as shown in the attachment -- at 90 degrees with respect to the existing un-coated area. You can tape the piece of paper without touching the mirror surface. Make sure the paper straight edge is comparable in length to the existing un-coated area. Look for a new spike at 90 degrees angle. If you see it then your theory has merits. If you don't then you can drop the theory and look somewhere else.

Jason

That is a logical suggestion. I'll mask off the mirror as above and see what happens next time I have an opportunity to do some more testing

[Jason D]

Just a clarification: When I said "Make sure the paper straight edge is comparable in length to the existing un-coated area" I meant the "apparent" length -- not the "actual" length -- as seen from the focuser end.

[dark knight]

Jason, can I pm u with another collimation question as don't want to hijack malc's thread

[Steve]

Jason, can I pm u with another collimation question as don't want to hijack malc's thread

If you think the solution will help others please consider starting another thread

[malc-c]

Well having spend hours on the net researching possible causes before I splash out around £90 on a new secondary, and getting conflicting information, one sight showed exactly the same issue which the author stated was due to the veins on the spider not being flat and square to each other. OK he was refurbishing a very old scope and it was obvious that the veins were out of line, and I'm using a new spider, which has the veins nice and tight and were squared using the edge of a set square so this shouldn't be the cause, but one I thought worth investigating.

Well the sky cleared and I had an had an hour or so before some friends were due round so I went out and slightly adjusted the veins at 90 degree's to the offending spike, taking test images between adjustments and managed to get the spike to be less noticeable. However the image (test008 attached) wasn't sharp so I parked the scope with the tube horizontal and re-collimated. Place the camera back in and slewed the scope to the same test star and took an image with the mask on to check focus. The mask was removed and a test image taken (test011 attached)..... I was back to square one

I'm going to strip the scope, blacken the rear and edge of the primary and secondary, check all 8 veins for flatness and make up the spider, checking for squareness, and then re-colimate the scope again. I'm predicting that I'll still have the same issue and I'll then try Jason's suggestion of masking the other edge of the secondary to see how the image is affected.

I've had an offer of having the scope's optics tested, but the cost runs in to hundreds of pounds, more than a replacement 200P OTA, and that doesn't include any remedial work or replacement mirrors.

One other option would be to replace the stock spider with one with curved veins, but again that is proving expensive, especially if shipping from the states gets subject to duty.

[JamesF]

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

[Jason D]

…and I'm using a new spider, which has the veins nice and tight and were squared using the edge of a set square so this shouldn't be the cause, but one I thought worth investigating.

..and secondary, check all 8 veins for flatness and make up the spider

Checking for flatness and straightness per vein is not enough. More importantly check to ensure each opposite spider vein pairs are in-line or at least parallel. See attachment.

Jason

[nightvision]

Malcolm, Have you considered drilling four new vane holes in the tube and placing the spider 45 degrees away from its current position, that way the vanes will no longer line up with the focuser or any secondary profile and that may reduce the chance of reinforcement of diffraction effects.

[malc-c]

Tony,

Fair point, but to be honest drilling holes in the tube isn't something I really want to entertain. I don't really trust myself to get the four holes perfectly lined up and spaced correctly so as not to induce the effect that Jason has commented on

[malc-c]

The top end of the scope was stripped and the new spider re-fitted. I used a set of digital calipers to centre the boss for the secondary, with all sides being equal to within 0.3mm. It will be interesting to see if doing it this way works better than the plasitcard and pencil method.

A length of threaded bar was attached to the spider boss with large washers and bolts either side. The Hotech was then fitted in the draw tube to check the alignment of the central axis, which was spot on, striking the bar right in the middle.

The new secondary was then fitted and wound tight and flat to the spider boss, and the collimation screws wound in until they touched the mirror support. The primary was then wound back as far as the screws would allow, and I proceeded to collimate the scope. I took my time and half an hour later collimation was complete, as text book as Jasons avatar

All I need now is another clear night and the enthusiasm to do some more testing

[Jason D]

Also stretch a string above opposite veins to ensure they are indeed in-lined. In addition, if you have "parallel" veins, ensure they are indeed parallel. These tests are more important than centering the secondary in the tube in reference to your extra spike issue.

Jason

[malc-c]

Jason, great minds think alike. I intend to thread some cotton thread through the gap where the spider is clamped to the tube and pull it tight so it gives me a datum line to work to, just to confirm that the veins are indeed true and in line.

[valefor]

If an object protrudes into the light path, it will introduce a spike regardless of color.

Jason

does that include the focuser barrel? because on my scope then focused in the barrel is partly inside the scope tube.

[valefor]

does that include the focuser barrel? because on my scope then focused in the barrel is partly inside the scope tube.

thats "when focused in"

[Moonshane]

yes, it does. I made this point at the start of the thread. ideally a scope should be set up so that the focuser drawtube is less than about 1cm into the drawtube when focused using your eyepiece with least outward travel required. unless doing critical imaging lke Malc it's unlikely to be an issue.