200P - colimation or poor optics

[malc-c]

To be honest Jason I thought I had fixed it. The test star was dead centre in the FOV and from the image taken the issue appeared to be resolved. The scope is housed in an observatory and simply parked at the end of the session. A day or so after there was an opportunity to do another session and to check focus used Arcturus as the star which to do so. When the mask was removed and a test shot taken this was the result.

I have no idea why it wasn't present in the image in the post you linked to. Theories range from the original star being dead centre, and Arcturus being off axis, to Arcturus being a lot brighter ?

[davies07]

Hi Jason,

Good to meet up. Sorry that I was not around yesterday.

OK. To answer your questions:

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?

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

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.

1. When Malcolm got his third mirror he collimated the scope with the primary mirror set as far back in the tube as he could. The idea was to make the light cone hitting the secondary as small as possible, thereby avoiding the mark. This would only work, of course, for an on-axis star. Any bright off-axis star still show a spike. I think Malcolm's tests has confirmed these predictions.

2. Malcolm has had three mirrors. The original had one mark, the second two and the third one. I predict that each mark will produce a spike. Here is a picture produced by mirror 1 (one spike).

Spike1stmirror_inverse by drdavies07, on Flickr

With my freshly sharpened pencil and kids protractor I make the angle 7.3 deg (not 8)

Here is a picture produced by mirror 2.

Spike2ndmirror_inverse by drdavies07, on Flickr

Now we have two spikes. One is at angle of 5 deg, the other at 6.3 deg; the average is 5.6 deg. Why the difference? I think it is because the secondary is rotated slightly about the holding bolt, so it is not quite square to the focuser tube. The tilt angle of the marks, as seen by the camera, will change if the secondary is rotated about the holding bolt. To see this. Here is a photo of the second mirror showing the two marks - ignore the large black area.

mirror2 by drdavies07, on Flickr

Malcolm has placed the mirror in the orientation as seen down the focuser tube. Note the two marks, left and right. Measuring these angles gives 4 deg and 8.3 deg - Ok the mirror is not quite in the correct orientation but the average is 6 deg. Not a bad comparison with the average of 5.6 deg spikes in the picture from this mirror.

3 - To be honest, I'm not sure what will happen with stars at the edge of the FOV. The image is formed by a whole bundle of parallel light waves coming into the tube and then being brought to focus. Stars at the edges will be vignetted by the secondary - the 100% illuminated field is only a few mm across for this size of secondary.

4 - The 3.7 mm offset for the secondary: I did my calculations to determine the offset as measured along the major axis of the diagonal (in order to mark the secondary mount with the required offset should Malcolm need to mount a new secondary himself). I can now see from the standard tools, e.g. Bartels

http://www.bbastrode...om/diagonal.htm

that the offset is quoted in focuser axis, i.e. towards the primary, or away from the focuser. I agree with your figure of 2.5 mm with this convention. I've checked my own spreadsheet and found an error. My figure should read 3.57 mm, measured along the major axis of the mirror. This equates to 2.52 mm in the axis of the focuser.

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?

I think I've now covered these questions.

So having made some corrections to my spreadsheets I predict that the marks would appear to the camera at an angle of 5.9 deg to the horizontal, and that the resultant diffraction spikes would be at this angle to the vertical. The angle will differ if the secondary is not absolutely square to the focuser - as in the picture from mirror 2.

Hope this helps,

Dave

[davies07]

Jason,

Ah, sorry, missed a picture.

Here is one of Malcolm's latest pictures produced by the third mirror. This has one mark and I measure the spike as 6.7 deg to the vertical. Other than that, the scope looks to me to be in pretty good collimation.

spike3rdmirror_inverse by drdavies07, on Flickr

Dave

[Jason D]

Thank for responding to my questions.

You are focusing on the angle value but what about the "quadrant?"

Assume the spike in the lower left quadrant is caused by mark X and the spike in the upper left quadrant is caused by mark Y then what about the spike in the lower right quadrant? If that is still caused by mark X then the secondary mirror will have to be significantly rotated. See attachment.

Jason

[davies07]

You are focusing on the angle value but what about the "quadrant?"

Assume the spike in the lower left quadrant is caused by mark X and the spike in the upper left quadrant is caused by mark Y then what about the spike in the lower right quadrant? If that is still caused by mark X then the secondary mirror will have to be significantly rotated.

Jason

Jason,

Yes, good question, I agree.

I have asked Malcolm for more information relating to the orientation of the spikes (images) in relation to the axis of the focuser. He thinks the camera could well have been rotated in one shot with respect to another but is not sure. So, we don't know the orientation of the spikes in relation to the secondary .

There are a couple of other bits of information I can add: When we had the scope on Es Reid's optical bench, we could see the spike (then with first mirror with one mark). We used an artificial star as the test object - a halogen lamp focussed on the tip of a tungsten ball.

At the very end of the time we had available, we made a simple mask using a small circle of paper on a fine wire and moved it around the mark on the mirror. The appearance of the extra spike was reduced as we moved the mask over the mark. Unfortunately, we had already run out of time and Es was late for another appointment. We thought we had seen enough to convince ourselves that the mark on the secondary was the culprit, and that led to Malcolm requesting a replacement mirror.

My problem was that I wanted to understand why the spike appeared as it did. I have only recently had the ideas as to the answers to that question, as I've tried to explain here.

Since the testing, Es has also been modelling the optical set up of the scope using a ray tracing program. Whilst he has had some success in reproducing what we saw, the resolution of the software was not fine enough to be conclusive. It would take hours of computing for one set-up to do the simulation properly.

Hope this helps.

Dave

[Jason D]

Referring to the following old post from Malcom:

http://stargazerslou...40#entry1624384

He uploaded two photos using the same secondary mirror: One photo with one spike and another with two spikes. The difference was re-collimation – not a mirror replacement. Unless I missed it, I did not see any prior posts to the above link that suggested Malcolm has changed his secondary mirror.

But how can recollimation add/remove a spike! Refer to the attachment. The photos came from the following page

http://www.cloudynig...l/fpart/20/vc/1

I was helping someone with a collimation issue. Both photos are from the same scope. You can see how in the first photo, 3 screw heads are exposed and in the second photo you can see 4 screw heads exposed. I expect an additional spike will be associated with that 4^th screw head. Re-collimation will expose/hide the 4^th screw head and subsequently will add/remove a spike from view associated with it.

Above is an example. I am not suggesting that is the culprit in Malcolm’s case but it is a good theory. The protrusion that is causing the spike could be around the secondary structure or around the OTA opening or around the primary mirror perimeter or somewhere else.

Jason

EDIT: Here is a link to a prior post I made abotu protrusions (bottom post in the page)

http://stargazerslou...60#entry1898956

[Jason D]

I have asked Malcolm for more information relating to the orientation of the spikes (images) in relation to the axis of the focuser. He thinks the camera could well have been rotated in one shot with respect to another but is not sure. So, we don't know the orientation of the spikes in relation to the secondary .

If the issue has to do with the secondary mirror or some scope structure then camera rotation should be irrelevant. Why do you expect the spike location to track camera rotation?

At the very end of the time we had available, we made a simple mask using a small circle of paper on a fine wire and moved it around the mark on the mirror. The appearance of the extra spike was reduced as we moved the mask over the mark. Unfortunately, we had already run out of time and Es was late for another appointment. We thought we had seen enough to convince ourselves that the mark on the secondary was the culprit, and that led to Malcolm requesting a replacement mirror.

Good experiment. I suggested the same experiment to Malcolm 5 months ago

http://stargazerslou...40#entry1740804

But you need to be careful. Maybe you hid a screw head protrusion inadvertently and assumed the disappearance of the spike was due to the mark – just a thought.

My problem was that I wanted to understand why the spike appeared as it did. I have only recently had the ideas as to the answers to that question, as I've tried to explain here.

I posted a theory in my prior post.

[Moonshane]

whatever the end result of this 'adventure' I think the calm and reasoned responses of all parties who apparently disagree (at this point) quite widely is admirable. this sort of attitude in trying to help others and debating reasons why one thing or other is not in accordance with their views is just fantastic as far as I am concerned and what this forum is all about.

quality posting guys.

[malc-c]

Guys, I must admit the maths and level of discussion is going over my head now

Let me try and answer one of the questions, that of the camera position. Of late the camera has been attached as per the attached. The minor axis with the uncoated flat is uppermost in the image as indicated by the yellow line.

This gave the attached result when Arcturus was used as a focusing target

I can't 100% vouch which orientation of the camera was for some of the early images - Sorry guys.

I was going to order a larger mirror today... but Murphy's law being what it is, the car has developed a fault which means the funds I set aside for the mirror will now have to be used for the repair on the car. Having said that I hope to organize the replacement with in the next few weeks.

I have to agree with Moonshane, the quality and caliber of posts in this thread is fantastic. Whilst a lot of the discussion has gone well and truly over my head, the fact that so many forum members have contributed to the plight with my scope to this degree has been a pleasing experience.

The support from you guys has been second to none

[Jason D]

I need to clarify that I was referring to the "quadrants" delimited by the spider vanes diffraction spikes; therefore, camera rotation is irrelevant.

Jason

[malc-c]

Well there has been some development. A further experiment is in the offering

Nigel PM'ed me the other day. He had a 63mm secondary blank that he had rejected due to damage in the processing, but the face was a perfect ellipse. He suggested that this may be handy to try before I committed to purchasing a new mirror. I was somewhat skeptical as whilst I know glass reflects, I couldn't see how having no coating would work. Anyway Nigel kindly posted to me and it arrived yesterday.

I've given the back and sides a good three or four coats of matt black paint and managed to prize off the boss of the spare mirror that OVL sent (the one with two flat edges). I've mounted the mirror using some double sided tape (used for sticking down carpets) and it seems secure enough, but the offset was purely guess work ! Anyway I've mounted the mirror in the scope and re-collimated. One thing I did notice, was that I could see the reflection of the whole of the primary including the clips, something I seemed to struggle with when using the stock mirror, even when the primary was wound a far back as it would go. I've taken a few test shots of the sun and it appears to work, with no obvious double reflections caused by light hitting the back of the blank, but that may well be more noticeable when tested on a bright star.

Anyway, I've now just got to sit and wait for a decent night to do some testing. If the rouge spike is still present then at least this has saved me the £100+ I would of spent on a new secondary, but it also means that something else is the cause and I'm running out of ideas.

I would like to thank Nigel for his support and for sending me the blank at no charge. Its people like him that make this forum such a wonderful community to be part of

[Astrobits]

Just a slight correction to Malc's post. The flat that I sent I rejected on the grounds of quality as you can see from the interferogram I sent to Malc. It was thrown into the re-working pile of flat blanks about 15 years ago and has since then acquired a small scratch which should not affect the outcome of this experiment. I just hope that it does a good job.

Nigel

[Freff]

Good luck Malcolm I hope you get results.

I would warn against using the carpet tape. It may appear fixed but it doesn't have the grab power to trust it from dropping onto your primary.

If you can get a piece of 3M 4611F Acrylic Foam tape it would guarantee holding. If you cant I can send you a piece.

[Jason D]

Malcolm,

You have already demonstrated a setup with a secondary mirror that has two uncoated marks without any additional spikes!

You have already demonstrated a setup with a secondary mirror that has only one uncoated mark which exhibited both a single spike and two spikes by recollimation!

If the new uncoated secondary mirror experiment shows the additional spike(s) then that proves once and for all that the uncoated marks have nothing to do with the spike(s).

If the new uncoated secondary mirror experiment does not show any additional spikes then the result is inconclusive.

Do you agree?

Jason

[malc-c]

Malcolm,

If the new uncoated secondary mirror experiment shows the additional spike(s) then that proves once and for all that the uncoated marks have nothing to do with the spike(s).

If the new uncoated secondary mirror experiment does not show any additional spikes then the result is inconclusive.

Do you agree?

Jason

Agreed, other than if the mirror does not show any additional spikes then the result could prove that my hunch that the uncoated area of the stock mirrors contributed to the affect, if not caused it

[Jason D]

"Could" implies inconclusiveness

So, I believe we are in agreement

Jason

[Jason D]

Anyway, I've now just got to sit and wait for a decent night to do some testing. If the rouge spike is still present then at least this has saved me the £100+ I would of spent on a new secondary, but it also means that something else is the cause and I'm running out of ideas.

Any updates?

[malc-c]

Well the blank proved a non starter - mainly as there wasn't enough reflectivity to pick out a star, and secondly there were internal reflections which made collimation a tad difficult. I've therefore opted to replace the stock mirror with a 57mm minor axis, but funds are a bit tight so nothing will happen for a couple of weeks.

I'll post a further update when that arrives and installed.

[Mr Flibble]

Any more developments?

[malc-c]

Any more developments?

Just waiting for Orion Optics to finish making a 57mm secondary for me. Hopefully it should be here within the next couple of weeks and then I'll do some collimation and see what happens. Hopefully this will resolve the issue and prove that this spike was caused by poor optics and not a collimation issue

[tich]

I've got the same spike with my 200P so I'll be interested in the outcome.

[Aenima]

You too tich?

Oh joy. )

Getting used to sseing it in my pics to be honest, still be interested to see how it turns out - best of luck malc.

Regards

Aenima

[Astrobits]

Have just come across this interesting blog that shows the same extra spikes that are the subject of this topic:

http://remanzacco.blogspot.it/

Scroll down to the movie of the Herschel Space Observatory and look at it enlarged. You will see two extra diffraction spikes on the bright star.

Nigel

[Earl]

Just a random thought here.

have you tried moving the primary up the OTA, it would be interesting to see if that makes any difference.

[malc-c]

Earl, I think it would need to be pulled back so the edges light cone fit the area of the secondary. Moving it closer to the secondary would mean the edges of the light cone overshoot the mirror. My gut feeling is that if the stock secondary was 100% elliptical with no flat un-coated area then the scope would perform how it should with no additional diffraction spikes. Anyway hopefully we'll see when the new 58mm secondary from Orion optics arrives, at the moment they are fine tuning the polishing to get the correct precision, which has delayed my order by a week or so.. but given the time I've spent trying to resolve this issue I'm prepared to wait a few more weeks.