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malc-c

200P - colimation or poor optics

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OK I'm quite amazed that I managed to get these images by holding the camera to the viewing hole in the auto-colimator. I've set the camera to close up and it's done the rest.

These were taken at 180 rotation. In one it looks like the traditional drawings showing the slight off-set but with everything else central (bear in mind I'm holding the camera's lens to the scope so take that into consideration). With it rotated 180 degrees the multiple reflections and off axis reflection of the focuser is clearly visible.

post-10726-0-71060500-1346600392_thumb.j

post-10726-0-11999200-1346600413_thumb.j

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These photos definitely look great for a hand-held camera.

The photos show several issues:

1- The first photo should show darkened background when collimation is achieved but it shows bright background.

2- The discrepancy between both photos is troubling. It should NOT be that significant. Regardless of the scope's collimation state and the quality of the focuser, when an autocollimator is rotated at 90 degree angles you should not see such a major discrepancy.

3- Something caught my attention. The mirror central unreflective area is very large. I estimated it to be around 6mm. That is very large and will have an impact on accuracy.

Jason

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These photos definitely look great for a hand-held camera.

The photos show several issues:

1- The first photo should show darkened background when collimation is achieved but it shows bright background.

2- The discrepancy between both photos is troubling. It should NOT be that significant. Regardless of the scope's collimation state and the quality of the focuser, when an autocollimator is rotated at 90 degree angles you should not see such a major discrepancy.

3- Something caught my attention. The mirror central unreflective area is very large. I estimated it to be around 6mm. That is very large and will have an impact on accuracy.

Jason

And again with larger fonts...

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Just a thought Malcolm, have you spotted the primary yourself or is it the original.

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3- Something caught my attention. The mirror central unreflective area is very large. I estimated it to be around 6mm. That is very large and will have an impact on accuracy.

Jason

Jason, can you indicate on the image what you mean ?

@Tony - The scope is as it came from the factory, I've not checked the centreing of the doughnut...

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Jason, can you indicate on the image what you mean ?

See attachment... Assuming I know who to attach photos with the new software of this website

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3- Something caught my attention. The mirror central unreflective area is very large. I estimated it to be around 6mm. That is very large and will have an impact on accuracy.

Jason

Are you meaning this area as shaded - If so what is the cause and how should I make the inner reflection larger and thus reduce the area indicated ?

post-10726-0-75530600-1346610243_thumb.j

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OK between clouds I've managed two test pictures of Vega. One with the CLS filter and one without. Whilst not as pronounced as before the two additional spikes are stil there, in both shots so it rules out the cls filter as being the cause.

To be honest I think I can live with this, however I still have Daves 450D to try... just need a gap in the clouds now :(

post-10726-0-79663800-1346617258_thumb.j

post-10726-0-67287600-1346617276_thumb.j

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Comparison between original and post re-collimation

Original

index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=70764

And post

index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=71140

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Last image for tonight as the gaps in the clouds are now getting smaller and smaller. This one taken with a Coma Corrector fitted - isit me or is there now a beam around the 2 O'clock position ??

Time to give up for now....

post-10726-0-22667900-1346618780_thumb.j

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Hi, it's the other David.

When I print these and measure the angles I thing the angle between the spikes has reduced form around 7.5 deg to around 5 deg. I still makes me think they are associated with the secondary support vanes, their securing screws etc. However, I see evidence for a whole family of fainter diffraction effects around 30 deg or so (and multiples of).

Did this effect first appear after you flocked the tube? (When I first flocked my 200P, it had a habit of pealing at a corner and made a shadow on the mirror.)

I agree with you there does seem to be a family of faint diffraction effects around the 2 o'clock position and at 90 deg to that around the star.

D

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Did this effect first appear after you flocked the tube? (When I first flocked my 200P, it had a habit of pealing at a corner and made a shadow on the mirror.)

David,

Hard to say when I first noticed it... I will have to see if I have still got any test images pre-flocking, but I'm sure wishing I had left things well alone. When I had the scope in bits on Friday night there was no sign of the flock lifting.

The spider was tightened up really tight when I was doing the centering but there is always a possibility its still out in some way.

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Did this effect first appear after you flocked the tube? (When I first flocked my 200P, it had a habit of pealing at a corner and made a shadow on the mirror.)

Well here's an old image of Sirus pre-flocking and it looks perfect to me... :(

post-10726-0-43995000-1346621012_thumb.p

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Which now begs a question.... when you dismantled for flocking did you remove the primary?

If so - did you make witness marks so that you could put it back into exactly the same place?

My 300P is secured by eight screws - which makes 8 different possible orientations when refitting unless I mark one hole and the relevant position on the mirror support before hand.

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Which now begs a question.... when you dismantled for flocking did you remove the primary?

If so - did you make witness marks so that you could put it back into exactly the same place?

My 300P is secured by eight screws - which makes 8 different possible orientations when refitting unless I mark one hole and the relevant position on the mirror support before hand.

Errrr... yes I removed the mirror cell... and don't recall making any reference mark :embarassed:

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I wouldn't worry too much about mirror orientation. it certainly would not create the artifacts in your images.

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Are you meaning this area as shaded - If so what is the cause and how should I make the inner reflection larger and thus reduce the area indicated ?

No... That is not what I was referencing. Here is a photo from my collimated scope. The area you have highlighted looks OK.

post-17988-133877743399_thumb.jpg

This is what I was referring to. The unreflective area around the central pupil. It is too large which will have an impact on accuracy.

farpoint1

And here is a photo from my autocollimator's central pupil. Note how the background is darkened.

post-17988-133877464515_thumb.jpg

Jason

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I'm just wondering if you have those spikes because your mirror "may" be in a different position now and so is picking up either your focuser, or some of the fixing bolts that it couldn't "see" before?

I've not actually had my own mirror cell out - but looking at the mechanics of it there's certainly the possibility of it not necessarily being centred in the OTA. I'm not at all sure how important the centring of the primary is when it comes to collimation - all the focus in tutorials seems to be on the secondary; but it seems to me to be sensible to have the darn thing in the middle. Doesn't it?

And THAT is pre-supposing that your OTA is truly round in the first place! Mine's mostly sheet metal and I know for sure that I can make the front end oval when tightening the secondary spider - it's one of the things I had to sort out when collimating recently. (While the secondary was initially centred N&S and also E&W - there was in fact a difference of 1mm in the diameter between the N&S axis and the E&W one. I had to relax the N&S as a pair and tighten the E&W as a pair to have the secondary centred AND the OTA round instead of oval)

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Be sure your spider vanes are inlined. If they are not then you will have double spikes. The following image illustrates my point.

post-17988-133877769366_thumb.png

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Be sure your spider vanes are inlined. If they are not then you will have double spikes..

Jason, your picture on the right is exactly the situation I would expect to result in the effects we're seeing here. I would check this by having the tube upright and sighting a straight edge along the vanes.

One mad thought.

Malc: did you clean the mirrors after flocking? I wonder if there is a mark or scratch on either one of them?

Does the angle of the spike change if you turn the main mirror through 90 deg?

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I recently bought the farpoint tool and used it on my Quattro. I did see changes to the stacking if I rotated the auto collimator in the focuser but nothing as dramatic as you are seeing. On my scope the stack clearly shows four images of the centre spot and when stacked the middle goes darker. When I rotate the alignment of the stack will separate but they all stay within view. Applying pressure at various places will bring it back in but the pressure and movement required is minimal and too be honest I don't think it's a huge problem as there is likely to be small errors with the focuser squareness or the secondary and vanes. Personally I think the farpoint tool is as good an alignment tool as my Cheshire as if it looks good in one it looks good in the other.

Regarding your refraction spikes I've never seen that myself and as it appeared after strip down and rebuild I think it must be connected to that. If it were me I'd check:

- the spiders are diametrically correct by placing a straight edge across the scope opening

- try changing/swapping the vanes of the spider assembly around to see if the spike follows

- check the primary is central flat and un pinched by any fixings.

- Also inspect the primary carefully and look to see if there is any sign of how it was originally aligned in its fixings. You should try and find the original orientation of the mirror

- check you haven't introduced anything into the light path. Longer screws or alterations to draw tube or spider.

- blacken all screws and the draw tube section that protrudes into the light path.

- did you also stealth( blacken) your mirror edges and back. If not do this

All I'm suggesting here is to make sure nothing you changed has introduced changes to the way light is reflected internally blackening screws etc ensures that anything which may have caused a minor issue before flocking does not become a major issue after flocking

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I spent a huge amount of time on similar issue with my 200PDS.. Was never happy with it. I have to say it really did boil down to the spider centralisation, and making sure the spider is tight but not over tight causing flexing!. Also make sure the flocking is not effecting the main mirror cell on reconnection.

Take the time to go over & over the whole rebuild and recollimation. Also I found I finally nailied it once I ditched the laser and did it all manual with cheshire and sight cap!

Rob

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There's also the Farpoint Autocollimator which, while expensive, is cheaper than the Catseye stuff.

You are probably comparing it to the higher specified Catseye XLK autocollimator with secondary offset pupil. The Farpoint product currently being promoted by Astronomy-Shed/Modern-Astronomy is a copy of Catseye's single-pupil XL autocollimator. We haven't imported the XL because the enquiries received so far have been for the XLK but if we did it would probably retail at a similar price to the Farpoint copy, and almost certainly have a superior mirror and alignment. We'll order a handful soon and add it to our website.

Be careful when purchasing an autocollimator. All have an eyepiece case, a mirror, and all show multiple reflections of the primary mirror center spot. But that does not say much about the precision of the tool.

An autocollimator has to meet a high standard of quality and precision. If it lacks precision then it will not provide any additional collimation improvement beyond a mass produced laser collimator.

That is a good summary :smiley:

HTH,

Steve

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