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Help with astigmatIc 14 inch mirror

mdstuart

By mdstuart
in DIY Astronomer

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mdstuart

mdstuart

Posted
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Help!

Is there anyone who has made there own mirror with a test bed that would test my mirror and re figure it?

I could then get galvoptics to re silver it

Anyone offer this service?

Mark - Bristol

Otherwise I will need someone to teach me!

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Peter Drew

Peter Drew

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There are many reasons, as Moonshane has pointed out, for a mirror to appear to be astigmatic, best to investigate these before suspecting the mirror. If it does eventually turn out to be astigmatic it is actually possible to mount it in such a way to "bend" it back into shape, in other words the reversal of the condition whereby the mounting system can introduce astigmatism into a good mirror. I would initially rotate the primary if possible to see if the astigmatic effect rotates with it. :smiley:

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mdstuart

mdstuart

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Thanks all..yes it existed from day 1. I don't think its the clips and the main mirror is not siliconed down..might be the secondry but I read the astigmatism normally is at right angles to the tube for secondry ast.

I am away this week. Will rotate the primary next..

Love the anti ast mount idea.

At the moment the mirror is held by just three clips and a flat foam base..it holds collimation well at all tube angles.

The ast is not visible in the 24mm eyepiece but becomes evident at 8mm with small airy disks..

Stars look like small messes rather than dots...

Thanks again for the input so far..will report back after rotation of the primary..

Mark

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Moonshane

Moonshane

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I'd change/mod your cell. a flat foam bed does not sound that ideal. try and make some sort of 6-9-18 point set up and I bet it would improve. even three 1" blobs of silicone on a solid flat surface would be better I think but I'd make a 9 point cell. it's easy to do.

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mdstuart

mdstuart

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Thanks.

The paturn is very distinctive..the airy disks go oval just before reaching focus and the reappear on the other side of focus at 90 degrees to the first oval..

Def...astigmatism...

So I will turn the primary and see if that moves the ovals then we know its the main mirror..

700 for a new one from G to beat!

Mark

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Astrobits

Astrobits

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Yup. That is classic astigmatism.

However, turning the mirror round might not show any change if it is being caused by the cell deforming a good mirror. Does the astigmatism change when viewing stars near the zenith and horizon. That might show up the cell contribution, if any.

As I mentioned previously You are welcome to bring the telescope down here and I can have a look at the mirrors. I am about 45 minutes from Thornbury.

Nigel

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mdstuart

mdstuart

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Moving the mirror today.

I am trying to use a simple led star and looking at the reflection using a knife edge but to be honest all I can see is the led magnified so that the whole mirror is white and then when I move my eye behind the knife edge then I only see part of the mirror..I am so no good at this..

Anyway mirror turn 45 deg clockwise next then a star test tonight.

Mark

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mdstuart

mdstuart

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Update..

Mirror rotated 45 degrees clockwise.

So with 0 degrees being down..90 degrees being towards the front and 180 degrees being up and 270 being towards the primary mirror..

Starting with the eye piece out as far as it will go and focusing in the nice circle of a bright star gets smaller and then goes oval with the long axis from about 030 to 150..

As you go further in and out of focus the oval shifts and the long axis is from 60 to 240..

Interestingly I estimated 160 and 070 before and its now 150 to 60 despite a 45 degree clockwise rotation of the mirror..so perhaps similar?

So maybe its the secondary...or the cell which has not been rotated...yet!

So I might try and use another small secondary I have to see if that helps tomorrow night..

I did try stars from low to overhead and the above observations were identical. The cell seems to keep collimation well.

I also tried various secondary angles and eye piece positions but that had no impact on the issue to eliminate a focuser / secondary alignment issue.

I feel a little better tonight given the mirror rotation had little effect.

Mark..

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mdstuart

mdstuart

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Update

I put a small secondary in from a 114mm scope but it was very difficult to get it in the centre of the tube etc...however it was worth it as the star images looked a lot better. No astigmatism of note... I need to work out how to rig it up so that its collimated etc but first results look better than the existing secondary...

Will report back after another evening of testing!

Mark

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John

John

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That sounds promising Mark.

I've read of pinched secondaries causing astigmatism with Meade Lightbridges. The solution there was to remove the secondary from it's holder and re-mount it with silicon on the back and no clips.

Alternatively the secondary could be of mediocre quality and a decent replacement could work wonders.

I hope you continue to make positive progress with this :smiley:

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Astrobits

Astrobits

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Yes, seems that you have narrowed down the problem to the flat/flat mounting.

Re: Johns suggestion to mount a flat with silicone. This is my normal method. You only need a blob of silicone in the centre of the flat ( approx half the diameter of the flat ) and make it about 1/8" ( 3mm ) thick. This prevents any flexing of the flat coming from the holder. No side clips are needed, the bond is very strong.

Nigel

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mdstuart

mdstuart

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Thanks Nigel.

i am going to retry tonight with the other secondry.

i will try to get the original secondry off. Looks like its silicon but very thin and all of the back looks to have been stuck to the holder.

mark

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mdstuart

mdstuart

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Secondary now taken off holder. It was siliconed on but all the way around the edge. I cannot believe that the way it was attached is an issue but I plan to create a new holder and go down the three silicon blob route.

now I need to make a new 45 deg holder as the existing hollow plastic one is not ideal. I would welcome ideas. Might get some aluminium disks and tubes and come up with something?

mark

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Moonshane

Moonshane

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a hollow secondary holder is not an issue. all my dobs have a ali pipe with one end sealed square and a hole tapped for the bolt, and a 45 degree cut at the other end leaving an elliptical shaped hole to allow three blobs to attach the flat to. gluing all the way round is a good way to induce astigmatism. either three points or a single point is best as the flat can flex with heating and cooling (eventually reaching a flat state at equilibrium) rather than being stretched and twisted by heat/cold and being unable to move.

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Astrobits

Astrobits

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+1 for Shane

If you are concerned about the plastic pipe being too poor to silicone to you can fill the pipe with Plastic Padding or similar to give a more solid surface. Just put two or three screws through the pipe and into the filler to make sure it doesn't fall out :eek:

Nigel

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Moonshane

Moonshane

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I should also add that it's important to use some spacers whichever way you glue it - 2mm is adequate - maybe match sticks or similar in between the blobs or outside of the single blob (to be removed when set). personally I prefer a safely rope too. I normally use a piece of good quality cord, tie a small washer to it and glue this with a blob of silicone to the back of the secondary. the other end tie around the nearest spider vane. it's unlikely to ever come off but if you want belt and braces........

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