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Pinched mirror or focuser drawtube?


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I'm trying to improve the shape of my stars and could do with some advice from you good folk.

I use a SkyWatcher 200PDS newt with the stock SW Crayford focuser powered by a Primaluce Sesto Senso.  My QHY268M is screwed to the end of the focuser draw tube.  When it's at focus, I have a couple of cm of drawtube sticking into the scope.  So, I'm thinking that is the cause of my problems.  But I'm reluctant to replace the focuser unless I'm sure it's causing me a problem.  I've considered the Baader Steeltrack but it's not a painless swap over as I'd need to drill new mounting holes in the scope.  I'll do it if I need to but I'd rather avoid it if it's not necessary.  I'm also happy with the stock focuser otherwise, it handles the weighty camera with filter wheel and OAG without any slippage problems.

Is it possible that my problem is more down to a pinched mirror?  I've read that it's a common problem on SW scopes for the mirror clips to be too tight to the primary.

I did a star test last night to try to diagnose the problem:

Extrafocal:

image.png.44b18dfa7bffcb80f22500c15f502167.png

Intrafocal:

image.png.98d1d7db0b8ce08e79558eae4ad0f469.png

 

The line on the right of the extrafocal test and on the left of the intrafocal test (a lot fainter but still visible) suggests to me that the drawtube is the problem, but TBH I don't have much of a clue of how to interpret these images.  I'm just guessing.

Any suggestions?

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Another easy enough test is to remove the mirror cell completely and check for mirror fit/clearance.
Just a few screws to remove from the periphery of the tube.
If you mark the tube and cell (masking tape) so you reassemble in the same orietation, it will minimise collimation effort.
You should have sub millimetre lateral clearance to the mirror at room temperature.
The clip screws should loosen the clips with a fraction of a turn. If not you are pinching the mirror.
If in doubt, re-assemble with the mirror definitely loose.
You may get an off collimation star test result - but no spiky bits this time will indicate you had mirror pinch.

HTH, David.

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5 minutes ago, Carbon Brush said:

Another easy enough test is to remove the mirror cell completely and check for mirror fit/clearance.
Just a few screws to remove from the periphery of the tube.
If you mark the tube and cell (masking tape) so you reassemble in the same orietation, it will minimise collimation effort.
You should have sub millimetre lateral clearance to the mirror at room temperature.
The clip screws should loosen the clips with a fraction of a turn. If not you are pinching the mirror.
If in doubt, re-assemble with the mirror definitely loose.
You may get an off collimation star test result - but no spiky bits this time will indicate you had mirror pinch.

HTH, David.

Thanks, David.  You make it sound so easy 😛 I might have a go at that when I'm feeling brave. 

So, having marked the position of the cell with the tape, I just remove the four crossheaded screws that are just up-tube from the white collar (I assume the white collar is the outer part of the mirror cell?)?  Then the cell will drop out of the tube?

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Hi Steve. Yes easy enough.

There are screws in the black paint just forward of the white ring. Shown in pic 8106.
This is a 200P and there are 6 screws. Indicated by the screwdriver.

Moving to pic 8107. This is a 250P. Same principles, just a bit more support for the mirror in the centre of the cell.

If you look in the for sale section, I have listed a 300PDS tube. Again 6 screws.
Slightly different again mirror cell centre. But the same principles.

Usually the screws are M4. I usually put a tiny bit of silicone (clear) grease on the screws on reassembly.
Work on the assumption (as I do) that the grease is made from a platinum/gold alloy and you are a miser😁.
The grease won't melt and run out on a sunny day. It prevents binding from corrosion in years to come.
You are of course leaving the scope out on dewy nights🤔.

HTH, David.

IMG_8106.JPG

IMG_8107.JPG

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It doesn't look like a mirror problem to me.  There is definitely something interfering with the light path, 20mm protrusion of the focuser shouldn't impact the light path though if it's shiny chrome it might well have an effect.     🙂

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I think Peter has a valid point.

The vertical artefacts differ significantly between intra and extra focal images.
Skywatcher have a habit of fitting shiny drawtubes to their (dark grey painted) tubes.

If this is happening, then black paint on the drawtube and/or tube flocking will help.

An interesting trial would be to put a bit of black card on the drawtube - held by an elastic band.

 

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5 hours ago, Carbon Brush said:

I think Peter has a valid point.

The vertical artefacts differ significantly between intra and extra focal images.
Skywatcher have a habit of fitting shiny drawtubes to their (dark grey painted) tubes.

If this is happening, then black paint on the drawtube and/or tube flocking will help.

An interesting trial would be to put a bit of black card on the drawtube - held by an elastic band.

 

Thanks, Peter & David.  That's my job sorted for the next clear night - I'll try the card test

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Rather than wasting clear nights - or having to wait for a clear night - why not use an artificial star?

These do need to be placed a long way from the scope. Some are usable in daylight.

I took a glance at FLO offerings and you can spend £19 or £117.
Probably worth looking on SGL for others experiences and comments on these.

 

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OK, it's official.  I'm an idiot.

Here is my test from last night

image.png.15ab48eda3dc8e58f14fa00ab14532eb.png

Quite an improvement, isn't it?  What did I do to get such an improvement, you ask?  All I did was chose a test star which was NOT obscured by the overhead powerlines above my garden!  Who'd've thunk it!

  • Haha 5
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