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Hi, I've been struggling with the stock focuser on the 130PDS for a while. The main problem is the tilt on the focuser tube.

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I ended up with stars like this:

1705216787_Stars1.thumb.jpg.6289e9aa9a18ed44b0f1a649ae6b29fc.jpg

There doesn't seem to be anyway I can find to get rid of the tilt in the focuser tube. Nothing is overtightened if I loosen the four shaft bolts it doesn't make a difference. The tilt only goes away when the focuser is completely slack

After re collimation and adjusting the tilt adjusters on the focuser I got this.

463179073_Stars2.thumb.jpg.636ba4ecc7e66a042417aa8d81509cbe.jpg

This is a iphone picture of the collimation:

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I'm just wondering what to do from here. The collimation looks pretty near to me. Using the tilt adjusters on the focuser don't seem to be helping that much and only one is accessible while using the focuser anyway. Very much open to ideas.

 

 

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Edited by cuivenion
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To me it looks like you have some rotational error in your secondary Mirror that will cause some problems, to set the focuser tilt properly you need to take the whole secondary assembly out and mark the point directly opposite the centre of the focuser hole in the tube it’s easier to take the whole focuser off aswell. Bit of a faff but once done it shouldn’t need doing again. Re the rotation error in your secondary can be sorted with a good Cheshire 

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24 minutes ago, johninderby said:

Or just use a Concentre which is the most accurate way to set up a secondary and doesn’t require taking anything apart.

That’s not entirely correct, squaring the focuser to the tube does need taking off and marking on the opposite side then putting the focuser back on with a Cheshire then adjusting the focuser so the cross in the Cheshire is pointed at the mark, a concenter won’t do that but will sort the secondary out with tilt or rotation, I use a concenter myself and agree it’s a must have particularly for the secondary but it won’t sort the focus tube out 

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Your stars look a bit triangular to me, indicating the primary mirror might be being pinched by the clips.

Edit: could also be the secondary mirror if it isn't attached to its holder properly.

Edited by CraigT82
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Posted (edited)
2 hours ago, CraigT82 said:

Your stars look a bit triangular to me, indicating the primary mirror might be being pinched by the clips.

Edit: could also be the secondary mirror if it isn't attached to its holder properly.

Hi, the stars are triangular in the second image but not the first and I've been careful with the primary mirror clips because I've made that mistake before. I think you're both right and the secondary is out. I'm not sure what you mean by not attached properly though.

Edited by cuivenion
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Posted (edited)
3 hours ago, Craig a said:

To me it looks like you have some rotational error in your secondary Mirror that will cause some problems, to set the focuser tilt properly you need to take the whole secondary assembly out and mark the point directly opposite the centre of the focuser hole in the tube it’s easier to take the whole focuser off aswell. Bit of a faff but once done it shouldn’t need doing again. Re the rotation error in your secondary can be sorted with a good Cheshire 

Hi, how did you recognise the rotational error? I do have a cheshire which one of the images above is from but I'm not sure whats indicating the rotational error.

Edited by cuivenion
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41 minutes ago, cuivenion said:

I'm not sure what you mean by not attached properly though.

If the secondary is glued across it's full width to the secondary holder it can cause stress in the mirror, when the glass and the holder material cool and contract at different rates at night.

This shouldn't be the case though if it hasn't been messed with by someone previously... it should be fine if it is as it came from the factory. 

If it is tilt you're concerned about you can confirm it by downloading CCD inspector (you get a 30 day trial period) and running the images through that. 

Edited by CraigT82
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47 minutes ago, cuivenion said:

Hi, how did you recognise the rotational error? I do have a cheshire which one of the images above is from but I'm not sure whats indicating the rotational error.

I can see the rotation error just by looking at the pic you posted of the view through the focuser, it’s laying left up right down in the view, hard to judge which way it’s rotated from the pic but I’m 99.9% certain there is a error there it’s not a circle but more oblong indicating a rotation 

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16 minutes ago, Craig a said:

I can see the rotation error just by looking at the pic you posted of the view through the focuser, it’s laying left up right down in the view, hard to judge which way it’s rotated from the pic but I’m 99.9% certain there is a error there it’s not a circle but more oblong indicating a rotation 

Just to be sure is area A or B you're looking at? Thanks for your help.

1239039022_IMG_02472.thumb.jpg.b0c9276615d8c3ab3100888b3124614a.jpg

 

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It’s A I’m looking at, that’s your secondary it’s not circular so that means it’s rotated, B is the reflection of the Cheshire’s silver diagonal 

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Just now, bottletopburly said:

Astap has a ccd inspector for measuring tilt built into its tools menu 

Nice, I didn't know that. I'll check it out.

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15 minutes ago, cuivenion said:

Nice, I didn't know that. I'll check it out.

Quote I read on astrobin from Astap wether tilt is within tolerance 

Sensor tilt as the percentage ratio between the best and worst corner median values. In addition as an graphical indication it draws an trapezium in the image based on the four median values.
There can be some variation in images of the same series, so a tilt of maybe 20% looks normal but anything more indicates a camera mounting problem. The lower the value the better. The unit if expressed in delta HFD.

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6 minutes ago, Spile said:

What is the result of a star test?

Just finishing up another collimation attempt. I'l hopfully do a star test tonight and show you an image.

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On 05/04/2021 at 17:13, Craig a said:

It’s A I’m looking at, that’s your secondary it’s not circular so that means it’s rotated, B is the reflection of the Cheshire’s silver diagonal 

That looks like secondary offset to me Craig because its F5 Example on here

Collimation - Newtonian Telescope - Reflector - Astro Baby's Guide to (astro-baby.com)

Example CN. (not mine)

The offset is exaggerated. Or even off. But the shadow is showing its effect which is the point

 

 

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Edited by neil phillips
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The offset is I believe how it looks as in the spider doesn’t look central and is offset slightly against the crosshairs of a Cheshire , the secondary should still look round, my own f4 scope which is faster is round when set using the concenter, maybe I’m wrong 

Edited by Craig a
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This is an image from the internet of a f5 correctly collimated, note the secondary edge, the third green circle inwards is round 

3DA13D85-D00A-48D1-A290-05897C3C68C3.png

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On 05/04/2021 at 17:13, Craig a said:

It’s A I’m looking at, that’s your secondary it’s not circular so that means it’s rotated, B is the reflection of the Cheshire’s silver diagonal 

 We may be on different pages craig ? i thought you was referring to the offset shape of A. Was worried the poster would interpret this comment to mean the shadow A Should be circular. Which in a F5 scope its offset from circular.  

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Has anyone mentioned Dion’s Astronomyshed tutorials on YouTube? A great addendum to Astrobabe’s document 

I successfully collimated an f4 reflector based on these 

Edited by iapa
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Well I did a star test but the results wouldnt be helpful as I realised that the coma corrector/camera was tilted after I took them. The stars were better after I corrected this but still not right.

The day before imaging I realigned the focuser using astrosheds video as a guide and also shimmed the bearing plate on the focuser to try and fix the tilt in the focuser tube. I also redid the collimation.  It all looked good until the focuser tube stated to tilt again the day after. Looks like this is going to be an ongoing process.

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