Having trouble collimating my Explore Scientific dobsonian

[Wilseus]

Hello, 

To cut a long story short, for a long time I have not been able to get my dobsonian collimated correctly. The best I've been able to get is the spot on the primary aligned with the middle of a collimation cap (dob1.jpg), but the whole lot is off centre of the crosshairs when I insert a cheshire eyepiece (dob2.jpg)

I contacted Explore Scientific's tech support who suggested adjusting the secondary mirror, which of course I have tried, but to no success. After quite a bit of fiddling, I discovered that the assembly that holds the secondary is clearly nowhere near perpendicular (dob3.jpg). It can clearly be seen that an imaginary line perpendicular the plane of the secondary mirror assembly points way off to the top right. It must be several degrees out.

I experimented with tightening the bolts at top right and bottom left and loosening the other two, to try and increase the tension bottom left to top right to rotate it anti-clockwise, but this was unsuccessful.

Can anyone shed any light on what is going on here? Am I doing something wrong? Is the scope faulty, bad QC perhaps?

Edited June 12, 2023 by Wilseus
Removed superfluous comma

[StarDuke82]

You adjust the angle of the secondary via the three knobs behind the secondary mirror, the spider vanes or assembly as you call it not being perpendicular has very little if no affect on the position of the secondary as long as they are relatively the same at both end points.

Edited June 12, 2023 by StarDuke82

[Wilseus]

OK, but whatever I do, I am unable to get the secondary centred in the middle of the focusing tube (pic 1.) Are you saying this doesn't matter?

[bosun21]

I'm not familiar with the ES secondary holder but looking at it it's the large central screw that moves the mirror forward and backward. You will need to turn it clockwise to move the mirror towards the front (open) end of the scope and vice versa. You will also need to loosen the other screws to allow this movement. Once centered in the focuser tube adjust the other screws so that the mirror appears perfectly round. Good luck.

[Kon]

I had a very similar issue with my 200P, regardless of how I was moving my secondary. If your focuser is not square with the secondary it will not collimate easily or it will 'not hold collimation'. In the 200P, the 1.25" extension adapter in the focuser is a bit wobbly and every time I would collimate and recheck it would appear as if it was off or I could not collimate it. The solution was to rotate the extension tube until it was square and getting the Cheshire on/off, rotate along did not change it. You may have a similar issue.

[JOC]

This is the definitive guide that SGL normally recommends https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/  It is worth noting that even when perfectly collimated some telescopes still visually appear to be 'off centre' per the description in that guide.  Perhaps you are spot on, but don't know it?

[doublevodka]

Defo follow the astro baby guide above, it is very good.

Manual for what looks like your scope attached which may help a little, specifically page 16.

6a779054279e1f0d788601fa8aba78ec.pdf

To my eye, it looks like the secondary vanes are very slightly off too, may be worth measuring from the screws marked in red, to the center marked in green.

 

 

Alternatively, there are quite a few astro clubs in the area, worth joining one and maybe there is a member that could help if you are really struggling with it?

http://www.astronomyclubs.co.uk/Clubs/Default.aspx?CountyId=55

[AstroKeith]

20 hours ago, Wilseus said:

OK, but whatever I do, I am unable to get the secondary centred in the middle of the focusing tube (pic 1.) Are you saying this doesn't matter?

You can stilll collimate with the secondary off centre, but it makes it more difficult and you will likely be losing quite a lot of light missing the mirror.

I've overlayed some equal squares over your image. which suggest the spider is OK as the squares match the suspension points.

But the red line suggest the focuser is mounted slightly to the left.

[StarDuke82]

22 hours ago, Wilseus said:

OK, but whatever I do, I am unable to get the secondary centred in the middle of the focusing tube (pic 1.) Are you saying this doesn't matter?

According to the manual the secondary is supposed to be offcenter slightly do you not have the manual for it? Hopefully the image is allowed and will be of some help, next check the center point of the main mirror in the secondary if the center point is not in the secondary adjust the secondary by using the three collimation screws at the back of the secondary. 

Edited June 15, 2023 by StarDuke82

[Peter Drew]

The image shown by AstroKeith perfectly shows the issue.  Although the radial offset of the focuser is relatively unimportant provided that the focuser is square to the tube, it's rather sloppy construction by the manufacturer as it means the buyer has to rotate the secondary to remedy it.  ES is correct in their description of the secondary offset, the cone of light from the main mirror is wider at the lower half of the secondary than it is in the fop half.  Ideally, a Newtonian secondary should be pear shaped but this would be more difficult to manufacture.   🙂

[AstroMuni]

3 hours ago, Peter Drew said:

Ideally, a Newtonian secondary should be pear shaped

Sorry, didnt understand this. Can you explain please?

[Wilseus]

Thanks all. I think I figured this out. However, when I said the secondary was offset, I meant it was too far to the "east" of my top down view, not too far away from the eyepiece or "south" as @StarDuke82's manual excerpt implies. Interestingly enough, I do have the manual for my scope, but it does not have this paragraph, and in fact it does not mention offsets at all. It's quite different from the one @doublevodka attached. I think mine's a later version of that scope, a "Generation II"

What I did was I adjusted the spider vane screws until the secondary was in the middle of the focuser. I got this more accurate by inserting a cheshire eyepiece and then adjusting the focuser so I could match up the outline of the secondary with the outline formed by the end of the cheshire.

Then I adjusted the secondary angle so that the reflection of the secondary in itself was centred on the cheshire crosshairs. After that I adjusted the primary so its central dot aligned with the centre of the crosshairs, then I repeated these list two steps until everything lined up.

[Spile]

I use a pair of compasses to level the focuser and to centre the spider as a preliminary check. 

[Peter Drew]

7 hours ago, AstroMuni said:

Sorry, didnt understand this. Can you explain please?

The light cone from the primary mirror gets larger as the cut off position of the secondary gets nearer to it.  A secondary just large enough to accept the full aperture of the primary at the centre line of the secondary would be slightly oversize above the top half of the secondary and slightly undersize on the bottom half.  Theoretically, a pear shaped secondary with wider portion towards the primary would correct this.  Due to the mentioned difficulty of manufacturing such an awkward shape, secondaries are usually elliptical and slightly oversized, offsetting them also helps to address the issue.       🙂

[Louis D]

@Peter Drew are you sure about that?  Isn't the secondary cutting an elliptical conic-section from the light cone, and aren't all ellipses symmetrical across both axes?

I assume secondaries are diagonally cut from cylinders and are also ellipses.  Shouldn't it be possible to get the major and minor axes to match that needed to match that of the cone's ellipse?

[Peter Drew]

Geometry suggests so.  If you draw an axial centre line and converging beams from the primary, cutting through the light cone at 45* shows that the cone radius is greater below the centre line of the diagonal than that above the centre line.  The "faster" the primary mirror is, the more this is exaggerated.     🙂

[AstroKeith]

The required secondary mirror shape is an ellipse. It's not obvious so, but it's classic conic sections. You might think the lower half has a different shape, but you need to consider that the position of minor axis diameter in such a conic section, moves off to one side. Hence the need to offset the mirror.

[AstroMuni]

32 minutes ago, Peter Drew said:

Geometry suggests so.  If you draw an axial centre line and converging beams from the primary, cutting through the light cone at 45* shows that the cone radius is greater below the centre line of the diagonal than that above the centre line.  The "faster" the primary mirror is, the more this is exaggerated.     🙂

My understanding was it should be an ellipse (atleast serving from memory of good old orthogonal projections 🙂 ). Like this https://en.wikipedia.org/wiki/Conic_section#/media/File:Conic_Sections.svg

Am I missing something here?

[Wilseus]

I always thought this meant that as others have said: the secondary is still an ellipse, but the centre of the field of view from the eyepiece would NOT be in the dead centre of the elliptical mirror if you looked at the mirror straight on. Or something like that.

[Peter Drew]

Precisely.  The ellipse is still the most effective solution for manufacturing purposes.   🙂