Collimation help sought

[pete_n_pugs]

I've read quite a few articles and watch videos, but I still can't quite get my head round it... I have a Explore Scientific Ultralight Dobsonian. This has two 'features' that I'm not entirely sure that I understand the impact of on collimation. Firstly there is no adjustment of the secondary up and down/along the primary axis. That is related I guess to the second item - the secondary is offset. So given those constraints, what should I be looking for in terms of alignment? I bought the Ocal collimator as I wanted to confirm the position of the secondary, and the Cheshire I have was confusing me as to where the secondary should appear. Attached is an image from the Ocal. Should the secondary be concentric with the focus tube? I should have taken another photo, but the red circle should be aligned to the focuser tube, and the green aligned on the outside of the secondary. When I properly checked collimation this week, the secondary seems to be offset from the center of the focuser tube. Is that expected? Adjusting the tilt of the secondary is the only option I have (although there is some possible adjustment in the spider - the implication in the manual is that this offset is set at the factory and should not be altered). Should adjusting that move the location of the secondary in the focuser tube? I will endeavour to get a better photo. The mirror doesn't seem to be perfectly round - is that what I need to adjust the tilt of the mirror for - to get it circular? And if it is off center by design, I can realign the crosshairs to the center of the secondary, and use that for the primary alignment. Perhaps I would be better off buying a decent quality laser collimator, but they don't seem to be in favour. 

Related to this, I tried to do a star test, but I've not been able to see any airy disks. I've a Baader zoom eyepiece, so perhaps I need to use the barlow as well? I can see a donut, with the 'hole' in the center is well centered either side of focus.

[dweller25]

Can you re-photograph with a piece of white paper opposite the focuser, it will help.

[dweller25]

It looks like the secondary is offset away from the primary mirror but you say you cannot adjust it, which is unusual.

 

Edited November 17, 2023 by dweller25

[Ricochet]

Does your secondary stalk look like this? 

If it does you have all of the normal secondary adjustment options. 

The offset refers to how the mirror is glued to the stalk. You do not need to think about it to collimate the secondary. All it means is that you only have to check that the central bolt is centred in the tube to start collimation. 

When you look through the focuser the secondary should appear to be round and circular. 

Once you get to collimating the primary the shadow of the secondary will appear to be offset but you don't need to worry about that yet. 

You also need to put some paper or card between the secondary and primary while you are doing this first step so that the reflections from the primary do not confuse the issue. You have placed the green circle in your image around the reflection of the primary, not the edge of your secondary. 

[dweller25]

Here is an image of the secondary support in the ES Dob, it’s not standard !

[dweller25]

Here is how it should and should not look, the problem could be that your secondary is not correctly positioned under the focuser - as in the “incorrect” image below.

Edited November 16, 2023 by dweller25

[Ricochet]

8 hours ago, dweller25 said:

Here is an image of the secondary support in the ES Dob, it’s not standard !

No, not standard at all. It does look like there really is no way to move the secondary towards the primary to centre it under the focuser. It looks like the secondary is oversized so there might be enough tilt adjustment to get all of the primary in view. 

[John]

With the Explore Scientific secondary mirror support design I am unsure how you would adjust the position of the diagonal along the optical axis of the primary, which seems to be what is needed to centre it under the focuser 🤔

The tilt of the diagonal is clearly adjusted by the 3 screws pictured in the photo that @dweller25 has posted above. But how can you move the secondary mirror closer to or further away from the primary with that design ? (which I think is what is needed)

Edit: I can see that @Ricochet is having similar thoughts !

 

Edited November 16, 2023 by John

[Ricochet]

18 minutes ago, John said:

Edit: I can see that @Ricochet is having similar thoughts !

Yes! I’ve looked through countless CN threads trying to find out how to collimate this model and all I can gather is that this is the first time anyone has used something other than a laser to check the collimation. 

[pete_n_pugs]

Thanks for the thoughts so far - glad I'm not being too obtuse. Here is an image with card behind, and the primary obscured. Looking at the schematic above, do the outer two circles represent the focuser tube and the secondary?

 

[pete_n_pugs]

This might be clearer. I notice that the spider has thumbscrews - I'm guessing it might be possible to move the secondary with those?

[pete_n_pugs]

This is one of the thumbwheels on the spider I am referring to, but on reflection (excuse the pun) I imagine it can only adjust it left to right, or towards and away from the focuser tube. Hopefully I am using the right nomenclature. 

[pete_n_pugs]

It may be that I can adjust the distance to the primary by sliding the whole secondary having loosened the screws clamping it to the spider - if the holes were slotted. But I would need to remove the secondary to access the third screw - which would make the process of aligning very tedious as it can only be checked with the secondary reattached. Is it worth pursuing - or should I just make sure the secondary and primary are aligned?

[John]

The movement that I think might be required to centre the secondary in the focuser tube is along the optical axis of the primary. I have indicated with red arrows that direction of movement:

 

[globular]

Is that a gap between the truss and the top - the cream colour looks like the door frame behind.  I'd expect this, and all the other trusses, to be fully flush fitting.  If not there could be some twisting. 

[dweller25]

9 hours ago, pete_n_pugs said:

do the outer two circles represent the focuser tube and the secondary?

 

Yes.

Your secondary needs to move towards the primary and slightly in the North/South direction.

Is there any wiggle room in the spider, secondary mount or trusses ?

Edited November 17, 2023 by dweller25

[dweller25]

9 hours ago, pete_n_pugs said:

Is it worth pursuing - or should I just make sure the secondary and primary are aligned?

 

It’s worth getting collimation as good as possible to get the best out of the optics.

Have you checked the collimation at high power (x300+) at perfect focus ?

If the scope is fully cooled and seeing is good you should be aiming to see a perfect airy disk.

Edited November 17, 2023 by dweller25

[pete_n_pugs]

I've struggled to get anything resembling an airy disk, but that may well be due to lack of skill, and possibly seeing conditions. I'll also check the secondary holder. I want to enclose it and also move the mount points for the trusses anyway. Thanks all for the help so far.

[dweller25]

1 hour ago, pete_n_pugs said:

I've struggled to get anything resembling an airy disk, but that may well be due to lack of skill, and possibly seeing conditions. 

It could be aperture related

[Spile]

Remove the piece of paper hiding the primary as the reflection of the secondary is a valid reference point.

[Captain Scarlet]

22 hours ago, pete_n_pugs said:

… Is it worth pursuing - or should I just make sure the secondary and primary are aligned?

Yes to the second part of that question. The _position_ of the secondary only affects illumination levels, it’s not really collimation proper, i.e. “alignment of the optical elements”, which are eyepiece and primary mirror. With a slightly misplaced secondary but otherwise collimated focus-tube and primary, you will still get views as aberration-free as they can possibly be. You will not notice the illumination difference visually with a secondary slightly out of position.

Ideally of course you’d like that secondary to be symmetrically straddling the primary light cone, but more important, by far, is to align the focus-tube axis with the primary mirror axis.

Magnus

Edited November 17, 2023 by Captain Scarlet

[pete_n_pugs]

Hello again, I have made progress... of sorts. I managed to move the focuser tube away from the primary - which should have the same effect as moving the secondary towards the primary, but not by enough to get it centered. I've been thinking about ways to get it actually central, but they would be quite mildly to quite drastic. I could make a different plate to hold the secondary, with the threads for the secondary adjusters further towards the primary, but currently the threads go through the spiders too, so the would need drilling or slotting appropriately. I could replace the entire spider and secondary holder with any adjustable one... For now I've decided to make do with what I've got. I have flocked the inside of the top part (not sure what it is called - that the trusses, focuser and spider attach to - the open frame), and I'm trying to make a shroud - all of which should help. 

However, I am now somewhat confused as to what I should be aiming for with my collimation, if the secondary is off center. Should I be adjusting for the primary to be in the center of the secondary, or the focuser tube. I went with the secondary - image below. I moved the yellow circle to be the outline of the secondary, got the primary (blue) in the center of that by adjusting the secondary, then adjusted to primary to get the purple cross on the center of the camera. This can be seen better in the second image (using a red led torch) to illuminate the camera. Is this almost collimated? I did get it slightly better, but rushed outside before taking another photo.

As an aside, how would I collimate this in the dark? If I went to a dark site, the whole thing would need reassembling, so would need re-collimating. I'm guessing that this is where the laser collimator would come in handy? I tried using the OCAL with a red torch, and that seemed to be OK - is that an option?

Thanks!

 

 

[Spile]

I would suggest placing a piece of paper behind the focus tube and, taking the photo through a cap with all of the edge of the sight tube visible .

aka…

[pete_n_pugs]

Thanks. I know the primary is covered in this pic, but this is about as good as I could get the secondary aligned (having moved the focuser tube since the secondary cannot be moved relative to the primary). I used the adjustment on the spider to get it as close as I could to centered left/right. Will have another go tomorrow. The OCAL is quite useful as it can at least take a picture - otherwise it's my phone which is rubbish!