Any advice on secondary collimation?

[billyharris72]

Hi all:

I'm carrying out a bit of an overhaul on my 8 inch Dobsonian, and am struggling to adjust the secondary collimation. Not helped by a secondary holder that appears to have a mind of its own.

Here's where I've got to.

1) The secondary is (more or less) aligned with the focuser tube and appears (approximately) circular.

2) When shining a (well collimated, I've checked) laser through the focuser and looking in the bottom of the tube (primary removed) the spot appears on the centre of the mirror.

3) With the primary in place, the laser dot hits the centre of the centre spot on the mirror.

4) Through a Cheshire, the eyehole and the primary spot are aligned.

My main concern is with steps 1-3, which, from what I have read, would seem to suggest the secondary is collimated. The problem is I don't think it is, because when I look through a collimation cap or a Cheshire the primary does not appear to the properly centred in the secondary. This is more obvious with the focuser racked in. The Cheshire seems to be aligned perfectly with the secondary spot, and I can see all three slips (in the cap, not always the Cheshire), but part of the mirror becomes obstructed as I rack the focus in.

I've pasted some photos in below to illustrate what i mean (shot with my phone though a collimation cap).

Any suggestions as to what could be going on here? Could it be a focuser issue or am I missing something obvious?

Thanks,

Billy.

[alanjgreen]

measure the veins holding the secondary. Are they all the same length? It doesnt look centered.

Either the whole assembly is off or maybe the focuser is not aligned correctly?

[SloMoe]

Here are some images that might help.

 

Image Credit : Jason D. 

[billyharris72]

Thanks for the replies. Looking at the images again and doing a bit of messing around in Gimp with the straight line tool seem to suggest that the focuser is reasonable square, at least in the "left-to"right" orientation. There's a line on the bottom of the focuser tube that should be pretty well aligned with the optical asis, and extending it takes me dead through the centre of the secondary holder.

Looking at the spider vanes showed a little bit of error - 91mm on one side to 93mm on the other, but correcting this actually moves the mirror out of the line of the focuser, not into it, so I'm thinking that difference is not a problem (tube may be slightly distorted).

What I also discovered when I went back to it, however, is that the secondary seems a bit high (i.e. too far from the secondary). When I loosened the secondary holder bolt I found I could get the mirror more centered, but the collimation bolts are now a fraction too short. These were replacement bolts (with thumbscrews) that I got a while ago and I'm wondering if, when I fitted them, I've "fixed" their shortness by tweaking the central bolt, pulling it out of position.

Anyway, new slightly longer bolts are now on order.

Billy

[Ricochet]

Looking at your photos I think you have gone wrong in step 1. I'll define "left" as towards the tube opening, "right" as towards the primary and then "up" and "down" as the directions perpendicular to that axis. Your screws being too short has given you the problem that your secondary is too far to the left but you also looks like it is too far down. In addition I think there is some rotational or tilt error because the secondary photo doesn't look round to me. 

In my opinion the best tool for the secondary is a sight tube or combined Cheshire and sight tube. With this you can set the sight tube at the right height in the focuser so that the circular bottom edge appears almost exactly the same size as the secondary. This makes judging the position and shape of the secondary much easier. 

I would completely avoid your step 2. Taking the primary out to look up at the secondary seems madness. Collimation of the secondary can be done with the Cheshire with a final check that the laser is hitting the centre of the primary. 

Additionally, in your photos I can see a bright crescent of reflected light off the bevelled edge of your secondary. I would suggest blackening this. 

Finally, something in my memory says you've got a bresser dob. If this is correct and you still use the original 2" eyepiece clamp make sure you collimate the telescope using the original 1.25"-2" adaptor or another adaptor without an undercut. The low profile nature of the 2" clamp is prone to tilting anything with an undercut which will throw off your collimation. 

[billyharris72]

Thanks Ricochet, some good advice there. To be honest i have always struggled with the secondary on this scope, as the holder does not respond in a predictable way to the screws being moved - often wonder why they don't use a countersprung or push-pull design, which i think would make the whole assembly behave better.

I'm hoping that getting the screws fixed will make the rest easier - for example, the rotational error may have been introduced or exacerbated in step 3 (if the secondary is out of position it will need to at something other than 45 degrees for the laser to hit the primary spot).

16 hours ago, Ricochet said:

I would completely avoid your step 2. Taking the primary out to look up at the secondary seems madness.

Neither would I - took it out to clean it, and once I'd done that I thought I may as well give the whole scope a once over. Agree that removing the primary to collimate the secondary is unnecessary.

16 hours ago, Ricochet said:

Finally, something in my memory says you've got a bresser dob.

You have a good memory. Already learned that lesson. I have a couple of adaptors but the one that came with the scope is the only one that will keep the laser steady as I rotate it. Now I know why!

Billy.

 

[Ricochet]

1 hour ago, billyharris72 said:

Thanks Ricochet, some good advice there. To be honest i have always struggled with the secondary on this scope, as the holder does not respond in a predictable way to the screws being moved - often wonder why they don't use a countersprung or push-pull design, which i think would make the whole assembly behave better.

I'm hoping that getting the screws fixed will make the rest easier - for example, the rotational error may have been introduced or exacerbated in step 3 (if the secondary is out of position it will need to at something other than 45 degrees for the laser to hit the primary spot).

I actually did mine yesterday too as I needed to remove the mirrors to move my finder shoe to a new position. I backed off the three outer screws and then put my left hand in the tube to hold the secondary stalk to hold the mirror in roughly the correct position, and then tightened each screw until it made contact. After that, there wasn't so far to adjust using the loosen-tighten-loosen-tighten process you have to go through. I did notice though that sometimes loosening a bolt would make the mirror appear to shift the wrong way but that then tightening one of the other screws would show that it had in fact been the right one to loosen (if that makes sense).

1 hour ago, billyharris72 said:

You have a good memory. Already learned that lesson. I have a couple of adaptors but the one that came with the scope is the only one that will keep the laser steady as I rotate it. Now I know why!

 

My solution was to replace the 2" clamp with a Clicklock. Fortunately, Baader were also able to make me an adaptor ring to convert from the M68 x 0.75mm thread on the Hexafoc to the M68 x 1mm thread on one of their Clicklocks. It's a bit shorter than the original clamp + extension so monoviewing is near the end of focuser travel (in fact my 2" Nirvana needed a bit more travel so I used parfocal rings as spacers), but I gambled that leaving it shorter might mean I could reach focus with a binoviewer (turns out I can) or add a Baader UFC to the chain.

[25585]

Hi Ricochet, I have a Bresser Messier 10 inch F5 dob..

Do you consider the OE focuser good enough for laser collimator precision generally?

As for eyepieces, I need to wear spectacles, so could you recommend the best type/design of collimator for me to use please?

 

[Ricochet]

1 hour ago, 25585 said:

Hi Ricochet, I have a Bresser Messier 10 inch F5 dob..

Do you consider the OE focuser good enough for laser collimator precision generally?

As for eyepieces, I need to wear spectacles, so could you recommend the best type/design of collimator for me to use please?

 

The focuser itself is very good I think. It's very solid and there's no play in it. The only problem is with the choice of eyepiece clamp "height" which doesn't play well with undercuts. If you use a smooth sided adaptor or 2" collimator I think it would be fine with the laser. 

With regards to collimation devices I think that the sight tube is always going to be the best option for centring the secondary under the focuser. I use an FLO Cheshire/sight tube and I can just about use it with my glasses on, however, it is aluminium and this forms a little ridge around the top edge that could potentially scratch glasses. In this case a plastic moulded sight tube might be safer or perhaps if you have flocked your telescope and have a bit left over you could stick a patch on top of the sight tube for protection. 

Aligning the secondary with the primary could then be done either using the sight tube or well collimated laser (remembering to only use smooth sided devices) depending on how easy you find using the sight tube with glasses. Primary alignment I always do with a barlowed laser as you can see it from the back of the scope providing your laser has a 45° reflecting face. You can then check primary alignment with a simple collimation cap.